Google Git Google Git
Sign in
llvm/llvm-project/a257e2aa4eb47ad340915136228cd28d918e8dc2/./llvm/lib/Transforms/Utils/SimplifyCFG.cpp
blob: 5bfd06affdb1feb4840a2e4ed458cf6939dd5cea [file]
//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// Peephole optimize the CFG.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/ConstantRange.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfo.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/NoFolder.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/ProfDataUtils.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/KnownBits.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LockstepReverseIterator.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <climits>
#include <cmath>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <map>
#include <optional>
#include <set>
#include <tuple>
#include <utility>
#include <vector>
using namespace llvm;
using namespace PatternMatch;
#define DEBUG_TYPE "simplifycfg"
namespace llvm {
cl::opt<bool> RequireAndPreserveDomTree(
"simplifycfg-require-and-preserve-domtree", cl::Hidden,
cl::desc(
"Temporary development switch used to gradually uplift SimplifyCFG "
"into preserving DomTree,"));
// Chosen as 2 so as to be cheap, but still to have enough power to fold
// a select, so the "clamp" idiom (of a min followed by a max) will be caught.
// To catch this, we need to fold a compare and a select, hence '2' being the
// minimum reasonable default.
static cl::opt<unsigned> PHINodeFoldingThreshold(
"phi-node-folding-threshold", cl::Hidden, cl::init(2),
cl::desc(
"Control the amount of phi node folding to perform (default = 2)"));
static cl::opt<unsigned> TwoEntryPHINodeFoldingThreshold(
"two-entry-phi-node-folding-threshold", cl::Hidden, cl::init(4),
cl::desc("Control the maximal total instruction cost that we are willing "
"to speculatively execute to fold a 2-entry PHI node into a "
"select (default = 4)"));
static cl::opt<bool>
HoistCommon("simplifycfg-hoist-common", cl::Hidden, cl::init(true),
cl::desc("Hoist common instructions up to the parent block"));
static cl::opt<bool> HoistLoadsWithCondFaulting(
"simplifycfg-hoist-loads-with-cond-faulting", cl::Hidden, cl::init(true),
cl::desc("Hoist loads if the target supports conditional faulting"));
static cl::opt<bool> HoistStoresWithCondFaulting(
"simplifycfg-hoist-stores-with-cond-faulting", cl::Hidden, cl::init(true),
cl::desc("Hoist stores if the target supports conditional faulting"));
static cl::opt<unsigned> HoistLoadsStoresWithCondFaultingThreshold(
"hoist-loads-stores-with-cond-faulting-threshold", cl::Hidden, cl::init(6),
cl::desc("Control the maximal conditional load/store that we are willing "
"to speculatively execute to eliminate conditional branch "
"(default = 6)"));
static cl::opt<unsigned>
HoistCommonSkipLimit("simplifycfg-hoist-common-skip-limit", cl::Hidden,
cl::init(20),
cl::desc("Allow reordering across at most this many "
"instructions when hoisting"));
static cl::opt<bool>
SinkCommon("simplifycfg-sink-common", cl::Hidden, cl::init(true),
cl::desc("Sink common instructions down to the end block"));
static cl::opt<bool> HoistCondStores(
"simplifycfg-hoist-cond-stores", cl::Hidden, cl::init(true),
cl::desc("Hoist conditional stores if an unconditional store precedes"));
static cl::opt<bool> MergeCondStores(
"simplifycfg-merge-cond-stores", cl::Hidden, cl::init(true),
cl::desc("Hoist conditional stores even if an unconditional store does not "
"precede - hoist multiple conditional stores into a single "
"predicated store"));
static cl::opt<bool> MergeCondStoresAggressively(
"simplifycfg-merge-cond-stores-aggressively", cl::Hidden, cl::init(false),
cl::desc("When merging conditional stores, do so even if the resultant "
"basic blocks are unlikely to be if-converted as a result"));
static cl::opt<bool> SpeculateOneExpensiveInst(
"speculate-one-expensive-inst", cl::Hidden, cl::init(true),
cl::desc("Allow exactly one expensive instruction to be speculatively "
"executed"));
static cl::opt<unsigned> MaxSpeculationDepth(
"max-speculation-depth", cl::Hidden, cl::init(10),
cl::desc("Limit maximum recursion depth when calculating costs of "
"speculatively executed instructions"));
static cl::opt<int>
MaxSmallBlockSize("simplifycfg-max-small-block-size", cl::Hidden,
cl::init(10),
cl::desc("Max size of a block which is still considered "
"small enough to thread through"));
// Two is chosen to allow one negation and a logical combine.
static cl::opt<unsigned>
BranchFoldThreshold("simplifycfg-branch-fold-threshold", cl::Hidden,
cl::init(2),
cl::desc("Maximum cost of combining conditions when "
"folding branches"));
static cl::opt<unsigned> BranchFoldToCommonDestVectorMultiplier(
"simplifycfg-branch-fold-common-dest-vector-multiplier", cl::Hidden,
cl::init(2),
cl::desc("Multiplier to apply to threshold when determining whether or not "
"to fold branch to common destination when vector operations are "
"present"));
static cl::opt<bool> EnableMergeCompatibleInvokes(
"simplifycfg-merge-compatible-invokes", cl::Hidden, cl::init(true),
cl::desc("Allow SimplifyCFG to merge invokes together when appropriate"));
static cl::opt<unsigned> MaxSwitchCasesPerResult(
"max-switch-cases-per-result", cl::Hidden, cl::init(16),
cl::desc("Limit cases to analyze when converting a switch to select"));
extern cl::opt<bool> ProfcheckDisableMetadataFixes;
} // end namespace llvm
STATISTIC(NumBitMaps, "Number of switch instructions turned into bitmaps");
STATISTIC(NumLinearMaps,
"Number of switch instructions turned into linear mapping");
STATISTIC(NumLookupTables,
"Number of switch instructions turned into lookup tables");
STATISTIC(
NumLookupTablesHoles,
"Number of switch instructions turned into lookup tables (holes checked)");
STATISTIC(NumTableCmpReuses, "Number of reused switch table lookup compares");
STATISTIC(NumFoldValueComparisonIntoPredecessors,
"Number of value comparisons folded into predecessor basic blocks");
STATISTIC(NumFoldBranchToCommonDest,
"Number of branches folded into predecessor basic block");
STATISTIC(
NumHoistCommonCode,
"Number of common instruction 'blocks' hoisted up to the begin block");
STATISTIC(NumHoistCommonInstrs,
"Number of common instructions hoisted up to the begin block");
STATISTIC(NumSinkCommonCode,
"Number of common instruction 'blocks' sunk down to the end block");
STATISTIC(NumSinkCommonInstrs,
"Number of common instructions sunk down to the end block");
STATISTIC(NumSpeculations, "Number of speculative executed instructions");
STATISTIC(NumInvokes,
"Number of invokes with empty resume blocks simplified into calls");
STATISTIC(NumInvokesMerged, "Number of invokes that were merged together");
STATISTIC(NumInvokeSetsFormed, "Number of invoke sets that were formed");
namespace {
// The first field contains the value that the switch produces when a certain
// case group is selected, and the second field is a vector containing the
// cases composing the case group.
using SwitchCaseResultVectorTy =
SmallVector<std::pair<Constant *, SmallVector<ConstantInt *, 4>>, 2>;
// The first field contains the phi node that generates a result of the switch
// and the second field contains the value generated for a certain case in the
// switch for that PHI.
using SwitchCaseResultsTy = SmallVector<std::pair<PHINode *, Constant *>, 4>;
/// ValueEqualityComparisonCase - Represents a case of a switch.
struct ValueEqualityComparisonCase {
ConstantInt *Value;
BasicBlock *Dest;
ValueEqualityComparisonCase(ConstantInt *Value, BasicBlock *Dest)
: Value(Value), Dest(Dest) {}
bool operator<(ValueEqualityComparisonCase RHS) const {
// Comparing pointers is ok as we only rely on the order for uniquing.
return Value < RHS.Value;
}
bool operator==(BasicBlock *RHSDest) const { return Dest == RHSDest; }
};
class SimplifyCFGOpt {
const TargetTransformInfo &TTI;
DomTreeUpdater *DTU;
const DataLayout &DL;
ArrayRef<WeakVH> LoopHeaders;
const SimplifyCFGOptions &Options;
bool Resimplify;
Value *isValueEqualityComparison(Instruction *TI);
BasicBlock *getValueEqualityComparisonCases(
Instruction *TI, std::vector<ValueEqualityComparisonCase> &Cases);
bool simplifyEqualityComparisonWithOnlyPredecessor(Instruction *TI,
BasicBlock *Pred,
IRBuilder<> &Builder);
bool performValueComparisonIntoPredecessorFolding(Instruction *TI, Value *&CV,
Instruction *PTI,
IRBuilder<> &Builder);
bool foldValueComparisonIntoPredecessors(Instruction *TI,
IRBuilder<> &Builder);
bool simplifyResume(ResumeInst *RI, IRBuilder<> &Builder);
bool simplifySingleResume(ResumeInst *RI);
bool simplifyCommonResume(ResumeInst *RI);
bool simplifyCleanupReturn(CleanupReturnInst *RI);
bool simplifyUnreachable(UnreachableInst *UI);
bool simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder);
bool simplifyDuplicateSwitchArms(SwitchInst *SI, DomTreeUpdater *DTU);
bool simplifyIndirectBr(IndirectBrInst *IBI);
bool simplifyUncondBranch(UncondBrInst *BI, IRBuilder<> &Builder);
bool simplifyCondBranch(CondBrInst *BI, IRBuilder<> &Builder);
bool foldCondBranchOnValueKnownInPredecessor(CondBrInst *BI);
bool tryToSimplifyUncondBranchWithICmpInIt(ICmpInst *ICI,
IRBuilder<> &Builder);
bool tryToSimplifyUncondBranchWithICmpSelectInIt(ICmpInst *ICI,
SelectInst *Select,
IRBuilder<> &Builder);
bool hoistCommonCodeFromSuccessors(Instruction *TI, bool AllInstsEqOnly);
bool hoistSuccIdenticalTerminatorToSwitchOrIf(
Instruction *TI, Instruction *I1,
SmallVectorImpl<Instruction *> &OtherSuccTIs,
ArrayRef<BasicBlock *> UniqueSuccessors);
bool speculativelyExecuteBB(CondBrInst *BI, BasicBlock *ThenBB);
bool simplifyTerminatorOnSelect(Instruction *OldTerm, Value *Cond,
BasicBlock *TrueBB, BasicBlock *FalseBB,
uint32_t TrueWeight, uint32_t FalseWeight);
bool simplifyBranchOnICmpChain(CondBrInst *BI, IRBuilder<> &Builder,
const DataLayout &DL);
bool simplifySwitchOnSelect(SwitchInst *SI, SelectInst *Select);
bool simplifyIndirectBrOnSelect(IndirectBrInst *IBI, SelectInst *SI);
bool turnSwitchRangeIntoICmp(SwitchInst *SI, IRBuilder<> &Builder);
bool simplifyDuplicatePredecessors(BasicBlock *Succ, DomTreeUpdater *DTU);
public:
SimplifyCFGOpt(const TargetTransformInfo &TTI, DomTreeUpdater *DTU,
const DataLayout &DL, ArrayRef<WeakVH> LoopHeaders,
const SimplifyCFGOptions &Opts)
: TTI(TTI), DTU(DTU), DL(DL), LoopHeaders(LoopHeaders), Options(Opts) {
assert((!DTU || !DTU->hasPostDomTree()) &&
"SimplifyCFG is not yet capable of maintaining validity of a "
"PostDomTree, so don't ask for it.");
}
bool simplifyOnce(BasicBlock *BB);
bool run(BasicBlock *BB);
// Helper to set Resimplify and return change indication.
bool requestResimplify() {
Resimplify = true;
return true;
}
};
// we synthesize a || b as select a, true, b
// we synthesize a && b as select a, b, false
// this function determines if SI is playing one of those roles.
[[maybe_unused]] bool
isSelectInRoleOfConjunctionOrDisjunction(const SelectInst *SI) {
return ((isa<ConstantInt>(SI->getTrueValue()) &&
(dyn_cast<ConstantInt>(SI->getTrueValue())->isOne())) ||
(isa<ConstantInt>(SI->getFalseValue()) &&
(dyn_cast<ConstantInt>(SI->getFalseValue())->isNullValue())));
}
} // end anonymous namespace
/// Return true if all the PHI nodes in the basic block \p BB
/// receive compatible (identical) incoming values when coming from
/// all of the predecessor blocks that are specified in \p IncomingBlocks.
///
/// Note that if the values aren't exactly identical, but \p EquivalenceSet
/// is provided, and *both* of the values are present in the set,
/// then they are considered equal.
static bool incomingValuesAreCompatible(
BasicBlock *BB, ArrayRef<BasicBlock *> IncomingBlocks,
SmallPtrSetImpl<Value *> *EquivalenceSet = nullptr) {
assert(IncomingBlocks.size() == 2 &&
"Only for a pair of incoming blocks at the time!");
// FIXME: it is okay if one of the incoming values is an `undef` value,
// iff the other incoming value is guaranteed to be a non-poison value.
// FIXME: it is okay if one of the incoming values is a `poison` value.
return all_of(BB->phis(), [IncomingBlocks, EquivalenceSet](PHINode &PN) {
Value *IV0 = PN.getIncomingValueForBlock(IncomingBlocks[0]);
Value *IV1 = PN.getIncomingValueForBlock(IncomingBlocks[1]);
if (IV0 == IV1)
return true;
if (EquivalenceSet && EquivalenceSet->contains(IV0) &&
EquivalenceSet->contains(IV1))
return true;
return false;
});
}
/// Return true if it is safe to merge these two
/// terminator instructions together.
static bool
safeToMergeTerminators(Instruction *SI1, Instruction *SI2,
SmallSetVector<BasicBlock *, 4> *FailBlocks = nullptr) {
if (SI1 == SI2)
return false; // Can't merge with self!
// It is not safe to merge these two switch instructions if they have a common
// successor, and if that successor has a PHI node, and if *that* PHI node has
// conflicting incoming values from the two switch blocks.
BasicBlock *SI1BB = SI1->getParent();
BasicBlock *SI2BB = SI2->getParent();
SmallPtrSet<BasicBlock *, 16> SI1Succs(llvm::from_range, successors(SI1BB));
bool Fail = false;
for (BasicBlock *Succ : successors(SI2BB)) {
if (!SI1Succs.count(Succ))
continue;
if (incomingValuesAreCompatible(Succ, {SI1BB, SI2BB}))
continue;
Fail = true;
if (FailBlocks)
FailBlocks->insert(Succ);
else
break;
}
return !Fail;
}
/// Update PHI nodes in Succ to indicate that there will now be entries in it
/// from the 'NewPred' block. The values that will be flowing into the PHI nodes
/// will be the same as those coming in from ExistPred, an existing predecessor
/// of Succ.
static void addPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
BasicBlock *ExistPred,
MemorySSAUpdater *MSSAU = nullptr) {
for (PHINode &PN : Succ->phis())
PN.addIncoming(PN.getIncomingValueForBlock(ExistPred), NewPred);
if (MSSAU)
if (auto *MPhi = MSSAU->getMemorySSA()->getMemoryAccess(Succ))
MPhi->addIncoming(MPhi->getIncomingValueForBlock(ExistPred), NewPred);
}
/// Compute an abstract "cost" of speculating the given instruction,
/// which is assumed to be safe to speculate. TCC_Free means cheap,
/// TCC_Basic means less cheap, and TCC_Expensive means prohibitively
/// expensive.
static InstructionCost computeSpeculationCost(const User *I,
const TargetTransformInfo &TTI) {
return TTI.getInstructionCost(I, TargetTransformInfo::TCK_SizeAndLatency);
}
/// If we have a merge point of an "if condition" as accepted above,
/// return true if the specified value dominates the block. We don't handle
/// the true generality of domination here, just a special case which works
/// well enough for us.
///
/// If AggressiveInsts is non-null, and if V does not dominate BB, we check to
/// see if V (which must be an instruction) and its recursive operands
/// that do not dominate BB have a combined cost lower than Budget and
/// are non-trapping. If both are true, the instruction is inserted into the
/// set and true is returned.
///
/// The cost for most non-trapping instructions is defined as 1 except for
/// Select whose cost is 2.
///
/// After this function returns, Cost is increased by the cost of
/// V plus its non-dominating operands. If that cost is greater than
/// Budget, false is returned and Cost is undefined.
static bool dominatesMergePoint(
Value *V, BasicBlock *BB, Instruction *InsertPt,
SmallPtrSetImpl<Instruction *> &AggressiveInsts, InstructionCost &Cost,
InstructionCost Budget, const TargetTransformInfo &TTI, AssumptionCache *AC,
SmallPtrSetImpl<Instruction *> &ZeroCostInstructions, unsigned Depth = 0) {
// It is possible to hit a zero-cost cycle (phi/gep instructions for example),
// so limit the recursion depth.
// TODO: While this recursion limit does prevent pathological behavior, it
// would be better to track visited instructions to avoid cycles.
if (Depth == MaxSpeculationDepth)
return false;
Instruction *I = dyn_cast<Instruction>(V);
if (!I) {
// Non-instructions dominate all instructions and can be executed
// unconditionally.
return true;
}
BasicBlock *PBB = I->getParent();
// We don't want to allow weird loops that might have the "if condition" in
// the bottom of this block.
if (PBB == BB)
return false;
// If this instruction is defined in a block that contains an unconditional
// branch to BB, then it must be in the 'conditional' part of the "if
// statement". If not, it definitely dominates the region.
UncondBrInst *BI = dyn_cast<UncondBrInst>(PBB->getTerminator());
if (!BI || BI->getSuccessor() != BB)
return true;
// If we have seen this instruction before, don't count it again.
if (AggressiveInsts.count(I))
return true;
// Okay, it looks like the instruction IS in the "condition". Check to
// see if it's a cheap instruction to unconditionally compute, and if it
// only uses stuff defined outside of the condition. If so, hoist it out.
if (!isSafeToSpeculativelyExecute(I, InsertPt, AC))
return false;
// Overflow arithmetic instruction plus extract value are usually generated
// when a division is being replaced. But, in this case, the zero check may
// still be kept in the code. In that case it would be worth to hoist these
// two instruction out of the basic block. Let's treat this pattern as one
// single cheap instruction here!
WithOverflowInst *OverflowInst;
if (match(I, m_ExtractValue<1>(m_OneUse(m_WithOverflowInst(OverflowInst))))) {
ZeroCostInstructions.insert(OverflowInst);
Cost += 1;
} else if (!ZeroCostInstructions.contains(I))
Cost += computeSpeculationCost(I, TTI);
// Allow exactly one instruction to be speculated regardless of its cost
// (as long as it is safe to do so).
// This is intended to flatten the CFG even if the instruction is a division
// or other expensive operation. The speculation of an expensive instruction
// is expected to be undone in CodeGenPrepare if the speculation has not
// enabled further IR optimizations.
if (Cost > Budget &&
(!SpeculateOneExpensiveInst || !AggressiveInsts.empty() || Depth > 0 ||
!Cost.isValid()))
return false;
// Okay, we can only really hoist these out if their operands do
// not take us over the cost threshold.
for (Use &Op : I->operands())
if (!dominatesMergePoint(Op, BB, InsertPt, AggressiveInsts, Cost, Budget,
TTI, AC, ZeroCostInstructions, Depth + 1))
return false;
// Okay, it's safe to do this! Remember this instruction.
AggressiveInsts.insert(I);
return true;
}
/// Extract ConstantInt from value, looking through IntToPtr
/// and PointerNullValue. Return NULL if value is not a constant int.
static ConstantInt *getConstantInt(Value *V, const DataLayout &DL) {
// Normal constant int.
ConstantInt *CI = dyn_cast<ConstantInt>(V);
if (CI || !isa<Constant>(V) || !V->getType()->isPointerTy())
return CI;
// It is not safe to look through inttoptr or ptrtoint when using unstable
// pointer types.
if (DL.hasUnstableRepresentation(V->getType()))
return nullptr;
// This is some kind of pointer constant. Turn it into a pointer-sized
// ConstantInt if possible.
IntegerType *IntPtrTy = cast<IntegerType>(DL.getIntPtrType(V->getType()));
// Null pointer means 0, see SelectionDAGBuilder::getValue(const Value*).
if (isa<ConstantPointerNull>(V))
return ConstantInt::get(IntPtrTy, 0);
// IntToPtr const int, we can look through this if the semantics of
// inttoptr for this address space are a simple (truncating) bitcast.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
if (CE->getOpcode() == Instruction::IntToPtr)
if (ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(0))) {
// The constant is very likely to have the right type already.
if (CI->getType() == IntPtrTy)
return CI;
else
return cast<ConstantInt>(
ConstantFoldIntegerCast(CI, IntPtrTy, /*isSigned=*/false, DL));
}
return nullptr;
}
namespace {
/// Given a chain of or (||) or and (&&) comparison of a value against a
/// constant, this will try to recover the information required for a switch
/// structure.
/// It will depth-first traverse the chain of comparison, seeking for patterns
/// like %a == 12 or %a < 4 and combine them to produce a set of integer
/// representing the different cases for the switch.
/// Note that if the chain is composed of '||' it will build the set of elements
/// that matches the comparisons (i.e. any of this value validate the chain)
/// while for a chain of '&&' it will build the set elements that make the test
/// fail.
struct ConstantComparesGatherer {
const DataLayout &DL;
/// Value found for the switch comparison
Value *CompValue = nullptr;
/// Extra clause to be checked before the switch
Value *Extra = nullptr;
/// Set of integers to match in switch
SmallVector<ConstantInt *, 8> Vals;
/// Number of comparisons matched in the and/or chain
unsigned UsedICmps = 0;
/// If the elements in Vals matches the comparisons
bool IsEq = false;
// Used to check if the first matched CompValue shall be the Extra check.
bool IgnoreFirstMatch = false;
bool MultipleMatches = false;
/// Construct and compute the result for the comparison instruction Cond
ConstantComparesGatherer(Instruction *Cond, const DataLayout &DL) : DL(DL) {
gather(Cond);
if (CompValue || !MultipleMatches)
return;
Extra = nullptr;
Vals.clear();
UsedICmps = 0;
IgnoreFirstMatch = true;
gather(Cond);
}
ConstantComparesGatherer(const ConstantComparesGatherer &) = delete;
ConstantComparesGatherer &
operator=(const ConstantComparesGatherer &) = delete;
private:
/// Try to set the current value used for the comparison, it succeeds only if
/// it wasn't set before or if the new value is the same as the old one
bool setValueOnce(Value *NewVal) {
if (IgnoreFirstMatch) {
IgnoreFirstMatch = false;
return false;
}
if (CompValue && CompValue != NewVal) {
MultipleMatches = true;
return false;
}
CompValue = NewVal;
return true;
}
/// Try to match Instruction "I" as a comparison against a constant and
/// populates the array Vals with the set of values that match (or do not
/// match depending on isEQ).
/// Return false on failure. On success, the Value the comparison matched
/// against is placed in CompValue.
/// If CompValue is already set, the function is expected to fail if a match
/// is found but the value compared to is different.
bool matchInstruction(Instruction *I, bool isEQ) {
if (match(I, m_Not(m_Instruction(I))))
isEQ = !isEQ;
Value *Val;
if (match(I, m_NUWTrunc(m_Value(Val)))) {
// If we already have a value for the switch, it has to match!
if (!setValueOnce(Val))
return false;
UsedICmps++;
Vals.push_back(ConstantInt::get(cast<IntegerType>(Val->getType()), isEQ));
return true;
}
// If this is an icmp against a constant, handle this as one of the cases.
ICmpInst *ICI;
ConstantInt *C;
if (!((ICI = dyn_cast<ICmpInst>(I)) &&
(C = getConstantInt(I->getOperand(1), DL)))) {
return false;
}
Value *RHSVal;
const APInt *RHSC;
// Pattern match a special case
// (x & ~2^z) == y --> x == y || x == y|2^z
// This undoes a transformation done by instcombine to fuse 2 compares.
if (ICI->getPredicate() == (isEQ ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE)) {
// It's a little bit hard to see why the following transformations are
// correct. Here is a CVC3 program to verify them for 64-bit values:
/*
ONE : BITVECTOR(64) = BVZEROEXTEND(0bin1, 63);
x : BITVECTOR(64);
y : BITVECTOR(64);
z : BITVECTOR(64);
mask : BITVECTOR(64) = BVSHL(ONE, z);
QUERY( (y & ~mask = y) =>
((x & ~mask = y) <=> (x = y OR x = (y | mask)))
);
QUERY( (y | mask = y) =>
((x | mask = y) <=> (x = y OR x = (y & ~mask)))
);
*/
// Please note that each pattern must be a dual implication (<--> or
// iff). One directional implication can create spurious matches. If the
// implication is only one-way, an unsatisfiable condition on the left
// side can imply a satisfiable condition on the right side. Dual
// implication ensures that satisfiable conditions are transformed to
// other satisfiable conditions and unsatisfiable conditions are
// transformed to other unsatisfiable conditions.
// Here is a concrete example of a unsatisfiable condition on the left
// implying a satisfiable condition on the right:
//
// mask = (1 << z)
// (x & ~mask) == y --> (x == y || x == (y | mask))
//
// Substituting y = 3, z = 0 yields:
// (x & -2) == 3 --> (x == 3 || x == 2)
// Pattern match a special case:
/*
QUERY( (y & ~mask = y) =>
((x & ~mask = y) <=> (x = y OR x = (y | mask)))
);
*/
if (match(ICI->getOperand(0),
m_And(m_Value(RHSVal), m_APInt(RHSC)))) {
APInt Mask = ~*RHSC;
if (Mask.isPowerOf2() && (C->getValue() & ~Mask) == C->getValue()) {
// If we already have a value for the switch, it has to match!
if (!setValueOnce(RHSVal))
return false;
Vals.push_back(C);
Vals.push_back(
ConstantInt::get(C->getContext(),
C->getValue() | Mask));
UsedICmps++;
return true;
}
}
// Pattern match a special case:
/*
QUERY( (y | mask = y) =>
((x | mask = y) <=> (x = y OR x = (y & ~mask)))
);
*/
if (match(ICI->getOperand(0),
m_Or(m_Value(RHSVal), m_APInt(RHSC)))) {
APInt Mask = *RHSC;
if (Mask.isPowerOf2() && (C->getValue() | Mask) == C->getValue()) {
// If we already have a value for the switch, it has to match!
if (!setValueOnce(RHSVal))
return false;
Vals.push_back(C);
Vals.push_back(ConstantInt::get(C->getContext(),
C->getValue() & ~Mask));
UsedICmps++;
return true;
}
}
// If we already have a value for the switch, it has to match!
if (!setValueOnce(ICI->getOperand(0)))
return false;
UsedICmps++;
Vals.push_back(C);
return true;
}
// If we have "x ult 3", for example, then we can add 0,1,2 to the set.
ConstantRange Span =
ConstantRange::makeExactICmpRegion(ICI->getPredicate(), C->getValue());
// Shift the range if the compare is fed by an add. This is the range
// compare idiom as emitted by instcombine.
Value *CandidateVal = I->getOperand(0);
if (match(I->getOperand(0), m_Add(m_Value(RHSVal), m_APInt(RHSC)))) {
Span = Span.subtract(*RHSC);
CandidateVal = RHSVal;
}
// If this is an and/!= check, then we are looking to build the set of
// value that *don't* pass the and chain. I.e. to turn "x ugt 2" into
// x != 0 && x != 1.
if (!isEQ)
Span = Span.inverse();
// If there are a ton of values, we don't want to make a ginormous switch.
if (Span.isSizeLargerThan(8) || Span.isEmptySet()) {
return false;
}
// If we already have a value for the switch, it has to match!
if (!setValueOnce(CandidateVal))
return false;
// Add all values from the range to the set
APInt Tmp = Span.getLower();
do
Vals.push_back(ConstantInt::get(I->getContext(), Tmp));
while (++Tmp != Span.getUpper());
UsedICmps++;
return true;
}
/// Given a potentially 'or'd or 'and'd together collection of icmp
/// eq/ne/lt/gt instructions that compare a value against a constant, extract
/// the value being compared, and stick the list constants into the Vals
/// vector.
/// One "Extra" case is allowed to differ from the other.
void gather(Value *V) {
Value *Op0, *Op1;
if (match(V, m_LogicalOr(m_Value(Op0), m_Value(Op1))))
IsEq = true;
else if (match(V, m_LogicalAnd(m_Value(Op0), m_Value(Op1))))
IsEq = false;
else
return;
// Keep a stack (SmallVector for efficiency) for depth-first traversal
SmallVector<Value *, 8> DFT{Op0, Op1};
SmallPtrSet<Value *, 8> Visited{V, Op0, Op1};
while (!DFT.empty()) {
V = DFT.pop_back_val();
if (Instruction *I = dyn_cast<Instruction>(V)) {
// If it is a || (or && depending on isEQ), process the operands.
if (IsEq ? match(I, m_LogicalOr(m_Value(Op0), m_Value(Op1)))
: match(I, m_LogicalAnd(m_Value(Op0), m_Value(Op1)))) {
if (Visited.insert(Op1).second)
DFT.push_back(Op1);
if (Visited.insert(Op0).second)
DFT.push_back(Op0);
continue;
}
// Try to match the current instruction
if (matchInstruction(I, IsEq))
// Match succeed, continue the loop
continue;
}
// One element of the sequence of || (or &&) could not be match as a
// comparison against the same value as the others.
// We allow only one "Extra" case to be checked before the switch
if (!Extra) {
Extra = V;
continue;
}
// Failed to parse a proper sequence, abort now
CompValue = nullptr;
break;
}
}
};
} // end anonymous namespace
static void eraseTerminatorAndDCECond(Instruction *TI,
MemorySSAUpdater *MSSAU = nullptr) {
Instruction *Cond = nullptr;
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Cond = dyn_cast<Instruction>(SI->getCondition());
} else if (CondBrInst *BI = dyn_cast<CondBrInst>(TI)) {
Cond = dyn_cast<Instruction>(BI->getCondition());
} else if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(TI)) {
Cond = dyn_cast<Instruction>(IBI->getAddress());
}
TI->eraseFromParent();
if (Cond)
RecursivelyDeleteTriviallyDeadInstructions(Cond, nullptr, MSSAU);
}
/// Return true if the specified terminator checks
/// to see if a value is equal to constant integer value.
Value *SimplifyCFGOpt::isValueEqualityComparison(Instruction *TI) {
Value *CV = nullptr;
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
// Do not permit merging of large switch instructions into their
// predecessors unless there is only one predecessor.
if (!SI->getParent()->hasNPredecessorsOrMore(128 / SI->getNumSuccessors()))
CV = SI->getCondition();
} else if (CondBrInst *BI = dyn_cast<CondBrInst>(TI))
if (BI->getCondition()->hasOneUse()) {
if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
if (ICI->isEquality() && getConstantInt(ICI->getOperand(1), DL))
CV = ICI->getOperand(0);
} else if (auto *Trunc = dyn_cast<TruncInst>(BI->getCondition())) {
if (Trunc->hasNoUnsignedWrap())
CV = Trunc->getOperand(0);
}
}
// Unwrap any lossless ptrtoint cast (except for unstable pointers).
if (CV) {
if (PtrToIntInst *PTII = dyn_cast<PtrToIntInst>(CV)) {
Value *Ptr = PTII->getPointerOperand();
if (DL.hasUnstableRepresentation(Ptr->getType()))
return CV;
if (PTII->getType() == DL.getIntPtrType(Ptr->getType()))
CV = Ptr;
}
}
return CV;
}
/// Given a value comparison instruction,
/// decode all of the 'cases' that it represents and return the 'default' block.
BasicBlock *SimplifyCFGOpt::getValueEqualityComparisonCases(
Instruction *TI, std::vector<ValueEqualityComparisonCase> &Cases) {
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Cases.reserve(SI->getNumCases());
for (auto Case : SI->cases())
Cases.push_back(ValueEqualityComparisonCase(Case.getCaseValue(),
Case.getCaseSuccessor()));
return SI->getDefaultDest();
}
CondBrInst *BI = cast<CondBrInst>(TI);
Value *Cond = BI->getCondition();
ICmpInst::Predicate Pred;
ConstantInt *C;
if (auto *ICI = dyn_cast<ICmpInst>(Cond)) {
Pred = ICI->getPredicate();
C = getConstantInt(ICI->getOperand(1), DL);
} else {
Pred = ICmpInst::ICMP_NE;
auto *Trunc = cast<TruncInst>(Cond);
C = ConstantInt::get(cast<IntegerType>(Trunc->getOperand(0)->getType()), 0);
}
BasicBlock *Succ = BI->getSuccessor(Pred == ICmpInst::ICMP_NE);
Cases.push_back(ValueEqualityComparisonCase(C, Succ));
return BI->getSuccessor(Pred == ICmpInst::ICMP_EQ);
}
/// Given a vector of bb/value pairs, remove any entries
/// in the list that match the specified block.
static void
eliminateBlockCases(BasicBlock *BB,
std::vector<ValueEqualityComparisonCase> &Cases) {
llvm::erase(Cases, BB);
}
/// Return true if there are any keys in C1 that exist in C2 as well.
static bool valuesOverlap(std::vector<ValueEqualityComparisonCase> &C1,
std::vector<ValueEqualityComparisonCase> &C2) {
std::vector<ValueEqualityComparisonCase> *V1 = &C1, *V2 = &C2;
// Make V1 be smaller than V2.
if (V1->size() > V2->size())
std::swap(V1, V2);
if (V1->empty())
return false;
if (V1->size() == 1) {
// Just scan V2.
ConstantInt *TheVal = (*V1)[0].Value;
for (const ValueEqualityComparisonCase &VECC : *V2)
if (TheVal == VECC.Value)
return true;
}
// Otherwise, just sort both lists and compare element by element.
array_pod_sort(V1->begin(), V1->end());
array_pod_sort(V2->begin(), V2->end());
unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
while (i1 != e1 && i2 != e2) {
if ((*V1)[i1].Value == (*V2)[i2].Value)
return true;
if ((*V1)[i1].Value < (*V2)[i2].Value)
++i1;
else
++i2;
}
return false;
}
/// If TI is known to be a terminator instruction and its block is known to
/// only have a single predecessor block, check to see if that predecessor is
/// also a value comparison with the same value, and if that comparison
/// determines the outcome of this comparison. If so, simplify TI. This does a
/// very limited form of jump threading.
bool SimplifyCFGOpt::simplifyEqualityComparisonWithOnlyPredecessor(
Instruction *TI, BasicBlock *Pred, IRBuilder<> &Builder) {
Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
if (!PredVal)
return false; // Not a value comparison in predecessor.
Value *ThisVal = isValueEqualityComparison(TI);
assert(ThisVal && "This isn't a value comparison!!");
if (ThisVal != PredVal)
return false; // Different predicates.
// TODO: Preserve branch weight metadata, similarly to how
// foldValueComparisonIntoPredecessors preserves it.
// Find out information about when control will move from Pred to TI's block.
std::vector<ValueEqualityComparisonCase> PredCases;
BasicBlock *PredDef =
getValueEqualityComparisonCases(Pred->getTerminator(), PredCases);
eliminateBlockCases(PredDef, PredCases); // Remove default from cases.
// Find information about how control leaves this block.
std::vector<ValueEqualityComparisonCase> ThisCases;
BasicBlock *ThisDef = getValueEqualityComparisonCases(TI, ThisCases);
eliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
// If TI's block is the default block from Pred's comparison, potentially
// simplify TI based on this knowledge.
if (PredDef == TI->getParent()) {
// If we are here, we know that the value is none of those cases listed in
// PredCases. If there are any cases in ThisCases that are in PredCases, we
// can simplify TI.
if (!valuesOverlap(PredCases, ThisCases))
return false;
if (isa<CondBrInst>(TI)) {
// Okay, one of the successors of this condbr is dead. Convert it to a
// uncond br.
assert(ThisCases.size() == 1 && "Branch can only have one case!");
// Insert the new branch.
Instruction *NI = Builder.CreateBr(ThisDef);
(void)NI;
// Remove PHI node entries for the dead edge.
ThisCases[0].Dest->removePredecessor(PredDef);
LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI << "Leaving: " << *NI
<< "\n");
eraseTerminatorAndDCECond(TI);
if (DTU)
DTU->applyUpdates(
{{DominatorTree::Delete, PredDef, ThisCases[0].Dest}});
return true;
}
SwitchInstProfUpdateWrapper SI = *cast<SwitchInst>(TI);
// Okay, TI has cases that are statically dead, prune them away.
SmallPtrSet<Constant *, 16> DeadCases;
for (const ValueEqualityComparisonCase &Case : PredCases)
DeadCases.insert(Case.Value);
LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI);
SmallDenseMap<BasicBlock *, int, 8> NumPerSuccessorCases;
for (SwitchInst::CaseIt i = SI->case_end(), e = SI->case_begin(); i != e;) {
--i;
auto *Successor = i->getCaseSuccessor();
if (DTU)
++NumPerSuccessorCases[Successor];
if (DeadCases.count(i->getCaseValue())) {
Successor->removePredecessor(PredDef);
SI.removeCase(i);
if (DTU)
--NumPerSuccessorCases[Successor];
}
}
if (DTU) {
std::vector<DominatorTree::UpdateType> Updates;
for (const std::pair<BasicBlock *, int> &I : NumPerSuccessorCases)
if (I.second == 0)
Updates.push_back({DominatorTree::Delete, PredDef, I.first});
DTU->applyUpdates(Updates);
}
LLVM_DEBUG(dbgs() << "Leaving: " << *TI << "\n");
return true;
}
// Otherwise, TI's block must correspond to some matched value. Find out
// which value (or set of values) this is.
ConstantInt *TIV = nullptr;
BasicBlock *TIBB = TI->getParent();
for (const auto &[Value, Dest] : PredCases)
if (Dest == TIBB) {
if (TIV)
return false; // Cannot handle multiple values coming to this block.
TIV = Value;
}
assert(TIV && "No edge from pred to succ?");
// Okay, we found the one constant that our value can be if we get into TI's
// BB. Find out which successor will unconditionally be branched to.
BasicBlock *TheRealDest = nullptr;
for (const auto &[Value, Dest] : ThisCases)
if (Value == TIV) {
TheRealDest = Dest;
break;
}
// If not handled by any explicit cases, it is handled by the default case.
if (!TheRealDest)
TheRealDest = ThisDef;
SmallPtrSet<BasicBlock *, 2> RemovedSuccs;
// Remove PHI node entries for dead edges.
BasicBlock *CheckEdge = TheRealDest;
for (BasicBlock *Succ : successors(TIBB))
if (Succ != CheckEdge) {
if (Succ != TheRealDest)
RemovedSuccs.insert(Succ);
Succ->removePredecessor(TIBB);
} else
CheckEdge = nullptr;
// Insert the new branch.
Instruction *NI = Builder.CreateBr(TheRealDest);
(void)NI;
LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI << "Leaving: " << *NI
<< "\n");
eraseTerminatorAndDCECond(TI);
if (DTU) {
SmallVector<DominatorTree::UpdateType, 2> Updates;
Updates.reserve(RemovedSuccs.size());
for (auto *RemovedSucc : RemovedSuccs)
Updates.push_back({DominatorTree::Delete, TIBB, RemovedSucc});
DTU->applyUpdates(Updates);
}
return true;
}
namespace {
/// This class implements a stable ordering of constant
/// integers that does not depend on their address. This is important for
/// applications that sort ConstantInt's to ensure uniqueness.
struct ConstantIntOrdering {
bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
return LHS->getValue().ult(RHS->getValue());
}
};
} // end anonymous namespace
static int constantIntSortPredicate(ConstantInt *const *P1,
ConstantInt *const *P2) {
const ConstantInt *LHS = *P1;
const ConstantInt *RHS = *P2;
if (LHS == RHS)
return 0;
return LHS->getValue().ult(RHS->getValue()) ? 1 : -1;
}
/// Get Weights of a given terminator, the default weight is at the front
/// of the vector. If TI is a conditional eq, we need to swap the branch-weight
/// metadata.
static void getBranchWeights(Instruction *TI,
SmallVectorImpl<uint64_t> &Weights) {
MDNode *MD = TI->getMetadata(LLVMContext::MD_prof);
assert(MD && "Invalid branch-weight metadata");
extractFromBranchWeightMD64(MD, Weights);
// If TI is a conditional eq, the default case is the false case,
// and the corresponding branch-weight data is at index 2. We swap the
// default weight to be the first entry.
if (CondBrInst *BI = dyn_cast<CondBrInst>(TI)) {
assert(Weights.size() == 2);
auto *ICI = dyn_cast<ICmpInst>(BI->getCondition());
if (!ICI)
return;
if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
std::swap(Weights.front(), Weights.back());
}
}
static void cloneInstructionsIntoPredecessorBlockAndUpdateSSAUses(
BasicBlock *BB, BasicBlock *PredBlock, ValueToValueMapTy &VMap) {
Instruction *PTI = PredBlock->getTerminator();
// If we have bonus instructions, clone them into the predecessor block.
// Note that there may be multiple predecessor blocks, so we cannot move
// bonus instructions to a predecessor block.
for (Instruction &BonusInst : *BB) {
if (BonusInst.isTerminator())
continue;
Instruction *NewBonusInst = BonusInst.clone();
if (!NewBonusInst->getDebugLoc().isSameSourceLocation(PTI->getDebugLoc())) {
// Unless the instruction has the same !dbg location as the original
// branch, drop it. When we fold the bonus instructions we want to make
// sure we reset their debug locations in order to avoid stepping on
// dead code caused by folding dead branches.
NewBonusInst->setDebugLoc(DebugLoc::getDropped());
} else if (const DebugLoc &DL = NewBonusInst->getDebugLoc()) {
mapAtomInstance(DL, VMap);
}
RemapInstruction(NewBonusInst, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
// If we speculated an instruction, we need to drop any metadata that may
// result in undefined behavior, as the metadata might have been valid
// only given the branch precondition.
// Similarly strip attributes on call parameters that may cause UB in
// location the call is moved to.
NewBonusInst->dropUBImplyingAttrsAndMetadata();
NewBonusInst->insertInto(PredBlock, PTI->getIterator());
auto Range = NewBonusInst->cloneDebugInfoFrom(&BonusInst);
RemapDbgRecordRange(NewBonusInst->getModule(), Range, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
NewBonusInst->takeName(&BonusInst);
BonusInst.setName(NewBonusInst->getName() + ".old");
VMap[&BonusInst] = NewBonusInst;
// Update (liveout) uses of bonus instructions,
// now that the bonus instruction has been cloned into predecessor.
// Note that we expect to be in a block-closed SSA form for this to work!
for (Use &U : make_early_inc_range(BonusInst.uses())) {
auto *UI = cast<Instruction>(U.getUser());
auto *PN = dyn_cast<PHINode>(UI);
if (!PN) {
assert(UI->getParent() == BB && BonusInst.comesBefore(UI) &&
"If the user is not a PHI node, then it should be in the same "
"block as, and come after, the original bonus instruction.");
continue; // Keep using the original bonus instruction.
