blob: c5534727385549c5a6e15074fb1476133e3426cd [file] [log] [blame]
//===- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop -------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This pass transforms loops that contain branches on loop-invariant conditions
// to multiple loops. For example, it turns the left into the right code:
// for (...) if (lic)
// A for (...)
// if (lic) A; B; C
// B else
// C for (...)
// A; C
// This can increase the size of the code exponentially (doubling it every time
// a loop is unswitched) so we only unswitch if the resultant code will be
// smaller than a threshold.
// This pass expects LICM to be run before it to hoist invariant conditions out
// of the loop, to make the unswitching opportunity obvious.
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LazyBlockFrequencyInfo.h"
#include "llvm/Analysis/LegacyDivergenceAnalysis.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/MustExecute.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.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/Intrinsics.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/LoopPassManager.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <map>
#include <set>
#include <tuple>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "loop-unswitch"
STATISTIC(NumBranches, "Number of branches unswitched");
STATISTIC(NumSwitches, "Number of switches unswitched");
STATISTIC(NumGuards, "Number of guards unswitched");
STATISTIC(NumSelects , "Number of selects unswitched");
STATISTIC(NumTrivial , "Number of unswitches that are trivial");
STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
STATISTIC(TotalInsts, "Total number of instructions analyzed");
// The specific value of 100 here was chosen based only on intuition and a
// few specific examples.
static cl::opt<unsigned>
Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
cl::init(100), cl::Hidden);
static cl::opt<unsigned>
cl::desc("Max number of memory uses to explore during "
"partial unswitching analysis"),
cl::init(100), cl::Hidden);
namespace {
class LUAnalysisCache {
using UnswitchedValsMap =
DenseMap<const SwitchInst *, SmallPtrSet<const Value *, 8>>;
using UnswitchedValsIt = UnswitchedValsMap::iterator;
struct LoopProperties {
unsigned CanBeUnswitchedCount;
unsigned WasUnswitchedCount;
unsigned SizeEstimation;
UnswitchedValsMap UnswitchedVals;
// Here we use std::map instead of DenseMap, since we need to keep valid
// LoopProperties pointer for current loop for better performance.
using LoopPropsMap = std::map<const Loop *, LoopProperties>;
using LoopPropsMapIt = LoopPropsMap::iterator;
LoopPropsMap LoopsProperties;
UnswitchedValsMap *CurLoopInstructions = nullptr;
LoopProperties *CurrentLoopProperties = nullptr;
// A loop unswitching with an estimated cost above this threshold
// is not performed. MaxSize is turned into unswitching quota for
// the current loop, and reduced correspondingly, though note that
// the quota is returned by releaseMemory() when the loop has been
// processed, so that MaxSize will return to its previous
// value. So in most cases MaxSize will equal the Threshold flag
// when a new loop is processed. An exception to that is that
// MaxSize will have a smaller value while processing nested loops
// that were introduced due to loop unswitching of an outer loop.
// FIXME: The way that MaxSize works is subtle and depends on the
// pass manager processing loops and calling releaseMemory() in a
// specific order. It would be good to find a more straightforward
// way of doing what MaxSize does.
unsigned MaxSize;
LUAnalysisCache() : MaxSize(Threshold) {}
// Analyze loop. Check its size, calculate is it possible to unswitch
// it. Returns true if we can unswitch this loop.
bool countLoop(const Loop *L, const TargetTransformInfo &TTI,
AssumptionCache *AC);
// Clean all data related to given loop.
void forgetLoop(const Loop *L);
// Mark case value as unswitched.
// Since SI instruction can be partly unswitched, in order to avoid
// extra unswitching in cloned loops keep track all unswitched values.
void setUnswitched(const SwitchInst *SI, const Value *V);
// Check was this case value unswitched before or not.
bool isUnswitched(const SwitchInst *SI, const Value *V);
// Returns true if another unswitching could be done within the cost
// threshold.
bool costAllowsUnswitching();
// Clone all loop-unswitch related loop properties.
// Redistribute unswitching quotas.