}
// Is this the block-closed SSA form PHI node?
if (PN->getIncomingBlock(U) == BB)
continue; // Great, keep using the original bonus instruction.
// The only other alternative is an "use" when coming from
// the predecessor block - here we should refer to the cloned bonus instr.
assert(PN->getIncomingBlock(U) == PredBlock &&
"Not in block-closed SSA form?");
U.set(NewBonusInst);
}
}
// Key Instructions: We may have propagated atom info into the pred. If the
// pred's terminator already has atom info do nothing as merging would drop
// one atom group anyway. If it doesn't, propagte the remapped atom group
// from BB's terminator.
if (auto &PredDL = PTI->getDebugLoc()) {
auto &DL = BB->getTerminator()->getDebugLoc();
if (!PredDL->getAtomGroup() && DL && DL->getAtomGroup() &&
PredDL.isSameSourceLocation(DL)) {
PTI->setDebugLoc(DL);
RemapSourceAtom(PTI, VMap);
}
}
}
bool SimplifyCFGOpt::performValueComparisonIntoPredecessorFolding(
Instruction *TI, Value *&CV, Instruction *PTI, IRBuilder<> &Builder) {
BasicBlock *BB = TI->getParent();
BasicBlock *Pred = PTI->getParent();
SmallVector<DominatorTree::UpdateType, 32> Updates;
// Figure out which 'cases' to copy from SI to PSI.
std::vector<ValueEqualityComparisonCase> BBCases;
BasicBlock *BBDefault = getValueEqualityComparisonCases(TI, BBCases);
std::vector<ValueEqualityComparisonCase> PredCases;
BasicBlock *PredDefault = getValueEqualityComparisonCases(PTI, PredCases);
// Based on whether the default edge from PTI goes to BB or not, fill in
// PredCases and PredDefault with the new switch cases we would like to
// build.
SmallMapVector<BasicBlock *, int, 8> NewSuccessors;
// Update the branch weight metadata along the way
SmallVector<uint64_t, 8> Weights;
bool PredHasWeights = hasBranchWeightMD(*PTI);
bool SuccHasWeights = hasBranchWeightMD(*TI);
if (PredHasWeights) {
getBranchWeights(PTI, Weights);
// branch-weight metadata is inconsistent here.
if (Weights.size() != 1 + PredCases.size())
PredHasWeights = SuccHasWeights = false;
} else if (SuccHasWeights)
// If there are no predecessor weights but there are successor weights,
// populate Weights with 1, which will later be scaled to the sum of
// successor's weights
Weights.assign(1 + PredCases.size(), 1);
SmallVector<uint64_t, 8> SuccWeights;
if (SuccHasWeights) {
getBranchWeights(TI, SuccWeights);
// branch-weight metadata is inconsistent here.
if (SuccWeights.size() != 1 + BBCases.size())
PredHasWeights = SuccHasWeights = false;
} else if (PredHasWeights)
SuccWeights.assign(1 + BBCases.size(), 1);
if (PredDefault == BB) {
// If this is the default destination from PTI, only the edges in TI
// that don't occur in PTI, or that branch to BB will be activated.
std::set<ConstantInt *, ConstantIntOrdering> PTIHandled;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].Dest != BB)
PTIHandled.insert(PredCases[i].Value);
else {
// The default destination is BB, we don't need explicit targets.
std::swap(PredCases[i], PredCases.back());
if (PredHasWeights || SuccHasWeights) {
// Increase weight for the default case.
Weights[0] += Weights[i + 1];
std::swap(Weights[i + 1], Weights.back());
Weights.pop_back();
}
PredCases.pop_back();
--i;
--e;
}
// Reconstruct the new switch statement we will be building.
if (PredDefault != BBDefault) {
PredDefault->removePredecessor(Pred);
if (DTU && PredDefault != BB)
Updates.push_back({DominatorTree::Delete, Pred, PredDefault});
PredDefault = BBDefault;
++NewSuccessors[BBDefault];
}
unsigned CasesFromPred = Weights.size();
uint64_t ValidTotalSuccWeight = 0;
for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
if (!PTIHandled.count(BBCases[i].Value) && BBCases[i].Dest != BBDefault) {
PredCases.push_back(BBCases[i]);
++NewSuccessors[BBCases[i].Dest];
if (SuccHasWeights || PredHasWeights) {
// The default weight is at index 0, so weight for the ith case
// should be at index i+1. Scale the cases from successor by
// PredDefaultWeight (Weights[0]).
Weights.push_back(Weights[0] * SuccWeights[i + 1]);
ValidTotalSuccWeight += SuccWeights[i + 1];
}
}
if (SuccHasWeights || PredHasWeights) {
ValidTotalSuccWeight += SuccWeights[0];
// Scale the cases from predecessor by ValidTotalSuccWeight.
for (unsigned i = 1; i < CasesFromPred; ++i)
Weights[i] *= ValidTotalSuccWeight;
// Scale the default weight by SuccDefaultWeight (SuccWeights[0]).
Weights[0] *= SuccWeights[0];
}
} else {
// If this is not the default destination from PSI, only the edges
// in SI that occur in PSI with a destination of BB will be
// activated.
std::set<ConstantInt *, ConstantIntOrdering> PTIHandled;
std::map<ConstantInt *, uint64_t> WeightsForHandled;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].Dest == BB) {
PTIHandled.insert(PredCases[i].Value);
if (PredHasWeights || SuccHasWeights) {
WeightsForHandled[PredCases[i].Value] = Weights[i + 1];
std::swap(Weights[i + 1], Weights.back());
Weights.pop_back();
}
std::swap(PredCases[i], PredCases.back());
PredCases.pop_back();
--i;
--e;
}
// Okay, now we know which constants were sent to BB from the
// predecessor. Figure out where they will all go now.
for (const ValueEqualityComparisonCase &Case : BBCases)
if (PTIHandled.count(Case.Value)) {
// If this is one we are capable of getting...
if (PredHasWeights || SuccHasWeights)
Weights.push_back(WeightsForHandled[Case.Value]);
PredCases.push_back(Case);
++NewSuccessors[Case.Dest];
PTIHandled.erase(Case.Value); // This constant is taken care of
}
// If there are any constants vectored to BB that TI doesn't handle,
// they must go to the default destination of TI.
for (ConstantInt *I : PTIHandled) {
if (PredHasWeights || SuccHasWeights)
Weights.push_back(WeightsForHandled[I]);
PredCases.push_back(ValueEqualityComparisonCase(I, BBDefault));
++NewSuccessors[BBDefault];
}
}
// Okay, at this point, we know which new successor Pred will get. Make
// sure we update the number of entries in the PHI nodes for these
// successors.
SmallPtrSet<BasicBlock *, 2> SuccsOfPred;
if (DTU) {
SuccsOfPred = {llvm::from_range, successors(Pred)};
Updates.reserve(Updates.size() + NewSuccessors.size());
}
for (const std::pair<BasicBlock *, int /*Num*/> &NewSuccessor :
NewSuccessors) {
for (auto I : seq(NewSuccessor.second)) {
(void)I;
addPredecessorToBlock(NewSuccessor.first, Pred, BB);
}
if (DTU && !SuccsOfPred.contains(NewSuccessor.first))
Updates.push_back({DominatorTree::Insert, Pred, NewSuccessor.first});
}
Builder.SetInsertPoint(PTI);
// Convert pointer to int before we switch.
if (CV->getType()->isPointerTy()) {
assert(!DL.hasUnstableRepresentation(CV->getType()) &&
"Should not end up here with unstable pointers");
CV =
Builder.CreatePtrToInt(CV, DL.getIntPtrType(CV->getType()), "magicptr");
}
// Now that the successors are updated, create the new Switch instruction.
SwitchInst *NewSI = Builder.CreateSwitch(CV, PredDefault, PredCases.size());
NewSI->setDebugLoc(PTI->getDebugLoc());
for (ValueEqualityComparisonCase &V : PredCases)
NewSI->addCase(V.Value, V.Dest);
if (PredHasWeights || SuccHasWeights)
setFittedBranchWeights(*NewSI, Weights, /*IsExpected=*/false,
/*ElideAllZero=*/true);
eraseTerminatorAndDCECond(PTI);
// Okay, last check. If BB is still a successor of PSI, then we must
// have an infinite loop case. If so, add an infinitely looping block
// to handle the case to preserve the behavior of the code.
BasicBlock *InfLoopBlock = nullptr;
for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
if (NewSI->getSuccessor(i) == BB) {
if (!InfLoopBlock) {
// Insert it at the end of the function, because it's either code,
// or it won't matter if it's hot. :)
InfLoopBlock =
BasicBlock::Create(BB->getContext(), "infloop", BB->getParent());
UncondBrInst::Create(InfLoopBlock, InfLoopBlock);
if (DTU)
Updates.push_back(
{DominatorTree::Insert, InfLoopBlock, InfLoopBlock});
}
NewSI->setSuccessor(i, InfLoopBlock);
}
if (DTU) {
if (InfLoopBlock)
Updates.push_back({DominatorTree::Insert, Pred, InfLoopBlock});
Updates.push_back({DominatorTree::Delete, Pred, BB});
DTU->applyUpdates(Updates);
}
++NumFoldValueComparisonIntoPredecessors;
return true;
}
/// The specified terminator is a value equality comparison instruction
/// (either a switch or a branch on "X == c").
/// See if any of the predecessors of the terminator block are value comparisons
/// on the same value. If so, and if safe to do so, fold them together.
bool SimplifyCFGOpt::foldValueComparisonIntoPredecessors(Instruction *TI,
IRBuilder<> &Builder) {
BasicBlock *BB = TI->getParent();
Value *CV = isValueEqualityComparison(TI); // CondVal
assert(CV && "Not a comparison?");
bool Changed = false;
SmallSetVector<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));
while (!Preds.empty()) {
BasicBlock *Pred = Preds.pop_back_val();
Instruction *PTI = Pred->getTerminator();
// Don't try to fold into itself.
if (Pred == BB)
continue;
// See if the predecessor is a comparison with the same value.
Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
if (PCV != CV)
continue;
SmallSetVector<BasicBlock *, 4> FailBlocks;
if (!safeToMergeTerminators(TI, PTI, &FailBlocks)) {
for (auto *Succ : FailBlocks) {
if (!SplitBlockPredecessors(Succ, TI->getParent(), ".fold.split", DTU))
return false;
}
}
performValueComparisonIntoPredecessorFolding(TI, CV, PTI, Builder);
Changed = true;
}
return Changed;
}
// If we would need to insert a select that uses the value of this invoke
// (comments in hoistSuccIdenticalTerminatorToSwitchOrIf explain why we would
// need to do this), we can't hoist the invoke, as there is nowhere to put the
// select in this case.
static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2,
Instruction *I1, Instruction *I2) {
for (BasicBlock *Succ : successors(BB1)) {
for (const PHINode &PN : Succ->phis()) {
Value *BB1V = PN.getIncomingValueForBlock(BB1);
Value *BB2V = PN.getIncomingValueForBlock(BB2);
if (BB1V != BB2V && (BB1V == I1 || BB2V == I2)) {
return false;
}
}
}
return true;
}
// Get interesting characteristics of instructions that
// `hoistCommonCodeFromSuccessors` didn't hoist. They restrict what kind of
// instructions can be reordered across.
enum SkipFlags {
SkipReadMem = 1,
SkipSideEffect = 2,
SkipImplicitControlFlow = 4
};
static unsigned skippedInstrFlags(Instruction *I) {
unsigned Flags = 0;
if (I->mayReadFromMemory())
Flags |= SkipReadMem;
// We can't arbitrarily move around allocas, e.g. moving allocas (especially
// inalloca) across stacksave/stackrestore boundaries.
if (I->mayHaveSideEffects() || isa<AllocaInst>(I))
Flags |= SkipSideEffect;
if (!isGuaranteedToTransferExecutionToSuccessor(I))
Flags |= SkipImplicitControlFlow;
return Flags;
}
// Returns true if it is safe to reorder an instruction across preceding
// instructions in a basic block.
static bool isSafeToHoistInstr(Instruction *I, unsigned Flags) {
// Don't reorder a store over a load.
if ((Flags & SkipReadMem) && I->mayWriteToMemory())
return false;
// If we have seen an instruction with side effects, it's unsafe to reorder an
// instruction which reads memory or itself has side effects.
if ((Flags & SkipSideEffect) &&
(I->mayReadFromMemory() || I->mayHaveSideEffects() || isa<AllocaInst>(I)))
return false;
// Reordering across an instruction which does not necessarily transfer
// control to the next instruction is speculation.
if ((Flags & SkipImplicitControlFlow) && !isSafeToSpeculativelyExecute(I))
return false;
// Hoisting of llvm.deoptimize is only legal together with the next return
// instruction, which this pass is not always able to do.
if (auto *CB = dyn_cast<CallBase>(I))
if (CB->getIntrinsicID() == Intrinsic::experimental_deoptimize)
return false;
// It's also unsafe/illegal to hoist an instruction above its instruction
// operands
BasicBlock *BB = I->getParent();
for (Value *Op : I->operands()) {
if (auto *J = dyn_cast<Instruction>(Op))
if (J->getParent() == BB)
return false;
}
return true;
}
static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValueMayBeModified = false);
/// Helper function for hoistCommonCodeFromSuccessors. Return true if identical
/// instructions \p I1 and \p I2 can and should be hoisted.
static bool shouldHoistCommonInstructions(Instruction *I1, Instruction *I2,
const TargetTransformInfo &TTI) {
// If we're going to hoist a call, make sure that the two instructions
// we're commoning/hoisting are both marked with musttail, or neither of
// them is marked as such. Otherwise, we might end up in a situation where
// we hoist from a block where the terminator is a `ret` to a block where
// the terminator is a `br`, and `musttail` calls expect to be followed by
// a return.
auto *C1 = dyn_cast<CallInst>(I1);
auto *C2 = dyn_cast<CallInst>(I2);
if (C1 && C2)
if (C1->isMustTailCall() != C2->isMustTailCall())
return false;
if (!TTI.isProfitableToHoist(I1) || !TTI.isProfitableToHoist(I2))
return false;
// If any of the two call sites has nomerge or convergent attribute, stop
// hoisting.
if (const auto *CB1 = dyn_cast<CallBase>(I1))
if (CB1->cannotMerge() || CB1->isConvergent())
return false;
if (const auto *CB2 = dyn_cast<CallBase>(I2))
if (CB2->cannotMerge() || CB2->isConvergent())
return false;
return true;
}
/// Hoists DbgVariableRecords from \p I1 and \p OtherInstrs that are identical
/// in lock-step to \p TI. This matches how dbg.* intrinsics are hoisting in
/// hoistCommonCodeFromSuccessors. e.g. The input:
/// I1 DVRs: { x, z },
/// OtherInsts: { I2 DVRs: { x, y, z } }
/// would result in hoisting only DbgVariableRecord x.
static void hoistLockstepIdenticalDbgVariableRecords(
Instruction *TI, Instruction *I1,
SmallVectorImpl<Instruction *> &OtherInsts) {
if (!I1->hasDbgRecords())
return;
using CurrentAndEndIt =
std::pair<DbgRecord::self_iterator, DbgRecord::self_iterator>;
// Vector of {Current, End} iterators.
SmallVector<CurrentAndEndIt> Itrs;
Itrs.reserve(OtherInsts.size() + 1);
// Helper lambdas for lock-step checks:
// Return true if this Current == End.
auto atEnd = [](const CurrentAndEndIt &Pair) {
return Pair.first == Pair.second;
};
// Return true if all Current are identical.
auto allIdentical = [](const SmallVector<CurrentAndEndIt> &Itrs) {
return all_of(make_first_range(ArrayRef(Itrs).drop_front()),
[&](DbgRecord::self_iterator I) {
return Itrs[0].first->isIdenticalToWhenDefined(*I);
});
};
// Collect the iterators.
Itrs.push_back(
{I1->getDbgRecordRange().begin(), I1->getDbgRecordRange().end()});
for (Instruction *Other : OtherInsts) {
if (!Other->hasDbgRecords())
return;
Itrs.push_back(
{Other->getDbgRecordRange().begin(), Other->getDbgRecordRange().end()});
}
// Iterate in lock-step until any of the DbgRecord lists are exausted. If
// the lock-step DbgRecord are identical, hoist all of them to TI.
// This replicates the dbg.* intrinsic behaviour in
// hoistCommonCodeFromSuccessors.
while (none_of(Itrs, atEnd)) {
bool HoistDVRs = allIdentical(Itrs);
for (CurrentAndEndIt &Pair : Itrs) {
// Increment Current iterator now as we may be about to move the
// DbgRecord.
DbgRecord &DR = *Pair.first++;
if (HoistDVRs) {
DR.removeFromParent();
TI->getParent()->insertDbgRecordBefore(&DR, TI->getIterator());
}
}
}
}
static bool areIdenticalUpToCommutativity(const Instruction *I1,
const Instruction *I2) {
if (I1->isIdenticalToWhenDefined(I2, /*IntersectAttrs=*/true))
return true;
if (auto *Cmp1 = dyn_cast<CmpInst>(I1))
if (auto *Cmp2 = dyn_cast<CmpInst>(I2))
return Cmp1->getPredicate() == Cmp2->getSwappedPredicate() &&
Cmp1->getOperand(0) == Cmp2->getOperand(1) &&
Cmp1->getOperand(1) == Cmp2->getOperand(0);
if (I1->isCommutative() && I1->isSameOperationAs(I2)) {
return I1->getOperand(0) == I2->getOperand(1) &&
I1->getOperand(1) == I2->getOperand(0) &&
equal(drop_begin(I1->operands(), 2), drop_begin(I2->operands(), 2));
}
return false;
}
/// If the target supports conditional faulting,
/// we look for the following pattern:
/// \code
/// BB:
/// ...
/// %cond = icmp ult %x, %y
/// br i1 %cond, label %TrueBB, label %FalseBB
/// FalseBB:
/// store i32 1, ptr %q, align 4
/// ...
/// TrueBB:
/// %maskedloadstore = load i32, ptr %b, align 4
/// store i32 %maskedloadstore, ptr %p, align 4
/// ...
/// \endcode
///
/// and transform it into:
///
/// \code
/// BB:
/// ...
/// %cond = icmp ult %x, %y
/// %maskedloadstore = cload i32, ptr %b, %cond
/// cstore i32 %maskedloadstore, ptr %p, %cond
/// cstore i32 1, ptr %q, ~%cond
/// br i1 %cond, label %TrueBB, label %FalseBB
/// FalseBB:
/// ...
/// TrueBB:
/// ...
/// \endcode
///
/// where cload/cstore are represented by llvm.masked.load/store intrinsics,
/// e.g.
///
/// \code
/// %vcond = bitcast i1 %cond to <1 x i1>
/// %v0 = call <1 x i32> @llvm.masked.load.v1i32.p0
/// (ptr %b, i32 4, <1 x i1> %vcond, <1 x i32> poison)
/// %maskedloadstore = bitcast <1 x i32> %v0 to i32
/// call void @llvm.masked.store.v1i32.p0
/// (<1 x i32> %v0, ptr %p, i32 4, <1 x i1> %vcond)
/// %cond.not = xor i1 %cond, true
/// %vcond.not = bitcast i1 %cond.not to <1 x i>
/// call void @llvm.masked.store.v1i32.p0
/// (<1 x i32> <i32 1>, ptr %q, i32 4, <1x i1> %vcond.not)
/// \endcode
///
/// So we need to turn hoisted load/store into cload/cstore.
///
/// \param BI The branch instruction.
/// \param SpeculatedConditionalLoadsStores The load/store instructions that
/// will be speculated.
/// \param Invert indicates if speculates FalseBB. Only used in triangle CFG.
static void hoistConditionalLoadsStores(
CondBrInst *BI,
SmallVectorImpl<Instruction *> &SpeculatedConditionalLoadsStores,
std::optional<bool> Invert, Instruction *Sel) {
auto &Context = BI->getParent()->getContext();
auto *VCondTy = FixedVectorType::get(Type::getInt1Ty(Context), 1);
auto *Cond = BI->getCondition();
// Construct the condition if needed.
BasicBlock *BB = BI->getParent();
Value *Mask = nullptr;
Value *MaskFalse = nullptr;
Value *MaskTrue = nullptr;
if (Invert.has_value()) {
IRBuilder<> Builder(Sel ? Sel : SpeculatedConditionalLoadsStores.back());
Mask = Builder.CreateBitCast(
*Invert ? Builder.CreateXor(Cond, ConstantInt::getTrue(Context)) : Cond,
VCondTy);
} else {
IRBuilder<> Builder(BI);
MaskFalse = Builder.CreateBitCast(
Builder.CreateXor(Cond, ConstantInt::getTrue(Context)), VCondTy);
MaskTrue = Builder.CreateBitCast(Cond, VCondTy);
}
auto PeekThroughBitcasts = [](Value *V) {
while (auto *BitCast = dyn_cast<BitCastInst>(V))
V = BitCast->getOperand(0);
return V;
};
for (auto *I : SpeculatedConditionalLoadsStores) {
IRBuilder<> Builder(Invert.has_value() ? I : BI);
if (!Invert.has_value())
Mask = I->getParent() == BI->getSuccessor(0) ? MaskTrue : MaskFalse;
// We currently assume conditional faulting load/store is supported for
// scalar types only when creating new instructions. This can be easily
// extended for vector types in the future.
assert(!getLoadStoreType(I)->isVectorTy() && "not implemented");
auto *Op0 = I->getOperand(0);
CallInst *MaskedLoadStore = nullptr;
if (auto *LI = dyn_cast<LoadInst>(I)) {
// Handle Load.
auto *Ty = I->getType();
PHINode *PN = nullptr;
Value *PassThru = nullptr;
if (Invert.has_value())
for (User *U : I->users()) {
if ((PN = dyn_cast<PHINode>(U))) {
PassThru = Builder.CreateBitCast(
PeekThroughBitcasts(PN->getIncomingValueForBlock(BB)),
FixedVectorType::get(Ty, 1));
} else if (auto *Ins = cast<Instruction>(U);
Sel && Ins->getParent() == BB) {
// This happens when store or/and a speculative instruction between
// load and store were hoisted to the BB. Make sure the masked load
// inserted before its use.
// We assume there's one of such use.
Builder.SetInsertPoint(Ins);
}
}
MaskedLoadStore = Builder.CreateMaskedLoad(
FixedVectorType::get(Ty, 1), Op0, LI->getAlign(), Mask, PassThru);
Value *NewLoadStore = Builder.CreateBitCast(MaskedLoadStore, Ty);
if (PN)
PN->setIncomingValue(PN->getBasicBlockIndex(BB), NewLoadStore);
I->replaceAllUsesWith(NewLoadStore);
} else {
// Handle Store.
auto *StoredVal = Builder.CreateBitCast(
PeekThroughBitcasts(Op0), FixedVectorType::get(Op0->getType(), 1));
MaskedLoadStore = Builder.CreateMaskedStore(
StoredVal, I->getOperand(1), cast<StoreInst>(I)->getAlign(), Mask);
}
// For non-debug metadata, only !annotation, !range, !nonnull and !align are
// kept when hoisting (see Instruction::dropUBImplyingAttrsAndMetadata).
//
// !nonnull, !align : Not support pointer type, no need to keep.
// !range: Load type is changed from scalar to vector, but the metadata on
// vector specifies a per-element range, so the semantics stay the
// same. Keep it.
// !annotation: Not impact semantics. Keep it.
if (const MDNode *Ranges = I->getMetadata(LLVMContext::MD_range))
MaskedLoadStore->addRangeRetAttr(getConstantRangeFromMetadata(*Ranges));
I->dropUBImplyingAttrsAndUnknownMetadata({LLVMContext::MD_annotation});
// FIXME: DIAssignID is not supported for masked store yet.
// (Verifier::visitDIAssignIDMetadata)
at::deleteAssignmentMarkers(I);
I->eraseMetadataIf([](unsigned MDKind, MDNode *Node) {
return Node->getMetadataID() == Metadata::DIAssignIDKind;
});
MaskedLoadStore->copyMetadata(*I);
I->eraseFromParent();
}
}
static bool isSafeCheapLoadStore(const Instruction *I,
const TargetTransformInfo &TTI) {
// Not handle volatile or atomic.
bool IsStore = false;
if (auto *L = dyn_cast<LoadInst>(I)) {
if (!L->isSimple() || !HoistLoadsWithCondFaulting)
return false;
} else if (auto *S = dyn_cast<StoreInst>(I)) {
if (!S->isSimple() || !HoistStoresWithCondFaulting)
return false;
IsStore = true;
} else
return false;
// llvm.masked.load/store use i32 for alignment while load/store use i64.
// That's why we have the alignment limitation.
// FIXME: Update the prototype of the intrinsics?
return TTI.hasConditionalLoadStoreForType(getLoadStoreType(I), IsStore) &&
getLoadStoreAlignment(I) < Value::MaximumAlignment;
}
/// Hoist any common code in the successor blocks up into the block. This
/// function guarantees that BB dominates all successors. If AllInstsEqOnly is
/// given, only perform hoisting in case all successors blocks contain matching
/// instructions only. In that case, all instructions can be hoisted and the
/// original branch will be replaced and selects for PHIs are added.
bool SimplifyCFGOpt::hoistCommonCodeFromSuccessors(Instruction *TI,
bool AllInstsEqOnly) {
// This does very trivial matching, with limited scanning, to find identical
// instructions in the two blocks. In particular, we don't want to get into
// O(N1*N2*...) situations here where Ni are the sizes of these successors. As
// such, we currently just scan for obviously identical instructions in an
// identical order, possibly separated by the same number of non-identical
// instructions.
BasicBlock *BB = TI->getParent();
unsigned int SuccSize = succ_size(BB);
if (SuccSize < 2)
return false;
// If either of the blocks has it's address taken, then we can't do this fold,
// because the code we'd hoist would no longer run when we jump into the block
// by it's address.
SmallSetVector<BasicBlock *, 4> UniqueSuccessors(from_range, successors(BB));
for (auto *Succ : UniqueSuccessors) {
if (Succ->hasAddressTaken())
return false;
// Use getUniquePredecessor instead of getSinglePredecessor to support
// multi-cases successors in switch.
if (Succ->getUniquePredecessor())
continue;
// If Succ has >1 predecessors, continue to check if the Succ contains only
// one `unreachable` inst. Since executing `unreachable` inst is an UB, we
// can relax the condition based on the assumptiom that the program would
// never enter Succ and trigger such an UB.
if (isa<UnreachableInst>(*Succ->begin()))
continue;
return false;
}
// The second of pair is a SkipFlags bitmask.
using SuccIterPair = std::pair<BasicBlock::iterator, unsigned>;
SmallVector<SuccIterPair, 8> SuccIterPairs;
for (auto *Succ : UniqueSuccessors) {
BasicBlock::iterator SuccItr = Succ->begin();
if (isa<PHINode>(*SuccItr))
return false;
SuccIterPairs.push_back(SuccIterPair(SuccItr, 0));
}
if (AllInstsEqOnly) {
// Check if all instructions in the successor blocks match. This allows
// hoisting all instructions and removing the blocks we are hoisting from,
// so does not add any new instructions.
// Check if sizes and terminators of all successors match.
unsigned Size0 = UniqueSuccessors[0]->size();
Instruction *Term0 = UniqueSuccessors[0]->getTerminator();
bool AllSame =
all_of(drop_begin(UniqueSuccessors), [Term0, Size0](BasicBlock *Succ) {
return Succ->getTerminator()->isIdenticalTo(Term0) &&
Succ->size() == Size0;
});
if (!AllSame)
return false;
LockstepReverseIterator<true> LRI(UniqueSuccessors.getArrayRef());
while (LRI.isValid()) {
Instruction *I0 = (*LRI)[0];
if (any_of(*LRI, [I0](Instruction *I) {
return !areIdenticalUpToCommutativity(I0, I);
})) {
return false;
}
--LRI;
}
// Now we know that all instructions in all successors can be hoisted. Let
// the loop below handle the hoisting.
}
// Count how many instructions were not hoisted so far. There's a limit on how
// many instructions we skip, serving as a compilation time control as well as
// preventing excessive increase of life ranges.
unsigned NumSkipped = 0;
// If we find an unreachable instruction at the beginning of a basic block, we
// can still hoist instructions from the rest of the basic blocks.
if (SuccIterPairs.size() > 2) {
erase_if(SuccIterPairs,
[](const auto &Pair) { return isa<UnreachableInst>(Pair.first); });
if (SuccIterPairs.size() < 2)
return false;
}
bool Changed = false;
for (;;) {
auto *SuccIterPairBegin = SuccIterPairs.begin();
auto &BB1ItrPair = *SuccIterPairBegin++;
auto OtherSuccIterPairRange =
iterator_range(SuccIterPairBegin, SuccIterPairs.end());
auto OtherSuccIterRange = make_first_range(OtherSuccIterPairRange);
Instruction *I1 = &*BB1ItrPair.first;
bool AllInstsAreIdentical = true;
bool HasTerminator = I1->isTerminator();
for (auto &SuccIter : OtherSuccIterRange) {
Instruction *I2 = &*SuccIter;
HasTerminator |= I2->isTerminator();
if (AllInstsAreIdentical && (!areIdenticalUpToCommutativity(I1, I2) ||
MMRAMetadata(*I1) != MMRAMetadata(*I2)))
AllInstsAreIdentical = false;
}
SmallVector<Instruction *, 8> OtherInsts;
for (auto &SuccIter : OtherSuccIterRange)
OtherInsts.push_back(&*SuccIter);
// If we are hoisting the terminator instruction, don't move one (making a
// broken BB), instead clone it, and remove BI.
if (HasTerminator) {
// Even if BB, which contains only one unreachable instruction, is ignored
// at the beginning of the loop, we can hoist the terminator instruction.
// If any instructions remain in the block, we cannot hoist terminators.
if (NumSkipped || !AllInstsAreIdentical) {
hoistLockstepIdenticalDbgVariableRecords(TI, I1, OtherInsts);
return Changed;
}
return hoistSuccIdenticalTerminatorToSwitchOrIf(
TI, I1, OtherInsts, UniqueSuccessors.getArrayRef()) ||
Changed;
}
if (AllInstsAreIdentical) {
unsigned SkipFlagsBB1 = BB1ItrPair.second;
AllInstsAreIdentical =
isSafeToHoistInstr(I1, SkipFlagsBB1) &&
all_of(OtherSuccIterPairRange, [=](const auto &Pair) {
Instruction *I2 = &*Pair.first;
unsigned SkipFlagsBB2 = Pair.second;
// Even if the instructions are identical, it may not
// be safe to hoist them if we have skipped over
// instructions with side effects or their operands
// weren't hoisted.
return isSafeToHoistInstr(I2, SkipFlagsBB2) &&
shouldHoistCommonInstructions(I1, I2, TTI);
});
}
if (AllInstsAreIdentical) {
BB1ItrPair.first++;
// For a normal instruction, we just move one to right before the
// branch, then replace all uses of the other with the first. Finally,
// we remove the now redundant second instruction.
hoistLockstepIdenticalDbgVariableRecords(TI, I1, OtherInsts);
// We've just hoisted DbgVariableRecords; move I1 after them (before TI)
// and leave any that were not hoisted behind (by calling moveBefore
// rather than moveBeforePreserving).
I1->moveBefore(TI->getIterator());
for (auto &SuccIter : OtherSuccIterRange) {
Instruction *I2 = &*SuccIter++;
assert(I2 != I1);
if (!I2->use_empty())
I2->replaceAllUsesWith(I1);
I1->andIRFlags(I2);
if (auto *CB = dyn_cast<CallBase>(I1)) {
bool Success = CB->tryIntersectAttributes(cast<CallBase>(I2));
assert(Success && "We should not be trying to hoist callbases "
"with non-intersectable attributes");
// For NDEBUG Compile.
(void)Success;
}
combineMetadataForCSE(I1, I2, true);
// I1 and I2 are being combined into a single instruction. Its debug
// location is the merged locations of the original instructions.
I1->applyMergedLocation(I1->getDebugLoc(), I2->getDebugLoc());
I2->eraseFromParent();
}
if (!Changed)
NumHoistCommonCode += SuccIterPairs.size();
Changed = true;
NumHoistCommonInstrs += SuccIterPairs.size();
} else {
if (NumSkipped >= HoistCommonSkipLimit) {
hoistLockstepIdenticalDbgVariableRecords(TI, I1, OtherInsts);
return Changed;
}
// We are about to skip over a pair of non-identical instructions. Record
// if any have characteristics that would prevent reordering instructions
// across them.
for (auto &SuccIterPair : SuccIterPairs) {
Instruction *I = &*SuccIterPair.first++;
SuccIterPair.second |= skippedInstrFlags(I);
}
++NumSkipped;
}
}
}
bool SimplifyCFGOpt::hoistSuccIdenticalTerminatorToSwitchOrIf(
Instruction *TI, Instruction *I1,
SmallVectorImpl<Instruction *> &OtherSuccTIs,
ArrayRef<BasicBlock *> UniqueSuccessors) {
auto *BI = dyn_cast<CondBrInst>(TI);
bool Changed = false;
BasicBlock *TIParent = TI->getParent();
BasicBlock *BB1 = I1->getParent();
// Use only for an if statement.
auto *I2 = *OtherSuccTIs.begin();
auto *BB2 = I2->getParent();
if (BI) {
assert(OtherSuccTIs.size() == 1);
assert(BI->getSuccessor(0) == I1->getParent());
assert(BI->getSuccessor(1) == I2->getParent());
}
// In the case of an if statement, we try to hoist an invoke.
// FIXME: Can we define a safety predicate for CallBr?
// FIXME: Test case llvm/test/Transforms/SimplifyCFG/2009-06-15-InvokeCrash.ll
// removed in 4c923b3b3fd0ac1edebf0603265ca3ba51724937 commit?
if (isa<InvokeInst>(I1) && (!BI || !isSafeToHoistInvoke(BB1, BB2, I1, I2)))
return false;
// TODO: callbr hoisting currently disabled pending further study.
if (isa<CallBrInst>(I1))
return false;
for (BasicBlock *Succ : successors(BB1)) {
for (PHINode &PN : Succ->phis()) {
Value *BB1V = PN.getIncomingValueForBlock(BB1);
for (Instruction *OtherSuccTI : OtherSuccTIs) {
Value *BB2V = PN.getIncomingValueForBlock(OtherSuccTI->getParent());
if (BB1V == BB2V)
continue;
// In the case of an if statement, check for
// passingValueIsAlwaysUndefined here because we would rather eliminate
// undefined control flow then converting it to a select.
if (!BI || passingValueIsAlwaysUndefined(BB1V, &PN) ||
passingValueIsAlwaysUndefined(BB2V, &PN))
return false;
}
}
}
// Hoist DbgVariableRecords attached to the terminator to match dbg.*
// intrinsic hoisting behaviour in hoistCommonCodeFromSuccessors.
hoistLockstepIdenticalDbgVariableRecords(TI, I1, OtherSuccTIs);
// Clone the terminator and hoist it into the pred, without any debug info.
Instruction *NT = I1->clone();
NT->insertInto(TIParent, TI->getIterator());
if (!NT->getType()->isVoidTy()) {
I1->replaceAllUsesWith(NT);
for (Instruction *OtherSuccTI : OtherSuccTIs)
OtherSuccTI->replaceAllUsesWith(NT);
NT->takeName(I1);
}
Changed = true;
NumHoistCommonInstrs += OtherSuccTIs.size() + 1;
// Ensure terminator gets a debug location, even an unknown one, in case
// it involves inlinable calls.
SmallVector<DebugLoc, 4> Locs;
Locs.push_back(I1->getDebugLoc());
for (auto *OtherSuccTI : OtherSuccTIs)
Locs.push_back(OtherSuccTI->getDebugLoc());
NT->setDebugLoc(DebugLoc::getMergedLocations(Locs));
// PHIs created below will adopt NT's merged DebugLoc.
IRBuilder<NoFolder> Builder(NT);
// In the case of an if statement, hoisting one of the terminators from our
// successor is a great thing. Unfortunately, the successors of the if/else
// blocks may have PHI nodes in them. If they do, all PHI entries for BB1/BB2
// must agree for all PHI nodes, so we insert select instruction to compute
// the final result.
if (BI) {
std::map<std::pair<Value *, Value *>, SelectInst *> InsertedSelects;
for (BasicBlock *Succ : successors(BB1)) {
for (PHINode &PN : Succ->phis()) {
Value *BB1V = PN.getIncomingValueForBlock(BB1);
Value *BB2V = PN.getIncomingValueForBlock(BB2);
if (BB1V == BB2V)
continue;
// These values do not agree. Insert a select instruction before NT
// that determines the right value.
SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
if (!SI) {
// Propagate fast-math-flags from phi node to its replacement select.
SI = cast<SelectInst>(Builder.CreateSelectFMF(
BI->getCondition(), BB1V, BB2V,
isa<FPMathOperator>(PN) ? &PN : nullptr,
BB1V->getName() + "." + BB2V->getName(), BI));
}
// Make the PHI node use the select for all incoming values for BB1/BB2
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
if (PN.getIncomingBlock(i) == BB1 || PN.getIncomingBlock(i) == BB2)
PN.setIncomingValue(i, SI);
}
}
}
SmallVector<DominatorTree::UpdateType, 4> Updates;
// Update any PHI nodes in our new successors.
for (BasicBlock *Succ : successors(BB1)) {
addPredecessorToBlock(Succ, TIParent, BB1);
if (DTU)
Updates.push_back({DominatorTree::Insert, TIParent, Succ});
}
if (DTU) {
// TI might be a switch with multi-cases destination, so we need to care for
// the duplication of successors.
for (BasicBlock *Succ : UniqueSuccessors)
Updates.push_back({DominatorTree::Delete, TIParent, Succ});
}
eraseTerminatorAndDCECond(TI);
if (DTU)
DTU->applyUpdates(Updates);
return Changed;
}
// TODO: Refine this. This should avoid cases like turning constant memcpy sizes
// into variables.
static bool replacingOperandWithVariableIsCheap(const Instruction *I,
int OpIdx) {
// Divide/Remainder by constant is typically much cheaper than by variable.
if (I->isIntDivRem())
return OpIdx != 1;
return !isa<IntrinsicInst>(I);
}
// All instructions in Insts belong to different blocks that all unconditionally
// branch to a common successor. Analyze each instruction and return true if it
// would be possible to sink them into their successor, creating one common
// instruction instead. For every value that would be required to be provided by
// PHI node (because an operand varies in each input block), add to PHIOperands.
static bool canSinkInstructions(
ArrayRef<Instruction *> Insts,
DenseMap<const Use *, SmallVector<Value *, 4>> &PHIOperands) {
// Prune out obviously bad instructions to move. Each instruction must have
// the same number of uses, and we check later that the uses are consistent.
std::optional<unsigned> NumUses;
for (auto *I : Insts) {
// These instructions may change or break semantics if moved.
if (isa<PHINode>(I) || I->isEHPad() || isa<AllocaInst>(I) ||
I->getType()->isTokenTy())
return false;
// Do not try to sink an instruction in an infinite loop - it can cause
// this algorithm to infinite loop.
if (I->getParent()->getSingleSuccessor() == I->getParent())
return false;
// Conservatively return false if I is an inline-asm instruction. Sinking
// and merging inline-asm instructions can potentially create arguments
// that cannot satisfy the inline-asm constraints.
// If the instruction has nomerge or convergent attribute, return false.
if (const auto *C = dyn_cast<CallBase>(I))
if (C->isInlineAsm() || C->cannotMerge() || C->isConvergent())
return false;
if (!NumUses)
NumUses = I->getNumUses();
else if (NumUses != I->getNumUses())
return false;
}
const Instruction *I0 = Insts.front();
const auto I0MMRA = MMRAMetadata(*I0);
for (auto *I : Insts) {
if (!I->isSameOperationAs(I0, Instruction::CompareUsingIntersectedAttrs))
return false;
// Treat MMRAs conservatively. This pass can be quite aggressive and
// could drop a lot of MMRAs otherwise.
if (MMRAMetadata(*I) != I0MMRA)
return false;
}
// Uses must be consistent: If I0 is used in a phi node in the sink target,
// then the other phi operands must match the instructions from Insts. This
// also has to hold true for any phi nodes that would be created as a result
// of sinking. Both of these cases are represented by PhiOperands.
for (const Use &U : I0->uses()) {
auto It = PHIOperands.find(&U);
if (It == PHIOperands.end())
// There may be uses in other blocks when sinking into a loop header.
return false;
if (!equal(Insts, It->second))
return false;
}
// For calls to be sinkable, they must all be indirect, or have same callee.
// I.e. if we have two direct calls to different callees, we don't want to
// turn that into an indirect call. Likewise, if we have an indirect call,
// and a direct call, we don't actually want to have a single indirect call.
if (isa<CallBase>(I0)) {
auto IsIndirectCall = [](const Instruction *I) {
return cast<CallBase>(I)->isIndirectCall();
};
bool HaveIndirectCalls = any_of(Insts, IsIndirectCall);
bool AllCallsAreIndirect = all_of(Insts, IsIndirectCall);
if (HaveIndirectCalls) {
if (!AllCallsAreIndirect)
return false;
} else {
// All callees must be identical.
Value *Callee = nullptr;
for (const Instruction *I : Insts) {
Value *CurrCallee = cast<CallBase>(I)->getCalledOperand();
if (!Callee)
Callee = CurrCallee;
else if (Callee != CurrCallee)
return false;
}
}
}
for (unsigned OI = 0, OE = I0->getNumOperands(); OI != OE; ++OI) {
Value *Op = I0->getOperand(OI);
auto SameAsI0 = [&I0, OI](const Instruction *I) {
assert(I->getNumOperands() == I0->getNumOperands());
return I->getOperand(OI) == I0->getOperand(OI);
};
if (!all_of(Insts, SameAsI0)) {
if ((isa<Constant>(Op) && !replacingOperandWithVariableIsCheap(I0, OI)) ||
!canReplaceOperandWithVariable(I0, OI))
// We can't create a PHI from this GEP.
return false;
auto &Ops = PHIOperands[&I0->getOperandUse(OI)];
for (auto *I : Insts)
Ops.push_back(I->getOperand(OI));
}
}
return true;
}
// Assuming canSinkInstructions(Blocks) has returned true, sink the last
// instruction of every block in Blocks to their common successor, commoning
// into one instruction.
static void sinkLastInstruction(ArrayRef<BasicBlock*> Blocks) {
auto *BBEnd = Blocks[0]->getTerminator()->getSuccessor(0);
// canSinkInstructions returning true guarantees that every block has at
// least one non-terminator instruction.
SmallVector<Instruction*,4> Insts;
for (auto *BB : Blocks) {
Instruction *I = BB->getTerminator();
I = I->getPrevNode();
Insts.push_back(I);
}
// We don't need to do any more checking here; canSinkInstructions should
// have done it all for us.
SmallVector<Value*, 4> NewOperands;
Instruction *I0 = Insts.front();
for (unsigned O = 0, E = I0->getNumOperands(); O != E; ++O) {
// This check is different to that in canSinkInstructions. There, we
// cared about the global view once simplifycfg (and instcombine) have
// completed - it takes into account PHIs that become trivially
// simplifiable. However here we need a more local view; if an operand
// differs we create a PHI and rely on instcombine to clean up the very
// small mess we may make.
bool NeedPHI = any_of(Insts, [&I0, O](const Instruction *I) {
return I->getOperand(O) != I0->getOperand(O);
});
if (!NeedPHI) {
NewOperands.push_back(I0->getOperand(O));
continue;
}
// Create a new PHI in the successor block and populate it.
auto *Op = I0->getOperand(O);
assert(!Op->getType()->isTokenTy() && "Can't PHI tokens!");
auto *PN =
PHINode::Create(Op->getType(), Insts.size(), Op->getName() + ".sink");
PN->insertBefore(BBEnd->begin());
for (auto *I : Insts)
PN->addIncoming(I->getOperand(O), I->getParent());
NewOperands.push_back(PN);
}
// Arbitrarily use I0 as the new "common" instruction; remap its operands
// and move it to the start of the successor block.
for (unsigned O = 0, E = I0->getNumOperands(); O != E; ++O)
I0->getOperandUse(O).set(NewOperands[O]);
I0->moveBefore(*BBEnd, BBEnd->getFirstInsertionPt());
// Update metadata and IR flags, and merge debug locations.
for (auto *I : Insts)
if (I != I0) {
// The debug location for the "common" instruction is the merged locations
// of all the commoned instructions. We start with the original location
// of the "common" instruction and iteratively merge each location in the
// loop below.
// This is an N-way merge, which will be inefficient if I0 is a CallInst.
// However, as N-way merge for CallInst is rare, so we use simplified API
// instead of using complex API for N-way merge.
I0->applyMergedLocation(I0->getDebugLoc(), I->getDebugLoc());
combineMetadataForCSE(I0, I, true);
I0->andIRFlags(I);
if (auto *CB = dyn_cast<CallBase>(I0)) {
bool Success = CB->tryIntersectAttributes(cast<CallBase>(I));
assert(Success && "We should not be trying to sink callbases "
"with non-intersectable attributes");
// For NDEBUG Compile.
(void)Success;
}
}
for (User *U : make_early_inc_range(I0->users())) {
// canSinkLastInstruction checked that all instructions are only used by
// phi nodes in a way that allows replacing the phi node with the common
// instruction.
auto *PN = cast<PHINode>(U);
PN->replaceAllUsesWith(I0);
PN->eraseFromParent();
}
// Finally nuke all instructions apart from the common instruction.
for (auto *I : Insts) {
if (I == I0)
continue;
// The remaining uses are debug users, replace those with the common inst.
// In most (all?) cases this just introduces a use-before-def.
assert(I->user_empty() && "Inst unexpectedly still has non-dbg users");
I->replaceAllUsesWith(I0);
I->eraseFromParent();
}
}
/// Check whether BB's predecessors end with unconditional branches. If it is
/// true, sink any common code from the predecessors to BB.
static bool sinkCommonCodeFromPredecessors(BasicBlock *BB,
DomTreeUpdater *DTU) {
// We support two situations:
// (1) all incoming arcs are unconditional
// (2) there are non-unconditional incoming arcs
//
// (2) is very common in switch defaults and
// else-if patterns;
//
// if (a) f(1);
// else if (b) f(2);
//
// produces:
//
// [if]
// / \
// [f(1)] [if]
// | | \
// | | |
// | [f(2)]|
// \ | /
// [ end ]
//
// [end] has two unconditional predecessor arcs and one conditional. The
// conditional refers to the implicit empty 'else' arc. This conditional
// arc can also be caused by an empty default block in a switch.
//
// In this case, we attempt to sink code from all *unconditional* arcs.
// If we can sink instructions from these arcs (determined during the scan
// phase below) we insert a common successor for all unconditional arcs and
// connect that to [end], to enable sinking:
//
// [if]
// / \
// [x(1)] [if]
// | | \
// | | \
// | [x(2)] |
// \ / |
// [sink.split] |
// \ /
// [ end ]
//
SmallVector<BasicBlock*,4> UnconditionalPreds;
bool HaveNonUnconditionalPredecessors = false;
for (auto *PredBB : predecessors(BB)) {
auto *PredBr = dyn_cast<UncondBrInst>(PredBB->getTerminator());
if (PredBr)
UnconditionalPreds.push_back(PredBB);
else
HaveNonUnconditionalPredecessors = true;
}
if (UnconditionalPreds.size() < 2)
return false;
// We take a two-step approach to tail sinking. First we scan from the end of
// each block upwards in lockstep. If the n'th instruction from the end of each
// block can be sunk, those instructions are added to ValuesToSink and we
// carry on. If we can sink an instruction but need to PHI-merge some operands
// (because they're not identical in each instruction) we add these to
// PHIOperands.
// We prepopulate PHIOperands with the phis that already exist in BB.
DenseMap<const Use *, SmallVector<Value *, 4>> PHIOperands;
for (PHINode &PN : BB->phis()) {
SmallDenseMap<BasicBlock *, const Use *, 4> IncomingVals;
for (const Use &U : PN.incoming_values())
IncomingVals.insert({PN.getIncomingBlock(U), &U});
auto &Ops = PHIOperands[IncomingVals[UnconditionalPreds[0]]];
for (BasicBlock *Pred : UnconditionalPreds)
Ops.push_back(*IncomingVals[Pred]);
}
int ScanIdx = 0;
SmallPtrSet<Value*,4> InstructionsToSink;
LockstepReverseIterator<true> LRI(UnconditionalPreds);
while (LRI.isValid() &&
canSinkInstructions(*LRI, PHIOperands)) {
LLVM_DEBUG(dbgs() << "SINK: instruction can be sunk: " << *(*LRI)[0]
<< "\n");
InstructionsToSink.insert_range(*LRI);
++ScanIdx;
--LRI;
}
// If no instructions can be sunk, early-return.
if (ScanIdx == 0)
return false;
bool followedByDeoptOrUnreachable = IsBlockFollowedByDeoptOrUnreachable(BB);
if (!followedByDeoptOrUnreachable) {
// Check whether this is the pointer operand of a load/store.
auto IsMemOperand = [](Use &U) {
auto *I = cast<Instruction>(U.getUser());
if (isa<LoadInst>(I))
return U.getOperandNo() == LoadInst::getPointerOperandIndex();
if (isa<StoreInst>(I))
return U.getOperandNo() == StoreInst::getPointerOperandIndex();
return false;
};
// Okay, we *could* sink last ScanIdx instructions. But how many can we
// actually sink before encountering instruction that is unprofitable to
// sink?
auto ProfitableToSinkInstruction = [&](LockstepReverseIterator<true> &LRI) {
unsigned NumPHIInsts = 0;
for (Use &U : (*LRI)[0]->operands()) {
auto It = PHIOperands.find(&U);
if (It != PHIOperands.end() && !all_of(It->second, [&](Value *V) {
return InstructionsToSink.contains(V);
})) {
++NumPHIInsts;
// Do not separate a load/store from the gep producing the address.
// The gep can likely be folded into the load/store as an addressing
// mode. Additionally, a load of a gep is easier to analyze than a
// load of a phi.
if (IsMemOperand(U) &&
any_of(It->second, [](Value *V) { return isa<GEPOperator>(V); }))
return false;
// FIXME: this check is overly optimistic. We may end up not sinking
// said instruction, due to the very same profitability check.
// See @creating_too_many_phis in sink-common-code.ll.
}
}
LLVM_DEBUG(dbgs() << "SINK: #phi insts: " << NumPHIInsts << "\n");
return NumPHIInsts <= 1;
};
// We've determined that we are going to sink last ScanIdx instructions,
// and recorded them in InstructionsToSink. Now, some instructions may be
// unprofitable to sink. But that determination depends on the instructions
// that we are going to sink.
// First, forward scan: find the first instruction unprofitable to sink,
// recording all the ones that are profitable to sink.
// FIXME: would it be better, after we detect that not all are profitable.
// to either record the profitable ones, or erase the unprofitable ones?
// Maybe we need to choose (at runtime) the one that will touch least
// instrs?
LRI.reset();
int Idx = 0;
SmallPtrSet<Value *, 4> InstructionsProfitableToSink;
while (Idx < ScanIdx) {
if (!ProfitableToSinkInstruction(LRI)) {
// Too many PHIs would be created.
LLVM_DEBUG(
dbgs() << "SINK: stopping here, too many PHIs would be created!\n");
break;
}
InstructionsProfitableToSink.insert_range(*LRI);
--LRI;
++Idx;
}
// If no instructions can be sunk, early-return.
if (Idx == 0)
return false;
// Did we determine that (only) some instructions are unprofitable to sink?
if (Idx < ScanIdx) {
// Okay, some instructions are unprofitable.
ScanIdx = Idx;
InstructionsToSink = InstructionsProfitableToSink;
// But, that may make other instructions unprofitable, too.
// So, do a backward scan, do any earlier instructions become
// unprofitable?
assert(
!ProfitableToSinkInstruction(LRI) &&
"We already know that the last instruction is unprofitable to sink");
++LRI;
--Idx;
while (Idx >= 0) {
// If we detect that an instruction becomes unprofitable to sink,
// all earlier instructions won't be sunk either,
// so preemptively keep InstructionsProfitableToSink in sync.
// FIXME: is this the most performant approach?
for (auto *I : *LRI)
InstructionsProfitableToSink.erase(I);
if (!ProfitableToSinkInstruction(LRI)) {
// Everything starting with this instruction won't be sunk.
ScanIdx = Idx;
InstructionsToSink = InstructionsProfitableToSink;
}
++LRI;
--Idx;
}
}
// If no instructions can be sunk, early-return.
if (ScanIdx == 0)
return false;
}
bool Changed = false;
if (HaveNonUnconditionalPredecessors) {
if (!followedByDeoptOrUnreachable) {
// It is always legal to sink common instructions from unconditional
// predecessors. However, if not all predecessors are unconditional,
// this transformation might be pessimizing. So as a rule of thumb,
// don't do it unless we'd sink at least one non-speculatable instruction.
// See https://bugs.llvm.org/show_bug.cgi?id=30244
LRI.reset();
int Idx = 0;
bool Profitable = false;
while (Idx < ScanIdx) {
if (!isSafeToSpeculativelyExecute((*LRI)[0])) {
Profitable = true;
break;
}
--LRI;
++Idx;
}
if (!Profitable)
return false;
}
LLVM_DEBUG(dbgs() << "SINK: Splitting edge\n");
// We have a conditional edge and we're going to sink some instructions.
// Insert a new block postdominating all blocks we're going to sink from.
if (!SplitBlockPredecessors(BB, UnconditionalPreds, ".sink.split", DTU))
// Edges couldn't be split.
return false;
Changed = true;
}
// Now that we've analyzed all potential sinking candidates, perform the
// actual sink. We iteratively sink the last non-terminator of the source
// blocks into their common successor unless doing so would require too
// many PHI instructions to be generated (currently only one PHI is allowed
// per sunk instruction).
//
// We can use InstructionsToSink to discount values needing PHI-merging that will
// actually be sunk in a later iteration. This allows us to be more
// aggressive in what we sink. This does allow a false positive where we
// sink presuming a later value will also be sunk, but stop half way through
// and never actually sink it which means we produce more PHIs than intended.
// This is unlikely in practice though.
int SinkIdx = 0;
for (; SinkIdx != ScanIdx; ++SinkIdx) {
LLVM_DEBUG(dbgs() << "SINK: Sink: "
<< *UnconditionalPreds[0]->getTerminator()->getPrevNode()
<< "\n");
// Because we've sunk every instruction in turn, the current instruction to
// sink is always at index 0.
LRI.reset();
sinkLastInstruction(UnconditionalPreds);
NumSinkCommonInstrs++;
Changed = true;
}
if (SinkIdx != 0)
++NumSinkCommonCode;
return Changed;
}
namespace {
struct CompatibleSets {
using SetTy = SmallVector<InvokeInst *, 2>;
SmallVector<SetTy, 1> Sets;
static bool shouldBelongToSameSet(ArrayRef<InvokeInst *> Invokes);
SetTy &getCompatibleSet(InvokeInst *II);
void insert(InvokeInst *II);
};
CompatibleSets::SetTy &CompatibleSets::getCompatibleSet(InvokeInst *II) {
// Perform a linear scan over all the existing sets, see if the new `invoke`
// is compatible with any particular set. Since we know that all the `invokes`
// within a set are compatible, only check the first `invoke` in each set.
// WARNING: at worst, this has quadratic complexity.
for (CompatibleSets::SetTy &Set : Sets) {
if (CompatibleSets::shouldBelongToSameSet({Set.front(), II}))
return Set;
}
// Otherwise, we either had no sets yet, or this invoke forms a new set.
return Sets.emplace_back();
}
void CompatibleSets::insert(InvokeInst *II) {
getCompatibleSet(II).emplace_back(II);
}
bool CompatibleSets::shouldBelongToSameSet(ArrayRef<InvokeInst *> Invokes) {
assert(Invokes.size() == 2 && "Always called with exactly two candidates.");
// Can we theoretically merge these `invoke`s?
auto IsIllegalToMerge = [](InvokeInst *II) {
return II->cannotMerge() || II->isInlineAsm();
};
if (any_of(Invokes, IsIllegalToMerge))
return false;
// Either both `invoke`s must be direct,
// or both `invoke`s must be indirect.
auto IsIndirectCall = [](InvokeInst *II) { return II->isIndirectCall(); };
bool HaveIndirectCalls = any_of(Invokes, IsIndirectCall);
bool AllCallsAreIndirect = all_of(Invokes, IsIndirectCall);
if (HaveIndirectCalls) {
if (!AllCallsAreIndirect)
return false;
} else {
// All callees must be identical.
Value *Callee = nullptr;
for (InvokeInst *II : Invokes) {
Value *CurrCallee = II->getCalledOperand();
assert(CurrCallee && "There is always a called operand.");
if (!Callee)
Callee = CurrCallee;
else if (Callee != CurrCallee)
return false;
}
}
// Either both `invoke`s must not have a normal destination,
// or both `invoke`s must have a normal destination,
auto HasNormalDest = [](InvokeInst *II) {
return !isa<UnreachableInst>(II->getNormalDest()->getFirstNonPHIOrDbg());
};
if (any_of(Invokes, HasNormalDest)) {
// Do not merge `invoke` that does not have a normal destination with one
// that does have a normal destination, even though doing so would be legal.
if (!all_of(Invokes, HasNormalDest))
return false;
// All normal destinations must be identical.
BasicBlock *NormalBB = nullptr;
for (InvokeInst *II : Invokes) {
BasicBlock *CurrNormalBB = II->getNormalDest();
assert(CurrNormalBB && "There is always a 'continue to' basic block.");
if (!NormalBB)
NormalBB = CurrNormalBB;
else if (NormalBB != CurrNormalBB)
return false;
}
// In the normal destination, the incoming values for these two `invoke`s
// must be compatible.
SmallPtrSet<Value *, 16> EquivalenceSet(llvm::from_range, Invokes);
if (!incomingValuesAreCompatible(
NormalBB, {Invokes[0]->getParent(), Invokes[1]->getParent()},
&EquivalenceSet))
return false;
}
#ifndef NDEBUG
// All unwind destinations must be identical.
// We know that because we have started from said unwind destination.
BasicBlock *UnwindBB = nullptr;
for (InvokeInst *II : Invokes) {
BasicBlock *CurrUnwindBB = II->getUnwindDest();
assert(CurrUnwindBB && "There is always an 'unwind to' basic block.");
if (!UnwindBB)
UnwindBB = CurrUnwindBB;
else
assert(UnwindBB == CurrUnwindBB && "Unexpected unwind destination.");
}
#endif
// In the unwind destination, the incoming values for these two `invoke`s
// must be compatible.
if (!incomingValuesAreCompatible(
Invokes.front()->getUnwindDest(),
{Invokes[0]->getParent(), Invokes[1]->getParent()}))
return false;
// Ignoring arguments, these `invoke`s must be identical,
// including operand bundles.
const InvokeInst *II0 = Invokes.front();
for (auto *II : Invokes.drop_front())
if (!II->isSameOperationAs(II0, Instruction::CompareUsingIntersectedAttrs))
return false;
// Can we theoretically form the data operands for the merged `invoke`?
auto IsIllegalToMergeArguments = [](auto Ops) {
Use &U0 = std::get<0>(Ops);
Use &U1 = std::get<1>(Ops);
if (U0 == U1)
return false;
return !canReplaceOperandWithVariable(cast<Instruction>(U0.getUser()),
U0.getOperandNo());
};
assert(Invokes.size() == 2 && "Always called with exactly two candidates.");
if (any_of(zip(Invokes[0]->data_ops(), Invokes[1]->data_ops()),
IsIllegalToMergeArguments))
return false;
return true;
}
} // namespace
// Merge all invokes in the provided set, all of which are compatible
// as per the `CompatibleSets::shouldBelongToSameSet()`.
static void mergeCompatibleInvokesImpl(ArrayRef<InvokeInst *> Invokes,
DomTreeUpdater *DTU) {
assert(Invokes.size() >= 2 && "Must have at least two invokes to merge.");
SmallVector<DominatorTree::UpdateType, 8> Updates;
if (DTU)
Updates.reserve(2 + 3 * Invokes.size());
bool HasNormalDest =
!isa<UnreachableInst>(Invokes[0]->getNormalDest()->getFirstNonPHIOrDbg());
// Clone one of the invokes into a new basic block.
// Since they are all compatible, it doesn't matter which invoke is cloned.
InvokeInst *MergedInvoke = [&Invokes, HasNormalDest]() {
InvokeInst *II0 = Invokes.front();
BasicBlock *II0BB = II0->getParent();
BasicBlock *InsertBeforeBlock =
II0->getParent()->getIterator()->getNextNode();
Function *Func = II0BB->getParent();
LLVMContext &Ctx = II0->getContext();
BasicBlock *MergedInvokeBB = BasicBlock::Create(
Ctx, II0BB->getName() + ".invoke", Func, InsertBeforeBlock);
auto *MergedInvoke = cast<InvokeInst>(II0->clone());
// NOTE: all invokes have the same attributes, so no handling needed.
MergedInvoke->insertInto(MergedInvokeBB, MergedInvokeBB->end());
if (!HasNormalDest) {
// This set does not have a normal destination,
// so just form a new block with unreachable terminator.
BasicBlock *MergedNormalDest = BasicBlock::Create(
Ctx, II0BB->getName() + ".cont", Func, InsertBeforeBlock);
auto *UI = new UnreachableInst(Ctx, MergedNormalDest);
UI->setDebugLoc(DebugLoc::getTemporary());
MergedInvoke->setNormalDest(MergedNormalDest);
}
// The unwind destination, however, remainds identical for all invokes here.
return MergedInvoke;
}();
if (DTU) {
// Predecessor blocks that contained these invokes will now branch to
// the new block that contains the merged invoke, ...
for (InvokeInst *II : Invokes)
Updates.push_back(
{DominatorTree::Insert, II->getParent(), MergedInvoke->getParent()});
// ... which has the new `unreachable` block as normal destination,
// or unwinds to the (same for all `invoke`s in this set) `landingpad`,
for (BasicBlock *SuccBBOfMergedInvoke : successors(MergedInvoke))
Updates.push_back({DominatorTree::Insert, MergedInvoke->getParent(),
SuccBBOfMergedInvoke});
// Since predecessor blocks now unconditionally branch to a new block,
// they no longer branch to their original successors.
for (InvokeInst *II : Invokes)
for (BasicBlock *SuccOfPredBB : successors(II->getParent()))
Updates.push_back(
{DominatorTree::Delete, II->getParent(), SuccOfPredBB});
}
bool IsIndirectCall = Invokes[0]->isIndirectCall();
// Form the merged operands for the merged invoke.
for (Use &U : MergedInvoke->operands()) {
// Only PHI together the indirect callees and data operands.
if (MergedInvoke->isCallee(&U)) {
if (!IsIndirectCall)
continue;
} else if (!MergedInvoke->isDataOperand(&U))
continue;
// Don't create trivial PHI's with all-identical incoming values.
bool NeedPHI = any_of(Invokes, [&U](InvokeInst *II) {
return II->getOperand(U.getOperandNo()) != U.get();
});
if (!NeedPHI)
continue;
// Form a PHI out of all the data ops under this index.
PHINode *PN = PHINode::Create(
U->getType(), /*NumReservedValues=*/Invokes.size(), "", MergedInvoke->getIterator());
for (InvokeInst *II : Invokes)
PN->addIncoming(II->getOperand(U.getOperandNo()), II->getParent());
U.set(PN);
}
// We've ensured that each PHI node has compatible (identical) incoming values
// when coming from each of the `invoke`s in the current merge set,
// so update the PHI nodes accordingly.
for (BasicBlock *Succ : successors(MergedInvoke))
addPredecessorToBlock(Succ, /*NewPred=*/MergedInvoke->getParent(),
/*ExistPred=*/Invokes.front()->getParent());
// And finally, replace the original `invoke`s with an unconditional branch
// to the block with the merged `invoke`. Also, give that merged `invoke`
// the merged debugloc of all the original `invoke`s.
DILocation *MergedDebugLoc = nullptr;
for (InvokeInst *II : Invokes) {
// Compute the debug location common to all the original `invoke`s.
if (!MergedDebugLoc)
MergedDebugLoc = II->getDebugLoc();
else
MergedDebugLoc =
DebugLoc::getMergedLocation(MergedDebugLoc, II->getDebugLoc());
// And replace the old `invoke` with an unconditionally branch
// to the block with the merged `invoke`.
for (BasicBlock *OrigSuccBB : successors(II->getParent()))
OrigSuccBB->removePredecessor(II->getParent());
auto *BI = UncondBrInst::Create(MergedInvoke->getParent(), II->getParent());
// The unconditional branch is part of the replacement for the original
// invoke, so should use its DebugLoc.
BI->setDebugLoc(II->getDebugLoc());
bool Success = MergedInvoke->tryIntersectAttributes(II);
assert(Success && "Merged invokes with incompatible attributes");
// For NDEBUG Compile
(void)Success;
II->replaceAllUsesWith(MergedInvoke);
II->eraseFromParent();
++NumInvokesMerged;
}
MergedInvoke->setDebugLoc(MergedDebugLoc);
++NumInvokeSetsFormed;
if (DTU)
DTU->applyUpdates(Updates);
}
/// If this block is a `landingpad` exception handling block, categorize all
/// the predecessor `invoke`s into sets, with all `invoke`s in each set
/// being "mergeable" together, and then merge invokes in each set together.
///
/// This is a weird mix of hoisting and sinking. Visually, it goes from:
/// [...] [...]
/// | |
/// [invoke0] [invoke1]
/// / \ / \
/// [cont0] [landingpad] [cont1]
/// to:
/// [...] [...]
/// \ /
/// [invoke]
/// / \
/// [cont] [landingpad]
///
/// But of course we can only do that if the invokes share the `landingpad`,
/// edges invoke0->cont0 and invoke1->cont1 are "compatible",
/// and the invoked functions are "compatible".
static bool mergeCompatibleInvokes(BasicBlock *BB, DomTreeUpdater *DTU) {
if (!EnableMergeCompatibleInvokes)
return false;
bool Changed = false;
// FIXME: generalize to all exception handling blocks?
if (!BB->isLandingPad())
return Changed;
CompatibleSets Grouper;
// Record all the predecessors of this `landingpad`. As per verifier,
// the only allowed predecessor is the unwind edge of an `invoke`.
// We want to group "compatible" `invokes` into the same set to be merged.
for (BasicBlock *PredBB : predecessors(BB))
Grouper.insert(cast<InvokeInst>(PredBB->getTerminator()));
// And now, merge `invoke`s that were grouped togeter.
for (ArrayRef<InvokeInst *> Invokes : Grouper.Sets) {
if (Invokes.size() < 2)
continue;
Changed = true;
mergeCompatibleInvokesImpl(Invokes, DTU);
}
return Changed;
}
namespace {
/// Track ephemeral values, which should be ignored for cost-modelling
/// purposes. Requires walking instructions in reverse order.
class EphemeralValueTracker {
SmallPtrSet<const Instruction *, 32> EphValues;
bool isEphemeral(const Instruction *I) {
if (isa<AssumeInst>(I))
return true;
return !I->mayHaveSideEffects() && !I->isTerminator() &&
all_of(I->users(), [&](const User *U) {
return EphValues.count(cast<Instruction>(U));
});
}
public:
bool track(const Instruction *I) {
if (isEphemeral(I)) {
EphValues.insert(I);
return true;
}
return false;
}
bool contains(const Instruction *I) const { return EphValues.contains(I); }
};
} // namespace
/// Determine if we can hoist sink a sole store instruction out of a
/// conditional block.
///
/// We are looking for code like the following:
/// BrBB:
/// store i32 %add, i32* %arrayidx2
/// ... // No other stores or function calls (we could be calling a memory
/// ... // function).
/// %cmp = icmp ult %x, %y
/// br i1 %cmp, label %EndBB, label %ThenBB
/// ThenBB:
/// store i32 %add5, i32* %arrayidx2
/// br label EndBB
/// EndBB:
/// ...
/// We are going to transform this into:
/// BrBB:
/// store i32 %add, i32* %arrayidx2
/// ... //
/// %cmp = icmp ult %x, %y
/// %add.add5 = select i1 %cmp, i32 %add, %add5
/// store i32 %add.add5, i32* %arrayidx2
/// ...
///
/// \return The pointer to the value of the previous store if the store can be
/// hoisted into the predecessor block. 0 otherwise.
static Value *isSafeToSpeculateStore(Instruction *I, BasicBlock *BrBB,
BasicBlock *StoreBB, BasicBlock *EndBB) {
StoreInst *StoreToHoist = dyn_cast<StoreInst>(I);
if (!StoreToHoist)
return nullptr;
// Volatile or atomic.
if (!StoreToHoist->isSimple())
return nullptr;
Value *StorePtr = StoreToHoist->getPointerOperand();
Type *StoreTy = StoreToHoist->getValueOperand()->getType();
// Look for a store to the same pointer in BrBB.
unsigned MaxNumInstToLookAt = 9;
// Skip pseudo probe intrinsic calls which are not really killing any memory
// accesses.
for (Instruction &CurI : reverse(*BrBB)) {
if (!MaxNumInstToLookAt)
break;
--MaxNumInstToLookAt;
if (isa<PseudoProbeInst>(CurI))
continue;
// Could be calling an instruction that affects memory like free().
if (CurI.mayWriteToMemory() && !isa<StoreInst>(CurI))
return nullptr;
if (auto *SI = dyn_cast<StoreInst>(&CurI)) {
// Found the previous store to same location and type. Make sure it is
// simple, to avoid introducing a spurious non-atomic write after an
// atomic write.
if (SI->getPointerOperand() == StorePtr &&
SI->getValueOperand()->getType() == StoreTy && SI->isSimple() &&
SI->getAlign() >= StoreToHoist->getAlign())
// Found the previous store, return its value operand.
return SI->getValueOperand();
return nullptr; // Unknown store.
}
if (auto *LI = dyn_cast<LoadInst>(&CurI)) {
if (LI->getPointerOperand() == StorePtr && LI->getType() == StoreTy &&
LI->isSimple() && LI->getAlign() >= StoreToHoist->getAlign()) {
Value *Obj = getUnderlyingObject(StorePtr);
bool ExplicitlyDereferenceableOnly;
if (isWritableObject(Obj, ExplicitlyDereferenceableOnly) &&
capturesNothing(
PointerMayBeCaptured(Obj, /*ReturnCaptures=*/false,
CaptureComponents::Provenance)) &&
(!ExplicitlyDereferenceableOnly ||
isDereferenceablePointer(StorePtr, StoreTy,
LI->getDataLayout()))) {
// Found a previous load, return it.
return LI;
}
}
// The load didn't work out, but we may still find a store.
}
}
return nullptr;
}
/// Estimate the cost of the insertion(s) and check that the PHI nodes can be
/// converted to selects.
static bool validateAndCostRequiredSelects(BasicBlock *BB, BasicBlock *ThenBB,
BasicBlock *EndBB,
unsigned &SpeculatedInstructions,
InstructionCost &Cost,
const TargetTransformInfo &TTI) {
TargetTransformInfo::TargetCostKind CostKind =
BB->getParent()->hasMinSize()
? TargetTransformInfo::TCK_CodeSize
: TargetTransformInfo::TCK_SizeAndLatency;
bool HaveRewritablePHIs = false;
for (PHINode &PN : EndBB->phis()) {
Value *OrigV = PN.getIncomingValueForBlock(BB);
Value *ThenV = PN.getIncomingValueForBlock(ThenBB);
// FIXME: Try to remove some of the duplication with
// hoistCommonCodeFromSuccessors. Skip PHIs which are trivial.
if (ThenV == OrigV)
continue;
Cost += TTI.getCmpSelInstrCost(Instruction::Select, PN.getType(),
CmpInst::makeCmpResultType(PN.getType()),
CmpInst::BAD_ICMP_PREDICATE, CostKind);
// Don't convert to selects if we could remove undefined behavior instead.
if (passingValueIsAlwaysUndefined(OrigV, &PN) ||
passingValueIsAlwaysUndefined(ThenV, &PN))
return false;
HaveRewritablePHIs = true;
ConstantExpr *OrigCE = dyn_cast<ConstantExpr>(OrigV);
ConstantExpr *ThenCE = dyn_cast<ConstantExpr>(ThenV);
if (!OrigCE && !ThenCE)
continue; // Known cheap (FIXME: Maybe not true for aggregates).
InstructionCost OrigCost = OrigCE ? computeSpeculationCost(OrigCE, TTI) : 0;
InstructionCost ThenCost = ThenCE ? computeSpeculationCost(ThenCE, TTI) : 0;
InstructionCost MaxCost =
2 * PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
if (OrigCost + ThenCost > MaxCost)
return false;
// Account for the cost of an unfolded ConstantExpr which could end up
// getting expanded into Instructions.
// FIXME: This doesn't account for how many operations are combined in the
// constant expression.
++SpeculatedInstructions;
if (SpeculatedInstructions > 1)
return false;
}
return HaveRewritablePHIs;
}
static bool isProfitableToSpeculate(const CondBrInst *BI,
std::optional<bool> Invert,
const TargetTransformInfo &TTI) {
// If the branch is non-unpredictable, and is predicted to *not* branch to
// the `then` block, then avoid speculating it.
if (BI->getMetadata(LLVMContext::MD_unpredictable))
return true;
uint64_t TWeight, FWeight;
if (!extractBranchWeights(*BI, TWeight, FWeight) || (TWeight + FWeight) == 0)
return true;
if (!Invert.has_value())
return false;
uint64_t EndWeight = *Invert ? TWeight : FWeight;
BranchProbability BIEndProb =
BranchProbability::getBranchProbability(EndWeight, TWeight + FWeight);
BranchProbability Likely = TTI.getPredictableBranchThreshold();
return BIEndProb < Likely;
}
/// Speculate a conditional basic block flattening the CFG.
///
/// Note that this is a very risky transform currently. Speculating
/// instructions like this is most often not desirable. Instead, there is an MI
/// pass which can do it with full awareness of the resource constraints.
/// However, some cases are "obvious" and we should do directly. An example of
/// this is speculating a single, reasonably cheap instruction.
///
/// There is only one distinct advantage to flattening the CFG at the IR level:
/// it makes very common but simplistic optimizations such as are common in
/// instcombine and the DAG combiner more powerful by removing CFG edges and
/// modeling their effects with easier to reason about SSA value graphs.
///
///
/// An illustration of this transform is turning this IR:
/// \code
/// BB:
/// %cmp = icmp ult %x, %y
/// br i1 %cmp, label %EndBB, label %ThenBB
/// ThenBB:
/// %sub = sub %x, %y
/// br label BB2
/// EndBB:
/// %phi = phi [ %sub, %ThenBB ], [ 0, %BB ]
/// ...
/// \endcode
///
/// Into this IR:
/// \code
/// BB:
/// %cmp = icmp ult %x, %y
/// %sub = sub %x, %y
/// %cond = select i1 %cmp, 0, %sub
/// ...
/// \endcode
///
/// \returns true if the conditional block is removed.
bool SimplifyCFGOpt::speculativelyExecuteBB(CondBrInst *BI,
BasicBlock *ThenBB) {
if (!Options.SpeculateBlocks)
return false;
// Be conservative for now. FP select instruction can often be expensive.
Value *BrCond = BI->getCondition();
if (isa<FCmpInst>(BrCond))
return false;
BasicBlock *BB = BI->getParent();
BasicBlock *EndBB = ThenBB->getTerminator()->getSuccessor(0);
InstructionCost Budget =
PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
// If ThenBB is actually on the false edge of the conditional branch, remember
// to swap the select operands later.
bool Invert = false;
if (ThenBB != BI->getSuccessor(0)) {
assert(ThenBB == BI->getSuccessor(1) && "No edge from 'if' block?");
Invert = true;
}
assert(EndBB == BI->getSuccessor(!Invert) && "No edge from to end block");
if (!isProfitableToSpeculate(BI, Invert, TTI))
return false;
// Keep a count of how many times instructions are used within ThenBB when
// they are candidates for sinking into ThenBB. Specifically:
// - They are defined in BB, and
// - They have no side effects, and
// - All of their uses are in ThenBB.
SmallDenseMap<Instruction *, unsigned, 4> SinkCandidateUseCounts;
SmallVector<Instruction *, 4> SpeculatedPseudoProbes;
unsigned SpeculatedInstructions = 0;
bool HoistLoadsStores = Options.HoistLoadsStoresWithCondFaulting;
SmallVector<Instruction *, 2> SpeculatedConditionalLoadsStores;
Value *SpeculatedStoreValue = nullptr;
StoreInst *SpeculatedStore = nullptr;
EphemeralValueTracker EphTracker;
for (Instruction &I : reverse(drop_end(*ThenBB))) {
// Skip pseudo probes. The consequence is we lose track of the branch
// probability for ThenBB, which is fine since the optimization here takes
// place regardless of the branch probability.
if (isa<PseudoProbeInst>(I)) {
// The probe should be deleted so that it will not be over-counted when
// the samples collected on the non-conditional path are counted towards
// the conditional path. We leave it for the counts inference algorithm to
// figure out a proper count for an unknown probe.
SpeculatedPseudoProbes.push_back(&I);
continue;
}
// Ignore ephemeral values, they will be dropped by the transform.
if (EphTracker.track(&I))
continue;
// Only speculatively execute a single instruction (not counting the
// terminator) for now.
bool IsSafeCheapLoadStore = HoistLoadsStores &&
isSafeCheapLoadStore(&I, TTI) &&
SpeculatedConditionalLoadsStores.size() <
HoistLoadsStoresWithCondFaultingThreshold;
// Not count load/store into cost if target supports conditional faulting
// b/c it's cheap to speculate it.
if (IsSafeCheapLoadStore)
SpeculatedConditionalLoadsStores.push_back(&I);
else
++SpeculatedInstructions;
if (SpeculatedInstructions > 1)
return false;
// Don't hoist the instruction if it's unsafe or expensive.
if (!IsSafeCheapLoadStore &&
!isSafeToSpeculativelyExecute(&I, BI, Options.AC) &&
!(HoistCondStores && !SpeculatedStoreValue &&
(SpeculatedStoreValue =
isSafeToSpeculateStore(&I, BB, ThenBB, EndBB))))
return false;
if (!IsSafeCheapLoadStore && !SpeculatedStoreValue &&
computeSpeculationCost(&I, TTI) >
PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic)
return false;
// Store the store speculation candidate.
if (!SpeculatedStore && SpeculatedStoreValue)
SpeculatedStore = cast<StoreInst>(&I);
// Do not hoist the instruction if any of its operands are defined but not
// used in BB. The transformation will prevent the operand from
// being sunk into the use block.
for (Use &Op : I.operands()) {
Instruction *OpI = dyn_cast<Instruction>(Op);
if (!OpI || OpI->getParent() != BB || OpI->mayHaveSideEffects())
continue; // Not a candidate for sinking.
++SinkCandidateUseCounts[OpI];
}
}
// Consider any sink candidates which are only used in ThenBB as costs for
// speculation. Note, while we iterate over a DenseMap here, we are summing
// and so iteration order isn't significant.
for (const auto &[Inst, Count] : SinkCandidateUseCounts)
if (Inst->hasNUses(Count)) {
++SpeculatedInstructions;
if (SpeculatedInstructions > 1)
return false;
}
// Check that we can insert the selects and that it's not too expensive to do
// so.
bool Convert =
SpeculatedStore != nullptr || !SpeculatedConditionalLoadsStores.empty();
InstructionCost Cost = 0;
Convert |= validateAndCostRequiredSelects(BB, ThenBB, EndBB,
SpeculatedInstructions, Cost, TTI);
if (!Convert || Cost > Budget)
return false;
// If we get here, we can hoist the instruction and if-convert.
LLVM_DEBUG(dbgs() << "SPECULATIVELY EXECUTING BB" << *ThenBB << "\n";);
Instruction *Sel = nullptr;
// Insert a select of the value of the speculated store.
if (SpeculatedStoreValue) {
IRBuilder<NoFolder> Builder(BI);
Value *OrigV = SpeculatedStore->getValueOperand();
Value *TrueV = SpeculatedStore->getValueOperand();
Value *FalseV = SpeculatedStoreValue;
if (Invert)
std::swap(TrueV, FalseV);
Value *S = Builder.CreateSelect(
BrCond, TrueV, FalseV, "spec.store.select", BI);
Sel = cast<Instruction>(S);
SpeculatedStore->setOperand(0, S);
SpeculatedStore->applyMergedLocation(BI->getDebugLoc(),
SpeculatedStore->getDebugLoc());
// The value stored is still conditional, but the store itself is now
// unconditonally executed, so we must be sure that any linked dbg.assign
// intrinsics are tracking the new stored value (the result of the
// select). If we don't, and the store were to be removed by another pass
// (e.g. DSE), then we'd eventually end up emitting a location describing
// the conditional value, unconditionally.
//
// === Before this transformation ===
// pred:
// store %one, %x.dest, !DIAssignID !1
// dbg.assign %one, "x", ..., !1, ...
// br %cond if.then
//
// if.then:
// store %two, %x.dest, !DIAssignID !2
// dbg.assign %two, "x", ..., !2, ...
//
// === After this transformation ===
// pred:
// store %one, %x.dest, !DIAssignID !1
// dbg.assign %one, "x", ..., !1
/// ...
// %merge = select %cond, %two, %one
// store %merge, %x.dest, !DIAssignID !2
// dbg.assign %merge, "x", ..., !2
for (DbgVariableRecord *DbgAssign :
at::getDVRAssignmentMarkers(SpeculatedStore))
if (llvm::is_contained(DbgAssign->location_ops(), OrigV))
DbgAssign->replaceVariableLocationOp(OrigV, S);
}
// Metadata can be dependent on the condition we are hoisting above.
// Strip all UB-implying metadata on the instruction. Drop the debug loc
// to avoid making it appear as if the condition is a constant, which would
// be misleading while debugging.
// Similarly strip attributes that maybe dependent on condition we are
// hoisting above.
for (auto &I : make_early_inc_range(*ThenBB)) {
if (!SpeculatedStoreValue || &I != SpeculatedStore) {
I.dropLocation();
}
I.dropUBImplyingAttrsAndMetadata();
// Drop ephemeral values.
if (EphTracker.contains(&I)) {
I.replaceAllUsesWith(PoisonValue::get(I.getType()));
I.eraseFromParent();
}
}
// Hoist the instructions.
// Drop DbgVariableRecords attached to these instructions.
for (auto &It : *ThenBB)
for (DbgRecord &DR : make_early_inc_range(It.getDbgRecordRange()))
// Drop all records except assign-kind DbgVariableRecords (dbg.assign
// equivalent).
if (DbgVariableRecord *DVR = dyn_cast<DbgVariableRecord>(&DR);
!DVR || !DVR->isDbgAssign())
It.dropOneDbgRecord(&DR);
BB->splice(BI->getIterator(), ThenBB, ThenBB->begin(),
std::prev(ThenBB->end()));
if (!SpeculatedConditionalLoadsStores.empty())
hoistConditionalLoadsStores(BI, SpeculatedConditionalLoadsStores, Invert,
Sel);
// Insert selects and rewrite the PHI operands.
IRBuilder<NoFolder> Builder(BI);
for (PHINode &PN : EndBB->phis()) {
unsigned OrigI = PN.getBasicBlockIndex(BB);
unsigned ThenI = PN.getBasicBlockIndex(ThenBB);
Value *OrigV = PN.getIncomingValue(OrigI);
Value *ThenV = PN.getIncomingValue(ThenI);
// Skip PHIs which are trivial.
if (OrigV == ThenV)
continue;
// Create a select whose true value is the speculatively executed value and
// false value is the pre-existing value. Swap them if the branch
// destinations were inverted.
Value *TrueV = ThenV, *FalseV = OrigV;
if (Invert)
std::swap(TrueV, FalseV);
Value *V = Builder.CreateSelect(BrCond, TrueV, FalseV, "spec.select", BI);
PN.setIncomingValue(OrigI, V);
PN.setIncomingValue(ThenI, V);
}
// Remove speculated pseudo probes.
for (Instruction *I : SpeculatedPseudoProbes)
I->eraseFromParent();
++NumSpeculations;
return true;
}
/// Return true if we can thread a branch across this block.
static bool blockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
int Size = 0;
EphemeralValueTracker EphTracker;
// Walk the loop in reverse so that we can identify ephemeral values properly
// (values only feeding assumes).
for (Instruction &I : reverse(*BB)) {
// Can't fold blocks that contain noduplicate or convergent calls.
if (CallInst *CI = dyn_cast<CallInst>(&I))
if (CI->cannotDuplicate() || CI->isConvergent())
return false;
// Ignore ephemeral values which are deleted during codegen.
// We will delete Phis while threading, so Phis should not be accounted in
// block's size.
if (!EphTracker.track(&I) && !isa<PHINode>(I)) {
if (Size++ > MaxSmallBlockSize)
return false; // Don't clone large BB's.
}
// We can only support instructions that do not define values that are
// live outside of the current basic block.
for (User *U : I.users()) {
Instruction *UI = cast<Instruction>(U);
if (UI->getParent() != BB || isa<PHINode>(UI))
return false;
}
// Looks ok, continue checking.