// Note, that new loop data is stored inside the VMap.
void cloneData(const Loop *NewLoop, const Loop *OldLoop,
const ValueToValueMapTy &VMap);
class LoopUnswitch : public LoopPass {
LoopInfo *LI; // Loop information
LPPassManager *LPM;
AssumptionCache *AC;
// Used to check if second loop needs processing after
// rewriteLoopBodyWithConditionConstant rewrites first loop.
std::vector<Loop*> LoopProcessWorklist;
LUAnalysisCache BranchesInfo;
bool OptimizeForSize;
bool RedoLoop = false;
Loop *CurrentLoop = nullptr;
DominatorTree *DT = nullptr;
MemorySSA *MSSA = nullptr;
AAResults *AA = nullptr;
std::unique_ptr<MemorySSAUpdater> MSSAU;
BasicBlock *LoopHeader = nullptr;
BasicBlock *LoopPreheader = nullptr;
bool SanitizeMemory;
SimpleLoopSafetyInfo SafetyInfo;
// LoopBlocks contains all of the basic blocks of the loop, including the
// preheader of the loop, the body of the loop, and the exit blocks of the
// loop, in that order.
std::vector<BasicBlock*> LoopBlocks;
// NewBlocks contained cloned copy of basic blocks from LoopBlocks.
std::vector<BasicBlock*> NewBlocks;
bool HasBranchDivergence;
static char ID; // Pass ID, replacement for typeid
explicit LoopUnswitch(bool Os = false, bool HasBranchDivergence = false)
: LoopPass(ID), OptimizeForSize(Os),
HasBranchDivergence(HasBranchDivergence) {
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
bool processCurrentLoop();
bool isUnreachableDueToPreviousUnswitching(BasicBlock *);
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG.
void getAnalysisUsage(AnalysisUsage &AU) const override {
// Lazy BFI and BPI are marked as preserved here so Loop Unswitching
// can remain part of the same loop pass as LICM
if (EnableMSSALoopDependency) {
if (HasBranchDivergence)
void releaseMemory() override { BranchesInfo.forgetLoop(CurrentLoop); }
void initLoopData() {
LoopHeader = CurrentLoop->getHeader();
LoopPreheader = CurrentLoop->getLoopPreheader();
/// Split all of the edges from inside the loop to their exit blocks.
/// Update the appropriate Phi nodes as we do so.
void splitExitEdges(Loop *L,
const SmallVectorImpl<BasicBlock *> &ExitBlocks);
bool tryTrivialLoopUnswitch(bool &Changed);
bool unswitchIfProfitable(Value *LoopCond, Constant *Val,
Instruction *TI = nullptr,
ArrayRef<Instruction *> ToDuplicate = {});
void unswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
BasicBlock *ExitBlock, Instruction *TI);
void unswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L,
Instruction *TI,
ArrayRef<Instruction *> ToDuplicate = {});
void rewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
Constant *Val, bool IsEqual);
emitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
BasicBlock *TrueDest, BasicBlock *FalseDest,
BranchInst *OldBranch, Instruction *TI,
ArrayRef<Instruction *> ToDuplicate = {});
void simplifyCode(std::vector<Instruction *> &Worklist, Loop *L);
/// Given that the Invariant is not equal to Val. Simplify instructions
/// in the loop.
Value *simplifyInstructionWithNotEqual(Instruction *Inst, Value *Invariant,
Constant *Val);
} // end anonymous namespace
// Analyze loop. Check its size, calculate is it possible to unswitch
// it. Returns true if we can unswitch this loop.
bool LUAnalysisCache::countLoop(const Loop *L, const TargetTransformInfo &TTI,
AssumptionCache *AC) {
LoopPropsMapIt PropsIt;
bool Inserted;
std::tie(PropsIt, Inserted) =
LoopsProperties.insert(std::make_pair(L, LoopProperties()));
LoopProperties &Props = PropsIt->second;
if (Inserted) {
// New loop.
// Limit the number of instructions to avoid causing significant code
// expansion, and the number of basic blocks, to avoid loops with
// large numbers of branches which cause loop unswitching to go crazy.
// This is a very ad-hoc heuristic.
SmallPtrSet<const Value *, 32> EphValues;
CodeMetrics::collectEphemeralValues(L, AC, EphValues);
// FIXME: This is overly conservative because it does not take into
// consideration code simplification opportunities and code that can
// be shared by the resultant unswitched loops.
CodeMetrics Metrics;
for (BasicBlock *BB : L->blocks())
Metrics.analyzeBasicBlock(BB, TTI, EphValues);
Props.SizeEstimation = Metrics.NumInsts;
Props.CanBeUnswitchedCount = MaxSize / (Props.SizeEstimation);
Props.WasUnswitchedCount = 0;
MaxSize -= Props.SizeEstimation * Props.CanBeUnswitchedCount;
if (Metrics.notDuplicatable) {
LLVM_DEBUG(dbgs() << "NOT unswitching loop %" << L->getHeader()->getName()
<< ", contents cannot be "
<< "duplicated!\n");
return false;
// Be careful. This links are good only before new loop addition.