}
return true;
}
static ConstantInt *getKnownValueOnEdge(Value *V, BasicBlock *From,
BasicBlock *To) {
// Don't look past the block defining the value, we might get the value from
// a previous loop iteration.
auto *I = dyn_cast<Instruction>(V);
if (I && I->getParent() == To)
return nullptr;
// We know the value if the From block branches on it.
auto *BI = dyn_cast<CondBrInst>(From->getTerminator());
if (BI && BI->getCondition() == V &&
BI->getSuccessor(0) != BI->getSuccessor(1))
return BI->getSuccessor(0) == To ? ConstantInt::getTrue(BI->getContext())
: ConstantInt::getFalse(BI->getContext());
return nullptr;
}
/// If we have a conditional branch on something for which we know the constant
/// value in predecessors (e.g. a phi node in the current block), thread edges
/// from the predecessor to their ultimate destination.
static std::optional<bool>
foldCondBranchOnValueKnownInPredecessorImpl(CondBrInst *BI, DomTreeUpdater *DTU,
const DataLayout &DL,
AssumptionCache *AC) {
SmallMapVector<ConstantInt *, SmallSetVector<BasicBlock *, 2>, 2> KnownValues;
BasicBlock *BB = BI->getParent();
Value *Cond = BI->getCondition();
PHINode *PN = dyn_cast<PHINode>(Cond);
if (PN && PN->getParent() == BB) {
// Degenerate case of a single entry PHI.
if (PN->getNumIncomingValues() == 1) {
FoldSingleEntryPHINodes(PN->getParent());
return true;
}
for (Use &U : PN->incoming_values())
if (auto *CB = dyn_cast<ConstantInt>(U))
KnownValues[CB].insert(PN->getIncomingBlock(U));
} else {
for (BasicBlock *Pred : predecessors(BB)) {
if (ConstantInt *CB = getKnownValueOnEdge(Cond, Pred, BB))
KnownValues[CB].insert(Pred);
}
}
if (KnownValues.empty())
return false;
// Now we know that this block has multiple preds and two succs.
// Check that the block is small enough and values defined in the block are
// not used outside of it.
if (!blockIsSimpleEnoughToThreadThrough(BB))
return false;
for (const auto &Pair : KnownValues) {
// Okay, we now know that all edges from PredBB should be revectored to
// branch to RealDest.
ConstantInt *CB = Pair.first;
ArrayRef<BasicBlock *> PredBBs = Pair.second.getArrayRef();
BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue());
if (RealDest == BB)
continue; // Skip self loops.
// Skip if the predecessor's terminator is an indirect branch.
if (any_of(PredBBs, [](BasicBlock *PredBB) {
return isa<IndirectBrInst>(PredBB->getTerminator());
}))
continue;
LLVM_DEBUG({
dbgs() << "Condition " << *Cond << " in " << BB->getName()
<< " has value " << *Pair.first << " in predecessors:\n";
for (const BasicBlock *PredBB : Pair.second)
dbgs() << " " << PredBB->getName() << "\n";
dbgs() << "Threading to destination " << RealDest->getName() << ".\n";
});
// Split the predecessors we are threading into a new edge block. We'll
// clone the instructions into this block, and then redirect it to RealDest.
BasicBlock *EdgeBB = SplitBlockPredecessors(BB, PredBBs, ".critedge", DTU);
if (!EdgeBB)
continue;
// TODO: These just exist to reduce test diff, we can drop them if we like.
EdgeBB->setName(RealDest->getName() + ".critedge");
EdgeBB->moveBefore(RealDest);
// Update PHI nodes.
addPredecessorToBlock(RealDest, EdgeBB, BB);
// BB may have instructions that are being threaded over. Clone these
// instructions into EdgeBB. We know that there will be no uses of the
// cloned instructions outside of EdgeBB.
BasicBlock::iterator InsertPt = EdgeBB->getFirstInsertionPt();
ValueToValueMapTy TranslateMap; // Track translated values.
TranslateMap[Cond] = CB;
// RemoveDIs: track instructions that we optimise away while folding, so
// that we can copy DbgVariableRecords from them later.
BasicBlock::iterator SrcDbgCursor = BB->begin();
for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
TranslateMap[PN] = PN->getIncomingValueForBlock(EdgeBB);
continue;
}
// Clone the instruction.
Instruction *N = BBI->clone();
// Insert the new instruction into its new home.
N->insertInto(EdgeBB, InsertPt);
if (BBI->hasName())
N->setName(BBI->getName() + ".c");
// Update operands due to translation.
// Key Instructions: Remap all the atom groups.
if (const DebugLoc &DL = BBI->getDebugLoc())
mapAtomInstance(DL, TranslateMap);
RemapInstruction(N, TranslateMap,
RF_IgnoreMissingLocals | RF_NoModuleLevelChanges);
// Check for trivial simplification.
if (Value *V = simplifyInstruction(N, {DL, nullptr, nullptr, AC})) {
if (!BBI->use_empty())
TranslateMap[&*BBI] = V;
if (!N->mayHaveSideEffects()) {
N->eraseFromParent(); // Instruction folded away, don't need actual
// inst
N = nullptr;
}
} else {
if (!BBI->use_empty())
TranslateMap[&*BBI] = N;
}
if (N) {
// Copy all debug-info attached to instructions from the last we
// successfully clone, up to this instruction (they might have been
// folded away).
for (; SrcDbgCursor != BBI; ++SrcDbgCursor)
N->cloneDebugInfoFrom(&*SrcDbgCursor);
SrcDbgCursor = std::next(BBI);
// Clone debug-info on this instruction too.
N->cloneDebugInfoFrom(&*BBI);
// Register the new instruction with the assumption cache if necessary.
if (auto *Assume = dyn_cast<AssumeInst>(N))
if (AC)
AC->registerAssumption(Assume);
}
}
for (; &*SrcDbgCursor != BI; ++SrcDbgCursor)
InsertPt->cloneDebugInfoFrom(&*SrcDbgCursor);
InsertPt->cloneDebugInfoFrom(BI);
BB->removePredecessor(EdgeBB);
UncondBrInst *EdgeBI = cast<UncondBrInst>(EdgeBB->getTerminator());
EdgeBI->setSuccessor(0, RealDest);
EdgeBI->setDebugLoc(BI->getDebugLoc());
if (DTU) {
SmallVector<DominatorTree::UpdateType, 2> Updates;
Updates.push_back({DominatorTree::Delete, EdgeBB, BB});
Updates.push_back({DominatorTree::Insert, EdgeBB, RealDest});
DTU->applyUpdates(Updates);
}
// For simplicity, we created a separate basic block for the edge. Merge
// it back into the predecessor if possible. This not only avoids
// unnecessary SimplifyCFG iterations, but also makes sure that we don't
// bypass the check for trivial cycles above.
MergeBlockIntoPredecessor(EdgeBB, DTU);
// Signal repeat, simplifying any other constants.
return std::nullopt;
}
return false;
}
bool SimplifyCFGOpt::foldCondBranchOnValueKnownInPredecessor(CondBrInst *BI) {
// Note: If BB is a loop header then there is a risk that threading introduces
// a non-canonical loop by moving a back edge. So we avoid this optimization
// for loop headers if NeedCanonicalLoop is set.
if (Options.NeedCanonicalLoop && is_contained(LoopHeaders, BI->getParent()))
return false;
std::optional<bool> Result;
bool EverChanged = false;
do {
// Note that None means "we changed things, but recurse further."
Result =
foldCondBranchOnValueKnownInPredecessorImpl(BI, DTU, DL, Options.AC);
EverChanged |= Result == std::nullopt || *Result;
} while (Result == std::nullopt);
return EverChanged;
}
/// Given a BB that starts with the specified two-entry PHI node,
/// see if we can eliminate it.
static bool foldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
DomTreeUpdater *DTU, AssumptionCache *AC,
const DataLayout &DL,
bool SpeculateUnpredictables) {
// Ok, this is a two entry PHI node. Check to see if this is a simple "if
// statement", which has a very simple dominance structure. Basically, we
// are trying to find the condition that is being branched on, which
// subsequently causes this merge to happen. We really want control
// dependence information for this check, but simplifycfg can't keep it up
// to date, and this catches most of the cases we care about anyway.
BasicBlock *BB = PN->getParent();
BasicBlock *IfTrue, *IfFalse;
CondBrInst *DomBI = GetIfCondition(BB, IfTrue, IfFalse);
if (!DomBI)
return false;
Value *IfCond = DomBI->getCondition();
// Don't bother if the branch will be constant folded trivially.
if (isa<ConstantInt>(IfCond))
return false;
BasicBlock *DomBlock = DomBI->getParent();
SmallVector<BasicBlock *, 2> IfBlocks;
llvm::copy_if(PN->blocks(), std::back_inserter(IfBlocks),
[](BasicBlock *IfBlock) {
return isa<UncondBrInst>(IfBlock->getTerminator());
});
assert((IfBlocks.size() == 1 || IfBlocks.size() == 2) &&
"Will have either one or two blocks to speculate.");
// If the branch is non-unpredictable, see if we either predictably jump to
// the merge bb (if we have only a single 'then' block), or if we predictably
// jump to one specific 'then' block (if we have two of them).
// It isn't beneficial to speculatively execute the code
// from the block that we know is predictably not entered.
bool IsUnpredictable = DomBI->getMetadata(LLVMContext::MD_unpredictable);
if (!IsUnpredictable) {
uint64_t TWeight, FWeight;
if (extractBranchWeights(*DomBI, TWeight, FWeight) &&
(TWeight + FWeight) != 0) {
BranchProbability BITrueProb =
BranchProbability::getBranchProbability(TWeight, TWeight + FWeight);
BranchProbability Likely = TTI.getPredictableBranchThreshold();
BranchProbability BIFalseProb = BITrueProb.getCompl();
if (IfBlocks.size() == 1) {
BranchProbability BIBBProb =
DomBI->getSuccessor(0) == BB ? BITrueProb : BIFalseProb;
if (BIBBProb >= Likely)
return false;
} else {
if (BITrueProb >= Likely || BIFalseProb >= Likely)
return false;
}
}
}
// Don't try to fold an unreachable block. For example, the phi node itself
// can't be the candidate if-condition for a select that we want to form.
if (auto *IfCondPhiInst = dyn_cast<PHINode>(IfCond))
if (IfCondPhiInst->getParent() == BB)
return false;
// Okay, we found that we can merge this two-entry phi node into a select.
// Doing so would require us to fold *all* two entry phi nodes in this block.
// At some point this becomes non-profitable (particularly if the target
// doesn't support cmov's). Only do this transformation if there are two or
// fewer PHI nodes in this block.
unsigned NumPhis = 0;
for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++NumPhis, ++I)
if (NumPhis > 2)
return false;
// Loop over the PHI's seeing if we can promote them all to select
// instructions. While we are at it, keep track of the instructions
// that need to be moved to the dominating block.
SmallPtrSet<Instruction *, 4> AggressiveInsts;
SmallPtrSet<Instruction *, 2> ZeroCostInstructions;
InstructionCost Cost = 0;
InstructionCost Budget =
TwoEntryPHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
if (SpeculateUnpredictables && IsUnpredictable)
Budget += TTI.getBranchMispredictPenalty();
bool Changed = false;
for (BasicBlock::iterator II = BB->begin(); isa<PHINode>(II);) {
PHINode *PN = cast<PHINode>(II++);
if (Value *V = simplifyInstruction(PN, {DL, PN})) {
PN->replaceAllUsesWith(V);
PN->eraseFromParent();
Changed = true;
continue;
}
if (!dominatesMergePoint(PN->getIncomingValue(0), BB, DomBI,
AggressiveInsts, Cost, Budget, TTI, AC,
ZeroCostInstructions) ||
!dominatesMergePoint(PN->getIncomingValue(1), BB, DomBI,
AggressiveInsts, Cost, Budget, TTI, AC,
ZeroCostInstructions))
return Changed;
}
// If we folded the first phi, PN dangles at this point. Refresh it. If
// we ran out of PHIs then we simplified them all.
PN = dyn_cast<PHINode>(BB->begin());
if (!PN)
return true;
// Don't fold i1 branches on PHIs which contain binary operators or
// (possibly inverted) select form of or/ands if their parameters are
// an equality test.
auto IsBinOpOrAndEq = [](Value *V) {
CmpPredicate Pred;
if (match(V, m_CombineOr(
m_CombineOr(
m_BinOp(m_Cmp(Pred, m_Value(), m_Value()), m_Value()),
m_BinOp(m_Value(), m_Cmp(Pred, m_Value(), m_Value()))),
m_c_Select(m_ImmConstant(),
m_Cmp(Pred, m_Value(), m_Value()))))) {
return CmpInst::isEquality(Pred);
}
return false;
};
if (PN->getType()->isIntegerTy(1) &&
(IsBinOpOrAndEq(PN->getIncomingValue(0)) ||
IsBinOpOrAndEq(PN->getIncomingValue(1)) || IsBinOpOrAndEq(IfCond)))
return Changed;
// If all PHI nodes are promotable, check to make sure that all instructions
// in the predecessor blocks can be promoted as well. If not, we won't be able
// to get rid of the control flow, so it's not worth promoting to select
// instructions.
for (BasicBlock *IfBlock : IfBlocks)
for (BasicBlock::iterator I = IfBlock->begin(); !I->isTerminator(); ++I)
if (!AggressiveInsts.count(&*I) && !I->isDebugOrPseudoInst()) {
// This is not an aggressive instruction that we can promote.
// Because of this, we won't be able to get rid of the control flow, so
// the xform is not worth it.
return Changed;
}
// If either of the blocks has it's address taken, we can't do this fold.
if (any_of(IfBlocks,
[](BasicBlock *IfBlock) { return IfBlock->hasAddressTaken(); }))
return Changed;
LLVM_DEBUG(dbgs() << "FOUND IF CONDITION! " << *IfCond;
if (IsUnpredictable) dbgs() << " (unpredictable)";
dbgs() << " T: " << IfTrue->getName()
<< " F: " << IfFalse->getName() << "\n");
// If we can still promote the PHI nodes after this gauntlet of tests,
// do all of the PHI's now.
// Move all 'aggressive' instructions, which are defined in the
// conditional parts of the if's up to the dominating block.
for (BasicBlock *IfBlock : IfBlocks)
hoistAllInstructionsInto(DomBlock, DomBI, IfBlock);
IRBuilder<NoFolder> Builder(DomBI);
// Propagate fast-math-flags from phi nodes to replacement selects.
while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
// Change the PHI node into a select instruction.
Value *TrueVal = PN->getIncomingValueForBlock(IfTrue);
Value *FalseVal = PN->getIncomingValueForBlock(IfFalse);
Value *Sel = Builder.CreateSelectFMF(IfCond, TrueVal, FalseVal,
isa<FPMathOperator>(PN) ? PN : nullptr,
"", DomBI);
PN->replaceAllUsesWith(Sel);
Sel->takeName(PN);
PN->eraseFromParent();
}
// At this point, all IfBlocks are empty, so our if statement
// has been flattened. Change DomBlock to jump directly to our new block to
// avoid other simplifycfg's kicking in on the diamond.
Builder.CreateBr(BB);
SmallVector<DominatorTree::UpdateType, 3> Updates;
if (DTU) {
Updates.push_back({DominatorTree::Insert, DomBlock, BB});
for (auto *Successor : successors(DomBlock))
Updates.push_back({DominatorTree::Delete, DomBlock, Successor});
}
DomBI->eraseFromParent();
if (DTU)
DTU->applyUpdates(Updates);
return true;
}
static Value *createLogicalOp(IRBuilderBase &Builder,
Instruction::BinaryOps Opc, Value *LHS,
Value *RHS, const Twine &Name = "") {
// Try to relax logical op to binary op.
if (impliesPoison(RHS, LHS))
return Builder.CreateBinOp(Opc, LHS, RHS, Name);
if (Opc == Instruction::And)
return Builder.CreateLogicalAnd(LHS, RHS, Name);
if (Opc == Instruction::Or)
return Builder.CreateLogicalOr(LHS, RHS, Name);
llvm_unreachable("Invalid logical opcode");
}
/// Return true if either PBI or BI has branch weight available, and store
/// the weights in {Pred|Succ}{True|False}Weight. If one of PBI and BI does
/// not have branch weight, use 1:1 as its weight.
static bool extractPredSuccWeights(CondBrInst *PBI, CondBrInst *BI,
uint64_t &PredTrueWeight,
uint64_t &PredFalseWeight,
uint64_t &SuccTrueWeight,
uint64_t &SuccFalseWeight) {
bool PredHasWeights =
extractBranchWeights(*PBI, PredTrueWeight, PredFalseWeight);
bool SuccHasWeights =
extractBranchWeights(*BI, SuccTrueWeight, SuccFalseWeight);
if (PredHasWeights || SuccHasWeights) {
if (!PredHasWeights)
PredTrueWeight = PredFalseWeight = 1;
if (!SuccHasWeights)
SuccTrueWeight = SuccFalseWeight = 1;
return true;
} else {
return false;
}
}
/// Determine if the two branches share a common destination and deduce a glue
/// that joins the branches' conditions to arrive at the common destination if
/// that would be profitable.
static std::optional<std::tuple<BasicBlock *, Instruction::BinaryOps, bool>>
shouldFoldCondBranchesToCommonDestination(CondBrInst *BI, CondBrInst *PBI,
const TargetTransformInfo *TTI) {
assert(BI && PBI && "Both blocks must end with a conditional branches.");
assert(is_contained(predecessors(BI->getParent()), PBI->getParent()) &&
"PredBB must be a predecessor of BB.");
// We have the potential to fold the conditions together, but if the
// predecessor branch is predictable, we may not want to merge them.
uint64_t PTWeight, PFWeight;
BranchProbability PBITrueProb, Likely;
if (TTI && !PBI->getMetadata(LLVMContext::MD_unpredictable) &&
extractBranchWeights(*PBI, PTWeight, PFWeight) &&
(PTWeight + PFWeight) != 0) {
PBITrueProb =
BranchProbability::getBranchProbability(PTWeight, PTWeight + PFWeight);
Likely = TTI->getPredictableBranchThreshold();
}
if (PBI->getSuccessor(0) == BI->getSuccessor(0)) {
// Speculate the 2nd condition unless the 1st is probably true.
if (PBITrueProb.isUnknown() || PBITrueProb < Likely)
return {{BI->getSuccessor(0), Instruction::Or, false}};
} else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) {
// Speculate the 2nd condition unless the 1st is probably false.
if (PBITrueProb.isUnknown() || PBITrueProb.getCompl() < Likely)
return {{BI->getSuccessor(1), Instruction::And, false}};
} else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) {
// Speculate the 2nd condition unless the 1st is probably true.
if (PBITrueProb.isUnknown() || PBITrueProb < Likely)
return {{BI->getSuccessor(1), Instruction::And, true}};
} else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) {
// Speculate the 2nd condition unless the 1st is probably false.
if (PBITrueProb.isUnknown() || PBITrueProb.getCompl() < Likely)
return {{BI->getSuccessor(0), Instruction::Or, true}};
}
return std::nullopt;
}
static bool performBranchToCommonDestFolding(CondBrInst *BI, CondBrInst *PBI,
DomTreeUpdater *DTU,
MemorySSAUpdater *MSSAU,
const TargetTransformInfo *TTI) {
BasicBlock *BB = BI->getParent();
BasicBlock *PredBlock = PBI->getParent();
// Determine if the two branches share a common destination.
BasicBlock *CommonSucc;
Instruction::BinaryOps Opc;
bool InvertPredCond;
std::tie(CommonSucc, Opc, InvertPredCond) =
*shouldFoldCondBranchesToCommonDestination(BI, PBI, TTI);
LLVM_DEBUG(dbgs() << "FOLDING BRANCH TO COMMON DEST:\n" << *PBI << *BB);
IRBuilder<> Builder(PBI);
// The builder is used to create instructions to eliminate the branch in BB.
// If BB's terminator has !annotation metadata, add it to the new
// instructions.
Builder.CollectMetadataToCopy(BB->getTerminator(),
{LLVMContext::MD_annotation});
// If we need to invert the condition in the pred block to match, do so now.
if (InvertPredCond) {
InvertBranch(PBI, Builder);
}
BasicBlock *UniqueSucc =
PBI->getSuccessor(0) == BB ? BI->getSuccessor(0) : BI->getSuccessor(1);
// Before cloning instructions, notify the successor basic block that it
// is about to have a new predecessor. This will update PHI nodes,
// which will allow us to update live-out uses of bonus instructions.
addPredecessorToBlock(UniqueSucc, PredBlock, BB, MSSAU);
// Try to update branch weights.
uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
SmallVector<uint64_t, 2> MDWeights;
if (extractPredSuccWeights(PBI, BI, PredTrueWeight, PredFalseWeight,
SuccTrueWeight, SuccFalseWeight)) {
if (PBI->getSuccessor(0) == BB) {
// PBI: br i1 %x, BB, FalseDest
// BI: br i1 %y, UniqueSucc, FalseDest
// TrueWeight is TrueWeight for PBI * TrueWeight for BI.
MDWeights.push_back(PredTrueWeight * SuccTrueWeight);
// FalseWeight is FalseWeight for PBI * TotalWeight for BI +
// TrueWeight for PBI * FalseWeight for BI.
// We assume that total weights of a CondBrInst can fit into 32 bits.
// Therefore, we will not have overflow using 64-bit arithmetic.
MDWeights.push_back(PredFalseWeight * (SuccFalseWeight + SuccTrueWeight) +
PredTrueWeight * SuccFalseWeight);
} else {
// PBI: br i1 %x, TrueDest, BB
// BI: br i1 %y, TrueDest, UniqueSucc
// TrueWeight is TrueWeight for PBI * TotalWeight for BI +
// FalseWeight for PBI * TrueWeight for BI.
MDWeights.push_back(PredTrueWeight * (SuccFalseWeight + SuccTrueWeight) +
PredFalseWeight * SuccTrueWeight);
// FalseWeight is FalseWeight for PBI * FalseWeight for BI.
MDWeights.push_back(PredFalseWeight * SuccFalseWeight);
}
setFittedBranchWeights(*PBI, MDWeights, /*IsExpected=*/false,
/*ElideAllZero=*/true);
// TODO: If BB is reachable from all paths through PredBlock, then we
// could replace PBI's branch probabilities with BI's.
} else
PBI->setMetadata(LLVMContext::MD_prof, nullptr);
// Now, update the CFG.
PBI->setSuccessor(PBI->getSuccessor(0) != BB, UniqueSucc);
if (DTU)
DTU->applyUpdates({{DominatorTree::Insert, PredBlock, UniqueSucc},
{DominatorTree::Delete, PredBlock, BB}});
// If BI was a loop latch, it may have had associated loop metadata.
// We need to copy it to the new latch, that is, PBI.
if (MDNode *LoopMD = BI->getMetadata(LLVMContext::MD_loop))
PBI->setMetadata(LLVMContext::MD_loop, LoopMD);
ValueToValueMapTy VMap; // maps original values to cloned values
cloneInstructionsIntoPredecessorBlockAndUpdateSSAUses(BB, PredBlock, VMap);
Module *M = BB->getModule();
PredBlock->getTerminator()->cloneDebugInfoFrom(BB->getTerminator());
for (DbgVariableRecord &DVR :
filterDbgVars(PredBlock->getTerminator()->getDbgRecordRange())) {
RemapDbgRecord(M, &DVR, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
}
// Now that the Cond was cloned into the predecessor basic block,
// or/and the two conditions together.
Value *BICond = VMap[BI->getCondition()];
PBI->setCondition(
createLogicalOp(Builder, Opc, PBI->getCondition(), BICond, "or.cond"));
if (!ProfcheckDisableMetadataFixes)
if (auto *SI = dyn_cast<SelectInst>(PBI->getCondition()))
if (!MDWeights.empty()) {
assert(isSelectInRoleOfConjunctionOrDisjunction(SI));
setFittedBranchWeights(*SI, {MDWeights[0], MDWeights[1]},
/*IsExpected=*/false, /*ElideAllZero=*/true);
}
++NumFoldBranchToCommonDest;
return true;
}
/// Return if an instruction's type or any of its operands' types are a vector
/// type.
static bool isVectorOp(Instruction &I) {
return I.getType()->isVectorTy() || any_of(I.operands(), [](Use &U) {
return U->getType()->isVectorTy();
});
}
/// If this basic block is simple enough, and if a predecessor branches to us
/// and one of our successors, fold the block into the predecessor and use
/// logical operations to pick the right destination.
bool llvm::foldBranchToCommonDest(CondBrInst *BI, DomTreeUpdater *DTU,
MemorySSAUpdater *MSSAU,
const TargetTransformInfo *TTI,
unsigned BonusInstThreshold) {
BasicBlock *BB = BI->getParent();
TargetTransformInfo::TargetCostKind CostKind =
BB->getParent()->hasMinSize() ? TargetTransformInfo::TCK_CodeSize
: TargetTransformInfo::TCK_SizeAndLatency;
Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
if (!Cond || !isa<CmpInst, BinaryOperator, SelectInst, TruncInst>(Cond) ||
Cond->getParent() != BB || !Cond->hasOneUse())
return false;
// Finally, don't infinitely unroll conditional loops.
if (is_contained(successors(BB), BB))
return false;
// With which predecessors will we want to deal with?
SmallVector<BasicBlock *, 8> Preds;
for (BasicBlock *PredBlock : predecessors(BB)) {
CondBrInst *PBI = dyn_cast<CondBrInst>(PredBlock->getTerminator());
// Check that we have two conditional branches. If there is a PHI node in
// the common successor, verify that the same value flows in from both
// blocks.
if (!PBI || !safeToMergeTerminators(BI, PBI))
continue;
// Determine if the two branches share a common destination.
BasicBlock *CommonSucc;
Instruction::BinaryOps Opc;
bool InvertPredCond;
if (auto Recipe = shouldFoldCondBranchesToCommonDestination(BI, PBI, TTI))
std::tie(CommonSucc, Opc, InvertPredCond) = *Recipe;
else
continue;
// Check the cost of inserting the necessary logic before performing the
// transformation.
if (TTI) {
Type *Ty = BI->getCondition()->getType();
InstructionCost Cost = TTI->getArithmeticInstrCost(Opc, Ty, CostKind);
if (InvertPredCond && (!PBI->getCondition()->hasOneUse() ||
!isa<CmpInst>(PBI->getCondition())))
Cost += TTI->getArithmeticInstrCost(Instruction::Xor, Ty, CostKind);
if (Cost > BranchFoldThreshold)
continue;
}
// Ok, we do want to deal with this predecessor. Record it.
Preds.emplace_back(PredBlock);
}
// If there aren't any predecessors into which we can fold,
// don't bother checking the cost.
if (Preds.empty())
return false;
// Only allow this transformation if computing the condition doesn't involve
// too many instructions and these involved instructions can be executed
// unconditionally. We denote all involved instructions except the condition
// as "bonus instructions", and only allow this transformation when the
// number of the bonus instructions we'll need to create when cloning into
// each predecessor does not exceed a certain threshold.
unsigned NumBonusInsts = 0;
bool SawVectorOp = false;
const unsigned PredCount = Preds.size();
for (Instruction &I : *BB) {
// Don't check the branch condition comparison itself.
if (&I == Cond)
continue;
// Ignore the terminator.
if (isa<UncondBrInst, CondBrInst>(I))
continue;
// I must be safe to execute unconditionally.
if (!isSafeToSpeculativelyExecute(&I))
return false;
SawVectorOp |= isVectorOp(I);
// Account for the cost of duplicating this instruction into each
// predecessor. Ignore free instructions.
if (!TTI || TTI->getInstructionCost(&I, CostKind) !=
TargetTransformInfo::TCC_Free) {
NumBonusInsts += PredCount;
// Early exits once we reach the limit.
if (NumBonusInsts >
BonusInstThreshold * BranchFoldToCommonDestVectorMultiplier)
return false;
}
auto IsBCSSAUse = [BB, &I](Use &U) {
auto *UI = cast<Instruction>(U.getUser());
if (auto *PN = dyn_cast<PHINode>(UI))
return PN->getIncomingBlock(U) == BB;
return UI->getParent() == BB && I.comesBefore(UI);
};
// Does this instruction require rewriting of uses?
if (!all_of(I.uses(), IsBCSSAUse))
return false;
}
if (NumBonusInsts >
BonusInstThreshold *
(SawVectorOp ? BranchFoldToCommonDestVectorMultiplier : 1))
return false;
// Ok, we have the budget. Perform the transformation.
for (BasicBlock *PredBlock : Preds) {
auto *PBI = cast<CondBrInst>(PredBlock->getTerminator());
return performBranchToCommonDestFolding(BI, PBI, DTU, MSSAU, TTI);
}
return false;
}
// If there is only one store in BB1 and BB2, return it, otherwise return
// nullptr.
static StoreInst *findUniqueStoreInBlocks(BasicBlock *BB1, BasicBlock *BB2) {
StoreInst *S = nullptr;
for (auto *BB : {BB1, BB2}) {
if (!BB)
continue;
for (auto &I : *BB)
if (auto *SI = dyn_cast<StoreInst>(&I)) {
if (S)
// Multiple stores seen.
return nullptr;
else
S = SI;
}
}
return S;
}
static Value *ensureValueAvailableInSuccessor(Value *V, BasicBlock *BB,
Value *AlternativeV = nullptr) {
// PHI is going to be a PHI node that allows the value V that is defined in
// BB to be referenced in BB's only successor.
//
// If AlternativeV is nullptr, the only value we care about in PHI is V. It
// doesn't matter to us what the other operand is (it'll never get used). We
// could just create a new PHI with an undef incoming value, but that could
// increase register pressure if EarlyCSE/InstCombine can't fold it with some
// other PHI. So here we directly look for some PHI in BB's successor with V
// as an incoming operand. If we find one, we use it, else we create a new
// one.
//
// If AlternativeV is not nullptr, we care about both incoming values in PHI.
// PHI must be exactly: phi <ty> [ %BB, %V ], [ %OtherBB, %AlternativeV]
// where OtherBB is the single other predecessor of BB's only successor.
PHINode *PHI = nullptr;
BasicBlock *Succ = BB->getSingleSuccessor();
for (auto I = Succ->begin(); isa<PHINode>(I); ++I)
if (cast<PHINode>(I)->getIncomingValueForBlock(BB) == V) {
PHI = cast<PHINode>(I);
if (!AlternativeV)
break;
assert(Succ->hasNPredecessors(2));
auto PredI = pred_begin(Succ);
BasicBlock *OtherPredBB = *PredI == BB ? *++PredI : *PredI;
if (PHI->getIncomingValueForBlock(OtherPredBB) == AlternativeV)
break;
PHI = nullptr;
}
if (PHI)
return PHI;
// If V is not an instruction defined in BB, just return it.
if (!AlternativeV &&
(!isa<Instruction>(V) || cast<Instruction>(V)->getParent() != BB))
return V;
PHI = PHINode::Create(V->getType(), 2, "simplifycfg.merge");
PHI->insertBefore(Succ->begin());
PHI->addIncoming(V, BB);
for (BasicBlock *PredBB : predecessors(Succ))
if (PredBB != BB)
PHI->addIncoming(
AlternativeV ? AlternativeV : PoisonValue::get(V->getType()), PredBB);
return PHI;
}
static bool mergeConditionalStoreToAddress(
BasicBlock *PTB, BasicBlock *PFB, BasicBlock *QTB, BasicBlock *QFB,
BasicBlock *PostBB, Value *Address, bool InvertPCond, bool InvertQCond,
DomTreeUpdater *DTU, const DataLayout &DL, const TargetTransformInfo &TTI) {
// For every pointer, there must be exactly two stores, one coming from
// PTB or PFB, and the other from QTB or QFB. We don't support more than one
// store (to any address) in PTB,PFB or QTB,QFB.
// FIXME: We could relax this restriction with a bit more work and performance
// testing.
StoreInst *PStore = findUniqueStoreInBlocks(PTB, PFB);
StoreInst *QStore = findUniqueStoreInBlocks(QTB, QFB);
if (!PStore || !QStore)
return false;
// Now check the stores are compatible.
if (!QStore->isUnordered() || !PStore->isUnordered() ||
PStore->getValueOperand()->getType() !=
QStore->getValueOperand()->getType())
return false;
// Check that sinking the store won't cause program behavior changes. Sinking
// the store out of the Q blocks won't change any behavior as we're sinking
// from a block to its unconditional successor. But we're moving a store from
// the P blocks down through the middle block (QBI) and past both QFB and QTB.
// So we need to check that there are no aliasing loads or stores in
// QBI, QTB and QFB. We also need to check there are no conflicting memory
// operations between PStore and the end of its parent block.
//
// The ideal way to do this is to query AliasAnalysis, but we don't
// preserve AA currently so that is dangerous. Be super safe and just
// check there are no other memory operations at all.
for (auto &I : *QFB->getSinglePredecessor())
if (I.mayReadOrWriteMemory())
return false;
for (auto &I : *QFB)
if (&I != QStore && I.mayReadOrWriteMemory())
return false;
if (QTB)
for (auto &I : *QTB)
if (&I != QStore && I.mayReadOrWriteMemory())
return false;
for (auto I = BasicBlock::iterator(PStore), E = PStore->getParent()->end();
I != E; ++I)
if (&*I != PStore && I->mayReadOrWriteMemory())
return false;
// If we're not in aggressive mode, we only optimize if we have some
// confidence that by optimizing we'll allow P and/or Q to be if-converted.
auto IsWorthwhile = [&](BasicBlock *BB, ArrayRef<StoreInst *> FreeStores) {
if (!BB)
return true;
// Heuristic: if the block can be if-converted/phi-folded and the
// instructions inside are all cheap (arithmetic/GEPs), it's worthwhile to
// thread this store.
InstructionCost Cost = 0;
InstructionCost Budget =
PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
for (auto &I : *BB) {
// Consider terminator instruction to be free.
if (I.isTerminator())
continue;
// If this is one the stores that we want to speculate out of this BB,
// then don't count it's cost, consider it to be free.
if (auto *S = dyn_cast<StoreInst>(&I))
if (llvm::find(FreeStores, S))
continue;
// Else, we have a white-list of instructions that we are ak speculating.
if (!isa<BinaryOperator>(I) && !isa<GetElementPtrInst>(I))
return false; // Not in white-list - not worthwhile folding.
// And finally, if this is a non-free instruction that we are okay
// speculating, ensure that we consider the speculation budget.
Cost +=
TTI.getInstructionCost(&I, TargetTransformInfo::TCK_SizeAndLatency);
if (Cost > Budget)
return false; // Eagerly refuse to fold as soon as we're out of budget.
}
assert(Cost <= Budget &&
"When we run out of budget we will eagerly return from within the "
"per-instruction loop.");
return true;
};
const std::array<StoreInst *, 2> FreeStores = {PStore, QStore};
if (!MergeCondStoresAggressively &&
(!IsWorthwhile(PTB, FreeStores) || !IsWorthwhile(PFB, FreeStores) ||
!IsWorthwhile(QTB, FreeStores) || !IsWorthwhile(QFB, FreeStores)))
return false;
// If PostBB has more than two predecessors, we need to split it so we can
// sink the store.
if (std::next(pred_begin(PostBB), 2) != pred_end(PostBB)) {
// We know that QFB's only successor is PostBB. And QFB has a single
// predecessor. If QTB exists, then its only successor is also PostBB.
// If QTB does not exist, then QFB's only predecessor has a conditional
// branch to QFB and PostBB.
BasicBlock *TruePred = QTB ? QTB : QFB->getSinglePredecessor();
BasicBlock *NewBB =
SplitBlockPredecessors(PostBB, {QFB, TruePred}, "condstore.split", DTU);
if (!NewBB)
return false;
PostBB = NewBB;
}
// OK, we're going to sink the stores to PostBB. The store has to be
// conditional though, so first create the predicate.
CondBrInst *PBranch =
cast<CondBrInst>(PFB->getSinglePredecessor()->getTerminator());
CondBrInst *QBranch =
cast<CondBrInst>(QFB->getSinglePredecessor()->getTerminator());
Value *PCond = PBranch->getCondition();
Value *QCond = QBranch->getCondition();
Value *PPHI = ensureValueAvailableInSuccessor(PStore->getValueOperand(),
PStore->getParent());
Value *QPHI = ensureValueAvailableInSuccessor(QStore->getValueOperand(),
QStore->getParent(), PPHI);
BasicBlock::iterator PostBBFirst = PostBB->getFirstInsertionPt();
IRBuilder<> QB(PostBB, PostBBFirst);
QB.SetCurrentDebugLocation(PostBBFirst->getStableDebugLoc());
InvertPCond ^= (PStore->getParent() != PTB);
InvertQCond ^= (QStore->getParent() != QTB);
Value *PPred = InvertPCond ? QB.CreateNot(PCond) : PCond;
Value *QPred = InvertQCond ? QB.CreateNot(QCond) : QCond;
Value *CombinedPred = QB.CreateOr(PPred, QPred);
BasicBlock::iterator InsertPt = QB.GetInsertPoint();
auto *T = SplitBlockAndInsertIfThen(CombinedPred, InsertPt,
/*Unreachable=*/false,
/*BranchWeights=*/nullptr, DTU);
if (hasBranchWeightMD(*PBranch) && hasBranchWeightMD(*QBranch) &&
!ProfcheckDisableMetadataFixes) {
SmallVector<uint32_t, 2> PWeights, QWeights;
extractBranchWeights(*PBranch, PWeights);
extractBranchWeights(*QBranch, QWeights);
if (InvertPCond)
std::swap(PWeights[0], PWeights[1]);
if (InvertQCond)
std::swap(QWeights[0], QWeights[1]);
auto CombinedWeights = getDisjunctionWeights(PWeights, QWeights);
setFittedBranchWeights(*PostBB->getTerminator(),
{CombinedWeights[0], CombinedWeights[1]},
/*IsExpected=*/false, /*ElideAllZero=*/true);
}
QB.SetInsertPoint(T);
StoreInst *SI = cast<StoreInst>(QB.CreateStore(QPHI, Address));
SI->setAAMetadata(PStore->getAAMetadata().merge(QStore->getAAMetadata()));
// Choose the minimum alignment. If we could prove both stores execute, we
// could use biggest one. In this case, though, we only know that one of the
// stores executes. And we don't know it's safe to take the alignment from a
// store that doesn't execute.
SI->setAlignment(std::min(PStore->getAlign(), QStore->getAlign()));
QStore->eraseFromParent();
PStore->eraseFromParent();
return true;
}
static bool mergeConditionalStores(CondBrInst *PBI, CondBrInst *QBI,
DomTreeUpdater *DTU, const DataLayout &DL,
const TargetTransformInfo &TTI) {
// The intention here is to find diamonds or triangles (see below) where each
// conditional block contains a store to the same address. Both of these
// stores are conditional, so they can't be unconditionally sunk. But it may
// be profitable to speculatively sink the stores into one merged store at the
// end, and predicate the merged store on the union of the two conditions of
// PBI and QBI.
//
// This can reduce the number of stores executed if both of the conditions are
// true, and can allow the blocks to become small enough to be if-converted.
// This optimization will also chain, so that ladders of test-and-set
// sequences can be if-converted away.
//
// We only deal with simple diamonds or triangles:
//
// PBI or PBI or a combination of the two
// / \ | \
// PTB PFB | PFB
// \ / | /
// QBI QBI
// / \ | \
// QTB QFB | QFB
// \ / | /
// PostBB PostBB
//
// We model triangles as a type of diamond with a nullptr "true" block.
// Triangles are canonicalized so that the fallthrough edge is represented by
// a true condition, as in the diagram above.
BasicBlock *PTB = PBI->getSuccessor(0);
BasicBlock *PFB = PBI->getSuccessor(1);
BasicBlock *QTB = QBI->getSuccessor(0);
BasicBlock *QFB = QBI->getSuccessor(1);
BasicBlock *PostBB = QFB->getSingleSuccessor();
// Make sure we have a good guess for PostBB. If QTB's only successor is
// QFB, then QFB is a better PostBB.
if (QTB->getSingleSuccessor() == QFB)
PostBB = QFB;
// If we couldn't find a good PostBB, stop.
if (!PostBB)
return false;
bool InvertPCond = false, InvertQCond = false;
// Canonicalize fallthroughs to the true branches.
if (PFB == QBI->getParent()) {
std::swap(PFB, PTB);
InvertPCond = true;
}
if (QFB == PostBB) {
std::swap(QFB, QTB);
InvertQCond = true;
}
// From this point on we can assume PTB or QTB may be fallthroughs but PFB
// and QFB may not. Model fallthroughs as a nullptr block.
if (PTB == QBI->getParent())
PTB = nullptr;
if (QTB == PostBB)
QTB = nullptr;
// Legality bailouts. We must have at least the non-fallthrough blocks and
// the post-dominating block, and the non-fallthroughs must only have one
// predecessor.
auto HasOnePredAndOneSucc = [](BasicBlock *BB, BasicBlock *P, BasicBlock *S) {
return BB->getSinglePredecessor() == P && BB->getSingleSuccessor() == S;
};
if (!HasOnePredAndOneSucc(PFB, PBI->getParent(), QBI->getParent()) ||
!HasOnePredAndOneSucc(QFB, QBI->getParent(), PostBB))
return false;
if ((PTB && !HasOnePredAndOneSucc(PTB, PBI->getParent(), QBI->getParent())) ||
(QTB && !HasOnePredAndOneSucc(QTB, QBI->getParent(), PostBB)))
return false;
if (!QBI->getParent()->hasNUses(2))
return false;
// OK, this is a sequence of two diamonds or triangles.
// Check if there are stores in PTB or PFB that are repeated in QTB or QFB.
SmallPtrSet<Value *, 4> PStoreAddresses, QStoreAddresses;
for (auto *BB : {PTB, PFB}) {
if (!BB)
continue;
for (auto &I : *BB)
if (StoreInst *SI = dyn_cast<StoreInst>(&I))
PStoreAddresses.insert(SI->getPointerOperand());
}
for (auto *BB : {QTB, QFB}) {
if (!BB)
continue;
for (auto &I : *BB)
if (StoreInst *SI = dyn_cast<StoreInst>(&I))
QStoreAddresses.insert(SI->getPointerOperand());
}
set_intersect(PStoreAddresses, QStoreAddresses);
// set_intersect mutates PStoreAddresses in place. Rename it here to make it
// clear what it contains.
auto &CommonAddresses = PStoreAddresses;
bool Changed = false;
for (auto *Address : CommonAddresses)
Changed |=
mergeConditionalStoreToAddress(PTB, PFB, QTB, QFB, PostBB, Address,
InvertPCond, InvertQCond, DTU, DL, TTI);
return Changed;
}
/// If the previous block ended with a widenable branch, determine if reusing
/// the target block is profitable and legal. This will have the effect of
/// "widening" PBI, but doesn't require us to reason about hosting safety.
static bool tryWidenCondBranchToCondBranch(CondBrInst *PBI, CondBrInst *BI,
DomTreeUpdater *DTU) {
// TODO: This can be generalized in two important ways:
// 1) We can allow phi nodes in IfFalseBB and simply reuse all the input
// values from the PBI edge.
// 2) We can sink side effecting instructions into BI's fallthrough
// successor provided they doesn't contribute to computation of
// BI's condition.
BasicBlock *IfTrueBB = PBI->getSuccessor(0);
BasicBlock *IfFalseBB = PBI->getSuccessor(1);
if (!isWidenableBranch(PBI) || IfTrueBB != BI->getParent() ||
!BI->getParent()->getSinglePredecessor())
return false;
if (!IfFalseBB->phis().empty())
return false; // TODO
// This helps avoid infinite loop with SimplifyCondBranchToCondBranch which
// may undo the transform done here.
// TODO: There might be a more fine-grained solution to this.
if (!llvm::succ_empty(IfFalseBB))
return false;
// Use lambda to lazily compute expensive condition after cheap ones.
auto NoSideEffects = [](BasicBlock &BB) {
return llvm::none_of(BB, [](const Instruction &I) {
return I.mayWriteToMemory() || I.mayHaveSideEffects();
});
};
if (BI->getSuccessor(1) != IfFalseBB && // no inf looping
BI->getSuccessor(1)->getTerminatingDeoptimizeCall() && // profitability
NoSideEffects(*BI->getParent())) {
auto *OldSuccessor = BI->getSuccessor(1);
OldSuccessor->removePredecessor(BI->getParent());
BI->setSuccessor(1, IfFalseBB);
if (DTU)
DTU->applyUpdates(
{{DominatorTree::Insert, BI->getParent(), IfFalseBB},
{DominatorTree::Delete, BI->getParent(), OldSuccessor}});
return true;
}
if (BI->getSuccessor(0) != IfFalseBB && // no inf looping
BI->getSuccessor(0)->getTerminatingDeoptimizeCall() && // profitability
NoSideEffects(*BI->getParent())) {
auto *OldSuccessor = BI->getSuccessor(0);
OldSuccessor->removePredecessor(BI->getParent());
BI->setSuccessor(0, IfFalseBB);
if (DTU)
DTU->applyUpdates(
{{DominatorTree::Insert, BI->getParent(), IfFalseBB},
{DominatorTree::Delete, BI->getParent(), OldSuccessor}});
return true;
}
return false;
}
/// If we have a conditional branch as a predecessor of another block,
/// this function tries to simplify it. We know
/// that PBI and BI are both conditional branches, and BI is in one of the
/// successor blocks of PBI - PBI branches to BI.
static bool SimplifyCondBranchToCondBranch(CondBrInst *PBI, CondBrInst *BI,
DomTreeUpdater *DTU,
const DataLayout &DL,
const TargetTransformInfo &TTI) {
BasicBlock *BB = BI->getParent();
// If this block ends with a branch instruction, and if there is a
// predecessor that ends on a branch of the same condition, make
// this conditional branch redundant.
if (PBI->getCondition() == BI->getCondition() &&
PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
// Okay, the outcome of this conditional branch is statically
// knowable. If this block had a single pred, handle specially, otherwise
// foldCondBranchOnValueKnownInPredecessor() will handle it.
if (BB->getSinglePredecessor()) {
// Turn this into a branch on constant.
bool CondIsTrue = PBI->getSuccessor(0) == BB;
BI->setCondition(
ConstantInt::get(Type::getInt1Ty(BB->getContext()), CondIsTrue));
return true; // Nuke the branch on constant.
}
}
// If the previous block ended with a widenable branch, determine if reusing
// the target block is profitable and legal. This will have the effect of
// "widening" PBI, but doesn't require us to reason about hosting safety.
if (tryWidenCondBranchToCondBranch(PBI, BI, DTU))
return true;
// If both branches are conditional and both contain stores to the same
// address, remove the stores from the conditionals and create a conditional
// merged store at the end.
if (MergeCondStores && mergeConditionalStores(PBI, BI, DTU, DL, TTI))
return true;
// If this is a conditional branch in an empty block, and if any
// predecessors are a conditional branch to one of our destinations,
// fold the conditions into logical ops and one cond br.
// Ignore dbg intrinsics.
if (&*BB->begin() != BI)
return false;
int PBIOp, BIOp;
if (PBI->getSuccessor(0) == BI->getSuccessor(0)) {
PBIOp = 0;
BIOp = 0;
} else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) {
PBIOp = 0;
BIOp = 1;
} else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) {
PBIOp = 1;
BIOp = 0;
} else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) {
PBIOp = 1;
BIOp = 1;
} else {
return false;
}
// Check to make sure that the other destination of this branch
// isn't BB itself. If so, this is an infinite loop that will
// keep getting unwound.
if (PBI->getSuccessor(PBIOp) == BB)
return false;
// If predecessor's branch probability to BB is too low don't merge branches.
SmallVector<uint32_t, 2> PredWeights;
if (!PBI->getMetadata(LLVMContext::MD_unpredictable) &&
extractBranchWeights(*PBI, PredWeights) &&
(static_cast<uint64_t>(PredWeights[0]) + PredWeights[1]) != 0) {
BranchProbability CommonDestProb = BranchProbability::getBranchProbability(
PredWeights[PBIOp],
static_cast<uint64_t>(PredWeights[0]) + PredWeights[1]);
BranchProbability Likely = TTI.getPredictableBranchThreshold();
if (CommonDestProb >= Likely)
return false;
}
// Do not perform this transformation if it would require
// insertion of a large number of select instructions. For targets
// without predication/cmovs, this is a big pessimization.
BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
BasicBlock *RemovedDest = PBI->getSuccessor(PBIOp ^ 1);
unsigned NumPhis = 0;
for (BasicBlock::iterator II = CommonDest->begin(); isa<PHINode>(II);
++II, ++NumPhis) {
if (NumPhis > 2) // Disable this xform.
return false;
}
// Finally, if everything is ok, fold the branches to logical ops.
BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
LLVM_DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent()
<< "AND: " << *BI->getParent());
SmallVector<DominatorTree::UpdateType, 5> Updates;
// If OtherDest *is* BB, then BB is a basic block with a single conditional
// branch in it, where one edge (OtherDest) goes back to itself but the other
// exits. We don't *know* that the program avoids the infinite loop
// (even though that seems likely). If we do this xform naively, we'll end up
// recursively unpeeling the loop. Since we know that (after the xform is
// done) that the block *is* infinite if reached, we just make it an obviously
// infinite loop with no cond branch.
if (OtherDest == BB) {
// Insert it at the end of the function, because it's either code,
// or it won't matter if it's hot. :)
BasicBlock *InfLoopBlock =
BasicBlock::Create(BB->getContext(), "infloop", BB->getParent());
UncondBrInst::Create(InfLoopBlock, InfLoopBlock);
if (DTU)
Updates.push_back({DominatorTree::Insert, InfLoopBlock, InfLoopBlock});
OtherDest = InfLoopBlock;
}
LLVM_DEBUG(dbgs() << *PBI->getParent()->getParent());
// BI may have other predecessors. Because of this, we leave
// it alone, but modify PBI.
// Make sure we get to CommonDest on True&True directions.
Value *PBICond = PBI->getCondition();
IRBuilder<NoFolder> Builder(PBI);
if (PBIOp)
PBICond = Builder.CreateNot(PBICond, PBICond->getName() + ".not");
Value *BICond = BI->getCondition();
if (BIOp)
BICond = Builder.CreateNot(BICond, BICond->getName() + ".not");
// Merge the conditions.
Value *Cond =
createLogicalOp(Builder, Instruction::Or, PBICond, BICond, "brmerge");
// Modify PBI to branch on the new condition to the new dests.
PBI->setCondition(Cond);
PBI->setSuccessor(0, CommonDest);
PBI->setSuccessor(1, OtherDest);
if (DTU) {
Updates.push_back({DominatorTree::Insert, PBI->getParent(), OtherDest});
Updates.push_back({DominatorTree::Delete, PBI->getParent(), RemovedDest});
DTU->applyUpdates(Updates);
}
// Update branch weight for PBI.
uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
uint64_t PredCommon, PredOther, SuccCommon, SuccOther;
bool HasWeights =
extractPredSuccWeights(PBI, BI, PredTrueWeight, PredFalseWeight,
SuccTrueWeight, SuccFalseWeight);
if (HasWeights) {
PredCommon = PBIOp ? PredFalseWeight : PredTrueWeight;
PredOther = PBIOp ? PredTrueWeight : PredFalseWeight;
SuccCommon = BIOp ? SuccFalseWeight : SuccTrueWeight;
SuccOther = BIOp ? SuccTrueWeight : SuccFalseWeight;
// The weight to CommonDest should be PredCommon * SuccTotal +
// PredOther * SuccCommon.
// The weight to OtherDest should be PredOther * SuccOther.
uint64_t NewWeights[2] = {PredCommon * (SuccCommon + SuccOther) +
PredOther * SuccCommon,
PredOther * SuccOther};
setFittedBranchWeights(*PBI, NewWeights, /*IsExpected=*/false,
/*ElideAllZero=*/true);
// Cond may be a select instruction with the first operand set to "true", or
// the second to "false" (see how createLogicalOp works for `and` and `or`)
if (!ProfcheckDisableMetadataFixes)
if (auto *SI = dyn_cast<SelectInst>(Cond)) {
assert(isSelectInRoleOfConjunctionOrDisjunction(SI));
// The select is predicated on PBICond
assert(SI->getCondition() == PBICond);
// The corresponding probabilities are what was referred to above as
// PredCommon and PredOther.
setFittedBranchWeights(*SI, {PredCommon, PredOther},
/*IsExpected=*/false, /*ElideAllZero=*/true);
}
}
// OtherDest may have phi nodes. If so, add an entry from PBI's
// block that are identical to the entries for BI's block.
addPredecessorToBlock(OtherDest, PBI->getParent(), BB);
// We know that the CommonDest already had an edge from PBI to
// it. If it has PHIs though, the PHIs may have different
// entries for BB and PBI's BB. If so, insert a select to make
// them agree.
for (PHINode &PN : CommonDest->phis()) {
Value *BIV = PN.getIncomingValueForBlock(BB);
unsigned PBBIdx = PN.getBasicBlockIndex(PBI->getParent());
Value *PBIV = PN.getIncomingValue(PBBIdx);
if (BIV != PBIV) {
// Insert a select in PBI to pick the right value.
SelectInst *NV = cast<SelectInst>(
Builder.CreateSelect(PBICond, PBIV, BIV, PBIV->getName() + ".mux"));
PN.setIncomingValue(PBBIdx, NV);
// The select has the same condition as PBI, in the same BB. The
// probabilities don't change.
if (HasWeights) {
uint64_t TrueWeight = PBIOp ? PredFalseWeight : PredTrueWeight;
uint64_t FalseWeight = PBIOp ? PredTrueWeight : PredFalseWeight;
setFittedBranchWeights(*NV, {TrueWeight, FalseWeight},
/*IsExpected=*/false, /*ElideAllZero=*/true);
}
}
}
LLVM_DEBUG(dbgs() << "INTO: " << *PBI->getParent());
LLVM_DEBUG(dbgs() << *PBI->getParent()->getParent());
// This basic block is probably dead. We know it has at least
// one fewer predecessor.
return true;
}
// Simplifies a terminator by replacing it with a branch to TrueBB if Cond is
// true or to FalseBB if Cond is false.
// Takes care of updating the successors and removing the old terminator.
// Also makes sure not to introduce new successors by assuming that edges to
// non-successor TrueBBs and FalseBBs aren't reachable.
bool SimplifyCFGOpt::simplifyTerminatorOnSelect(Instruction *OldTerm,
Value *Cond, BasicBlock *TrueBB,
BasicBlock *FalseBB,
uint32_t TrueWeight,
uint32_t FalseWeight) {
auto *BB = OldTerm->getParent();
// Remove any superfluous successor edges from the CFG.
// First, figure out which successors to preserve.
// If TrueBB and FalseBB are equal, only try to preserve one copy of that
// successor.
BasicBlock *KeepEdge1 = TrueBB;
BasicBlock *KeepEdge2 = TrueBB != FalseBB ? FalseBB : nullptr;
SmallSetVector<BasicBlock *, 2> RemovedSuccessors;
// Then remove the rest.
for (BasicBlock *Succ : successors(OldTerm)) {
// Make sure only to keep exactly one copy of each edge.
if (Succ == KeepEdge1)
KeepEdge1 = nullptr;
else if (Succ == KeepEdge2)
KeepEdge2 = nullptr;
else {
Succ->removePredecessor(BB,
/*KeepOneInputPHIs=*/true);
if (Succ != TrueBB && Succ != FalseBB)
RemovedSuccessors.insert(Succ);
}
}
IRBuilder<> Builder(OldTerm);
Builder.SetCurrentDebugLocation(OldTerm->getDebugLoc());
// Insert an appropriate new terminator.
if (!KeepEdge1 && !KeepEdge2) {
if (TrueBB == FalseBB) {
// We were only looking for one successor, and it was present.
// Create an unconditional branch to it.
Builder.CreateBr(TrueBB);
} else {
// We found both of the successors we were looking for.
// Create a conditional branch sharing the condition of the select.
CondBrInst *NewBI = Builder.CreateCondBr(Cond, TrueBB, FalseBB);
setBranchWeights(*NewBI, {TrueWeight, FalseWeight},
/*IsExpected=*/false, /*ElideAllZero=*/true);
}
} else if (KeepEdge1 && (KeepEdge2 || TrueBB == FalseBB)) {
// Neither of the selected blocks were successors, so this
// terminator must be unreachable.
new UnreachableInst(OldTerm->getContext(), OldTerm->getIterator());
} else {
// One of the selected values was a successor, but the other wasn't.
// Insert an unconditional branch to the one that was found;
// the edge to the one that wasn't must be unreachable.
if (!KeepEdge1) {
// Only TrueBB was found.
Builder.CreateBr(TrueBB);
} else {
// Only FalseBB was found.
Builder.CreateBr(FalseBB);
}
}
eraseTerminatorAndDCECond(OldTerm);
if (DTU) {
SmallVector<DominatorTree::UpdateType, 2> Updates;
Updates.reserve(RemovedSuccessors.size());
for (auto *RemovedSuccessor : RemovedSuccessors)
Updates.push_back({DominatorTree::Delete, BB, RemovedSuccessor});
DTU->applyUpdates(Updates);
}
return true;
}
// Replaces
// (switch (select cond, X, Y)) on constant X, Y
// with a branch - conditional if X and Y lead to distinct BBs,
// unconditional otherwise.
bool SimplifyCFGOpt::simplifySwitchOnSelect(SwitchInst *SI,
SelectInst *Select) {
// Check for constant integer values in the select.
ConstantInt *TrueVal = dyn_cast<ConstantInt>(Select->getTrueValue());
ConstantInt *FalseVal = dyn_cast<ConstantInt>(Select->getFalseValue());
if (!TrueVal || !FalseVal)
return false;
// Find the relevant condition and destinations.
Value *Condition = Select->getCondition();
BasicBlock *TrueBB = SI->findCaseValue(TrueVal)->getCaseSuccessor();
BasicBlock *FalseBB = SI->findCaseValue(FalseVal)->getCaseSuccessor();
// Get weight for TrueBB and FalseBB.
uint32_t TrueWeight = 0, FalseWeight = 0;
SmallVector<uint64_t, 8> Weights;
bool HasWeights = hasBranchWeightMD(*SI);
if (HasWeights) {
getBranchWeights(SI, Weights);
if (Weights.size() == 1 + SI->getNumCases()) {
TrueWeight =
(uint32_t)Weights[SI->findCaseValue(TrueVal)->getSuccessorIndex()];
FalseWeight =
(uint32_t)Weights[SI->findCaseValue(FalseVal)->getSuccessorIndex()];
}
}
// Perform the actual simplification.
return simplifyTerminatorOnSelect(SI, Condition, TrueBB, FalseBB, TrueWeight,
FalseWeight);
}
// Replaces
// (indirectbr (select cond, blockaddress(@fn, BlockA),
// blockaddress(@fn, BlockB)))
// with
// (br cond, BlockA, BlockB).
bool SimplifyCFGOpt::simplifyIndirectBrOnSelect(IndirectBrInst *IBI,
SelectInst *SI) {
// Check that both operands of the select are block addresses.
BlockAddress *TBA = dyn_cast<BlockAddress>(SI->getTrueValue());
BlockAddress *FBA = dyn_cast<BlockAddress>(SI->getFalseValue());
if (!TBA || !FBA)
return false;
// Extract the actual blocks.
BasicBlock *TrueBB = TBA->getBasicBlock();
BasicBlock *FalseBB = FBA->getBasicBlock();
// The select's profile becomes the profile of the conditional branch that
// replaces the indirect branch.
SmallVector<uint32_t> SelectBranchWeights(2);
if (!ProfcheckDisableMetadataFixes)
extractBranchWeights(*SI, SelectBranchWeights);
// Perform the actual simplification.
return simplifyTerminatorOnSelect(IBI, SI->getCondition(), TrueBB, FalseBB,
SelectBranchWeights[0],
SelectBranchWeights[1]);
}
/// This is called when we find an icmp instruction
/// (a seteq/setne with a constant) as the only instruction in a
/// block that ends with an uncond branch. We are looking for a very specific
/// pattern that occurs when "A == 1 || A == 2 || A == 3" gets simplified. In
/// this case, we merge the first two "or's of icmp" into a switch, but then the
/// default value goes to an uncond block with a seteq in it, we get something
/// like:
///
/// switch i8 %A, label %DEFAULT [ i8 1, label %end i8 2, label %end ]
/// DEFAULT:
/// %tmp = icmp eq i8 %A, 92
/// br label %end
/// end:
/// ... = phi i1 [ true, %entry ], [ %tmp, %DEFAULT ], [ true, %entry ]
///
/// We prefer to split the edge to 'end' so that there is a true/false entry to
/// the PHI, merging the third icmp into the switch.
bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpInIt(
ICmpInst *ICI, IRBuilder<> &Builder) {
// Select == nullptr means we assume that there is a hidden no-op select
// instruction of `_ = select %icmp, true, false` after `%icmp = icmp ...`
return tryToSimplifyUncondBranchWithICmpSelectInIt(ICI, nullptr, Builder);
}
/// Similar to tryToSimplifyUncondBranchWithICmpInIt, but handle a more generic
/// case. This is called when we find an icmp instruction (a seteq/setne with a
/// constant) and its following select instruction as the only TWO instructions
/// in a block that ends with an uncond branch. We are looking for a very
/// specific pattern that occurs when "
/// if (A == 1) return C1;
/// if (A == 2) return C2;
/// if (A < 3) return C3;
/// return C4;
/// " gets simplified. In this case, we merge the first two "branches of icmp"
/// into a switch, but then the default value goes to an uncond block with a lt
/// icmp and select in it, as InstCombine can not simplify "A < 3" as "A == 2".
/// After SimplifyCFG and other subsequent optimizations (e.g., SCCP), we might
/// get something like:
///
/// case1:
/// switch i8 %A, label %DEFAULT [ i8 0, label %end i8 1, label %case2 ]
/// case2:
/// br label %end
/// DEFAULT:
/// %tmp = icmp eq i8 %A, 2
/// %val = select i1 %tmp, i8 C3, i8 C4
/// br label %end
/// end:
/// _ = phi i8 [ C1, %case1 ], [ C2, %case2 ], [ %val, %DEFAULT ]
///
/// We prefer to split the edge to 'end' so that there are TWO entries of V3/V4
/// to the PHI, merging the icmp & select into the switch, as follows:
///
/// case1:
/// switch i8 %A, label %DEFAULT [
/// i8 0, label %end
/// i8 1, label %case2
/// i8 2, label %case3
/// ]
/// case2:
/// br label %end
/// case3:
/// br label %end
/// DEFAULT:
/// br label %end
/// end:
/// _ = phi i8 [ C1, %case1 ], [ C2, %case2 ], [ C3, %case2 ], [ C4, %DEFAULT]
bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpSelectInIt(
ICmpInst *ICI, SelectInst *Select, IRBuilder<> &Builder) {
BasicBlock *BB = ICI->getParent();
// If the block has any PHIs in it or the icmp/select has multiple uses, it is
// too complex.
/// TODO: support multi-phis in succ BB of select's BB.
if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse() ||
(Select && !Select->hasOneUse()))
return false;
// The pattern we're looking for is where our only predecessor is a switch on
// 'V' and this block is the default case for the switch. In this case we can
// fold the compared value into the switch to simplify things.
BasicBlock *Pred = BB->getSinglePredecessor();
if (!Pred || !isa<SwitchInst>(Pred->getTerminator()))
return false;
Value *IcmpCond;
ConstantInt *NewCaseVal;
CmpPredicate Predicate;
// Match icmp X, C
if (!match(ICI,
m_ICmp(Predicate, m_Value(IcmpCond), m_ConstantInt(NewCaseVal))))
return false;
Value *SelectCond, *SelectTrueVal, *SelectFalseVal;
Instruction *User;
if (!Select) {
// If Select == nullptr, we can assume that there is a hidden no-op select
// just after icmp
SelectCond = ICI;
SelectTrueVal = Builder.getTrue();
SelectFalseVal = Builder.getFalse();
User = ICI->user_back();
} else {
SelectCond = Select->getCondition();
// Check if the select condition is the same as the icmp condition.
if (SelectCond != ICI)
return false;
SelectTrueVal = Select->getTrueValue();
SelectFalseVal = Select->getFalseValue();
User = Select->user_back();
}
SwitchInst *SI = cast<SwitchInst>(Pred->getTerminator());
if (SI->getCondition() != IcmpCond)
return false;
// If BB is reachable on a non-default case, then we simply know the value of
// V in this block. Substitute it and constant fold the icmp instruction
// away.
if (SI->getDefaultDest() != BB) {
ConstantInt *VVal = SI->findCaseDest(BB);
assert(VVal && "Should have a unique destination value");
ICI->setOperand(0, VVal);
if (Value *V = simplifyInstruction(ICI, {DL, ICI})) {
ICI->replaceAllUsesWith(V);
ICI->eraseFromParent();
}
// BB is now empty, so it is likely to simplify away.
return requestResimplify();
}
// Ok, the block is reachable from the default dest. If the constant we're
// comparing exists in one of the other edges, then we can constant fold ICI
// and zap it.
if (SI->findCaseValue(NewCaseVal) != SI->case_default()) {
Value *V;
if (Predicate == ICmpInst::ICMP_EQ)
V = ConstantInt::getFalse(BB->getContext());
else
V = ConstantInt::getTrue(BB->getContext());
ICI->replaceAllUsesWith(V);
ICI->eraseFromParent();
// BB is now empty, so it is likely to simplify away.
return requestResimplify();
}
// The use of the select has to be in the 'end' block, by the only PHI node in
// the block.
BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0);
PHINode *PHIUse = dyn_cast<PHINode>(User);
if (PHIUse == nullptr || PHIUse != &SuccBlock->front() ||
isa<PHINode>(++BasicBlock::iterator(PHIUse)))
return false;
// If the icmp is a SETEQ, then the default dest gets SelectFalseVal, the new
// edge gets SelectTrueVal in the PHI.
Value *DefaultCst = SelectFalseVal;
Value *NewCst = SelectTrueVal;
if (ICI->getPredicate() == ICmpInst::ICMP_NE)
std::swap(DefaultCst, NewCst);
// Replace Select (which is used by the PHI for the default value) with
// SelectFalseVal or SelectTrueVal depending on if ICI is EQ or NE.
if (Select) {
Select->replaceAllUsesWith(DefaultCst);
Select->eraseFromParent();
} else {
ICI->replaceAllUsesWith(DefaultCst);
}
ICI->eraseFromParent();
SmallVector<DominatorTree::UpdateType, 2> Updates;
// Okay, the switch goes to this block on a default value. Add an edge from
// the switch to the merge point on the compared value.
BasicBlock *NewBB =
BasicBlock::Create(BB->getContext(), "switch.edge", BB->getParent(), BB);
{
SwitchInstProfUpdateWrapper SIW(*SI);
auto W0 = SIW.getSuccessorWeight(0);
SwitchInstProfUpdateWrapper::CaseWeightOpt NewW;
if (W0) {
NewW = ((uint64_t(*W0) + 1) >> 1);
SIW.setSuccessorWeight(0, *NewW);
}
SIW.addCase(NewCaseVal, NewBB, NewW);
if (DTU)
Updates.push_back({DominatorTree::Insert, Pred, NewBB});
}
// NewBB branches to the phi block, add the uncond branch and the phi entry.
Builder.SetInsertPoint(NewBB);
Builder.SetCurrentDebugLocation(SI->getDebugLoc());
Builder.CreateBr(SuccBlock);
PHIUse->addIncoming(NewCst, NewBB);
if (DTU) {
Updates.push_back({DominatorTree::Insert, NewBB, SuccBlock});
DTU->applyUpdates(Updates);
}
return true;
}
/// Check to see if it is branching on an or/and chain of icmp instructions, and
/// fold it into a switch instruction if so.
bool SimplifyCFGOpt::simplifyBranchOnICmpChain(CondBrInst *BI,
IRBuilder<> &Builder,
const DataLayout &DL) {
Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
if (!Cond)
return false;
// Change br (X == 0 | X == 1), T, F into a switch instruction.
// If this is a bunch of seteq's or'd together, or if it's a bunch of
// 'setne's and'ed together, collect them.
// Try to gather values from a chain of and/or to be turned into a switch
ConstantComparesGatherer ConstantCompare(Cond, DL);
// Unpack the result
SmallVectorImpl<ConstantInt *> &Values = ConstantCompare.Vals;
Value *CompVal = ConstantCompare.CompValue;
unsigned UsedICmps = ConstantCompare.UsedICmps;
Value *ExtraCase = ConstantCompare.Extra;
bool TrueWhenEqual = ConstantCompare.IsEq;
// If we didn't have a multiply compared value, fail.
if (!CompVal)
return false;
// Avoid turning single icmps into a switch.
if (UsedICmps <= 1)
return false;
// There might be duplicate constants in the list, which the switch
// instruction can't handle, remove them now.
array_pod_sort(Values.begin(), Values.end(), constantIntSortPredicate);
Values.erase(llvm::unique(Values), Values.end());
// If Extra was used, we require at least two switch values to do the
// transformation. A switch with one value is just a conditional branch.
if (ExtraCase && Values.size() < 2)
return false;
SmallVector<uint32_t> BranchWeights;
const bool HasProfile = !ProfcheckDisableMetadataFixes &&
extractBranchWeights(*BI, BranchWeights);
// Figure out which block is which destination.
BasicBlock *DefaultBB = BI->getSuccessor(1);
BasicBlock *EdgeBB = BI->getSuccessor(0);
if (!TrueWhenEqual) {
std::swap(DefaultBB, EdgeBB);
if (HasProfile)
std::swap(BranchWeights[0], BranchWeights[1]);
}
BasicBlock *BB = BI->getParent();
LLVM_DEBUG(dbgs() << "Converting 'icmp' chain with " << Values.size()
<< " cases into SWITCH. BB is:\n"
<< *BB);
SmallVector<DominatorTree::UpdateType, 2> Updates;
// If there are any extra values that couldn't be folded into the switch
// then we evaluate them with an explicit branch first. Split the block
// right before the condbr to handle it.
if (ExtraCase) {
BasicBlock *NewBB = SplitBlock(BB, BI, DTU, /*LI=*/nullptr,
/*MSSAU=*/nullptr, "switch.early.test");
// Remove the uncond branch added to the old block.
Instruction *OldTI = BB->getTerminator();
Builder.SetInsertPoint(OldTI);
// There can be an unintended UB if extra values are Poison. Before the
// transformation, extra values may not be evaluated according to the
// condition, and it will not raise UB. But after transformation, we are
// evaluating extra values before checking the condition, and it will raise
// UB. It can be solved by adding freeze instruction to extra values.
AssumptionCache *AC = Options.AC;
if (!isGuaranteedNotToBeUndefOrPoison(ExtraCase, AC, BI, nullptr))
ExtraCase = Builder.CreateFreeze(ExtraCase);
// We don't have any info about this condition.
auto *Br = TrueWhenEqual ? Builder.CreateCondBr(ExtraCase, EdgeBB, NewBB)
: Builder.CreateCondBr(ExtraCase, NewBB, EdgeBB);
setExplicitlyUnknownBranchWeightsIfProfiled(*Br, DEBUG_TYPE);
OldTI->eraseFromParent();
if (DTU)
Updates.push_back({DominatorTree::Insert, BB, EdgeBB});
// If there are PHI nodes in EdgeBB, then we need to add a new entry to them
// for the edge we just added.
addPredecessorToBlock(EdgeBB, BB, NewBB);
LLVM_DEBUG(dbgs() << " ** 'icmp' chain unhandled condition: " << *ExtraCase
<< "\nEXTRABB = " << *BB);
BB = NewBB;
}
Builder.SetInsertPoint(BI);
// Convert pointer to int before we switch.
if (CompVal->getType()->isPointerTy()) {
assert(!DL.hasUnstableRepresentation(CompVal->getType()) &&
"Should not end up here with unstable pointers");
CompVal = Builder.CreatePtrToInt(
CompVal, DL.getIntPtrType(CompVal->getType()), "magicptr");
}
// Check if we can represent the values as a contiguous range. If so, we use a
// range check + conditional branch instead of a switch.
if (Values.front()->getValue() - Values.back()->getValue() ==
Values.size() - 1) {
ConstantRange RangeToCheck = ConstantRange::getNonEmpty(
Values.back()->getValue(), Values.front()->getValue() + 1);
APInt Offset, RHS;
ICmpInst::Predicate Pred;
RangeToCheck.getEquivalentICmp(Pred, RHS, Offset);
Value *X = CompVal;
if (!Offset.isZero())
X = Builder.CreateAdd(X, ConstantInt::get(CompVal->getType(), Offset));
Value *Cond =
Builder.CreateICmp(Pred, X, ConstantInt::get(CompVal->getType(), RHS));
CondBrInst *NewBI = Builder.CreateCondBr(Cond, EdgeBB, DefaultBB);
if (HasProfile)
setBranchWeights(*NewBI, BranchWeights, /*IsExpected=*/false);
// We don't need to update PHI nodes since we don't add any new edges.
} else {
// Create the new switch instruction now.
SwitchInst *New = Builder.CreateSwitch(CompVal, DefaultBB, Values.size());
if (HasProfile) {
// We know the weight of the default case. We don't know the weight of the
// other cases, but rather than completely lose profiling info, we split
// the remaining probability equally over them.
SmallVector<uint32_t> NewWeights(Values.size() + 1);
NewWeights[0] = BranchWeights[1]; // this is the default, and we swapped
// if TrueWhenEqual.
for (auto &V : drop_begin(NewWeights))
V = BranchWeights[0] / Values.size();
setBranchWeights(*New, NewWeights, /*IsExpected=*/false);
}
// Add all of the 'cases' to the switch instruction.
for (ConstantInt *Val : Values)
New->addCase(Val, EdgeBB);
// We added edges from PI to the EdgeBB. As such, if there were any
// PHI nodes in EdgeBB, they need entries to be added corresponding to
// the number of edges added.
for (BasicBlock::iterator BBI = EdgeBB->begin(); isa<PHINode>(BBI); ++BBI) {
PHINode *PN = cast<PHINode>(BBI);
Value *InVal = PN->getIncomingValueForBlock(BB);
for (unsigned i = 0, e = Values.size() - 1; i != e; ++i)
PN->addIncoming(InVal, BB);
}
}
// Erase the old branch instruction.
eraseTerminatorAndDCECond(BI);
if (DTU)
DTU->applyUpdates(Updates);
LLVM_DEBUG(dbgs() << " ** 'icmp' chain result is:\n" << *BB << '\n');
return true;
}
bool SimplifyCFGOpt::simplifyResume(ResumeInst *RI, IRBuilder<> &Builder) {
if (isa<PHINode>(RI->getValue()))
return simplifyCommonResume(RI);
else if (isa<LandingPadInst>(RI->getParent()->getFirstNonPHIIt()) &&
RI->getValue() == &*RI->getParent()->getFirstNonPHIIt())
// The resume must unwind the exception that caused control to branch here.
return simplifySingleResume(RI);
return false;
}
// Check if cleanup block is empty
static bool isCleanupBlockEmpty(iterator_range<BasicBlock::iterator> R) {
for (Instruction &I : R) {
auto *II = dyn_cast<IntrinsicInst>(&I);
if (!II)
return false;
Intrinsic::ID IntrinsicID = II->getIntrinsicID();
switch (IntrinsicID) {
case Intrinsic::dbg_declare:
case Intrinsic::dbg_value:
case Intrinsic::dbg_label:
case Intrinsic::lifetime_end:
break;
default:
return false;
}
}
return true;
}
// Simplify resume that is shared by several landing pads (phi of landing pad).
bool SimplifyCFGOpt::simplifyCommonResume(ResumeInst *RI) {
BasicBlock *BB = RI->getParent();
// Check that there are no other instructions except for debug and lifetime
// intrinsics between the phi's and resume instruction.
if (!isCleanupBlockEmpty(make_range(RI->getParent()->getFirstNonPHIIt(),
BB->getTerminator()->getIterator())))
return false;
SmallSetVector<BasicBlock *, 4> TrivialUnwindBlocks;
auto *PhiLPInst = cast<PHINode>(RI->getValue());
// Check incoming blocks to see if any of them are trivial.
for (unsigned Idx = 0, End = PhiLPInst->getNumIncomingValues(); Idx != End;
Idx++) {
auto *IncomingBB = PhiLPInst->getIncomingBlock(Idx);
auto *IncomingValue = PhiLPInst->getIncomingValue(Idx);
// If the block has other successors, we can not delete it because
// it has other dependents.
if (IncomingBB->getUniqueSuccessor() != BB)
continue;
auto *LandingPad = dyn_cast<LandingPadInst>(IncomingBB->getFirstNonPHIIt());
// Not the landing pad that caused the control to branch here.
if (IncomingValue != LandingPad)
continue;
if (isCleanupBlockEmpty(
make_range(LandingPad->getNextNode(), IncomingBB->getTerminator())))
TrivialUnwindBlocks.insert(IncomingBB);
}
// If no trivial unwind blocks, don't do any simplifications.
if (TrivialUnwindBlocks.empty())
return false;
// Turn all invokes that unwind here into calls.
for (auto *TrivialBB : TrivialUnwindBlocks) {
// Blocks that will be simplified should be removed from the phi node.
// Note there could be multiple edges to the resume block, and we need
// to remove them all.
while (PhiLPInst->getBasicBlockIndex(TrivialBB) != -1)
BB->removePredecessor(TrivialBB, true);
for (BasicBlock *Pred :
llvm::make_early_inc_range(predecessors(TrivialBB))) {
removeUnwindEdge(Pred, DTU);
++NumInvokes;
}
// In each SimplifyCFG run, only the current processed block can be erased.
// Otherwise, it will break the iteration of SimplifyCFG pass. So instead
// of erasing TrivialBB, we only remove the branch to the common resume
// block so that we can later erase the resume block since it has no
// predecessors.
TrivialBB->getTerminator()->eraseFromParent();
new UnreachableInst(RI->getContext(), TrivialBB);
if (DTU)
DTU->applyUpdates({{DominatorTree::Delete, TrivialBB, BB}});
}
// Delete the resume block if all its predecessors have been removed.
if (pred_empty(BB))
DeleteDeadBlock(BB, DTU);
return !TrivialUnwindBlocks.empty();
}
// Simplify resume that is only used by a single (non-phi) landing pad.
bool SimplifyCFGOpt::simplifySingleResume(ResumeInst *RI) {
BasicBlock *BB = RI->getParent();
auto *LPInst = cast<LandingPadInst>(BB->getFirstNonPHIIt());
assert(RI->getValue() == LPInst &&
"Resume must unwind the exception that caused control to here");
// Check that there are no other instructions except for debug intrinsics.
if (!isCleanupBlockEmpty(
make_range<Instruction *>(LPInst->getNextNode(), RI)))
return false;
// Turn all invokes that unwind here into calls and delete the basic block.
for (BasicBlock *Pred : llvm::make_early_inc_range(predecessors(BB))) {
removeUnwindEdge(Pred, DTU);
++NumInvokes;
}
// The landingpad is now unreachable. Zap it.
DeleteDeadBlock(BB, DTU);
return true;
}
static bool removeEmptyCleanup(CleanupReturnInst *RI, DomTreeUpdater *DTU) {
// If this is a trivial cleanup pad that executes no instructions, it can be
// eliminated. If the cleanup pad continues to the caller, any predecessor
// that is an EH pad will be updated to continue to the caller and any
// predecessor that terminates with an invoke instruction will have its invoke
// instruction converted to a call instruction. If the cleanup pad being
// simplified does not continue to the caller, each predecessor will be
// updated to continue to the unwind destination of the cleanup pad being
// simplified.
BasicBlock *BB = RI->getParent();
CleanupPadInst *CPInst = RI->getCleanupPad();
if (CPInst->getParent() != BB)
// This isn't an empty cleanup.
return false;
// We cannot kill the pad if it has multiple uses. This typically arises
// from unreachable basic blocks.
if (!CPInst->hasOneUse())
return false;
// Check that there are no other instructions except for benign intrinsics.
if (!isCleanupBlockEmpty(
make_range<Instruction *>(CPInst->getNextNode(), RI)))
return false;
// If the cleanup return we are simplifying unwinds to the caller, this will
// set UnwindDest to nullptr.
BasicBlock *UnwindDest = RI->getUnwindDest();
// We're about to remove BB from the control flow. Before we do, sink any
// PHINodes into the unwind destination. Doing this before changing the
// control flow avoids some potentially slow checks, since we can currently
// be certain that UnwindDest and BB have no common predecessors (since they
// are both EH pads).
if (UnwindDest) {
// First, go through the PHI nodes in UnwindDest and update any nodes that
// reference the block we are removing
for (PHINode &DestPN : UnwindDest->phis()) {
int Idx = DestPN.getBasicBlockIndex(BB);
// Since BB unwinds to UnwindDest, it has to be in the PHI node.
assert(Idx != -1);
// This PHI node has an incoming value that corresponds to a control
// path through the cleanup pad we are removing. If the incoming
// value is in the cleanup pad, it must be a PHINode (because we
// verified above that the block is otherwise empty). Otherwise, the
// value is either a constant or a value that dominates the cleanup
// pad being removed.
//
// Because BB and UnwindDest are both EH pads, all of their
// predecessors must unwind to these blocks, and since no instruction
// can have multiple unwind destinations, there will be no overlap in
// incoming blocks between SrcPN and DestPN.
Value *SrcVal = DestPN.getIncomingValue(Idx);
PHINode *SrcPN = dyn_cast<PHINode>(SrcVal);
bool NeedPHITranslation = SrcPN && SrcPN->getParent() == BB;
for (auto *Pred : predecessors(BB)) {
Value *Incoming =
NeedPHITranslation ? SrcPN->getIncomingValueForBlock(Pred) : SrcVal;
DestPN.addIncoming(Incoming, Pred);
}
}
// Sink any remaining PHI nodes directly into UnwindDest.
BasicBlock::iterator InsertPt = UnwindDest->getFirstNonPHIIt();
for (PHINode &PN : make_early_inc_range(BB->phis())) {
if (PN.use_empty() || !PN.isUsedOutsideOfBlock(BB))
// If the PHI node has no uses or all of its uses are in this basic
// block (meaning they are debug or lifetime intrinsics), just leave
// it. It will be erased when we erase BB below.
continue;
// Otherwise, sink this PHI node into UnwindDest.
// Any predecessors to UnwindDest which are not already represented
// must be back edges which inherit the value from the path through
// BB. In this case, the PHI value must reference itself.
for (auto *pred : predecessors(UnwindDest))
if (pred != BB)
PN.addIncoming(&PN, pred);
PN.moveBefore(InsertPt);
// Also, add a dummy incoming value for the original BB itself,
// so that the PHI is well-formed until we drop said predecessor.
PN.addIncoming(PoisonValue::get(PN.getType()), BB);
}
}
std::vector<DominatorTree::UpdateType> Updates;
// We use make_early_inc_range here because we will remove all predecessors.
for (BasicBlock *PredBB : llvm::make_early_inc_range(predecessors(BB))) {
if (UnwindDest == nullptr) {
if (DTU) {
DTU->applyUpdates(Updates);
Updates.clear();
}
removeUnwindEdge(PredBB, DTU);
++NumInvokes;
} else {
BB->removePredecessor(PredBB);
Instruction *TI = PredBB->getTerminator();
TI->replaceUsesOfWith(BB, UnwindDest);
if (DTU) {
Updates.push_back({DominatorTree::Insert, PredBB, UnwindDest});
Updates.push_back({DominatorTree::Delete, PredBB, BB});
}
}
}
if (DTU)
DTU->applyUpdates(Updates);
DeleteDeadBlock(BB, DTU);
return true;
}
// Try to merge two cleanuppads together.
static bool mergeCleanupPad(CleanupReturnInst *RI) {
// Skip any cleanuprets which unwind to caller, there is nothing to merge
// with.
BasicBlock *UnwindDest = RI->getUnwindDest();
if (!UnwindDest)
return false;
// This cleanupret isn't the only predecessor of this cleanuppad, it wouldn't
// be safe to merge without code duplication.
if (UnwindDest->getSinglePredecessor() != RI->getParent())
return false;
// Verify that our cleanuppad's unwind destination is another cleanuppad.
auto *SuccessorCleanupPad = dyn_cast<CleanupPadInst>(&UnwindDest->front());
if (!SuccessorCleanupPad)
return false;
CleanupPadInst *PredecessorCleanupPad = RI->getCleanupPad();
// Replace any uses of the successor cleanupad with the predecessor pad
// The only cleanuppad uses should be this cleanupret, it's cleanupret and
// funclet bundle operands.
SuccessorCleanupPad->replaceAllUsesWith(PredecessorCleanupPad);
// Remove the old cleanuppad.
SuccessorCleanupPad->eraseFromParent();
// Now, we simply replace the cleanupret with a branch to the unwind
// destination.
UncondBrInst::Create(UnwindDest, RI->getParent());
RI->eraseFromParent();
return true;
}
bool SimplifyCFGOpt::simplifyCleanupReturn(CleanupReturnInst *RI) {
// It is possible to transiantly have an undef cleanuppad operand because we
// have deleted some, but not all, dead blocks.
// Eventually, this block will be deleted.
if (isa<UndefValue>(RI->getOperand(0)))
return false;
if (mergeCleanupPad(RI))
return true;
if (removeEmptyCleanup(RI, DTU))
return true;
return false;
}
// WARNING: keep in sync with InstCombinerImpl::visitUnreachableInst()!
bool SimplifyCFGOpt::simplifyUnreachable(UnreachableInst *UI) {
BasicBlock *BB = UI->getParent();
bool Changed = false;
// Ensure that any debug-info records that used to occur after the Unreachable
// are moved to in front of it -- otherwise they'll "dangle" at the end of
// the block.
BB->flushTerminatorDbgRecords();
// Debug-info records on the unreachable inst itself should be deleted, as
// below we delete everything past the final executable instruction.
UI->dropDbgRecords();
// If there are any instructions immediately before the unreachable that can
// be removed, do so.
while (UI->getIterator() != BB->begin()) {
BasicBlock::iterator BBI = UI->getIterator();
--BBI;
if (!isGuaranteedToTransferExecutionToSuccessor(&*BBI))
break; // Can not drop any more instructions. We're done here.
// Otherwise, this instruction can be freely erased,
// even if it is not side-effect free.