CurrentLoopProperties = &Props;
CurLoopInstructions = &Props.UnswitchedVals;
return true;
// Clean all data related to given loop.
void LUAnalysisCache::forgetLoop(const Loop *L) {
LoopPropsMapIt LIt = LoopsProperties.find(L);
if (LIt != LoopsProperties.end()) {
LoopProperties &Props = LIt->second;
MaxSize += (Props.CanBeUnswitchedCount + Props.WasUnswitchedCount) *
CurrentLoopProperties = nullptr;
CurLoopInstructions = nullptr;
// Mark case value as unswitched.
// Since SI instruction can be partly unswitched, in order to avoid
// extra unswitching in cloned loops keep track all unswitched values.
void LUAnalysisCache::setUnswitched(const SwitchInst *SI, const Value *V) {
// Check was this case value unswitched before or not.
bool LUAnalysisCache::isUnswitched(const SwitchInst *SI, const Value *V) {
return (*CurLoopInstructions)[SI].count(V);
bool LUAnalysisCache::costAllowsUnswitching() {
return CurrentLoopProperties->CanBeUnswitchedCount > 0;
// Clone all loop-unswitch related loop properties.
// Redistribute unswitching quotas.
// Note, that new loop data is stored inside the VMap.
void LUAnalysisCache::cloneData(const Loop *NewLoop, const Loop *OldLoop,
const ValueToValueMapTy &VMap) {
LoopProperties &NewLoopProps = LoopsProperties[NewLoop];
LoopProperties &OldLoopProps = *CurrentLoopProperties;
UnswitchedValsMap &Insts = OldLoopProps.UnswitchedVals;
// Reallocate "can-be-unswitched quota"
NewLoopProps.WasUnswitchedCount = 0;
unsigned Quota = OldLoopProps.CanBeUnswitchedCount;
NewLoopProps.CanBeUnswitchedCount = Quota / 2;
OldLoopProps.CanBeUnswitchedCount = Quota - Quota / 2;
NewLoopProps.SizeEstimation = OldLoopProps.SizeEstimation;
// Clone unswitched values info:
// for new loop switches we clone info about values that was
// already unswitched and has redundant successors.
for (const auto &I : Insts) {
const SwitchInst *OldInst = I.first;
Value *NewI = VMap.lookup(OldInst);
const SwitchInst *NewInst = cast_or_null<SwitchInst>(NewI);
assert(NewInst && "All instructions that are in SrcBB must be in VMap.");
NewLoopProps.UnswitchedVals[NewInst] = OldLoopProps.UnswitchedVals[OldInst];
char LoopUnswitch::ID = 0;
INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops",
false, false)
INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops",
false, false)
Pass *llvm::createLoopUnswitchPass(bool Os, bool HasBranchDivergence) {
return new LoopUnswitch(Os, HasBranchDivergence);
/// Operator chain lattice.
enum OperatorChain {
OC_OpChainNone, ///< There is no operator.
OC_OpChainOr, ///< There are only ORs.
OC_OpChainAnd, ///< There are only ANDs.
OC_OpChainMixed ///< There are ANDs and ORs.
/// Cond is a condition that occurs in L. If it is invariant in the loop, or has
/// an invariant piece, return the invariant. Otherwise, return null.
/// NOTE: findLIVLoopCondition will not return a partial LIV by walking up a
/// mixed operator chain, as we can not reliably find a value which will
/// simplify the operator chain. If the chain is AND-only or OR-only, we can use
/// 0 or ~0 to simplify the chain.
/// NOTE: In case a partial LIV and a mixed operator chain, we may be able to
/// simplify the condition itself to a loop variant condition, but at the
/// cost of creating an entirely new loop.
static Value *findLIVLoopCondition(Value *Cond, Loop *L, bool &Changed,
OperatorChain &ParentChain,
DenseMap<Value *, Value *> &Cache,
MemorySSAUpdater *MSSAU) {
auto CacheIt = Cache.find(Cond);
if (CacheIt != Cache.end())
return CacheIt->second;
// We started analyze new instruction, increment scanned instructions counter.