// Note that deleting EH's here is in fact okay, although it involves a bit
// of subtle reasoning. If this inst is an EH, all the predecessors of this
// block will be the unwind edges of Invoke/CatchSwitch/CleanupReturn,
// and we can therefore guarantee this block will be erased.
// If we're deleting this, we're deleting any subsequent debug info, so
// delete DbgRecords.
BBI->dropDbgRecords();
// Delete this instruction (any uses are guaranteed to be dead)
BBI->replaceAllUsesWith(PoisonValue::get(BBI->getType()));
BBI->eraseFromParent();
Changed = true;
}
// If the unreachable instruction is the first in the block, take a gander
// at all of the predecessors of this instruction, and simplify them.
if (&BB->front() != UI)
return Changed;
std::vector<DominatorTree::UpdateType> Updates;
SmallSetVector<BasicBlock *, 8> Preds(pred_begin(BB), pred_end(BB));
for (BasicBlock *Predecessor : Preds) {
Instruction *TI = Predecessor->getTerminator();
IRBuilder<> Builder(TI);
if (isa<UncondBrInst>(TI)) {
new UnreachableInst(TI->getContext(), TI->getIterator());
TI->eraseFromParent();
Changed = true;
if (DTU)
Updates.push_back({DominatorTree::Delete, Predecessor, BB});
} else if (auto *BI = dyn_cast<CondBrInst>(TI)) {
// We could either have a proper unconditional branch,
// or a degenerate conditional branch with matching destinations.
if (BI->getSuccessor(0) == BI->getSuccessor(1)) {
new UnreachableInst(TI->getContext(), TI->getIterator());
TI->eraseFromParent();
Changed = true;
} else {
Value* Cond = BI->getCondition();
assert(BI->getSuccessor(0) != BI->getSuccessor(1) &&
"The destinations are guaranteed to be different here.");
CallInst *Assumption;
if (BI->getSuccessor(0) == BB) {
Assumption = Builder.CreateAssumption(Builder.CreateNot(Cond));
Builder.CreateBr(BI->getSuccessor(1));
} else {
assert(BI->getSuccessor(1) == BB && "Incorrect CFG");
Assumption = Builder.CreateAssumption(Cond);
Builder.CreateBr(BI->getSuccessor(0));
}
if (Options.AC)
Options.AC->registerAssumption(cast<AssumeInst>(Assumption));
eraseTerminatorAndDCECond(BI);
Changed = true;
}
if (DTU)
Updates.push_back({DominatorTree::Delete, Predecessor, BB});
} else if (auto *SI = dyn_cast<SwitchInst>(TI)) {
SwitchInstProfUpdateWrapper SU(*SI);
for (auto i = SU->case_begin(), e = SU->case_end(); i != e;) {
if (i->getCaseSuccessor() != BB) {
++i;
continue;
}
BB->removePredecessor(SU->getParent());
i = SU.removeCase(i);
e = SU->case_end();
Changed = true;
}
// Note that the default destination can't be removed!
if (DTU && SI->getDefaultDest() != BB)
Updates.push_back({DominatorTree::Delete, Predecessor, BB});
} else if (auto *II = dyn_cast<InvokeInst>(TI)) {
if (II->getUnwindDest() == BB) {
if (DTU) {
DTU->applyUpdates(Updates);
Updates.clear();
}
auto *CI = cast<CallInst>(removeUnwindEdge(TI->getParent(), DTU));
if (!CI->doesNotThrow())
CI->setDoesNotThrow();
Changed = true;
}
} else if (auto *CSI = dyn_cast<CatchSwitchInst>(TI)) {
if (CSI->getUnwindDest() == BB) {
if (DTU) {
DTU->applyUpdates(Updates);
Updates.clear();
}
removeUnwindEdge(TI->getParent(), DTU);
Changed = true;
continue;
}
for (CatchSwitchInst::handler_iterator I = CSI->handler_begin(),
E = CSI->handler_end();
I != E; ++I) {
if (*I == BB) {
CSI->removeHandler(I);
--I;
--E;
Changed = true;
}
}
if (DTU)
Updates.push_back({DominatorTree::Delete, Predecessor, BB});
if (CSI->getNumHandlers() == 0) {
if (CSI->hasUnwindDest()) {
// Redirect all predecessors of the block containing CatchSwitchInst
// to instead branch to the CatchSwitchInst's unwind destination.
if (DTU) {
for (auto *PredecessorOfPredecessor : predecessors(Predecessor)) {
Updates.push_back({DominatorTree::Insert,
PredecessorOfPredecessor,
CSI->getUnwindDest()});
Updates.push_back({DominatorTree::Delete,
PredecessorOfPredecessor, Predecessor});
}
}
Predecessor->replaceAllUsesWith(CSI->getUnwindDest());
} else {
// Rewrite all preds to unwind to caller (or from invoke to call).
if (DTU) {
DTU->applyUpdates(Updates);
Updates.clear();
}
SmallVector<BasicBlock *, 8> EHPreds(predecessors(Predecessor));
for (BasicBlock *EHPred : EHPreds)
removeUnwindEdge(EHPred, DTU);
}
// The catchswitch is no longer reachable.
new UnreachableInst(CSI->getContext(), CSI->getIterator());
CSI->eraseFromParent();
Changed = true;
}
} else if (auto *CRI = dyn_cast<CleanupReturnInst>(TI)) {
(void)CRI;
assert(CRI->hasUnwindDest() && CRI->getUnwindDest() == BB &&
"Expected to always have an unwind to BB.");
if (DTU)
Updates.push_back({DominatorTree::Delete, Predecessor, BB});
new UnreachableInst(TI->getContext(), TI->getIterator());
TI->eraseFromParent();
Changed = true;
}
}
if (DTU)
DTU->applyUpdates(Updates);
// If this block is now dead, remove it.
if (pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()) {
DeleteDeadBlock(BB, DTU);
return true;
}
return Changed;
}
struct ContiguousCasesResult {
ConstantInt *Min;
ConstantInt *Max;
BasicBlock *Dest;
BasicBlock *OtherDest;
SmallVectorImpl<ConstantInt *> *Cases;
SmallVectorImpl<ConstantInt *> *OtherCases;
};
static std::optional<ContiguousCasesResult>
findContiguousCases(Value *Condition, SmallVectorImpl<ConstantInt *> &Cases,
SmallVectorImpl<ConstantInt *> &OtherCases,
BasicBlock *Dest, BasicBlock *OtherDest) {
assert(Cases.size() >= 1);
array_pod_sort(Cases.begin(), Cases.end(), constantIntSortPredicate);
const APInt &Min = Cases.back()->getValue();
const APInt &Max = Cases.front()->getValue();
APInt Offset = Max - Min;
size_t ContiguousOffset = Cases.size() - 1;
if (Offset == ContiguousOffset) {
return ContiguousCasesResult{
/*Min=*/Cases.back(),
/*Max=*/Cases.front(),
/*Dest=*/Dest,
/*OtherDest=*/OtherDest,
/*Cases=*/&Cases,
/*OtherCases=*/&OtherCases,
};
}
ConstantRange CR = computeConstantRange(Condition, /*ForSigned=*/false,
SimplifyQuery(Dest->getDataLayout()));
// If this is a wrapping contiguous range, that is, [Min, OtherMin] +
// [OtherMax, Max] (also [OtherMax, OtherMin]), [OtherMin+1, OtherMax-1] is a
// contiguous range for the other destination. N.B. If CR is not a full range,
// Max+1 is not equal to Min. It's not continuous in arithmetic.
if (Max == CR.getUnsignedMax() && Min == CR.getUnsignedMin()) {
assert(Cases.size() >= 2);
auto *It =
std::adjacent_find(Cases.begin(), Cases.end(), [](auto L, auto R) {
return L->getValue() != R->getValue() + 1;
});
if (It == Cases.end())
return std::nullopt;
auto [OtherMax, OtherMin] = std::make_pair(*It, *std::next(It));
if ((Max - OtherMax->getValue()) + (OtherMin->getValue() - Min) ==
Cases.size() - 2) {
return ContiguousCasesResult{
/*Min=*/cast<ConstantInt>(
ConstantInt::get(OtherMin->getType(), OtherMin->getValue() + 1)),
/*Max=*/
cast<ConstantInt>(
ConstantInt::get(OtherMax->getType(), OtherMax->getValue() - 1)),
/*Dest=*/OtherDest,
/*OtherDest=*/Dest,
/*Cases=*/&OtherCases,
/*OtherCases=*/&Cases,
};
}
}
return std::nullopt;
}
static void createUnreachableSwitchDefault(SwitchInst *Switch,
DomTreeUpdater *DTU,
bool RemoveOrigDefaultBlock = true) {
LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
auto *BB = Switch->getParent();
auto *OrigDefaultBlock = Switch->getDefaultDest();
if (RemoveOrigDefaultBlock)
OrigDefaultBlock->removePredecessor(BB);
BasicBlock *NewDefaultBlock = BasicBlock::Create(
BB->getContext(), BB->getName() + ".unreachabledefault", BB->getParent(),
OrigDefaultBlock);
auto *UI = new UnreachableInst(Switch->getContext(), NewDefaultBlock);
UI->setDebugLoc(DebugLoc::getTemporary());
Switch->setDefaultDest(&*NewDefaultBlock);
if (DTU) {
SmallVector<DominatorTree::UpdateType, 2> Updates;
Updates.push_back({DominatorTree::Insert, BB, &*NewDefaultBlock});
if (RemoveOrigDefaultBlock &&
!is_contained(successors(BB), OrigDefaultBlock))
Updates.push_back({DominatorTree::Delete, BB, &*OrigDefaultBlock});
DTU->applyUpdates(Updates);
}
}
/// Turn a switch into an integer range comparison and branch.
/// Switches with more than 2 destinations are ignored.
/// Switches with 1 destination are also ignored.
bool SimplifyCFGOpt::turnSwitchRangeIntoICmp(SwitchInst *SI,
IRBuilder<> &Builder) {
assert(SI->getNumCases() > 1 && "Degenerate switch?");
bool HasDefault = !SI->defaultDestUnreachable();
auto *BB = SI->getParent();
// Partition the cases into two sets with different destinations.
BasicBlock *DestA = HasDefault ? SI->getDefaultDest() : nullptr;
BasicBlock *DestB = nullptr;
SmallVector<ConstantInt *, 16> CasesA;
SmallVector<ConstantInt *, 16> CasesB;
for (auto Case : SI->cases()) {
BasicBlock *Dest = Case.getCaseSuccessor();
if (!DestA)
DestA = Dest;
if (Dest == DestA) {
CasesA.push_back(Case.getCaseValue());
continue;
}
if (!DestB)
DestB = Dest;
if (Dest == DestB) {
CasesB.push_back(Case.getCaseValue());
continue;
}
return false; // More than two destinations.
}
if (!DestB)
return false; // All destinations are the same and the default is unreachable
assert(DestA && DestB &&
"Single-destination switch should have been folded.");
assert(DestA != DestB);
assert(DestB != SI->getDefaultDest());
assert(!CasesB.empty() && "There must be non-default cases.");
assert(!CasesA.empty() || HasDefault);
// Figure out if one of the sets of cases form a contiguous range.
std::optional<ContiguousCasesResult> ContiguousCases;
// Only one icmp is needed when there is only one case.
if (!HasDefault && CasesA.size() == 1)
ContiguousCases = ContiguousCasesResult{
/*Min=*/CasesA[0],
/*Max=*/CasesA[0],
/*Dest=*/DestA,
/*OtherDest=*/DestB,
/*Cases=*/&CasesA,
/*OtherCases=*/&CasesB,
};
else if (CasesB.size() == 1)
ContiguousCases = ContiguousCasesResult{
/*Min=*/CasesB[0],
/*Max=*/CasesB[0],
/*Dest=*/DestB,
/*OtherDest=*/DestA,
/*Cases=*/&CasesB,
/*OtherCases=*/&CasesA,
};
// Correctness: Cases to the default destination cannot be contiguous cases.
else if (!HasDefault)
ContiguousCases =
findContiguousCases(SI->getCondition(), CasesA, CasesB, DestA, DestB);
if (!ContiguousCases)
ContiguousCases =
findContiguousCases(SI->getCondition(), CasesB, CasesA, DestB, DestA);
if (!ContiguousCases)
return false;
auto [Min, Max, Dest, OtherDest, Cases, OtherCases] = *ContiguousCases;
// Start building the compare and branch.
Constant *Offset = ConstantExpr::getNeg(Min);
Constant *NumCases = ConstantInt::get(Offset->getType(),
Max->getValue() - Min->getValue() + 1);
Instruction *NewBI;
if (NumCases->isOneValue()) {
assert(Max->getValue() == Min->getValue());
Value *Cmp = Builder.CreateICmpEQ(SI->getCondition(), Min);
NewBI = Builder.CreateCondBr(Cmp, Dest, OtherDest);
}
// If NumCases overflowed, then all possible values jump to the successor.
else if (NumCases->isNullValue() && !Cases->empty()) {
NewBI = Builder.CreateBr(Dest);
} else {
Value *Sub = SI->getCondition();
if (!Offset->isNullValue())
Sub = Builder.CreateAdd(Sub, Offset, Sub->getName() + ".off");
Value *Cmp = Builder.CreateICmpULT(Sub, NumCases, "switch");
NewBI = Builder.CreateCondBr(Cmp, Dest, OtherDest);
}
// Update weight for the newly-created conditional branch.
if (hasBranchWeightMD(*SI) && isa<CondBrInst>(NewBI)) {
SmallVector<uint64_t, 8> Weights;
getBranchWeights(SI, Weights);
if (Weights.size() == 1 + SI->getNumCases()) {
uint64_t TrueWeight = 0;
uint64_t FalseWeight = 0;
for (size_t I = 0, E = Weights.size(); I != E; ++I) {
if (SI->getSuccessor(I) == Dest)
TrueWeight += Weights[I];
else
FalseWeight += Weights[I];
}
while (TrueWeight > UINT32_MAX || FalseWeight > UINT32_MAX) {
TrueWeight /= 2;
FalseWeight /= 2;
}
setFittedBranchWeights(*NewBI, {TrueWeight, FalseWeight},
/*IsExpected=*/false, /*ElideAllZero=*/true);
}
}
// Prune obsolete incoming values off the successors' PHI nodes.
for (auto &PHI : make_early_inc_range(Dest->phis())) {
unsigned PreviousEdges = Cases->size();
if (Dest == SI->getDefaultDest())
++PreviousEdges;
for (unsigned I = 0, E = PreviousEdges - 1; I != E; ++I)
PHI.removeIncomingValue(SI->getParent());
}
for (auto &PHI : make_early_inc_range(OtherDest->phis())) {
unsigned PreviousEdges = OtherCases->size();
if (OtherDest == SI->getDefaultDest())
++PreviousEdges;
unsigned E = PreviousEdges - 1;
// Remove all incoming values from OtherDest if OtherDest is unreachable.
if (isa<UncondBrInst>(NewBI))
++E;
for (unsigned I = 0; I != E; ++I)
PHI.removeIncomingValue(SI->getParent());
}
// Clean up the default block - it may have phis or other instructions before
// the unreachable terminator.
if (!HasDefault)
createUnreachableSwitchDefault(SI, DTU);
auto *UnreachableDefault = SI->getDefaultDest();
// Drop the switch.
SI->eraseFromParent();
if (!HasDefault && DTU)
DTU->applyUpdates({{DominatorTree::Delete, BB, UnreachableDefault}});
return true;
}
/// Compute masked bits for the condition of a switch
/// and use it to remove dead cases.
static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
AssumptionCache *AC,
const DataLayout &DL) {
Value *Cond = SI->getCondition();
KnownBits Known = computeKnownBits(Cond, DL, AC, SI);
SmallPtrSet<const Constant *, 4> KnownValues;
bool IsKnownValuesValid = collectPossibleValues(Cond, KnownValues, 4);
// We can also eliminate cases by determining that their values are outside of
// the limited range of the condition based on how many significant (non-sign)
// bits are in the condition value.
unsigned MaxSignificantBitsInCond =
ComputeMaxSignificantBits(Cond, DL, AC, SI);
// Gather dead cases.
SmallVector<ConstantInt *, 8> DeadCases;
SmallDenseMap<BasicBlock *, int, 8> NumPerSuccessorCases;
SmallVector<BasicBlock *, 8> UniqueSuccessors;
for (const auto &Case : SI->cases()) {
auto *Successor = Case.getCaseSuccessor();
if (DTU) {
auto [It, Inserted] = NumPerSuccessorCases.try_emplace(Successor);
if (Inserted)
UniqueSuccessors.push_back(Successor);
++It->second;
}
ConstantInt *CaseC = Case.getCaseValue();
const APInt &CaseVal = CaseC->getValue();
if (Known.Zero.intersects(CaseVal) || !Known.One.isSubsetOf(CaseVal) ||
(CaseVal.getSignificantBits() > MaxSignificantBitsInCond) ||
(IsKnownValuesValid && !KnownValues.contains(CaseC))) {
DeadCases.push_back(CaseC);
if (DTU)
--NumPerSuccessorCases[Successor];
LLVM_DEBUG(dbgs() << "SimplifyCFG: switch case " << CaseVal
<< " is dead.\n");
} else if (IsKnownValuesValid)
KnownValues.erase(CaseC);
}
// If we can prove that the cases must cover all possible values, the
// default destination becomes dead and we can remove it. If we know some
// of the bits in the value, we can use that to more precisely compute the
// number of possible unique case values.
bool HasDefault = !SI->defaultDestUnreachable();
const unsigned NumUnknownBits =
Known.getBitWidth() - (Known.Zero | Known.One).popcount();
assert(NumUnknownBits <= Known.getBitWidth());
if (HasDefault && DeadCases.empty()) {
if (IsKnownValuesValid && all_of(KnownValues, IsaPred<UndefValue>)) {
createUnreachableSwitchDefault(SI, DTU);
return true;
}
if (NumUnknownBits < 64 /* avoid overflow */) {
uint64_t AllNumCases = 1ULL << NumUnknownBits;
if (SI->getNumCases() == AllNumCases) {
createUnreachableSwitchDefault(SI, DTU);
return true;
}
// When only one case value is missing, replace default with that case.
// Eliminating the default branch will provide more opportunities for
// optimization, such as lookup tables.
if (SI->getNumCases() == AllNumCases - 1) {
assert(NumUnknownBits > 1 && "Should be canonicalized to a branch");
IntegerType *CondTy = cast<IntegerType>(Cond->getType());
if (CondTy->getIntegerBitWidth() > 64 ||
!DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
return false;
uint64_t MissingCaseVal = 0;
for (const auto &Case : SI->cases())
MissingCaseVal ^= Case.getCaseValue()->getValue().getLimitedValue();
auto *MissingCase = cast<ConstantInt>(
ConstantInt::get(Cond->getType(), MissingCaseVal));
SwitchInstProfUpdateWrapper SIW(*SI);
SIW.addCase(MissingCase, SI->getDefaultDest(),
SIW.getSuccessorWeight(0));
createUnreachableSwitchDefault(SI, DTU,
/*RemoveOrigDefaultBlock*/ false);
SIW.setSuccessorWeight(0, 0);
return true;
}
}
}
if (DeadCases.empty())
return false;
SwitchInstProfUpdateWrapper SIW(*SI);
for (ConstantInt *DeadCase : DeadCases) {
SwitchInst::CaseIt CaseI = SI->findCaseValue(DeadCase);
assert(CaseI != SI->case_default() &&
"Case was not found. Probably mistake in DeadCases forming.");
// Prune unused values from PHI nodes.
CaseI->getCaseSuccessor()->removePredecessor(SI->getParent());
SIW.removeCase(CaseI);
}
if (DTU) {
std::vector<DominatorTree::UpdateType> Updates;
for (auto *Successor : UniqueSuccessors)
if (NumPerSuccessorCases[Successor] == 0)
Updates.push_back({DominatorTree::Delete, SI->getParent(), Successor});
DTU->applyUpdates(Updates);
}
return true;
}
/// If BB would be eligible for simplification by
/// TryToSimplifyUncondBranchFromEmptyBlock (i.e. it is empty and terminated
/// by an unconditional branch), look at the phi node for BB in the successor
/// block and see if the incoming value is equal to CaseValue. If so, return
/// the phi node, and set PhiIndex to BB's index in the phi node.
static PHINode *findPHIForConditionForwarding(ConstantInt *CaseValue,
BasicBlock *BB, int *PhiIndex) {
if (&*BB->getFirstNonPHIIt() != BB->getTerminator())
return nullptr; // BB must be empty to be a candidate for simplification.
if (!BB->getSinglePredecessor())
return nullptr; // BB must be dominated by the switch.
UncondBrInst *Branch = dyn_cast<UncondBrInst>(BB->getTerminator());
if (!Branch)
return nullptr; // Terminator must be unconditional branch.
BasicBlock *Succ = Branch->getSuccessor();
for (PHINode &PHI : Succ->phis()) {
int Idx = PHI.getBasicBlockIndex(BB);
assert(Idx >= 0 && "PHI has no entry for predecessor?");
Value *InValue = PHI.getIncomingValue(Idx);
if (InValue != CaseValue)
continue;
*PhiIndex = Idx;
return &PHI;
}
return nullptr;
}
/// Try to forward the condition of a switch instruction to a phi node
/// dominated by the switch, if that would mean that some of the destination
/// blocks of the switch can be folded away. Return true if a change is made.
static bool forwardSwitchConditionToPHI(SwitchInst *SI) {
using ForwardingNodesMap = DenseMap<PHINode *, SmallVector<int, 4>>;
ForwardingNodesMap ForwardingNodes;
BasicBlock *SwitchBlock = SI->getParent();
bool Changed = false;
for (const auto &Case : SI->cases()) {
ConstantInt *CaseValue = Case.getCaseValue();
BasicBlock *CaseDest = Case.getCaseSuccessor();
// Replace phi operands in successor blocks that are using the constant case
// value rather than the switch condition variable:
// switchbb:
// switch i32 %x, label %default [
// i32 17, label %succ
// ...
// succ:
// %r = phi i32 ... [ 17, %switchbb ] ...
// -->
// %r = phi i32 ... [ %x, %switchbb ] ...
for (PHINode &Phi : CaseDest->phis()) {
// This only works if there is exactly 1 incoming edge from the switch to
// a phi. If there is >1, that means multiple cases of the switch map to 1
// value in the phi, and that phi value is not the switch condition. Thus,
// this transform would not make sense (the phi would be invalid because
// a phi can't have different incoming values from the same block).
int SwitchBBIdx = Phi.getBasicBlockIndex(SwitchBlock);
if (Phi.getIncomingValue(SwitchBBIdx) == CaseValue &&
count(Phi.blocks(), SwitchBlock) == 1) {
Phi.setIncomingValue(SwitchBBIdx, SI->getCondition());
Changed = true;
}
}
// Collect phi nodes that are indirectly using this switch's case constants.
int PhiIdx;
if (auto *Phi = findPHIForConditionForwarding(CaseValue, CaseDest, &PhiIdx))
ForwardingNodes[Phi].push_back(PhiIdx);
}
for (auto &ForwardingNode : ForwardingNodes) {
PHINode *Phi = ForwardingNode.first;
SmallVectorImpl<int> &Indexes = ForwardingNode.second;
// Check if it helps to fold PHI.
if (Indexes.size() < 2 && !llvm::is_contained(Phi->incoming_values(), SI->getCondition()))
continue;
for (int Index : Indexes)
Phi->setIncomingValue(Index, SI->getCondition());
Changed = true;
}
return Changed;
}
/// Return true if the backend will be able to handle
/// initializing an array of constants like C.
static bool validLookupTableConstant(Constant *C, const TargetTransformInfo &TTI) {
if (C->isThreadDependent())
return false;
if (C->isDLLImportDependent())
return false;
if (!isa<ConstantDataVector, ConstantExpr, ConstantFP, ConstantInt,
ConstantPointerNull, GlobalValue, UndefValue>(C))
return false;
// Globals cannot contain scalable types.
if (C->getType()->isScalableTy())
return false;
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
// Pointer casts and in-bounds GEPs will not prohibit the backend from
// materializing the array of constants.
Constant *StrippedC = cast<Constant>(CE->stripInBoundsConstantOffsets());
if (StrippedC == C || !validLookupTableConstant(StrippedC, TTI))
return false;
}
if (!TTI.shouldBuildLookupTablesForConstant(C))
return false;
return true;
}
/// If V is a Constant, return it. Otherwise, try to look up
/// its constant value in ConstantPool, returning 0 if it's not there.
static Constant *
lookupConstant(Value *V,
const SmallDenseMap<Value *, Constant *> &ConstantPool) {
if (Constant *C = dyn_cast<Constant>(V))
return C;
return ConstantPool.lookup(V);
}
/// Try to fold instruction I into a constant. This works for
/// simple instructions such as binary operations where both operands are
/// constant or can be replaced by constants from the ConstantPool. Returns the
/// resulting constant on success, 0 otherwise.
static Constant *
constantFold(Instruction *I, const DataLayout &DL,
const SmallDenseMap<Value *, Constant *> &ConstantPool) {
if (SelectInst *Select = dyn_cast<SelectInst>(I)) {
Constant *A = lookupConstant(Select->getCondition(), ConstantPool);
if (!A)
return nullptr;
if (A->isAllOnesValue())
return lookupConstant(Select->getTrueValue(), ConstantPool);
if (A->isNullValue())
return lookupConstant(Select->getFalseValue(), ConstantPool);
return nullptr;
}
SmallVector<Constant *, 4> COps;
for (unsigned N = 0, E = I->getNumOperands(); N != E; ++N) {
if (Constant *A = lookupConstant(I->getOperand(N), ConstantPool))
COps.push_back(A);
else
return nullptr;
}
return ConstantFoldInstOperands(I, COps, DL);
}
/// Try to determine the resulting constant values in phi nodes
/// at the common destination basic block, *CommonDest, for one of the case
/// destinations CaseDest corresponding to value CaseVal (nullptr for the
/// default case), of a switch instruction SI.
static bool
getCaseResults(SwitchInst *SI, ConstantInt *CaseVal, BasicBlock *CaseDest,
BasicBlock **CommonDest,
SmallVectorImpl<std::pair<PHINode *, Constant *>> &Res,
const DataLayout &DL, const TargetTransformInfo &TTI) {
// The block from which we enter the common destination.
BasicBlock *Pred = SI->getParent();
// If CaseDest is empty except for some side-effect free instructions through
// which we can constant-propagate the CaseVal, continue to its successor.
SmallDenseMap<Value *, Constant *> ConstantPool;
ConstantPool.insert(std::make_pair(SI->getCondition(), CaseVal));
for (Instruction &I : *CaseDest) {
if (I.isTerminator()) {
// If the terminator is a simple branch, continue to the next block.
if (I.getNumSuccessors() != 1 || I.isSpecialTerminator())
return false;
Pred = CaseDest;
CaseDest = I.getSuccessor(0);
} else if (Constant *C = constantFold(&I, DL, ConstantPool)) {
// Instruction is side-effect free and constant.
// If the instruction has uses outside this block or a phi node slot for
// the block, it is not safe to bypass the instruction since it would then
// no longer dominate all its uses.
for (auto &Use : I.uses()) {
User *User = Use.getUser();
if (Instruction *I = dyn_cast<Instruction>(User))
if (I->getParent() == CaseDest)
continue;
if (PHINode *Phi = dyn_cast<PHINode>(User))
if (Phi->getIncomingBlock(Use) == CaseDest)
continue;
return false;
}
ConstantPool.insert(std::make_pair(&I, C));
} else {
break;
}
}
// If we did not have a CommonDest before, use the current one.
if (!*CommonDest)
*CommonDest = CaseDest;
// If the destination isn't the common one, abort.
if (CaseDest != *CommonDest)
return false;
// Get the values for this case from phi nodes in the destination block.
for (PHINode &PHI : (*CommonDest)->phis()) {
int Idx = PHI.getBasicBlockIndex(Pred);
if (Idx == -1)
continue;
Constant *ConstVal =
lookupConstant(PHI.getIncomingValue(Idx), ConstantPool);
if (!ConstVal)
return false;
// Be conservative about which kinds of constants we support.
if (!validLookupTableConstant(ConstVal, TTI))
return false;
Res.push_back(std::make_pair(&PHI, ConstVal));
}
return Res.size() > 0;
}
// Helper function used to add CaseVal to the list of cases that generate
// Result. Returns the updated number of cases that generate this result.
static size_t mapCaseToResult(ConstantInt *CaseVal,
SwitchCaseResultVectorTy &UniqueResults,
Constant *Result) {
for (auto &I : UniqueResults) {
if (I.first == Result) {
I.second.push_back(CaseVal);
return I.second.size();
}
}
UniqueResults.push_back(
std::make_pair(Result, SmallVector<ConstantInt *, 4>(1, CaseVal)));
return 1;
}
// Helper function that initializes a map containing
// results for the PHI node of the common destination block for a switch
// instruction. Returns false if multiple PHI nodes have been found or if
// there is not a common destination block for the switch.
static bool initializeUniqueCases(SwitchInst *SI, PHINode *&PHI,
BasicBlock *&CommonDest,
SwitchCaseResultVectorTy &UniqueResults,
Constant *&DefaultResult,
const DataLayout &DL,
const TargetTransformInfo &TTI,
uintptr_t MaxUniqueResults) {
for (const auto &I : SI->cases()) {
ConstantInt *CaseVal = I.getCaseValue();
// Resulting value at phi nodes for this case value.
SwitchCaseResultsTy Results;
if (!getCaseResults(SI, CaseVal, I.getCaseSuccessor(), &CommonDest, Results,
DL, TTI))
return false;
// Only one value per case is permitted.
if (Results.size() > 1)
return false;
// Add the case->result mapping to UniqueResults.
const size_t NumCasesForResult =
mapCaseToResult(CaseVal, UniqueResults, Results.begin()->second);
// Early out if there are too many cases for this result.
if (NumCasesForResult > MaxSwitchCasesPerResult)
return false;
// Early out if there are too many unique results.
if (UniqueResults.size() > MaxUniqueResults)
return false;
// Check the PHI consistency.
if (!PHI)
PHI = Results[0].first;
else if (PHI != Results[0].first)
return false;
}
// Find the default result value.
SmallVector<std::pair<PHINode *, Constant *>, 1> DefaultResults;
getCaseResults(SI, nullptr, SI->getDefaultDest(), &CommonDest, DefaultResults,
DL, TTI);
// If the default value is not found abort unless the default destination
// is unreachable.
DefaultResult =
DefaultResults.size() == 1 ? DefaultResults.begin()->second : nullptr;
return DefaultResult || SI->defaultDestUnreachable();
}
// Helper function that checks if it is possible to transform a switch with only
// two cases (or two cases + default) that produces a result into a select.
// TODO: Handle switches with more than 2 cases that map to the same result.
// The branch weights correspond to the provided Condition (i.e. if Condition is
// modified from the original SwitchInst, the caller must adjust the weights)
static Value *foldSwitchToSelect(const SwitchCaseResultVectorTy &ResultVector,
Constant *DefaultResult, Value *Condition,
IRBuilder<> &Builder, const DataLayout &DL,
ArrayRef<uint32_t> BranchWeights) {
// If we are selecting between only two cases transform into a simple
// select or a two-way select if default is possible.
// Example:
// switch (a) { %0 = icmp eq i32 %a, 10
// case 10: return 42; %1 = select i1 %0, i32 42, i32 4
// case 20: return 2; ----> %2 = icmp eq i32 %a, 20
// default: return 4; %3 = select i1 %2, i32 2, i32 %1
// }
const bool HasBranchWeights =
!BranchWeights.empty() && !ProfcheckDisableMetadataFixes;
if (ResultVector.size() == 2 && ResultVector[0].second.size() == 1 &&
ResultVector[1].second.size() == 1) {
ConstantInt *FirstCase = ResultVector[0].second[0];
ConstantInt *SecondCase = ResultVector[1].second[0];
Value *SelectValue = ResultVector[1].first;
if (DefaultResult) {
Value *ValueCompare =
Builder.CreateICmpEQ(Condition, SecondCase, "switch.selectcmp");
SelectValue = Builder.CreateSelect(ValueCompare, ResultVector[1].first,
DefaultResult, "switch.select");
if (auto *SI = dyn_cast<SelectInst>(SelectValue);
SI && HasBranchWeights) {
// We start with 3 probabilities, where the numerator is the
// corresponding BranchWeights[i], and the denominator is the sum over
// BranchWeights. We want the probability and negative probability of
// Condition == SecondCase.
assert(BranchWeights.size() == 3);
setBranchWeights(
*SI, {BranchWeights[2], BranchWeights[0] + BranchWeights[1]},
/*IsExpected=*/false, /*ElideAllZero=*/true);
}
}
Value *ValueCompare =
Builder.CreateICmpEQ(Condition, FirstCase, "switch.selectcmp");
Value *Ret = Builder.CreateSelect(ValueCompare, ResultVector[0].first,
SelectValue, "switch.select");
if (auto *SI = dyn_cast<SelectInst>(Ret); SI && HasBranchWeights) {
// We may have had a DefaultResult. Base the position of the first and
// second's branch weights accordingly. Also the proability that Condition
// != FirstCase needs to take that into account.
assert(BranchWeights.size() >= 2);
size_t FirstCasePos = (Condition != nullptr);
size_t SecondCasePos = FirstCasePos + 1;
uint32_t DefaultCase = (Condition != nullptr) ? BranchWeights[0] : 0;
setBranchWeights(*SI,
{BranchWeights[FirstCasePos],
DefaultCase + BranchWeights[SecondCasePos]},
/*IsExpected=*/false, /*ElideAllZero=*/true);
}
return Ret;
}
// Handle the degenerate case where two cases have the same result value.
if (ResultVector.size() == 1 && DefaultResult) {
ArrayRef<ConstantInt *> CaseValues = ResultVector[0].second;
unsigned CaseCount = CaseValues.size();
// n bits group cases map to the same result:
// case 0,4 -> Cond & 0b1..1011 == 0 ? result : default
// case 0,2,4,6 -> Cond & 0b1..1001 == 0 ? result : default
// case 0,2,8,10 -> Cond & 0b1..0101 == 0 ? result : default
if (isPowerOf2_32(CaseCount)) {
ConstantInt *MinCaseVal = CaseValues[0];
// If there are bits that are set exclusively by CaseValues, we
// can transform the switch into a select if the conjunction of
// all the values uniquely identify CaseValues.
APInt AndMask = APInt::getAllOnes(MinCaseVal->getBitWidth());
// Find the minimum value and compute the and of all the case values.
for (auto *Case : CaseValues) {
if (Case->getValue().slt(MinCaseVal->getValue()))
MinCaseVal = Case;
AndMask &= Case->getValue();
}
KnownBits Known = computeKnownBits(Condition, DL);
if (!AndMask.isZero() && Known.getMaxValue().uge(AndMask)) {
// Compute the number of bits that are free to vary.
unsigned FreeBits = Known.countMaxActiveBits() - AndMask.popcount();
// Check if the number of values covered by the mask is equal
// to the number of cases.
if (FreeBits == Log2_32(CaseCount)) {
Value *And = Builder.CreateAnd(Condition, AndMask);
Value *Cmp = Builder.CreateICmpEQ(
And, Constant::getIntegerValue(And->getType(), AndMask));
Value *Ret =
Builder.CreateSelect(Cmp, ResultVector[0].first, DefaultResult);
if (auto *SI = dyn_cast<SelectInst>(Ret); SI && HasBranchWeights) {
// We know there's a Default case. We base the resulting branch
// weights off its probability.
assert(BranchWeights.size() >= 2);
setBranchWeights(
*SI,
{accumulate(drop_begin(BranchWeights), 0U), BranchWeights[0]},
/*IsExpected=*/false, /*ElideAllZero=*/true);
}
return Ret;
}
}
// Mark the bits case number touched.
APInt BitMask = APInt::getZero(MinCaseVal->getBitWidth());
for (auto *Case : CaseValues)
BitMask |= (Case->getValue() - MinCaseVal->getValue());
// Check if cases with the same result can cover all number
// in touched bits.
if (BitMask.popcount() == Log2_32(CaseCount)) {
if (!MinCaseVal->isNullValue())
Condition = Builder.CreateSub(Condition, MinCaseVal);
Value *And = Builder.CreateAnd(Condition, ~BitMask, "switch.and");
Value *Cmp = Builder.CreateICmpEQ(
And, Constant::getNullValue(And->getType()), "switch.selectcmp");
Value *Ret =
Builder.CreateSelect(Cmp, ResultVector[0].first, DefaultResult);
if (auto *SI = dyn_cast<SelectInst>(Ret); SI && HasBranchWeights) {
assert(BranchWeights.size() >= 2);
setBranchWeights(
*SI,
{accumulate(drop_begin(BranchWeights), 0U), BranchWeights[0]},
/*IsExpected=*/false, /*ElideAllZero=*/true);
}
return Ret;
}
}
// Handle the degenerate case where two cases have the same value.
if (CaseValues.size() == 2) {
Value *Cmp1 = Builder.CreateICmpEQ(Condition, CaseValues[0],
"switch.selectcmp.case1");
Value *Cmp2 = Builder.CreateICmpEQ(Condition, CaseValues[1],
"switch.selectcmp.case2");
Value *Cmp = Builder.CreateOr(Cmp1, Cmp2, "switch.selectcmp");
Value *Ret =
Builder.CreateSelect(Cmp, ResultVector[0].first, DefaultResult);
if (auto *SI = dyn_cast<SelectInst>(Ret); SI && HasBranchWeights) {
assert(BranchWeights.size() >= 2);
setBranchWeights(
*SI, {accumulate(drop_begin(BranchWeights), 0U), BranchWeights[0]},
/*IsExpected=*/false, /*ElideAllZero=*/true);
}
return Ret;
}
}
return nullptr;
}
// Helper function to cleanup a switch instruction that has been converted into
// a select, fixing up PHI nodes and basic blocks.
static void removeSwitchAfterSelectFold(SwitchInst *SI, PHINode *PHI,
Value *SelectValue,
IRBuilder<> &Builder,
DomTreeUpdater *DTU) {
std::vector<DominatorTree::UpdateType> Updates;
BasicBlock *SelectBB = SI->getParent();
BasicBlock *DestBB = PHI->getParent();
if (DTU && !is_contained(predecessors(DestBB), SelectBB))
Updates.push_back({DominatorTree::Insert, SelectBB, DestBB});
Builder.CreateBr(DestBB);
// Remove the switch.
PHI->removeIncomingValueIf(
[&](unsigned Idx) { return PHI->getIncomingBlock(Idx) == SelectBB; });
PHI->addIncoming(SelectValue, SelectBB);
SmallPtrSet<BasicBlock *, 4> RemovedSuccessors;
for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) {
BasicBlock *Succ = SI->getSuccessor(i);
if (Succ == DestBB)
continue;
Succ->removePredecessor(SelectBB);
if (DTU && RemovedSuccessors.insert(Succ).second)
Updates.push_back({DominatorTree::Delete, SelectBB, Succ});
}
SI->eraseFromParent();
if (DTU)
DTU->applyUpdates(Updates);
}
/// If a switch is only used to initialize one or more phi nodes in a common
/// successor block with only two different constant values, try to replace the
/// switch with a select. Returns true if the fold was made.
static bool trySwitchToSelect(SwitchInst *SI, IRBuilder<> &Builder,
DomTreeUpdater *DTU, const DataLayout &DL,
const TargetTransformInfo &TTI) {
Value *const Cond = SI->getCondition();
PHINode *PHI = nullptr;
BasicBlock *CommonDest = nullptr;
Constant *DefaultResult;
SwitchCaseResultVectorTy UniqueResults;
// Collect all the cases that will deliver the same value from the switch.
if (!initializeUniqueCases(SI, PHI, CommonDest, UniqueResults, DefaultResult,
DL, TTI, /*MaxUniqueResults*/ 2))
return false;
assert(PHI != nullptr && "PHI for value select not found");
Builder.SetInsertPoint(SI);
SmallVector<uint32_t, 4> BranchWeights;
if (!ProfcheckDisableMetadataFixes) {
[[maybe_unused]] auto HasWeights =
extractBranchWeights(getBranchWeightMDNode(*SI), BranchWeights);
assert(!HasWeights == (BranchWeights.empty()));
}
assert(BranchWeights.empty() ||
(BranchWeights.size() >=
UniqueResults.size() + (DefaultResult != nullptr)));
Value *SelectValue = foldSwitchToSelect(UniqueResults, DefaultResult, Cond,
Builder, DL, BranchWeights);
if (!SelectValue)
return false;
removeSwitchAfterSelectFold(SI, PHI, SelectValue, Builder, DTU);
return true;
}
namespace {
/// This class finds alternatives for switches to ultimately
/// replace the switch.
class SwitchReplacement {
public:
/// Create a helper for optimizations to use as a switch replacement.
/// Find a better representation for the content of Values,
/// using DefaultValue to fill any holes in the table.
SwitchReplacement(
Module &M, uint64_t TableSize, ConstantInt *Offset,
const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values,
Constant *DefaultValue, const DataLayout &DL, const StringRef &FuncName);
/// Build instructions with Builder to retrieve values using Index
/// and replace the switch.
Value *replaceSwitch(Value *Index, IRBuilder<> &Builder, const DataLayout &DL,
Function *Func);
/// Return true if a table with TableSize elements of
/// type ElementType would fit in a target-legal register.
static bool wouldFitInRegister(const DataLayout &DL, uint64_t TableSize,
Type *ElementType);
/// Return the default value of the switch.
Constant *getDefaultValue();
/// Return true if the replacement is a lookup table.
bool isLookupTable();
/// Return true if the replacement is a bit map.
bool isBitMap();
private:
// Depending on the switch, there are different alternatives.
enum {
// For switches where each case contains the same value, we just have to
// store that single value and return it for each lookup.
SingleValueKind,
// For switches where there is a linear relationship between table index
// and values. We calculate the result with a simple multiplication
// and addition instead of a table lookup.
LinearMapKind,
// For small tables with integer elements, we can pack them into a bitmap
// that fits into a target-legal register. Values are retrieved by
// shift and mask operations.
BitMapKind,
// The table is stored as an array of values. Values are retrieved by load
// instructions from the table.
LookupTableKind
} Kind;
// The default value of the switch.
Constant *DefaultValue;
// The type of the output values.
Type *ValueType;
// For SingleValueKind, this is the single value.
Constant *SingleValue = nullptr;
// For BitMapKind, this is the bitmap.
ConstantInt *BitMap = nullptr;
IntegerType *BitMapElementTy = nullptr;
// For LinearMapKind, these are the constants used to derive the value.
ConstantInt *LinearOffset = nullptr;
ConstantInt *LinearMultiplier = nullptr;
bool LinearMapValWrapped = false;
// For LookupTableKind, this is the table.
Constant *Initializer = nullptr;
};
} // end anonymous namespace
SwitchReplacement::SwitchReplacement(
Module &M, uint64_t TableSize, ConstantInt *Offset,
const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values,
Constant *DefaultValue, const DataLayout &DL, const StringRef &FuncName)
: DefaultValue(DefaultValue) {
assert(Values.size() && "Can't build lookup table without values!");
assert(TableSize >= Values.size() && "Can't fit values in table!");
// If all values in the table are equal, this is that value.
SingleValue = Values.begin()->second;
ValueType = Values.begin()->second->getType();
// Build up the table contents.
SmallVector<Constant *, 64> TableContents(TableSize);
for (const auto &[CaseVal, CaseRes] : Values) {
assert(CaseRes->getType() == ValueType);
uint64_t Idx = (CaseVal->getValue() - Offset->getValue()).getLimitedValue();
TableContents[Idx] = CaseRes;
if (SingleValue && !isa<PoisonValue>(CaseRes) && CaseRes != SingleValue)
SingleValue = isa<PoisonValue>(SingleValue) ? CaseRes : nullptr;
}
// Fill in any holes in the table with the default result.
if (Values.size() < TableSize) {
assert(DefaultValue &&
"Need a default value to fill the lookup table holes.");
assert(DefaultValue->getType() == ValueType);
for (uint64_t I = 0; I < TableSize; ++I) {
if (!TableContents[I])
TableContents[I] = DefaultValue;
}
// If the default value is poison, all the holes are poison.
bool DefaultValueIsPoison = isa<PoisonValue>(DefaultValue);
if (DefaultValue != SingleValue && !DefaultValueIsPoison)
SingleValue = nullptr;
}
// If each element in the table contains the same value, we only need to store
// that single value.
if (SingleValue) {
Kind = SingleValueKind;
return;
}
// Check if we can derive the value with a linear transformation from the
// table index.
if (isa<IntegerType>(ValueType)) {
bool LinearMappingPossible = true;
APInt PrevVal;
APInt DistToPrev;
// When linear map is monotonic and signed overflow doesn't happen on
// maximum index, we can attach nsw on Add and Mul.
bool NonMonotonic = false;
assert(TableSize >= 2 && "Should be a SingleValue table.");
// Check if there is the same distance between two consecutive values.
for (uint64_t I = 0; I < TableSize; ++I) {
ConstantInt *ConstVal = dyn_cast<ConstantInt>(TableContents[I]);
if (!ConstVal && isa<PoisonValue>(TableContents[I])) {
// This is an poison, so it's (probably) a lookup table hole.
// To prevent any regressions from before we switched to using poison as
// the default value, holes will fall back to using the first value.
// This can be removed once we add proper handling for poisons in lookup
// tables.
ConstVal = dyn_cast<ConstantInt>(Values[0].second);
}
if (!ConstVal) {
// This is an undef. We could deal with it, but undefs in lookup tables
// are very seldom. It's probably not worth the additional complexity.
LinearMappingPossible = false;
break;
}
const APInt &Val = ConstVal->getValue();
if (I != 0) {
APInt Dist = Val - PrevVal;
if (I == 1) {
DistToPrev = Dist;
} else if (Dist != DistToPrev) {
LinearMappingPossible = false;
break;
}
NonMonotonic |=
Dist.isStrictlyPositive() ? Val.sle(PrevVal) : Val.sgt(PrevVal);
}
PrevVal = Val;
}
if (LinearMappingPossible) {
LinearOffset = cast<ConstantInt>(TableContents[0]);
LinearMultiplier = ConstantInt::get(M.getContext(), DistToPrev);
APInt M = LinearMultiplier->getValue();
bool MayWrap = true;
if (isIntN(M.getBitWidth(), TableSize - 1))
(void)M.smul_ov(APInt(M.getBitWidth(), TableSize - 1), MayWrap);
LinearMapValWrapped = NonMonotonic || MayWrap;
Kind = LinearMapKind;
return;
}
}
// If the type is integer and the table fits in a register, build a bitmap.
if (wouldFitInRegister(DL, TableSize, ValueType)) {
IntegerType *IT = cast<IntegerType>(ValueType);
APInt TableInt(TableSize * IT->getBitWidth(), 0);
for (uint64_t I = TableSize; I > 0; --I) {
TableInt <<= IT->getBitWidth();
// Insert values into the bitmap. Undef values are set to zero.
if (!isa<UndefValue>(TableContents[I - 1])) {
ConstantInt *Val = cast<ConstantInt>(TableContents[I - 1]);
TableInt |= Val->getValue().zext(TableInt.getBitWidth());
}
}
BitMap = ConstantInt::get(M.getContext(), TableInt);
BitMapElementTy = IT;
Kind = BitMapKind;
return;
}
// Store the table in an array.
auto *TableTy = ArrayType::get(ValueType, TableSize);
Initializer = ConstantArray::get(TableTy, TableContents);
Kind = LookupTableKind;
}
Value *SwitchReplacement::replaceSwitch(Value *Index, IRBuilder<> &Builder,
const DataLayout &DL, Function *Func) {
switch (Kind) {
case SingleValueKind:
return SingleValue;
case LinearMapKind: {
++NumLinearMaps;
// Derive the result value from the input value.
Value *Result = Builder.CreateIntCast(Index, LinearMultiplier->getType(),
false, "switch.idx.cast");
if (!LinearMultiplier->isOne())
Result = Builder.CreateMul(Result, LinearMultiplier, "switch.idx.mult",
/*HasNUW = */ false,
/*HasNSW = */ !LinearMapValWrapped);
if (!LinearOffset->isZero())
Result = Builder.CreateAdd(Result, LinearOffset, "switch.offset",
/*HasNUW = */ false,
/*HasNSW = */ !LinearMapValWrapped);
return Result;
}
case BitMapKind: {
++NumBitMaps;
// Type of the bitmap (e.g. i59).
IntegerType *MapTy = BitMap->getIntegerType();
// Cast Index to the same type as the bitmap.
// Note: The Index is <= the number of elements in the table, so
// truncating it to the width of the bitmask is safe.
Value *ShiftAmt = Builder.CreateZExtOrTrunc(Index, MapTy, "switch.cast");
// Multiply the shift amount by the element width. NUW/NSW can always be
// set, because wouldFitInRegister guarantees Index * ShiftAmt is in
// BitMap's bit width.
ShiftAmt = Builder.CreateMul(
ShiftAmt, ConstantInt::get(MapTy, BitMapElementTy->getBitWidth()),
"switch.shiftamt",/*HasNUW =*/true,/*HasNSW =*/true);
// Shift down.
Value *DownShifted =
Builder.CreateLShr(BitMap, ShiftAmt, "switch.downshift");
// Mask off.
return Builder.CreateTrunc(DownShifted, BitMapElementTy, "switch.masked");
}
case LookupTableKind: {
++NumLookupTables;
auto *Table =
new GlobalVariable(*Func->getParent(), Initializer->getType(),
/*isConstant=*/true, GlobalVariable::PrivateLinkage,
Initializer, "switch.table." + Func->getName());
Table->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
// Set the alignment to that of an array items. We will be only loading one
// value out of it.
Table->setAlignment(DL.getPrefTypeAlign(ValueType));
Type *IndexTy = DL.getIndexType(Table->getType());
auto *ArrayTy = cast<ArrayType>(Table->getValueType());
if (Index->getType() != IndexTy) {
unsigned OldBitWidth = Index->getType()->getIntegerBitWidth();
Index = Builder.CreateZExtOrTrunc(Index, IndexTy);
if (auto *Zext = dyn_cast<ZExtInst>(Index))
Zext->setNonNeg(
isUIntN(OldBitWidth - 1, ArrayTy->getNumElements() - 1));
}
Value *GEPIndices[] = {ConstantInt::get(IndexTy, 0), Index};
Value *GEP =
Builder.CreateInBoundsGEP(ArrayTy, Table, GEPIndices, "switch.gep");
return Builder.CreateLoad(ArrayTy->getElementType(), GEP, "switch.load");
}
}
llvm_unreachable("Unknown helper kind!");
}
bool SwitchReplacement::wouldFitInRegister(const DataLayout &DL,
uint64_t TableSize,
Type *ElementType) {
auto *IT = dyn_cast<IntegerType>(ElementType);
if (!IT)
return false;
// FIXME: If the type is wider than it needs to be, e.g. i8 but all values
// are <= 15, we could try to narrow the type.
// Avoid overflow, fitsInLegalInteger uses unsigned int for the width.
if (TableSize >= UINT_MAX / IT->getBitWidth())
return false;
return DL.fitsInLegalInteger(TableSize * IT->getBitWidth());
}
static bool isTypeLegalForLookupTable(Type *Ty, const TargetTransformInfo &TTI,
const DataLayout &DL) {
// Allow any legal type.
if (TTI.isTypeLegal(Ty))
return true;
auto *IT = dyn_cast<IntegerType>(Ty);
if (!IT)
return false;
// Also allow power of 2 integer types that have at least 8 bits and fit in
// a register. These types are common in frontend languages and targets
// usually support loads of these types.
// TODO: We could relax this to any integer that fits in a register and rely
// on ABI alignment and padding in the table to allow the load to be widened.
// Or we could widen the constants and truncate the load.
unsigned BitWidth = IT->getBitWidth();
return BitWidth >= 8 && isPowerOf2_32(BitWidth) &&
DL.fitsInLegalInteger(IT->getBitWidth());
}
Constant *SwitchReplacement::getDefaultValue() { return DefaultValue; }
bool SwitchReplacement::isLookupTable() { return Kind == LookupTableKind; }
bool SwitchReplacement::isBitMap() { return Kind == BitMapKind; }
static bool isSwitchDense(uint64_t NumCases, uint64_t CaseRange) {
// 40% is the default density for building a jump table in optsize/minsize
// mode. See also TargetLoweringBase::isSuitableForJumpTable(), which this
// function was based on.
const uint64_t MinDensity = 40;
if (CaseRange >= UINT64_MAX / 100)
return false; // Avoid multiplication overflows below.
return NumCases * 100 >= CaseRange * MinDensity;
}
static bool isSwitchDense(ArrayRef<int64_t> Values) {
uint64_t Diff = (uint64_t)Values.back() - (uint64_t)Values.front();
uint64_t Range = Diff + 1;
if (Range < Diff)
return false; // Overflow.
return isSwitchDense(Values.size(), Range);
}
/// Determine whether a lookup table should be built for this switch, based on
/// the number of cases, size of the table, and the types of the results.
// TODO: We could support larger than legal types by limiting based on the
// number of loads required and/or table size. If the constants are small we
// could use smaller table entries and extend after the load.
static bool shouldBuildLookupTable(SwitchInst *SI, uint64_t TableSize,
const TargetTransformInfo &TTI,
const DataLayout &DL,
const SmallVector<Type *> &ResultTypes) {
if (SI->getNumCases() > TableSize)
return false; // TableSize overflowed.
bool AllTablesFitInRegister = true;
bool HasIllegalType = false;
for (const auto &Ty : ResultTypes) {
// Saturate this flag to true.
HasIllegalType = HasIllegalType || !isTypeLegalForLookupTable(Ty, TTI, DL);
// Saturate this flag to false.
AllTablesFitInRegister =
AllTablesFitInRegister &&
SwitchReplacement::wouldFitInRegister(DL, TableSize, Ty);
// If both flags saturate, we're done. NOTE: This *only* works with
// saturating flags, and all flags have to saturate first due to the
// non-deterministic behavior of iterating over a dense map.
if (HasIllegalType && !AllTablesFitInRegister)
break;
}
// If each table would fit in a register, we should build it anyway.
if (AllTablesFitInRegister)
return true;
// Don't build a table that doesn't fit in-register if it has illegal types.
if (HasIllegalType)
return false;
return isSwitchDense(SI->getNumCases(), TableSize);
}
static bool shouldUseSwitchConditionAsTableIndex(
ConstantInt &MinCaseVal, const ConstantInt &MaxCaseVal,
bool HasDefaultResults, const SmallVector<Type *> &ResultTypes,
const DataLayout &DL, const TargetTransformInfo &TTI) {
if (MinCaseVal.isNullValue())
return true;
if (MinCaseVal.isNegative() ||
MaxCaseVal.getLimitedValue() == std::numeric_limits<uint64_t>::max() ||
!HasDefaultResults)
return false;
return all_of(ResultTypes, [&](const auto &ResultType) {
return SwitchReplacement::wouldFitInRegister(
DL, MaxCaseVal.getLimitedValue() + 1 /* TableSize */, ResultType);
});
}
/// Try to reuse the switch table index compare. Following pattern:
/// \code
/// if (idx < tablesize)
/// r = table[idx]; // table does not contain default_value
/// else
/// r = default_value;
/// if (r != default_value)
/// ...
/// \endcode
/// Is optimized to:
/// \code
/// cond = idx < tablesize;
/// if (cond)
/// r = table[idx];
/// else
/// r = default_value;
/// if (cond)
/// ...
/// \endcode
/// Jump threading will then eliminate the second if(cond).
static void reuseTableCompare(
User *PhiUser, BasicBlock *PhiBlock, CondBrInst *RangeCheckBranch,
Constant *DefaultValue,
const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values) {
ICmpInst *CmpInst = dyn_cast<ICmpInst>(PhiUser);
if (!CmpInst)
return;
// We require that the compare is in the same block as the phi so that jump
// threading can do its work afterwards.
if (CmpInst->getParent() != PhiBlock)
return;
Constant *CmpOp1 = dyn_cast<Constant>(CmpInst->getOperand(1));
if (!CmpOp1)
return;
Value *RangeCmp = RangeCheckBranch->getCondition();
Constant *TrueConst = ConstantInt::getTrue(RangeCmp->getType());
Constant *FalseConst = ConstantInt::getFalse(RangeCmp->getType());
// Check if the compare with the default value is constant true or false.
const DataLayout &DL = PhiBlock->getDataLayout();
Constant *DefaultConst = ConstantFoldCompareInstOperands(
CmpInst->getPredicate(), DefaultValue, CmpOp1, DL);
if (DefaultConst != TrueConst && DefaultConst != FalseConst)
return;
// Check if the compare with the case values is distinct from the default
// compare result.
for (auto ValuePair : Values) {
Constant *CaseConst = ConstantFoldCompareInstOperands(
CmpInst->getPredicate(), ValuePair.second, CmpOp1, DL);
if (!CaseConst || CaseConst == DefaultConst ||
(CaseConst != TrueConst && CaseConst != FalseConst))
return;
}
// Check if the branch instruction dominates the phi node. It's a simple
// dominance check, but sufficient for our needs.
// Although this check is invariant in the calling loops, it's better to do it
// at this late stage. Practically we do it at most once for a switch.
BasicBlock *BranchBlock = RangeCheckBranch->getParent();
for (BasicBlock *Pred : predecessors(PhiBlock)) {
if (Pred != BranchBlock && Pred->getUniquePredecessor() != BranchBlock)
return;
}
if (DefaultConst == FalseConst) {
// The compare yields the same result. We can replace it.
CmpInst->replaceAllUsesWith(RangeCmp);
++NumTableCmpReuses;
} else {
// The compare yields the same result, just inverted. We can replace it.
Value *InvertedTableCmp = BinaryOperator::CreateXor(
RangeCmp, ConstantInt::get(RangeCmp->getType(), 1), "inverted.cmp",
RangeCheckBranch->getIterator());
CmpInst->replaceAllUsesWith(InvertedTableCmp);
++NumTableCmpReuses;
}
}
/// If the switch is only used to initialize one or more phi nodes in a common
/// successor block with different constant values, replace the switch with
/// lookup tables.
static bool simplifySwitchLookup(SwitchInst *SI, IRBuilder<> &Builder,
DomTreeUpdater *DTU, const DataLayout &DL,
const TargetTransformInfo &TTI,
bool ConvertSwitchToLookupTable) {
assert(SI->getNumCases() > 1 && "Degenerate switch?");
BasicBlock *BB = SI->getParent();
Function *Fn = BB->getParent();
// FIXME: If the switch is too sparse for a lookup table, perhaps we could
// split off a dense part and build a lookup table for that.
// FIXME: This creates arrays of GEPs to constant strings, which means each
// GEP needs a runtime relocation in PIC code. We should just build one big
// string and lookup indices into that.
// Ignore switches with less than three cases. Lookup tables will not make
// them faster, so we don't analyze them.
if (SI->getNumCases() < 3)
return false;
// Figure out the corresponding result for each case value and phi node in the
// common destination, as well as the min and max case values.
assert(!SI->cases().empty());
SwitchInst::CaseIt CI = SI->case_begin();
ConstantInt *MinCaseVal = CI->getCaseValue();
ConstantInt *MaxCaseVal = CI->getCaseValue();
BasicBlock *CommonDest = nullptr;
using ResultListTy = SmallVector<std::pair<ConstantInt *, Constant *>, 4>;
SmallDenseMap<PHINode *, ResultListTy> ResultLists;
SmallDenseMap<PHINode *, Constant *> DefaultResults;
SmallVector<Type *> ResultTypes;
SmallVector<PHINode *, 4> PHIs;
for (SwitchInst::CaseIt E = SI->case_end(); CI != E; ++CI) {
ConstantInt *CaseVal = CI->getCaseValue();
if (CaseVal->getValue().slt(MinCaseVal->getValue()))
MinCaseVal = CaseVal;
if (CaseVal->getValue().sgt(MaxCaseVal->getValue()))
MaxCaseVal = CaseVal;
// Resulting value at phi nodes for this case value.
using ResultsTy = SmallVector<std::pair<PHINode *, Constant *>, 4>;
ResultsTy Results;
if (!getCaseResults(SI, CaseVal, CI->getCaseSuccessor(), &CommonDest,
Results, DL, TTI))
return false;
// Append the result and result types from this case to the list for each
// phi.
for (const auto &I : Results) {
PHINode *PHI = I.first;
Constant *Value = I.second;
auto [It, Inserted] = ResultLists.try_emplace(PHI);
if (Inserted)
PHIs.push_back(PHI);
It->second.push_back(std::make_pair(CaseVal, Value));
ResultTypes.push_back(PHI->getType());
}
}
// If the table has holes, we need a constant result for the default case
// or a bitmask that fits in a register.
SmallVector<std::pair<PHINode *, Constant *>, 4> DefaultResultsList;
bool HasDefaultResults =
getCaseResults(SI, nullptr, SI->getDefaultDest(), &CommonDest,
DefaultResultsList, DL, TTI);
for (const auto &I : DefaultResultsList) {
PHINode *PHI = I.first;
Constant *Result = I.second;
DefaultResults[PHI] = Result;
}
bool UseSwitchConditionAsTableIndex = shouldUseSwitchConditionAsTableIndex(
*MinCaseVal, *MaxCaseVal, HasDefaultResults, ResultTypes, DL, TTI);
uint64_t TableSize;
ConstantInt *TableIndexOffset;
if (UseSwitchConditionAsTableIndex) {
TableSize = MaxCaseVal->getLimitedValue() + 1;
TableIndexOffset = ConstantInt::get(MaxCaseVal->getIntegerType(), 0);
} else {
TableSize =
(MaxCaseVal->getValue() - MinCaseVal->getValue()).getLimitedValue() + 1;
TableIndexOffset = MinCaseVal;
}
// If the default destination is unreachable, or if the lookup table covers
// all values of the conditional variable, branch directly to the lookup table
// BB. Otherwise, check that the condition is within the case range.
uint64_t NumResults = ResultLists[PHIs[0]].size();
bool DefaultIsReachable = !SI->defaultDestUnreachable();
bool TableHasHoles = (NumResults < TableSize);
// If the table has holes but the default destination doesn't produce any
// constant results, the lookup table entries corresponding to the holes will
// contain poison.
bool AllHolesArePoison = TableHasHoles && !HasDefaultResults;
// If the default destination doesn't produce a constant result but is still
// reachable, and the lookup table has holes, we need to use a mask to
// determine if the current index should load from the lookup table or jump
// to the default case.
// The mask is unnecessary if the table has holes but the default destination
// is unreachable, as in that case the holes must also be unreachable.
bool NeedMask = AllHolesArePoison && DefaultIsReachable;
if (NeedMask) {
// As an extra penalty for the validity test we require more cases.
if (SI->getNumCases() < 4) // FIXME: Find best threshold value (benchmark).
return false;
if (!DL.fitsInLegalInteger(TableSize))
return false;
}
if (!shouldBuildLookupTable(SI, TableSize, TTI, DL, ResultTypes))
return false;
// Compute the table index value.
Value *TableIndex;
if (UseSwitchConditionAsTableIndex) {
TableIndex = SI->getCondition();
if (HasDefaultResults) {
// Grow the table to cover all possible index values to avoid the range
// check. It will use the default result to fill in the table hole later,
// so make sure it exist.
ConstantRange CR = computeConstantRange(TableIndex, /*ForSigned=*/false,
SimplifyQuery(DL));
// Grow the table shouldn't have any size impact by checking
// wouldFitInRegister.
// TODO: Consider growing the table also when it doesn't fit in a register
// if no optsize is specified.
const uint64_t UpperBound = CR.getUpper().getLimitedValue();
if (!CR.isUpperWrapped() &&
all_of(ResultTypes, [&](const auto &ResultType) {
return SwitchReplacement::wouldFitInRegister(DL, UpperBound,
ResultType);
})) {
// There may be some case index larger than the UpperBound (unreachable
// case), so make sure the table size does not get smaller.
TableSize = std::max(UpperBound, TableSize);
// The default branch is unreachable after we enlarge the lookup table.
// Adjust DefaultIsReachable to reuse code path.
DefaultIsReachable = false;
}
}
}
// Keep track of the switch replacement for each phi
SmallDenseMap<PHINode *, SwitchReplacement> PhiToReplacementMap;
for (PHINode *PHI : PHIs) {
const auto &ResultList = ResultLists[PHI];
Type *ResultType = ResultList.begin()->second->getType();
// Use any value to fill the lookup table holes.
Constant *DefaultVal =
AllHolesArePoison ? PoisonValue::get(ResultType) : DefaultResults[PHI];
StringRef FuncName = Fn->getName();
SwitchReplacement Replacement(*Fn->getParent(), TableSize, TableIndexOffset,
ResultList, DefaultVal, DL, FuncName);
PhiToReplacementMap.insert({PHI, Replacement});
}
bool AnyLookupTables = any_of(
PhiToReplacementMap, [](auto &KV) { return KV.second.isLookupTable(); });
bool AnyBitMaps = any_of(PhiToReplacementMap,
[](auto &KV) { return KV.second.isBitMap(); });
// A few conditions prevent the generation of lookup tables:
// 1. The target does not support lookup tables.
// 2. The "no-jump-tables" function attribute is set.
// However, these objections do not apply to other switch replacements, like
// the bitmap, so we only stop here if any of these conditions are met and we
// want to create a LUT. Otherwise, continue with the switch replacement.
if (AnyLookupTables &&
(!TTI.shouldBuildLookupTables() ||
Fn->getFnAttribute("no-jump-tables").getValueAsBool()))
return false;
// In the early optimization pipeline, disable formation of lookup tables,
// bit maps and mask checks, as they may inhibit further optimization.
if (!ConvertSwitchToLookupTable &&
(AnyLookupTables || AnyBitMaps || NeedMask))
return false;
Builder.SetInsertPoint(SI);
// TableIndex is the switch condition - TableIndexOffset if we don't
// use the condition directly
if (!UseSwitchConditionAsTableIndex) {
// If the default is unreachable, all case values are s>= MinCaseVal. Then
// we can try to attach nsw.
bool MayWrap = true;
if (!DefaultIsReachable) {
APInt Res =
MaxCaseVal->getValue().ssub_ov(MinCaseVal->getValue(), MayWrap);
(void)Res;
}
TableIndex = Builder.CreateSub(SI->getCondition(), TableIndexOffset,
"switch.tableidx", /*HasNUW =*/false,
/*HasNSW =*/!MayWrap);
}
std::vector<DominatorTree::UpdateType> Updates;
// Compute the maximum table size representable by the integer type we are
// switching upon.
unsigned CaseSize = MinCaseVal->getType()->getPrimitiveSizeInBits();
uint64_t MaxTableSize = CaseSize > 63 ? UINT64_MAX : 1ULL << CaseSize;
assert(MaxTableSize >= TableSize &&
"It is impossible for a switch to have more entries than the max "
"representable value of its input integer type's size.");
// Create the BB that does the lookups.
Module &Mod = *CommonDest->getParent()->getParent();
BasicBlock *LookupBB = BasicBlock::Create(
Mod.getContext(), "switch.lookup", CommonDest->getParent(), CommonDest);
CondBrInst *RangeCheckBranch = nullptr;
CondBrInst *CondBranch = nullptr;
Builder.SetInsertPoint(SI);
const bool GeneratingCoveredLookupTable = (MaxTableSize == TableSize);
if (!DefaultIsReachable || GeneratingCoveredLookupTable) {
Builder.CreateBr(LookupBB);
if (DTU)
Updates.push_back({DominatorTree::Insert, BB, LookupBB});
// Note: We call removeProdecessor later since we need to be able to get the
// PHI value for the default case in case we're using a bit mask.
} else {
Value *Cmp = Builder.CreateICmpULT(
TableIndex, ConstantInt::get(MinCaseVal->getType(), TableSize));
RangeCheckBranch =
Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
CondBranch = RangeCheckBranch;
if (DTU)
Updates.push_back({DominatorTree::Insert, BB, LookupBB});
}
// Populate the BB that does the lookups.
Builder.SetInsertPoint(LookupBB);
if (NeedMask) {
// Before doing the lookup, we do the hole check. The LookupBB is therefore
// re-purposed to do the hole check, and we create a new LookupBB.
BasicBlock *MaskBB = LookupBB;
MaskBB->setName("switch.hole_check");
LookupBB = BasicBlock::Create(Mod.getContext(), "switch.lookup",
CommonDest->getParent(), CommonDest);
// Make the mask's bitwidth at least 8-bit and a power-of-2 to avoid
// unnecessary illegal types.
uint64_t TableSizePowOf2 = NextPowerOf2(std::max(7ULL, TableSize - 1ULL));
APInt MaskInt(TableSizePowOf2, 0);
APInt One(TableSizePowOf2, 1);
// Build bitmask; fill in a 1 bit for every case.
const ResultListTy &ResultList = ResultLists[PHIs[0]];
for (const auto &Result : ResultList) {
uint64_t Idx = (Result.first->getValue() - TableIndexOffset->getValue())
.getLimitedValue();
MaskInt |= One << Idx;
}
ConstantInt *TableMask = ConstantInt::get(Mod.getContext(), MaskInt);
// Get the TableIndex'th bit of the bitmask.
// If this bit is 0 (meaning hole) jump to the default destination,
// else continue with table lookup.
IntegerType *MapTy = TableMask->getIntegerType();
Value *MaskIndex =
Builder.CreateZExtOrTrunc(TableIndex, MapTy, "switch.maskindex");
Value *Shifted = Builder.CreateLShr(TableMask, MaskIndex, "switch.shifted");
Value *LoBit = Builder.CreateTrunc(
Shifted, Type::getInt1Ty(Mod.getContext()), "switch.lobit");
CondBranch = Builder.CreateCondBr(LoBit, LookupBB, SI->getDefaultDest());
if (DTU) {
Updates.push_back({DominatorTree::Insert, MaskBB, LookupBB});
Updates.push_back({DominatorTree::Insert, MaskBB, SI->getDefaultDest()});
}
Builder.SetInsertPoint(LookupBB);
addPredecessorToBlock(SI->getDefaultDest(), MaskBB, BB);
}
if (!DefaultIsReachable || GeneratingCoveredLookupTable) {
// We cached PHINodes in PHIs. To avoid accessing deleted PHINodes later,
// do not delete PHINodes here.
SI->getDefaultDest()->removePredecessor(BB,
/*KeepOneInputPHIs=*/true);
if (DTU)
Updates.push_back({DominatorTree::Delete, BB, SI->getDefaultDest()});
}
for (PHINode *PHI : PHIs) {
const ResultListTy &ResultList = ResultLists[PHI];
auto Replacement = PhiToReplacementMap.at(PHI);
auto *Result = Replacement.replaceSwitch(TableIndex, Builder, DL, Fn);
// Do a small peephole optimization: re-use the switch table compare if
// possible.
if (!TableHasHoles && HasDefaultResults && RangeCheckBranch) {
BasicBlock *PhiBlock = PHI->getParent();
// Search for compare instructions which use the phi.
for (auto *User : PHI->users()) {
reuseTableCompare(User, PhiBlock, RangeCheckBranch,
Replacement.getDefaultValue(), ResultList);
}
}
PHI->addIncoming(Result, LookupBB);
}
Builder.CreateBr(CommonDest);
if (DTU)
Updates.push_back({DominatorTree::Insert, LookupBB, CommonDest});
SmallVector<uint32_t> BranchWeights;
const bool HasBranchWeights = CondBranch && !ProfcheckDisableMetadataFixes &&
extractBranchWeights(*SI, BranchWeights);
uint64_t ToLookupWeight = 0;
uint64_t ToDefaultWeight = 0;
// Remove the switch.
SmallPtrSet<BasicBlock *, 8> RemovedSuccessors;
for (unsigned I = 0, E = SI->getNumSuccessors(); I < E; ++I) {
BasicBlock *Succ = SI->getSuccessor(I);
if (Succ == SI->getDefaultDest()) {
if (HasBranchWeights)
ToDefaultWeight += BranchWeights[I];
continue;
}
Succ->removePredecessor(BB);
if (DTU && RemovedSuccessors.insert(Succ).second)
Updates.push_back({DominatorTree::Delete, BB, Succ});
if (HasBranchWeights)
ToLookupWeight += BranchWeights[I];
}
SI->eraseFromParent();
if (HasBranchWeights)
setFittedBranchWeights(*CondBranch, {ToLookupWeight, ToDefaultWeight},
/*IsExpected=*/false);
if (DTU)
DTU->applyUpdates(Updates);
if (NeedMask)
++NumLookupTablesHoles;
return true;
}
/// Try to transform a switch that has "holes" in it to a contiguous sequence
/// of cases.
///
/// A switch such as: switch(i) {case 5: case 9: case 13: case 17:} can be
/// range-reduced to: switch ((i-5) / 4) {case 0: case 1: case 2: case 3:}.
///
/// This converts a sparse switch into a dense switch which allows better
/// lowering and could also allow transforming into a lookup table.
static bool reduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
const DataLayout &DL,
const TargetTransformInfo &TTI) {
auto *CondTy = cast<IntegerType>(SI->getCondition()->getType());
if (CondTy->getIntegerBitWidth() > 64 ||
!DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
return false;
// Only bother with this optimization if there are more than 3 switch cases;
// SDAG will only bother creating jump tables for 4 or more cases.
if (SI->getNumCases() < 4)
return false;
// This transform is agnostic to the signedness of the input or case values. We
// can treat the case values as signed or unsigned. We can optimize more common
// cases such as a sequence crossing zero {-4,0,4,8} if we interpret case values
// as signed.
SmallVector<int64_t,4> Values;
for (const auto &C : SI->cases())
Values.push_back(C.getCaseValue()->getValue().getSExtValue());
llvm::sort(Values);
// If the switch is already dense, there's nothing useful to do here.
if (isSwitchDense(Values))
return false;
// First, transform the values such that they start at zero and ascend.
int64_t Base = Values[0];
for (auto &V : Values)
V -= (uint64_t)(Base);
// Now we have signed numbers that have been shifted so that, given enough
// precision, there are no negative values. Since the rest of the transform
// is bitwise only, we switch now to an unsigned representation.
// This transform can be done speculatively because it is so cheap - it
// results in a single rotate operation being inserted.
// countTrailingZeros(0) returns 64. As Values is guaranteed to have more than
// one element and LLVM disallows duplicate cases, Shift is guaranteed to be
// less than 64.
unsigned Shift = 64;
for (auto &V : Values)
Shift = std::min(Shift, (unsigned)llvm::countr_zero((uint64_t)V));
assert(Shift < 64);
if (Shift > 0)
for (auto &V : Values)
V = (int64_t)((uint64_t)V >> Shift);
if (!isSwitchDense(Values))
// Transform didn't create a dense switch.
return false;
// The obvious transform is to shift the switch condition right and emit a
// check that the condition actually cleanly divided by GCD, i.e.
// C & (1 << Shift - 1) == 0
// inserting a new CFG edge to handle the case where it didn't divide cleanly.
//
// A cheaper way of doing this is a simple ROTR(C, Shift). This performs the
// shift and puts the shifted-off bits in the uppermost bits. If any of these
// are nonzero then the switch condition will be very large and will hit the
// default case.
auto *Ty = cast<IntegerType>(SI->getCondition()->getType());
Builder.SetInsertPoint(SI);
Value *Sub =
Builder.CreateSub(SI->getCondition(), ConstantInt::getSigned(Ty, Base));
Value *Rot = Builder.CreateIntrinsic(
Ty, Intrinsic::fshl,
{Sub, Sub, ConstantInt::get(Ty, Ty->getBitWidth() - Shift)});
SI->replaceUsesOfWith(SI->getCondition(), Rot);
for (auto Case : SI->cases()) {
auto *Orig = Case.getCaseValue();
auto Sub = Orig->getValue() - APInt(Ty->getBitWidth(), Base, true);
Case.setValue(cast<ConstantInt>(ConstantInt::get(Ty, Sub.lshr(Shift))));
}
return true;
}
/// Tries to transform the switch when the condition is umin with a constant.
/// In that case, the default branch can be replaced by the constant's branch.
/// This method also removes dead cases when the simplification cannot replace
/// the default branch.
///
/// For example:
/// switch(umin(a, 3)) {
/// case 0:
/// case 1:
/// case 2:
/// case 3:
/// case 4:
/// // ...
/// default:
/// unreachable
/// }
///
/// Transforms into:
///
/// switch(a) {
/// case 0:
/// case 1:
/// case 2:
/// default:
/// // This is case 3
/// }
static bool simplifySwitchWhenUMin(SwitchInst *SI, DomTreeUpdater *DTU) {
Value *A;
ConstantInt *Constant;
if (!match(SI->getCondition(), m_UMin(m_Value(A), m_ConstantInt(Constant))))
return false;
SmallVector<DominatorTree::UpdateType> Updates;
SwitchInstProfUpdateWrapper SIW(*SI);
BasicBlock *BB = SIW->getParent();
// Dead cases are removed even when the simplification fails.
// A case is dead when its value is higher than the Constant.
for (auto I = SI->case_begin(), E = SI->case_end(); I != E;) {
if (!I->getCaseValue()->getValue().ugt(Constant->getValue())) {
++I;
continue;
}
BasicBlock *DeadCaseBB = I->getCaseSuccessor();
DeadCaseBB->removePredecessor(BB);
Updates.push_back({DominatorTree::Delete, BB, DeadCaseBB});
I = SIW.removeCase(I);
E = SIW->case_end();
}
auto Case = SI->findCaseValue(Constant);
// If the case value is not found, `findCaseValue` returns the default case.
// In this scenario, since there is no explicit `case 3:`, the simplification
// fails. The simplification also fails when the switch’s default destination
// is reachable.
if (!SI->defaultDestUnreachable() || Case == SI->case_default()) {
if (DTU)
DTU->applyUpdates(Updates);
return !Updates.empty();
}
BasicBlock *Unreachable = SI->getDefaultDest();
SIW.replaceDefaultDest(Case);
SIW.removeCase(Case);
SIW->setCondition(A);
Updates.push_back({DominatorTree::Delete, BB, Unreachable});
if (DTU)
DTU->applyUpdates(Updates);
return true;
}
/// Tries to transform switch of powers of two to reduce switch range.
/// For example, switch like:
/// switch (C) { case 1: case 2: case 64: case 128: }
/// will be transformed to:
/// switch (count_trailing_zeros(C)) { case 0: case 1: case 6: case 7: }
///
/// This transformation allows better lowering and may transform the switch
/// instruction into a sequence of bit manipulation and a smaller
/// log2(C)-indexed value table (instead of traditionally emitting a load of the
/// address of the jump target, and indirectly jump to it).
static bool simplifySwitchOfPowersOfTwo(SwitchInst *SI, IRBuilder<> &Builder,
DomTreeUpdater *DTU,
const DataLayout &DL,
const TargetTransformInfo &TTI) {
Value *Condition = SI->getCondition();
LLVMContext &Context = SI->getContext();
auto *CondTy = cast<IntegerType>(Condition->getType());
if (CondTy->getIntegerBitWidth() > 64 ||
!DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
return false;
// Ensure trailing zeroes count intrinsic emission is not too expensive.
IntrinsicCostAttributes Attrs(Intrinsic::cttz, CondTy,
{Condition, ConstantInt::getTrue(Context)});
if (TTI.getIntrinsicInstrCost(Attrs, TTI::TCK_SizeAndLatency) >
TTI::TCC_Basic * 2)
return false;
// Only bother with this optimization if there are more than 3 switch cases.
// SDAG will start emitting jump tables for 4 or more cases.
if (SI->getNumCases() < 4)
return false;
// Check that switch cases are powers of two.
SmallVector<uint64_t, 4> Values;
for (const auto &Case : SI->cases()) {
uint64_t CaseValue = Case.getCaseValue()->getValue().getZExtValue();
if (llvm::has_single_bit(CaseValue))
Values.push_back(CaseValue);
else
return false;
}
// isSwichDense requires case values to be sorted.
llvm::sort(Values);
if (!isSwitchDense(Values.size(), llvm::countr_zero(Values.back()) -
llvm::countr_zero(Values.front()) + 1))
// Transform is unable to generate dense switch.
return false;
Builder.SetInsertPoint(SI);
if (!SI->defaultDestUnreachable()) {
// Let non-power-of-two inputs jump to the default case, when the latter is
// reachable.
auto *PopC = Builder.CreateUnaryIntrinsic(Intrinsic::ctpop, Condition);
auto *IsPow2 = Builder.CreateICmpEQ(PopC, ConstantInt::get(CondTy, 1));
auto *OrigBB = SI->getParent();
auto *DefaultCaseBB = SI->getDefaultDest();
BasicBlock *SplitBB = SplitBlock(OrigBB, SI, DTU);
auto It = OrigBB->getTerminator()->getIterator();
SmallVector<uint32_t> Weights;
auto HasWeights =
!ProfcheckDisableMetadataFixes && extractBranchWeights(*SI, Weights);
auto *BI = CondBrInst::Create(IsPow2, SplitBB, DefaultCaseBB, It);
if (HasWeights && any_of(Weights, not_equal_to(0))) {
// IsPow2 covers a subset of the cases in which we'd go to the default
// label. The other is those powers of 2 that don't appear in the case
// statement. We don't know the distribution of the values coming in, so
// the safest is to split 50-50 the original probability to `default`.
uint64_t OrigDenominator =
sum_of(map_range(Weights, StaticCastTo<uint64_t>));
SmallVector<uint64_t> NewWeights(2);
NewWeights[1] = Weights[0] / 2;
NewWeights[0] = OrigDenominator - NewWeights[1];
setFittedBranchWeights(*BI, NewWeights, /*IsExpected=*/false);
// The probability of executing the default block stays constant. It was
// p_d = Weights[0] / OrigDenominator
// we rewrite as W/D
// We want to find the probability of the default branch of the switch
// statement. Let's call it X. We have W/D = W/2D + X * (1-W/2D)
// i.e. the original probability is the probability we go to the default
// branch from the BI branch, or we take the default branch on the SI.
// Meaning X = W / (2D - W), or (W/2) / (D - W/2)
// This matches using W/2 for the default branch probability numerator and
// D-W/2 as the denominator.
Weights[0] = NewWeights[1];
uint64_t CasesDenominator = OrigDenominator - Weights[0];
for (auto &W : drop_begin(Weights))
W = NewWeights[0] * static_cast<double>(W) / CasesDenominator;
setBranchWeights(*SI, Weights, /*IsExpected=*/false);
}
// BI is handling the default case for SI, and so should share its DebugLoc.
BI->setDebugLoc(SI->getDebugLoc());
It->eraseFromParent();
addPredecessorToBlock(DefaultCaseBB, OrigBB, SplitBB);
if (DTU)
DTU->applyUpdates({{DominatorTree::Insert, OrigBB, DefaultCaseBB}});
}
// Replace each case with its trailing zeros number.
for (auto &Case : SI->cases()) {
auto *OrigValue = Case.getCaseValue();
Case.setValue(ConstantInt::get(OrigValue->getIntegerType(),
OrigValue->getValue().countr_zero()));
}
// Replace condition with its trailing zeros number.
auto *ConditionTrailingZeros = Builder.CreateIntrinsic(
Intrinsic::cttz, {CondTy}, {Condition, ConstantInt::getTrue(Context)});
SI->setCondition(ConditionTrailingZeros);
return true;
}
/// Fold switch over ucmp/scmp intrinsic to br if two of the switch arms have
/// the same destination.
static bool simplifySwitchOfCmpIntrinsic(SwitchInst *SI, IRBuilderBase &Builder,
DomTreeUpdater *DTU) {
auto *Cmp = dyn_cast<CmpIntrinsic>(SI->getCondition());
if (!Cmp || !Cmp->hasOneUse())
return false;
SmallVector<uint32_t, 4> Weights;
bool HasWeights = extractBranchWeights(getBranchWeightMDNode(*SI), Weights);
if (!HasWeights)
Weights.resize(4); // Avoid checking HasWeights everywhere.
// Normalize to [us]cmp == Res ? Succ : OtherSucc.
int64_t Res;
BasicBlock *Succ, *OtherSucc;
uint32_t SuccWeight = 0, OtherSuccWeight = 0;
BasicBlock *Unreachable = nullptr;
if (SI->getNumCases() == 2) {
// Find which of 1, 0 or -1 is missing (handled by default dest).
SmallSet<int64_t, 3> Missing;
Missing.insert(1);
Missing.insert(0);
Missing.insert(-1);
Succ = SI->getDefaultDest();
SuccWeight = Weights[0];
OtherSucc = nullptr;
for (auto &Case : SI->cases()) {
std::optional<int64_t> Val =
Case.getCaseValue()->getValue().trySExtValue();
if (!Val)
return false;
if (!Missing.erase(*Val))
return false;
if (OtherSucc && OtherSucc != Case.getCaseSuccessor())
return false;
OtherSucc = Case.getCaseSuccessor();
OtherSuccWeight += Weights[Case.getSuccessorIndex()];
}
assert(Missing.size() == 1 && "Should have one case left");
Res = *Missing.begin();
} else if (SI->getNumCases() == 3 && SI->defaultDestUnreachable()) {
// Normalize so that Succ is taken once and OtherSucc twice.
Unreachable = SI->getDefaultDest();
Succ = OtherSucc = nullptr;
for (auto &Case : SI->cases()) {
BasicBlock *NewSucc = Case.getCaseSuccessor();
uint32_t Weight = Weights[Case.getSuccessorIndex()];
if (!OtherSucc || OtherSucc == NewSucc) {
OtherSucc = NewSucc;
OtherSuccWeight += Weight;
} else if (!Succ) {
Succ = NewSucc;
SuccWeight = Weight;
} else if (Succ == NewSucc) {
std::swap(Succ, OtherSucc);
std::swap(SuccWeight, OtherSuccWeight);
} else
return false;
}
for (auto &Case : SI->cases()) {
std::optional<int64_t> Val =
Case.getCaseValue()->getValue().trySExtValue();
if (!Val || (Val != 1 && Val != 0 && Val != -1))
return false;
if (Case.getCaseSuccessor() == Succ) {
Res = *Val;
break;
}
}
} else {
return false;
}
// Determine predicate for the missing case.