// We can never unswitch on vector conditions.
if (Cond->getType()->isVectorTy())
return nullptr;
// Constants should be folded, not unswitched on!
if (isa<Constant>(Cond)) return nullptr;
// Hoist simple values out.
if (L->makeLoopInvariant(Cond, Changed, nullptr, MSSAU)) {
Cache[Cond] = Cond;
return Cond;
// Walk up the operator chain to find partial invariant conditions.
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
if (BO->getOpcode() == Instruction::And ||
BO->getOpcode() == Instruction::Or) {
// Given the previous operator, compute the current operator chain status.
OperatorChain NewChain;
switch (ParentChain) {
case OC_OpChainNone:
NewChain = BO->getOpcode() == Instruction::And ? OC_OpChainAnd :
case OC_OpChainOr:
NewChain = BO->getOpcode() == Instruction::Or ? OC_OpChainOr :
case OC_OpChainAnd:
NewChain = BO->getOpcode() == Instruction::And ? OC_OpChainAnd :
case OC_OpChainMixed:
NewChain = OC_OpChainMixed;
// If we reach a Mixed state, we do not want to keep walking up as we can not
// reliably find a value that will simplify the chain. With this check, we
// will return null on the first sight of mixed chain and the caller will
// either backtrack to find partial LIV in other operand or return null.
if (NewChain != OC_OpChainMixed) {
// Update the current operator chain type before we search up the chain.
ParentChain = NewChain;
// If either the left or right side is invariant, we can unswitch on this,
// which will cause the branch to go away in one loop and the condition to
// simplify in the other one.
if (Value *LHS = findLIVLoopCondition(BO->getOperand(0), L, Changed,
ParentChain, Cache, MSSAU)) {
Cache[Cond] = LHS;
return LHS;
// We did not manage to find a partial LIV in operand(0). Backtrack and try
// operand(1).
ParentChain = NewChain;
if (Value *RHS = findLIVLoopCondition(BO->getOperand(1), L, Changed,
ParentChain, Cache, MSSAU)) {
Cache[Cond] = RHS;
return RHS;
Cache[Cond] = nullptr;
return nullptr;
/// Cond is a condition that occurs in L. If it is invariant in the loop, or has
/// an invariant piece, return the invariant along with the operator chain type.
/// Otherwise, return null.
static std::pair<Value *, OperatorChain>
findLIVLoopCondition(Value *Cond, Loop *L, bool &Changed,
MemorySSAUpdater *MSSAU) {
DenseMap<Value *, Value *> Cache;
OperatorChain OpChain = OC_OpChainNone;
Value *FCond = findLIVLoopCondition(Cond, L, Changed, OpChain, Cache, MSSAU);
// In case we do find a LIV, it can not be obtained by walking up a mixed
// operator chain.
assert((!FCond || OpChain != OC_OpChainMixed) &&
"Do not expect a partial LIV with mixed operator chain");
return {FCond, OpChain};
bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPMRef) {
if (skipLoop(L))
return false;
AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
LPM = &LPMRef;
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
if (EnableMSSALoopDependency) {
MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA();
MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
assert(DT && "Cannot update MemorySSA without a valid DomTree.");
CurrentLoop = L;
Function *F = CurrentLoop->getHeader()->getParent();
SanitizeMemory = F->hasFnAttribute(Attribute::SanitizeMemory);
if (SanitizeMemory)
if (MSSA && VerifyMemorySSA)
bool Changed = false;
do {
if (MSSA && VerifyMemorySSA)
RedoLoop = false;
Changed |= processCurrentLoop();
} while (RedoLoop);
if (MSSA && VerifyMemorySSA)
return Changed;
// Return true if the BasicBlock BB is unreachable from the loop header.
// Return false, otherwise.
bool LoopUnswitch::isUnreachableDueToPreviousUnswitching(BasicBlock *BB) {
auto *Node = DT->getNode(BB)->getIDom();
BasicBlock *DomBB = Node->getBlock();
while (CurrentLoop->contains(DomBB)) {
BranchInst *BInst = dyn_cast<BranchInst>(DomBB->getTerminator());
Node = DT->getNode(DomBB)->getIDom();
DomBB = Node->getBlock();
if (!BInst || !BInst->isConditional())
Value *Cond = BInst->getCondition();
if (!isa<ConstantInt>(Cond))
BasicBlock *UnreachableSucc =
Cond == ConstantInt::getTrue(Cond->getContext())
? BInst->getSuccessor(1)
: BInst->getSuccessor(0);
if (DT->dominates(UnreachableSucc, BB))
return true;
return false;
/// FIXME: Remove this workaround when freeze related patches are done.