ICmpInst::Predicate Pred;
switch (Res) {
case 1:
Pred = ICmpInst::ICMP_UGT;
break;
case 0:
Pred = ICmpInst::ICMP_EQ;
break;
case -1:
Pred = ICmpInst::ICMP_ULT;
break;
}
if (Cmp->isSigned())
Pred = ICmpInst::getSignedPredicate(Pred);
MDNode *NewWeights = nullptr;
if (HasWeights)
NewWeights = MDBuilder(SI->getContext())
.createBranchWeights(SuccWeight, OtherSuccWeight);
BasicBlock *BB = SI->getParent();
Builder.SetInsertPoint(SI->getIterator());
Value *ICmp = Builder.CreateICmp(Pred, Cmp->getLHS(), Cmp->getRHS());
Builder.CreateCondBr(ICmp, Succ, OtherSucc, NewWeights,
SI->getMetadata(LLVMContext::MD_unpredictable));
OtherSucc->removePredecessor(BB);
if (Unreachable)
Unreachable->removePredecessor(BB);
SI->eraseFromParent();
Cmp->eraseFromParent();
if (DTU && Unreachable)
DTU->applyUpdates({{DominatorTree::Delete, BB, Unreachable}});
return true;
}
/// Checking whether two BBs are equal depends on the contents of the
/// BasicBlock and the incoming values of their successor PHINodes.
/// PHINode::getIncomingValueForBlock is O(|Preds|), so we'd like to avoid
/// calling this function on each BasicBlock every time isEqual is called,
/// especially since the same BasicBlock may be passed as an argument multiple
/// times. To do this, we can precompute a map of PHINode -> Pred BasicBlock ->
/// IncomingValue and add it in the Wrapper so isEqual can do O(1) checking
/// of the incoming values.
struct EqualBBWrapper {
BasicBlock *BB;
// One Phi usually has < 8 incoming values.
using BB2ValueMap = SmallDenseMap<BasicBlock *, Value *, 8>;
using Phi2IVsMap = DenseMap<PHINode *, BB2ValueMap>;
Phi2IVsMap *PhiPredIVs;
// We only merge the identical non-entry BBs with
// - terminator unconditional br to Succ (pending relaxation),
// - does not have address taken / weird control.
static bool canBeMerged(const BasicBlock *BB) {
assert(BB && "Expected non-null BB");
// Entry block cannot be eliminated or have predecessors.
if (BB->isEntryBlock())
return false;
// Single successor and must be Succ.
// FIXME: Relax that the terminator is a BranchInst by checking for equality
// on other kinds of terminators. We decide to only support unconditional
// branches for now for compile time reasons.
auto *BI = dyn_cast<UncondBrInst>(BB->getTerminator());
if (!BI)
return false;
// Avoid blocks that are "address-taken" (blockaddress) or have unusual
// uses.
if (BB->hasAddressTaken() || BB->isEHPad())
return false;
// TODO: relax this condition to merge equal blocks with >1 instructions?
// Here, we use a O(1) form of the O(n) comparison of `size() != 1`.
if (&BB->front() != &BB->back())
return false;
// The BB must have at least one predecessor.
if (pred_empty(BB))
return false;
return true;
}
};
template <> struct llvm::DenseMapInfo<const EqualBBWrapper *> {
static const EqualBBWrapper *getEmptyKey() {
return static_cast<EqualBBWrapper *>(DenseMapInfo<void *>::getEmptyKey());
}
static const EqualBBWrapper *getTombstoneKey() {
return static_cast<EqualBBWrapper *>(
DenseMapInfo<void *>::getTombstoneKey());
}
static unsigned getHashValue(const EqualBBWrapper *EBW) {
BasicBlock *BB = EBW->BB;
UncondBrInst *BI = cast<UncondBrInst>(BB->getTerminator());
assert(BB->size() == 1 && "Expected just a single branch in the BB");
// Since we assume the BB is just a single UncondBrInst with a single
// successor, we hash as the BB and the incoming Values of its successor
// PHIs. Initially, we tried to just use the successor BB as the hash, but
// including the incoming PHI values leads to better performance.
// We also tried to build a map from BB -> Succs.IncomingValues ahead of
// time and passing it in EqualBBWrapper, but this slowed down the average
// compile time without having any impact on the worst case compile time.
BasicBlock *Succ = BI->getSuccessor();
auto PhiValsForBB = map_range(Succ->phis(), [&](PHINode &Phi) {
return (*EBW->PhiPredIVs)[&Phi][BB];
});
return hash_combine(Succ, hash_combine_range(PhiValsForBB));
}
static bool isEqual(const EqualBBWrapper *LHS, const EqualBBWrapper *RHS) {
auto *EKey = DenseMapInfo<EqualBBWrapper *>::getEmptyKey();
auto *TKey = DenseMapInfo<EqualBBWrapper *>::getTombstoneKey();
if (LHS == EKey || RHS == EKey || LHS == TKey || RHS == TKey)
return LHS == RHS;
BasicBlock *A = LHS->BB;
BasicBlock *B = RHS->BB;
// FIXME: we checked that the size of A and B are both 1 in
// mergeIdenticalUncondBBs to make the Case list smaller to
// improve performance. If we decide to support BasicBlocks with more
// than just a single instruction, we need to check that A.size() ==
// B.size() here, and we need to check more than just the BranchInsts
// for equality.
UncondBrInst *ABI = cast<UncondBrInst>(A->getTerminator());
UncondBrInst *BBI = cast<UncondBrInst>(B->getTerminator());
if (ABI->getSuccessor() != BBI->getSuccessor())
return false;
// Need to check that PHIs in successor have matching values.
BasicBlock *Succ = ABI->getSuccessor();
auto IfPhiIVMatch = [&](PHINode &Phi) {
// Replace O(|Pred|) Phi.getIncomingValueForBlock with this O(1) hashmap
// query.
auto &PredIVs = (*LHS->PhiPredIVs)[&Phi];
return PredIVs[A] == PredIVs[B];
};
return all_of(Succ->phis(), IfPhiIVMatch);
}
};
// Merge identical BBs into one of them.
static bool mergeIdenticalBBs(ArrayRef<BasicBlock *> Candidates,
DomTreeUpdater *DTU) {
if (Candidates.size() < 2)
return false;
// Build Cases. Skip BBs that are not candidates for simplification. Mark
// PHINodes which need to be processed into PhiPredIVs. We decide to process
// an entire PHI at once after the loop, opposed to calling
// getIncomingValueForBlock inside this loop, since each call to
// getIncomingValueForBlock is O(|Preds|).
EqualBBWrapper::Phi2IVsMap PhiPredIVs;
SmallVector<EqualBBWrapper> BBs2Merge;
BBs2Merge.reserve(Candidates.size());
SmallSetVector<PHINode *, 8> Phis;
for (BasicBlock *BB : Candidates) {
BasicBlock *Succ = BB->getSingleSuccessor();
assert(Succ && "Expected unconditional BB");
BBs2Merge.emplace_back(EqualBBWrapper{BB, &PhiPredIVs});
Phis.insert_range(make_pointer_range(Succ->phis()));
}
// Precompute a data structure to improve performance of isEqual for
// EqualBBWrapper.
PhiPredIVs.reserve(Phis.size());
for (PHINode *Phi : Phis) {
auto &IVs =
PhiPredIVs.try_emplace(Phi, Phi->getNumIncomingValues()).first->second;
// Pre-fill all incoming for O(1) lookup as Phi.getIncomingValueForBlock is
// O(|Pred|).
for (auto &IV : Phi->incoming_values())
IVs.insert({Phi->getIncomingBlock(IV), IV.get()});
}
// Group duplicates using DenseSet with custom equality/hashing.
// Build a set such that if the EqualBBWrapper exists in the set and another
// EqualBBWrapper isEqual, then the equivalent EqualBBWrapper which is not in
// the set should be replaced with the one in the set. If the EqualBBWrapper
// is not in the set, then it should be added to the set so other
// EqualBBWrapper can check against it in the same manner. We use
// EqualBBWrapper instead of just BasicBlock because we'd like to pass around
// information to isEquality, getHashValue, and when doing the replacement
// with better performance.
DenseSet<const EqualBBWrapper *> Keep;
Keep.reserve(BBs2Merge.size());
SmallVector<DominatorTree::UpdateType> Updates;
Updates.reserve(BBs2Merge.size() * 2);
bool MadeChange = false;
// Helper: redirect all edges X -> DeadPred to X -> LivePred.
auto RedirectIncomingEdges = [&](BasicBlock *Dead, BasicBlock *Live) {
SmallSetVector<BasicBlock *, 8> DeadPreds(llvm::from_range,
predecessors(Dead));
if (DTU) {
// All predecessors of DeadPred (except the common predecessor) will be
// moved to LivePred.
Updates.reserve(Updates.size() + DeadPreds.size() * 2);
SmallPtrSet<BasicBlock *, 16> LivePreds(llvm::from_range,
predecessors(Live));
for (BasicBlock *PredOfDead : DeadPreds) {
// Do not modify those common predecessors of DeadPred and LivePred.
if (!LivePreds.contains(PredOfDead))
Updates.push_back({DominatorTree::Insert, PredOfDead, Live});
Updates.push_back({DominatorTree::Delete, PredOfDead, Dead});
}
}
LLVM_DEBUG(dbgs() << "Replacing duplicate pred BB ";
Dead->printAsOperand(dbgs()); dbgs() << " with pred ";
Live->printAsOperand(dbgs()); dbgs() << " for ";
Live->getSingleSuccessor()->printAsOperand(dbgs());
dbgs() << "\n");
// Replace successors in all predecessors of DeadPred.
for (BasicBlock *PredOfDead : DeadPreds) {
Instruction *T = PredOfDead->getTerminator();
T->replaceSuccessorWith(Dead, Live);
}
};
// Try to eliminate duplicate predecessors.
for (const auto &EBW : BBs2Merge) {
// EBW is a candidate for simplification. If we find a duplicate BB,
// replace it.
const auto &[It, Inserted] = Keep.insert(&EBW);
if (Inserted)
continue;
// Found duplicate: merge P into canonical predecessor It->Pred.
BasicBlock *KeepBB = (*It)->BB;
BasicBlock *DeadBB = EBW.BB;
// Avoid merging a BB with itself.
if (KeepBB == DeadBB)
continue;
// Redirect all edges into DeadPred to KeepPred.
RedirectIncomingEdges(DeadBB, KeepBB);
// Now DeadBB should become unreachable; leave DCE to later,
// but we can try to simplify it if it only branches to Succ.
// (We won't erase here to keep the routine simple and DT-safe.)
assert(pred_empty(DeadBB) && "DeadBB should be unreachable.");
MadeChange = true;
}
if (DTU && !Updates.empty())
DTU->applyUpdates(Updates);
return MadeChange;
}
bool SimplifyCFGOpt::simplifyDuplicateSwitchArms(SwitchInst *SI,
DomTreeUpdater *DTU) {
// Collect candidate switch-arms top-down.
SmallSetVector<BasicBlock *, 16> FilteredArms(
llvm::from_range,
make_filter_range(successors(SI), EqualBBWrapper::canBeMerged));
return mergeIdenticalBBs(FilteredArms.getArrayRef(), DTU);
}
bool SimplifyCFGOpt::simplifyDuplicatePredecessors(BasicBlock *BB,
DomTreeUpdater *DTU) {
// Need at least 2 predecessors to do anything.
if (!BB || !BB->hasNPredecessorsOrMore(2))
return false;
// Compilation time consideration: retain the canonical loop, otherwise, we
// require more time in the later loop canonicalization.
if (Options.NeedCanonicalLoop && is_contained(LoopHeaders, BB))
return false;
// Collect candidate predecessors bottom-up.
SmallSetVector<BasicBlock *, 8> FilteredPreds(
llvm::from_range,
make_filter_range(predecessors(BB), EqualBBWrapper::canBeMerged));
return mergeIdenticalBBs(FilteredPreds.getArrayRef(), DTU);
}
bool SimplifyCFGOpt::simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
BasicBlock *BB = SI->getParent();
if (isValueEqualityComparison(SI)) {
// If we only have one predecessor, and if it is a branch on this value,
// see if that predecessor totally determines the outcome of this switch.
if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
if (simplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred, Builder))
return requestResimplify();
Value *Cond = SI->getCondition();
if (SelectInst *Select = dyn_cast<SelectInst>(Cond))
if (simplifySwitchOnSelect(SI, Select))
return requestResimplify();
// If the block only contains the switch, see if we can fold the block
// away into any preds.
if (SI == &*BB->begin())
if (foldValueComparisonIntoPredecessors(SI, Builder))
return requestResimplify();
}
// Try to transform the switch into an icmp and a branch.
// The conversion from switch to comparison may lose information on
// impossible switch values, so disable it early in the pipeline.
if (Options.ConvertSwitchRangeToICmp && turnSwitchRangeIntoICmp(SI, Builder))
return requestResimplify();
// Remove unreachable cases.
if (eliminateDeadSwitchCases(SI, DTU, Options.AC, DL))
return requestResimplify();
if (simplifySwitchOfCmpIntrinsic(SI, Builder, DTU))
return requestResimplify();
if (trySwitchToSelect(SI, Builder, DTU, DL, TTI))
return requestResimplify();
if (Options.ForwardSwitchCondToPhi && forwardSwitchConditionToPHI(SI))
return requestResimplify();
// The conversion of switches to arithmetic or lookup table is disabled in
// the early optimization pipeline, as it may lose information or make the
// resulting code harder to analyze.
if (Options.ConvertSwitchToArithmetic || Options.ConvertSwitchToLookupTable)
if (simplifySwitchLookup(SI, Builder, DTU, DL, TTI,
Options.ConvertSwitchToLookupTable))
return requestResimplify();
if (simplifySwitchOfPowersOfTwo(SI, Builder, DTU, DL, TTI))
return requestResimplify();
if (reduceSwitchRange(SI, Builder, DL, TTI))
return requestResimplify();
if (HoistCommon &&
hoistCommonCodeFromSuccessors(SI, !Options.HoistCommonInsts))
return requestResimplify();
// We can merge identical switch arms early to enhance more aggressive
// optimization on switch.
if (simplifyDuplicateSwitchArms(SI, DTU))
return requestResimplify();
if (simplifySwitchWhenUMin(SI, DTU))
return requestResimplify();
return false;
}
bool SimplifyCFGOpt::simplifyIndirectBr(IndirectBrInst *IBI) {
BasicBlock *BB = IBI->getParent();
bool Changed = false;
SmallVector<uint32_t> BranchWeights;
const bool HasBranchWeights = !ProfcheckDisableMetadataFixes &&
extractBranchWeights(*IBI, BranchWeights);
DenseMap<const BasicBlock *, uint64_t> TargetWeight;
if (HasBranchWeights)
for (size_t I = 0, E = IBI->getNumDestinations(); I < E; ++I)
TargetWeight[IBI->getDestination(I)] += BranchWeights[I];
// Eliminate redundant destinations.
SmallPtrSet<Value *, 8> Succs;
SmallSetVector<BasicBlock *, 8> RemovedSuccs;
for (unsigned I = 0, E = IBI->getNumDestinations(); I != E; ++I) {
BasicBlock *Dest = IBI->getDestination(I);
if (!Dest->hasAddressTaken() || !Succs.insert(Dest).second) {
if (!Dest->hasAddressTaken())
RemovedSuccs.insert(Dest);
Dest->removePredecessor(BB);
IBI->removeDestination(I);
--I;
--E;
Changed = true;
}
}
if (DTU) {
std::vector<DominatorTree::UpdateType> Updates;
Updates.reserve(RemovedSuccs.size());
for (auto *RemovedSucc : RemovedSuccs)
Updates.push_back({DominatorTree::Delete, BB, RemovedSucc});
DTU->applyUpdates(Updates);
}
if (IBI->getNumDestinations() == 0) {
// If the indirectbr has no successors, change it to unreachable.
new UnreachableInst(IBI->getContext(), IBI->getIterator());
eraseTerminatorAndDCECond(IBI);
return true;
}
if (IBI->getNumDestinations() == 1) {
// If the indirectbr has one successor, change it to a direct branch.
UncondBrInst::Create(IBI->getDestination(0), IBI->getIterator());
eraseTerminatorAndDCECond(IBI);
return true;
}
if (HasBranchWeights) {
SmallVector<uint64_t> NewBranchWeights(IBI->getNumDestinations());
for (size_t I = 0, E = IBI->getNumDestinations(); I < E; ++I)
NewBranchWeights[I] += TargetWeight.find(IBI->getDestination(I))->second;
setFittedBranchWeights(*IBI, NewBranchWeights, /*IsExpected=*/false);
}
if (SelectInst *SI = dyn_cast<SelectInst>(IBI->getAddress())) {
if (simplifyIndirectBrOnSelect(IBI, SI))
return requestResimplify();
}
return Changed;
}
/// Given an block with only a single landing pad and a unconditional branch
/// try to find another basic block which this one can be merged with. This
/// handles cases where we have multiple invokes with unique landing pads, but
/// a shared handler.
///
/// We specifically choose to not worry about merging non-empty blocks
/// here. That is a PRE/scheduling problem and is best solved elsewhere. In
/// practice, the optimizer produces empty landing pad blocks quite frequently
/// when dealing with exception dense code. (see: instcombine, gvn, if-else
/// sinking in this file)
///
/// This is primarily a code size optimization. We need to avoid performing
/// any transform which might inhibit optimization (such as our ability to
/// specialize a particular handler via tail commoning). We do this by not
/// merging any blocks which require us to introduce a phi. Since the same
/// values are flowing through both blocks, we don't lose any ability to
/// specialize. If anything, we make such specialization more likely.
///
/// TODO - This transformation could remove entries from a phi in the target
/// block when the inputs in the phi are the same for the two blocks being
/// merged. In some cases, this could result in removal of the PHI entirely.
static bool tryToMergeLandingPad(LandingPadInst *LPad, UncondBrInst *BI,
BasicBlock *BB, DomTreeUpdater *DTU) {
auto Succ = BB->getUniqueSuccessor();
assert(Succ);
// If there's a phi in the successor block, we'd likely have to introduce
// a phi into the merged landing pad block.
if (isa<PHINode>(*Succ->begin()))
return false;
for (BasicBlock *OtherPred : predecessors(Succ)) {
if (BB == OtherPred)
continue;
BasicBlock::iterator I = OtherPred->begin();
LandingPadInst *LPad2 = dyn_cast<LandingPadInst>(I);
if (!LPad2 || !LPad2->isIdenticalTo(LPad))
continue;
++I;
UncondBrInst *BI2 = dyn_cast<UncondBrInst>(I);
if (!BI2 || !BI2->isIdenticalTo(BI))
continue;
std::vector<DominatorTree::UpdateType> Updates;
// We've found an identical block. Update our predecessors to take that
// path instead and make ourselves dead.
SmallSetVector<BasicBlock *, 16> UniquePreds(pred_begin(BB), pred_end(BB));
for (BasicBlock *Pred : UniquePreds) {
InvokeInst *II = cast<InvokeInst>(Pred->getTerminator());
assert(II->getNormalDest() != BB && II->getUnwindDest() == BB &&
"unexpected successor");
II->setUnwindDest(OtherPred);
if (DTU) {
Updates.push_back({DominatorTree::Insert, Pred, OtherPred});
Updates.push_back({DominatorTree::Delete, Pred, BB});
}
}
SmallSetVector<BasicBlock *, 16> UniqueSuccs(succ_begin(BB), succ_end(BB));
for (BasicBlock *Succ : UniqueSuccs) {
Succ->removePredecessor(BB);
if (DTU)
Updates.push_back({DominatorTree::Delete, BB, Succ});
}
IRBuilder<> Builder(BI);
Builder.CreateUnreachable();
BI->eraseFromParent();
if (DTU)
DTU->applyUpdates(Updates);
return true;
}
return false;
}
bool SimplifyCFGOpt::simplifyUncondBranch(UncondBrInst *BI,
IRBuilder<> &Builder) {
BasicBlock *BB = BI->getParent();
BasicBlock *Succ = BI->getSuccessor(0);
// If the Terminator is the only non-phi instruction, simplify the block.
// If LoopHeader is provided, check if the block or its successor is a loop
// header. (This is for early invocations before loop simplify and
// vectorization to keep canonical loop forms for nested loops. These blocks
// can be eliminated when the pass is invoked later in the back-end.)
// Note that if BB has only one predecessor then we do not introduce new
// backedge, so we can eliminate BB.
bool NeedCanonicalLoop =
Options.NeedCanonicalLoop &&
(!LoopHeaders.empty() && BB->hasNPredecessorsOrMore(2) &&
(is_contained(LoopHeaders, BB) || is_contained(LoopHeaders, Succ)));
BasicBlock::iterator I = BB->getFirstNonPHIOrDbg();
if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() &&
!NeedCanonicalLoop && TryToSimplifyUncondBranchFromEmptyBlock(BB, DTU))
return true;
// If the only instruction in the block is a seteq/setne comparison against a
// constant, try to simplify the block.
if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
if (ICI->isEquality() && isa<ConstantInt>(ICI->getOperand(1))) {
++I;
if (I->isTerminator() &&
tryToSimplifyUncondBranchWithICmpInIt(ICI, Builder))
return true;
if (isa<SelectInst>(I) && I->getNextNode()->isTerminator() &&
tryToSimplifyUncondBranchWithICmpSelectInIt(ICI, cast<SelectInst>(I),
Builder))
return true;
}
}
// See if we can merge an empty landing pad block with another which is
// equivalent.
if (LandingPadInst *LPad = dyn_cast<LandingPadInst>(I)) {
++I;
if (I->isTerminator() && tryToMergeLandingPad(LPad, BI, BB, DTU))
return true;
}
return false;
}
static BasicBlock *allPredecessorsComeFromSameSource(BasicBlock *BB) {
BasicBlock *PredPred = nullptr;
for (auto *P : predecessors(BB)) {
BasicBlock *PPred = P->getSinglePredecessor();
if (!PPred || (PredPred && PredPred != PPred))
return nullptr;
PredPred = PPred;
}
return PredPred;
}
/// Fold the following pattern:
/// bb0:
/// br i1 %cond1, label %bb1, label %bb2
/// bb1:
/// br i1 %cond2, label %bb3, label %bb4
/// bb2:
/// br i1 %cond2, label %bb4, label %bb3
/// bb3:
/// ...
/// bb4:
/// ...
/// into
/// bb0:
/// %cond = xor i1 %cond1, %cond2
/// br i1 %cond, label %bb4, label %bb3
/// bb3:
/// ...
/// bb4:
/// ...
/// NOTE: %cond2 always dominates the terminator of bb0.
static bool mergeNestedCondBranch(CondBrInst *BI, DomTreeUpdater *DTU) {
BasicBlock *BB = BI->getParent();
BasicBlock *BB1 = BI->getSuccessor(0);
BasicBlock *BB2 = BI->getSuccessor(1);
auto IsSimpleSuccessor = [BB](BasicBlock *Succ, CondBrInst *&SuccBI) {
if (Succ == BB)
return false;
if (&Succ->front() != Succ->getTerminator())
return false;
SuccBI = dyn_cast<CondBrInst>(Succ->getTerminator());
if (!SuccBI)
return false;
BasicBlock *Succ1 = SuccBI->getSuccessor(0);
BasicBlock *Succ2 = SuccBI->getSuccessor(1);
return Succ1 != Succ && Succ2 != Succ && Succ1 != BB && Succ2 != BB &&
!isa<PHINode>(Succ1->front()) && !isa<PHINode>(Succ2->front());
};
CondBrInst *BB1BI, *BB2BI;
if (!IsSimpleSuccessor(BB1, BB1BI) || !IsSimpleSuccessor(BB2, BB2BI))
return false;
if (BB1BI->getCondition() != BB2BI->getCondition() ||
BB1BI->getSuccessor(0) != BB2BI->getSuccessor(1) ||
BB1BI->getSuccessor(1) != BB2BI->getSuccessor(0))
return false;
BasicBlock *BB3 = BB1BI->getSuccessor(0);
BasicBlock *BB4 = BB1BI->getSuccessor(1);
IRBuilder<> Builder(BI);
BI->setCondition(
Builder.CreateXor(BI->getCondition(), BB1BI->getCondition()));
BB1->removePredecessor(BB);
BI->setSuccessor(0, BB4);
BB2->removePredecessor(BB);
BI->setSuccessor(1, BB3);
if (DTU) {
SmallVector<DominatorTree::UpdateType, 4> Updates;
Updates.push_back({DominatorTree::Delete, BB, BB1});
Updates.push_back({DominatorTree::Insert, BB, BB4});
Updates.push_back({DominatorTree::Delete, BB, BB2});
Updates.push_back({DominatorTree::Insert, BB, BB3});
DTU->applyUpdates(Updates);
}
bool HasWeight = false;
uint64_t BBTWeight, BBFWeight;
if (extractBranchWeights(*BI, BBTWeight, BBFWeight))
HasWeight = true;
else
BBTWeight = BBFWeight = 1;
uint64_t BB1TWeight, BB1FWeight;
if (extractBranchWeights(*BB1BI, BB1TWeight, BB1FWeight))
HasWeight = true;
else
BB1TWeight = BB1FWeight = 1;
uint64_t BB2TWeight, BB2FWeight;
if (extractBranchWeights(*BB2BI, BB2TWeight, BB2FWeight))
HasWeight = true;
else
BB2TWeight = BB2FWeight = 1;
if (HasWeight) {
uint64_t Weights[2] = {BBTWeight * BB1FWeight + BBFWeight * BB2TWeight,
BBTWeight * BB1TWeight + BBFWeight * BB2FWeight};
setFittedBranchWeights(*BI, Weights, /*IsExpected=*/false,
/*ElideAllZero=*/true);
}
return true;
}
bool SimplifyCFGOpt::simplifyCondBranch(CondBrInst *BI, IRBuilder<> &Builder) {
assert(
!isa<ConstantInt>(BI->getCondition()) &&
BI->getSuccessor(0) != BI->getSuccessor(1) &&
"Tautological conditional branch should have been eliminated already.");
BasicBlock *BB = BI->getParent();
if (!Options.SimplifyCondBranch ||
BI->getFunction()->hasFnAttribute(Attribute::OptForFuzzing))
return false;
// Conditional branch
if (isValueEqualityComparison(BI)) {
// If we only have one predecessor, and if it is a branch on this value,
// see if that predecessor totally determines the outcome of this
// switch.
if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
if (simplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred, Builder))
return requestResimplify();
// This block must be empty, except for the setcond inst, if it exists.
// Ignore pseudo intrinsics.
for (auto &I : *BB) {
if (isa<PseudoProbeInst>(I) ||
&I == cast<Instruction>(BI->getCondition()))
continue;
if (&I == BI)
if (foldValueComparisonIntoPredecessors(BI, Builder))
return requestResimplify();
break;
}
}
// Try to turn "br (X == 0 | X == 1), T, F" into a switch instruction.
if (simplifyBranchOnICmpChain(BI, Builder, DL))
return true;
// If this basic block has dominating predecessor blocks and the dominating
// blocks' conditions imply BI's condition, we know the direction of BI.
std::optional<bool> Imp = isImpliedByDomCondition(BI->getCondition(), BI, DL);
if (Imp) {
// Turn this into a branch on constant.
auto *OldCond = BI->getCondition();
ConstantInt *TorF = *Imp ? ConstantInt::getTrue(BB->getContext())
: ConstantInt::getFalse(BB->getContext());
BI->setCondition(TorF);
RecursivelyDeleteTriviallyDeadInstructions(OldCond);
return requestResimplify();
}
// If this basic block is ONLY a compare and a branch, and if a predecessor
// branches to us and one of our successors, fold the comparison into the
// predecessor and use logical operations to pick the right destination.
if (Options.SpeculateBlocks &&
foldBranchToCommonDest(BI, DTU, /*MSSAU=*/nullptr, &TTI,
Options.BonusInstThreshold))
return requestResimplify();
// We have a conditional branch to two blocks that are only reachable
// from BI. We know that the condbr dominates the two blocks, so see if
// there is any identical code in the "then" and "else" blocks. If so, we
// can hoist it up to the branching block.
if (BI->getSuccessor(0)->getSinglePredecessor()) {
if (BI->getSuccessor(1)->getSinglePredecessor()) {
if (HoistCommon &&
hoistCommonCodeFromSuccessors(BI, !Options.HoistCommonInsts))
return requestResimplify();
if (BI && Options.HoistLoadsStoresWithCondFaulting &&
isProfitableToSpeculate(BI, std::nullopt, TTI)) {
SmallVector<Instruction *, 2> SpeculatedConditionalLoadsStores;
auto CanSpeculateConditionalLoadsStores = [&]() {
for (auto *Succ : successors(BB)) {
for (Instruction &I : *Succ) {
if (I.isTerminator()) {
if (I.getNumSuccessors() > 1)
return false;
continue;
} else if (!isSafeCheapLoadStore(&I, TTI) ||
SpeculatedConditionalLoadsStores.size() ==
HoistLoadsStoresWithCondFaultingThreshold) {
return false;
}
SpeculatedConditionalLoadsStores.push_back(&I);
}
}
return !SpeculatedConditionalLoadsStores.empty();
};
if (CanSpeculateConditionalLoadsStores()) {
hoistConditionalLoadsStores(BI, SpeculatedConditionalLoadsStores,
std::nullopt, nullptr);
return requestResimplify();
}
}
} else {
// If Successor #1 has multiple preds, we may be able to conditionally
// execute Successor #0 if it branches to Successor #1.
Instruction *Succ0TI = BI->getSuccessor(0)->getTerminator();
if (Succ0TI->getNumSuccessors() == 1 &&
Succ0TI->getSuccessor(0) == BI->getSuccessor(1))
if (speculativelyExecuteBB(BI, BI->getSuccessor(0)))
return requestResimplify();
}
} else if (BI->getSuccessor(1)->getSinglePredecessor()) {
// If Successor #0 has multiple preds, we may be able to conditionally
// execute Successor #1 if it branches to Successor #0.
Instruction *Succ1TI = BI->getSuccessor(1)->getTerminator();
if (Succ1TI->getNumSuccessors() == 1 &&
Succ1TI->getSuccessor(0) == BI->getSuccessor(0))
if (speculativelyExecuteBB(BI, BI->getSuccessor(1)))
return requestResimplify();
}
// If this is a branch on something for which we know the constant value in
// predecessors (e.g. a phi node in the current block), thread control
// through this block.
if (foldCondBranchOnValueKnownInPredecessor(BI))
return requestResimplify();
// Scan predecessor blocks for conditional branches.
for (BasicBlock *Pred : predecessors(BB))
if (CondBrInst *PBI = dyn_cast<CondBrInst>(Pred->getTerminator()))
if (PBI != BI)
if (SimplifyCondBranchToCondBranch(PBI, BI, DTU, DL, TTI))
return requestResimplify();
// Look for diamond patterns.
if (MergeCondStores)
if (BasicBlock *PrevBB = allPredecessorsComeFromSameSource(BB))
if (CondBrInst *PBI = dyn_cast<CondBrInst>(PrevBB->getTerminator()))
if (PBI != BI)
if (mergeConditionalStores(PBI, BI, DTU, DL, TTI))
return requestResimplify();
// Look for nested conditional branches.
if (mergeNestedCondBranch(BI, DTU))
return requestResimplify();
return false;
}
/// Check if passing a value to an instruction will cause undefined behavior.
static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValueMayBeModified) {
assert(V->getType() == I->getType() && "Mismatched types");
Constant *C = dyn_cast<Constant>(V);
if (!C)
return false;
if (I->use_empty())
return false;
if (C->isNullValue() || isa<UndefValue>(C)) {
// Only look at the first use we can handle, avoid hurting compile time with
// long uselists
auto FindUse = llvm::find_if(I->uses(), [](auto &U) {
auto *Use = cast<Instruction>(U.getUser());
// Change this list when we want to add new instructions.
switch (Use->getOpcode()) {
default:
return false;
case Instruction::GetElementPtr:
case Instruction::Ret:
case Instruction::BitCast:
case Instruction::Load:
case Instruction::Store:
case Instruction::Call:
case Instruction::CallBr:
case Instruction::Invoke:
case Instruction::UDiv:
case Instruction::URem:
// Note: signed div/rem of INT_MIN / -1 is also immediate UB, not
// implemented to avoid code complexity as it is unclear how useful such
// logic is.
case Instruction::SDiv:
case Instruction::SRem:
return true;
}
});
if (FindUse == I->use_end())
return false;
auto &Use = *FindUse;
auto *User = cast<Instruction>(Use.getUser());
// Bail out if User is not in the same BB as I or User == I or User comes
// before I in the block. The latter two can be the case if User is a
// PHI node.
if (User->getParent() != I->getParent() || User == I ||
User->comesBefore(I))
return false;
// Now make sure that there are no instructions in between that can alter
// control flow (eg. calls)
auto InstrRange =
make_range(std::next(I->getIterator()), User->getIterator());
if (any_of(InstrRange, [](Instruction &I) {
return !isGuaranteedToTransferExecutionToSuccessor(&I);
}))
return false;
// Look through GEPs. A load from a GEP derived from NULL is still undefined
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(User))
if (GEP->getPointerOperand() == I) {
// The type of GEP may differ from the type of base pointer.
// Bail out on vector GEPs, as they are not handled by other checks.
if (GEP->getType()->isVectorTy())
return false;
// The current base address is null, there are four cases to consider:
// getelementptr (TY, null, 0) -> null
// getelementptr (TY, null, not zero) -> may be modified
// getelementptr inbounds (TY, null, 0) -> null
// getelementptr inbounds (TY, null, not zero) -> poison iff null is
// undefined?
if (!GEP->hasAllZeroIndices() &&
(!GEP->isInBounds() ||
NullPointerIsDefined(GEP->getFunction(),
GEP->getPointerAddressSpace())))
PtrValueMayBeModified = true;
return passingValueIsAlwaysUndefined(V, GEP, PtrValueMayBeModified);
}
// Look through return.
if (ReturnInst *Ret = dyn_cast<ReturnInst>(User)) {
bool HasNoUndefAttr =
Ret->getFunction()->hasRetAttribute(Attribute::NoUndef);
// Return undefined to a noundef return value is undefined.
if (isa<UndefValue>(C) && HasNoUndefAttr)
return true;
// Return null to a nonnull+noundef return value is undefined.
if (C->isNullValue() && HasNoUndefAttr &&
Ret->getFunction()->hasRetAttribute(Attribute::NonNull)) {
return !PtrValueMayBeModified;
}
}
// Load from null is undefined.
if (LoadInst *LI = dyn_cast<LoadInst>(User))
if (!LI->isVolatile())
return !NullPointerIsDefined(LI->getFunction(),
LI->getPointerAddressSpace());
// Store to null is undefined.
if (StoreInst *SI = dyn_cast<StoreInst>(User))
if (!SI->isVolatile())
return (!NullPointerIsDefined(SI->getFunction(),
SI->getPointerAddressSpace())) &&
SI->getPointerOperand() == I;
// llvm.assume(false/undef) always triggers immediate UB.
if (auto *Assume = dyn_cast<AssumeInst>(User)) {
// Ignore assume operand bundles.
if (I == Assume->getArgOperand(0))
return true;
}
if (auto *CB = dyn_cast<CallBase>(User)) {
if (C->isNullValue() && NullPointerIsDefined(CB->getFunction()))
return false;
// A call to null is undefined.
if (CB->getCalledOperand() == I)
return true;
if (CB->isArgOperand(&Use)) {
unsigned ArgIdx = CB->getArgOperandNo(&Use);
// Passing null to a nonnnull+noundef argument is undefined.
if (isa<ConstantPointerNull>(C) &&
CB->paramHasNonNullAttr(ArgIdx, /*AllowUndefOrPoison=*/false))
return !PtrValueMayBeModified;
// Passing undef to a noundef argument is undefined.
if (isa<UndefValue>(C) && CB->isPassingUndefUB(ArgIdx))
return true;
}
}
// Div/Rem by zero is immediate UB
if (match(User, m_BinOp(m_Value(), m_Specific(I))) && User->isIntDivRem())
return true;
}
return false;
}
/// If BB has an incoming value that will always trigger undefined behavior
/// (eg. null pointer dereference), remove the branch leading here.
static bool removeUndefIntroducingPredecessor(BasicBlock *BB,
DomTreeUpdater *DTU,
AssumptionCache *AC) {
for (PHINode &PHI : BB->phis())
for (unsigned i = 0, e = PHI.getNumIncomingValues(); i != e; ++i)
if (passingValueIsAlwaysUndefined(PHI.getIncomingValue(i), &PHI)) {
BasicBlock *Predecessor = PHI.getIncomingBlock(i);
Instruction *T = Predecessor->getTerminator();
IRBuilder<> Builder(T);
if (isa<UncondBrInst>(T)) {
BB->removePredecessor(Predecessor);
// Turn unconditional branches into unreachables.
Builder.CreateUnreachable();
T->eraseFromParent();
if (DTU)
DTU->applyUpdates({{DominatorTree::Delete, Predecessor, BB}});
return true;
} else if (CondBrInst *BI = dyn_cast<CondBrInst>(T)) {
BB->removePredecessor(Predecessor);
// Preserve guarding condition in assume, because it might not be
// inferrable from any dominating condition.
Value *Cond = BI->getCondition();
CallInst *Assumption;
if (BI->getSuccessor(0) == BB)
Assumption = Builder.CreateAssumption(Builder.CreateNot(Cond));
else
Assumption = Builder.CreateAssumption(Cond);
if (AC)
AC->registerAssumption(cast<AssumeInst>(Assumption));
Builder.CreateBr(BI->getSuccessor(0) == BB ? BI->getSuccessor(1)
: BI->getSuccessor(0));
BI->eraseFromParent();
if (DTU)
DTU->applyUpdates({{DominatorTree::Delete, Predecessor, BB}});
return true;
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
// Redirect all branches leading to UB into
// a newly created unreachable block.
BasicBlock *Unreachable = BasicBlock::Create(
Predecessor->getContext(), "unreachable", BB->getParent(), BB);
Builder.SetInsertPoint(Unreachable);
// The new block contains only one instruction: Unreachable
Builder.CreateUnreachable();
for (const auto &Case : SI->cases())
if (Case.getCaseSuccessor() == BB) {
BB->removePredecessor(Predecessor);
Case.setSuccessor(Unreachable);
}
if (SI->getDefaultDest() == BB) {
BB->removePredecessor(Predecessor);
SI->setDefaultDest(Unreachable);
}
if (DTU)
DTU->applyUpdates(
{ { DominatorTree::Insert, Predecessor, Unreachable },
{ DominatorTree::Delete, Predecessor, BB } });
return true;
}
}
return false;
}
bool SimplifyCFGOpt::simplifyOnce(BasicBlock *BB) {
bool Changed = false;
assert(BB && BB->getParent() && "Block not embedded in function!");
assert(BB->getTerminator() && "Degenerate basic block encountered!");
// Remove basic blocks that have no predecessors (except the entry block)...
// or that just have themself as a predecessor. These are unreachable.
if ((pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()) ||
BB->getSinglePredecessor() == BB) {
LLVM_DEBUG(dbgs() << "Removing BB: \n" << *BB);
DeleteDeadBlock(BB, DTU);
return true;
}
// Check to see if we can constant propagate this terminator instruction
// away...
Changed |= ConstantFoldTerminator(BB, /*DeleteDeadConditions=*/true,
/*TLI=*/nullptr, DTU);
// Check for and eliminate duplicate PHI nodes in this block.
Changed |= EliminateDuplicatePHINodes(BB);
// Check for and remove branches that will always cause undefined behavior.
if (removeUndefIntroducingPredecessor(BB, DTU, Options.AC))
return requestResimplify();
// Merge basic blocks into their predecessor if there is only one distinct
// pred, and if there is only one distinct successor of the predecessor, and
// if there are no PHI nodes.
if (MergeBlockIntoPredecessor(BB, DTU))
return true;
if (SinkCommon && Options.SinkCommonInsts) {
if (sinkCommonCodeFromPredecessors(BB, DTU) ||
mergeCompatibleInvokes(BB, DTU)) {
// sinkCommonCodeFromPredecessors() does not automatically CSE PHI's,
// so we may now how duplicate PHI's.
// Let's rerun EliminateDuplicatePHINodes() first,
// before foldTwoEntryPHINode() potentially converts them into select's,
// after which we'd need a whole EarlyCSE pass run to cleanup them.
return true;
}
// Merge identical predecessors of this block.
if (simplifyDuplicatePredecessors(BB, DTU))
return true;
}
if (Options.SpeculateBlocks &&
!BB->getParent()->hasFnAttribute(Attribute::OptForFuzzing)) {
// If there is a trivial two-entry PHI node in this basic block, and we can
// eliminate it, do so now.
if (auto *PN = dyn_cast<PHINode>(BB->begin()))
if (PN->getNumIncomingValues() == 2)
if (foldTwoEntryPHINode(PN, TTI, DTU, Options.AC, DL,
Options.SpeculateUnpredictables))
return true;
}
IRBuilder<> Builder(BB);
Instruction *Terminator = BB->getTerminator();
Builder.SetInsertPoint(Terminator);
switch (Terminator->getOpcode()) {
case Instruction::UncondBr:
Changed |= simplifyUncondBranch(cast<UncondBrInst>(Terminator), Builder);
break;
case Instruction::CondBr:
Changed |= simplifyCondBranch(cast<CondBrInst>(Terminator), Builder);
break;
case Instruction::Resume:
Changed |= simplifyResume(cast<ResumeInst>(Terminator), Builder);
break;
case Instruction::CleanupRet:
Changed |= simplifyCleanupReturn(cast<CleanupReturnInst>(Terminator));
break;
case Instruction::Switch:
Changed |= simplifySwitch(cast<SwitchInst>(Terminator), Builder);
break;
case Instruction::Unreachable:
Changed |= simplifyUnreachable(cast<UnreachableInst>(Terminator));
break;
case Instruction::IndirectBr:
Changed |= simplifyIndirectBr(cast<IndirectBrInst>(Terminator));
break;
}
return Changed;
}
bool SimplifyCFGOpt::run(BasicBlock *BB) {
bool Changed = false;
// Repeated simplify BB as long as resimplification is requested.
do {
Resimplify = false;
// Perform one round of simplifcation. Resimplify flag will be set if
// another iteration is requested.
Changed |= simplifyOnce(BB);
} while (Resimplify);
return Changed;
}
bool llvm::simplifyCFG(BasicBlock *BB, const TargetTransformInfo &TTI,
DomTreeUpdater *DTU, const SimplifyCFGOptions &Options,
ArrayRef<WeakVH> LoopHeaders) {
return SimplifyCFGOpt(TTI, DTU, BB->getDataLayout(), LoopHeaders,
Options)
.run(BB);
}