/// LoopUnswitch and Equality propagation in GVN have discrepancy about
/// whether branch on undef/poison has undefine behavior. Here it is to
/// rule out some common cases that we found such discrepancy already
/// causing problems. Detail could be found in PR31652. Note if the
/// func returns true, it is unsafe. But if it is false, it doesn't mean
/// it is necessarily safe.
static bool equalityPropUnSafe(Value &LoopCond) {
ICmpInst *CI = dyn_cast<ICmpInst>(&LoopCond);
if (!CI || !CI->isEquality())
return false;
Value *LHS = CI->getOperand(0);
Value *RHS = CI->getOperand(1);
if (isa<UndefValue>(LHS) || isa<UndefValue>(RHS))
return true;
auto HasUndefInPHI = [](PHINode &PN) {
for (Value *Opd : PN.incoming_values()) {
if (isa<UndefValue>(Opd))
return true;
return false;
PHINode *LPHI = dyn_cast<PHINode>(LHS);
PHINode *RPHI = dyn_cast<PHINode>(RHS);
if ((LPHI && HasUndefInPHI(*LPHI)) || (RPHI && HasUndefInPHI(*RPHI)))
return true;
auto HasUndefInSelect = [](SelectInst &SI) {
if (isa<UndefValue>(SI.getTrueValue()) ||
return true;
return false;
SelectInst *LSI = dyn_cast<SelectInst>(LHS);
SelectInst *RSI = dyn_cast<SelectInst>(RHS);
if ((LSI && HasUndefInSelect(*LSI)) || (RSI && HasUndefInSelect(*RSI)))
return true;
return false;
namespace {
/// Struct to hold information about a partially invariant condition.
struct IVConditionInfo {
/// Instructions that need to be duplicated and checked for the unswitching
/// condition.
SmallVector<Instruction *, 4> InstToDuplicate;
/// Constant to indicate for which value the condition is invariant.
Constant *KnownValue = nullptr;
/// True if the partially invariant path is no-op (=does not have any
/// side-effects and no loop value is used outside the loop).
bool PathIsNoop = true;
/// If the partially invariant path reaches a single exit block, ExitForPath
/// is set to that block. Otherwise it is nullptr.
BasicBlock *ExitForPath = nullptr;
} // namespace
/// Check if the loop header has a conditional branch that is not
/// loop-invariant, because it involves load instructions. If all paths from
/// either the true or false successor to the header or loop exists do not
/// modify the memory feeding the condition, perform 'partial unswitching'. That
/// is, duplicate the instructions feeding the condition in the pre-header. Then
/// unswitch on the duplicated condition. The condition is now known in the
/// unswitched version for the 'invariant' path through the original loop.
/// If the branch condition of the header is partially invariant, return a pair
/// containing the instructions to duplicate and a boolean Constant to update
/// the condition in the loops created for the true or false successors.
static Optional<IVConditionInfo> hasPartialIVCondition(Loop *L, MemorySSA &MSSA,
AAResults *AA) {
auto *TI = dyn_cast<BranchInst>(L->getHeader()->getTerminator());
if (!TI || !TI->isConditional())
return {};
auto *CondI = dyn_cast<CmpInst>(TI->getCondition());
// The case with the condition outside the loop should already be handled
// earlier.
if (!CondI || !L->contains(CondI))
return {};
SmallVector<Instruction *, 4> InstToDuplicate;
SmallVector<Value *, 4> WorkList;
WorkList.append(CondI->op_begin(), CondI->op_end());
SmallVector<MemoryAccess *, 4> AccessesToCheck;
SmallVector<MemoryLocation, 4> AccessedLocs;
while (!WorkList.empty()) {
Instruction *I = dyn_cast<Instruction>(WorkList.pop_back_val());
if (!I || !L->contains(I))
// TODO: support additional instructions.
if (!isa<LoadInst>(I) && !isa<GetElementPtrInst>(I))
return {};
// Do not duplicate volatile and atomic loads.
if (auto *LI = dyn_cast<LoadInst>(I))
if (LI->isVolatile() || LI->isAtomic())
return {};
if (MemoryAccess *MA = MSSA.getMemoryAccess(I)) {
if (auto *MemUse = dyn_cast_or_null<MemoryUse>(MA)) {
// Queue the defining access to check for alias checks.
} else {
// MemoryDefs may clobber the location or may be atomic memory
// operations. Bail out.
return {};
WorkList.append(I->op_begin(), I->op_end());
if (InstToDuplicate.size() <= 1)
return {};
SmallVector<BasicBlock *, 4> ExitingBlocks;
auto HasNoClobbersOnPath =
[L, AA, &AccessedLocs, &ExitingBlocks,
&InstToDuplicate](BasicBlock *Succ, BasicBlock *Header,
SmallVector<MemoryAccess *, 4> AccessesToCheck)
-> Optional<IVConditionInfo> {
IVConditionInfo Info;
// First, collect all blocks in the loop that are on a patch from Succ
// to the header.
SmallVector<BasicBlock *, 4> WorkList;
SmallPtrSet<BasicBlock *, 4> Seen;
Info.PathIsNoop &=
all_of(*Header, [](Instruction &I) { return !I.mayHaveSideEffects(); });
while (!WorkList.empty()) {
BasicBlock *Current = WorkList.pop_back_val();
if (!L->contains(Current))
const auto &SeenIns = Seen.insert(Current);
if (!SeenIns.second)
Info.PathIsNoop &= all_of(
*Current, [](Instruction &I) { return !I.mayHaveSideEffects(); });
WorkList.append(succ_begin(Current), succ_end(Current));
// Require at least 2 blocks on a path through the loop. This skips
// paths that directly exit the loop.
if (Seen.size() < 2)
return {};
// Next, check if there are any MemoryDefs that are on the path through
// the loop (in the Seen set) and they may-alias any of the locations in
// AccessedLocs. If that is the case, they may modify the condition and
// partial unswitching is not possible.
SmallPtrSet<MemoryAccess *, 4> SeenAccesses;
while (!AccessesToCheck.empty()) {
MemoryAccess *Current = AccessesToCheck.pop_back_val();
auto SeenI = SeenAccesses.insert(Current);
if (!SeenI.second || !Seen.contains(Current->getBlock()))
// Bail out if exceeded the threshold.
if (SeenAccesses.size() >= MSSAThreshold)
return {};
// MemoryUse are read-only accesses.
if (isa<MemoryUse>(Current))
// For a MemoryDef, check if is aliases any of the location feeding
// the original condition.
if (auto *CurrentDef = dyn_cast<MemoryDef>(Current)) {
if (any_of(AccessedLocs, [AA, CurrentDef](MemoryLocation &Loc) {
return isModSet(
AA->getModRefInfo(CurrentDef->getMemoryInst(), Loc));
return {};
for (Use &U : Current->uses())
// We could also allow loops with known trip counts without mustprogress,
// but ScalarEvolution may not be available.
Info.PathIsNoop &=
L->getHeader()->getParent()->mustProgress() || hasMustProgress(L);
// If the path is considered a no-op so far, check if it reaches a
// single exit block without any phis. This ensures no values from the
// loop are used outside of the loop.
if (Info.PathIsNoop) {
for (auto *Exiting : ExitingBlocks) {
if (!Seen.contains(Exiting))
for (auto *Succ : successors(Exiting)) {
if (L->contains(Succ))
Info.PathIsNoop &= llvm::empty(Succ->phis()) &&
(!Info.ExitForPath || Info.ExitForPath == Succ);
if (!Info.PathIsNoop)
assert((!Info.ExitForPath || Info.ExitForPath == Succ) &&
"cannot have multiple exit blocks");
Info.ExitForPath = Succ;
if (!Info.ExitForPath)
Info.PathIsNoop = false;
Info.InstToDuplicate = InstToDuplicate;
return Info;
// If we branch to the same successor, partial unswitching will not be
// beneficial.
if (TI->getSuccessor(0) == TI->getSuccessor(1))
return {};
if (auto Info = HasNoClobbersOnPath(TI->getSuccessor(0), L->getHeader(),
AccessesToCheck)) {
Info->KnownValue = ConstantInt::getTrue(TI->getContext());
return Info;
if (auto Info = HasNoClobbersOnPath(TI->getSuccessor(1), L->getHeader(),
AccessesToCheck)) {
Info->KnownValue = ConstantInt::getFalse(TI->getContext());
return Info;
return {};
/// Do actual work and unswitch loop if possible and profitable.
bool LoopUnswitch::processCurrentLoop() {
bool Changed = false;
// If LoopSimplify was unable to form a preheader, don't do any unswitching.
if (!LoopPreheader)
return false;
// Loops with indirectbr cannot be cloned.
if (!CurrentLoop->isSafeToClone())
return false;
// Without dedicated exits, splitting the exit edge may fail.
if (!CurrentLoop->hasDedicatedExits())
return false;
LLVMContext &Context = LoopHeader->getContext();
// Analyze loop cost, and stop unswitching if loop content can not be duplicated.
if (!BranchesInfo.countLoop(
return false;
// Try trivial unswitch first before loop over other basic blocks in the loop.
if (tryTrivialLoopUnswitch(Changed)) {
return true;
// Do not do non-trivial unswitch while optimizing for size.
// FIXME: Use Function::hasOptSize().
if (OptimizeForSize ||
return Changed;
// Run through the instructions in the loop, keeping track of three things:
// - That we do not unswitch loops containing convergent operations, as we
// might be making them control dependent on the unswitch value when they
// were not before.
// FIXME: This could be refined to only bail if the convergent operation is
// not already control-dependent on the unswitch value.
// - That basic blocks in the loop contain invokes whose predecessor edges we
// cannot split.
// - The set of guard intrinsics encountered (these are non terminator
// instructions that are also profitable to be unswitched).
SmallVector<IntrinsicInst *, 4> Guards;
for (const auto BB : CurrentLoop->blocks()) {
for (auto &I : *BB) {
auto *CB = dyn_cast<CallBase>(&I);
if (!CB)
if (CB->isConvergent())
return Changed;
if (auto *II = dyn_cast<InvokeInst>(&I))
if (!II->getUnwindDest()->canSplitPredecessors())
return Changed;
if (auto *II = dyn_cast<IntrinsicInst>(&I))
if (II->getIntrinsicID() == Intrinsic::experimental_guard)
for (IntrinsicInst *Guard : Guards) {
Value *LoopCond = findLIVLoopCondition(Guard->getOperand(0), CurrentLoop,
Changed, MSSAU.get())
if (LoopCond &&
unswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context))) {
// NB! Unswitching (if successful) could have erased some of the
// instructions in Guards leaving dangling pointers there. This is fine
// because we're returning now, and won't look at Guards again.
return true;
// Loop over all of the basic blocks in the loop. If we find an interior
// block that is branching on a loop-invariant condition, we can unswitch this
// loop.
for (Loop::block_iterator I = CurrentLoop->block_begin(),
E = CurrentLoop->block_end();
I != E; ++I) {
Instruction *TI = (*I)->getTerminator();
// Unswitching on a potentially uninitialized predicate is not
// MSan-friendly. Limit this to the cases when the original predicate is
// guaranteed to execute, to avoid creating a use-of-uninitialized-value
// in the code that did not have one.
// This is a workaround for the discrepancy between LLVM IR and MSan
// semantics. See PR28054 for more details.
if (SanitizeMemory &&
!SafetyInfo.isGuaranteedToExecute(*TI, DT, CurrentLoop))
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
// Some branches may be rendered unreachable because of previous
// unswitching.
// Unswitch only those branches that are reachable.
if (isUnreachableDueToPreviousUnswitching(*I))
// If this isn't branching on an invariant condition, we can't unswitch
// it.
if (BI->isConditional()) {
// See if this, or some part of it, is loop invariant. If so, we can
// unswitch on it if we desire.
Value *LoopCond = findLIVLoopCondition(BI->getCondition(), CurrentLoop,
Changed, MSSAU.get())
if (LoopCond && !equalityPropUnSafe(*LoopCond) &&
unswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context), TI)) {
return true;
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Value *SC = SI->getCondition();
Value *LoopCond;
OperatorChain OpChain;
std::tie(LoopCond, OpChain) =
findLIVLoopCondition(SC, CurrentLoop, Changed, MSSAU.get());
unsigned NumCases = SI->getNumCases();
if (LoopCond && NumCases) {
// Find a value to unswitch on:
// FIXME: this should chose the most expensive case!
// FIXME: scan for a case with a non-critical edge?
Constant *UnswitchVal = nullptr;
// Find a case value such that at least one case value is unswitched
// out.
if (OpChain == OC_OpChainAnd) {
// If the chain only has ANDs and the switch has a case value of 0.
// Dropping in a 0 to the chain will unswitch out the 0-casevalue.
auto *AllZero = cast<ConstantInt>(Constant::getNullValue(SC->getType()));
if (BranchesInfo.isUnswitched(SI, AllZero))
// We are unswitching 0 out.
UnswitchVal = AllZero;
} else if (OpChain == OC_OpChainOr) {
// If the chain only has ORs and the switch has a case value of ~0.
// Dropping in a ~0 to the chain will unswitch out the ~0-casevalue.
auto *AllOne = cast<ConstantInt>(Constant::getAllOnesValue(SC->getType()));
if (BranchesInfo.isUnswitched(SI, AllOne))
// We are unswitching ~0 out.
UnswitchVal = AllOne;
} else {
assert(OpChain == OC_OpChainNone &&
"Expect to unswitch on trivial chain");
// Do not process same value again and again.
// At this point we have some cases already unswitched and
// some not yet unswitched. Let's find the first not yet unswitched one.
for (auto Case : SI->cases()) {
Constant *UnswitchValCandidate = Case.getCaseValue();
if (!BranchesInfo.isUnswitched(SI, UnswitchValCandidate)) {
UnswitchVal = UnswitchValCandidate;
if (!UnswitchVal)
if (unswitchIfProfitable(LoopCond, UnswitchVal)) {
// In case of a full LIV, UnswitchVal is the value we unswitched out.
// In case of a partial LIV, we only unswitch when its an AND-chain
// or OR-chain. In both cases switch input value simplifies to
// UnswitchVal.
BranchesInfo.setUnswitched(SI, UnswitchVal);
return true;
// Scan the instructions to check for unswitchable values.
for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
BBI != E; ++BBI)
if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
Value *LoopCond = findLIVLoopCondition(SI->getCondition(), CurrentLoop,
Changed, MSSAU.get())
if (LoopCond &&
unswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context))) {
return true;
// Check if there is a header condition that is invariant along the patch from
// either the true or false successors to the header. This allows unswitching
// conditions depending on memory accesses, if there's a path not clobbering
// the memory locations. Check if this transform has been disabled using
// metadata, to avoid unswitching the same loop multiple times.
if (MSSA &&
!findOptionMDForLoop(CurrentLoop, "llvm.loop.unswitch.partial.disable")) {
if (auto Info = hasPartialIVCondition(CurrentLoop, *MSSA, AA)) {
assert(!Info->InstToDuplicate.empty() &&
"need at least a partially invariant condition");
LLVM_DEBUG(dbgs() << "loop-unswitch: Found partially invariant condition "
<< *Info->InstToDuplicate[0] << "\n");
Instruction *TI = CurrentLoop->getHeader()->getTerminator();
Value *LoopCond = Info->InstToDuplicate[0];
// If the partially unswitched path is a no-op and has a single exit
// block, we do not need to do full unswitching. Instead, we can directly
// branch to the exit.
// TODO: Instead of duplicating the checks, we could also just directly
// branch to the exit from the conditional branch in the loop.
if (Info->PathIsNoop) {
if (HasBranchDivergence &&
getAnalysis<LegacyDivergenceAnalysis>().isDivergent(LoopCond)) {
LLVM_DEBUG(dbgs() << "NOT unswitching loop %"
<< CurrentLoop->getHeader()->getName()
<< " at non-trivial condition '"
<< *Info->KnownValue << "' == " << *LoopCond << "\n"
<< ". Condition is divergent.\n");
return false;
BasicBlock *TrueDest = LoopHeader;
BasicBlock *FalseDest = Info->ExitForPath;
if (Info->KnownValue->isOneValue())
std::swap(TrueDest, FalseDest);
auto *OldBr =
emitPreheaderBranchOnCondition(LoopCond, Info->KnownValue, TrueDest,
FalseDest, OldBr, TI,
delete OldBr;
RedoLoop = false;
return true;
// Otherwise, the path is not a no-op. Run regular unswitching.
if (unswitchIfProfitable(LoopCond, Info->KnownValue,
Info->InstToDuplicate)) {
RedoLoop = false;
return true;
return Changed;
/// Check to see if all paths from BB exit the loop with no side effects
/// (including infinite loops).
/// If true, we return true and set ExitBB to the block we
/// exit through.
static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
BasicBlock *&ExitBB,
std::set<BasicBlock*> &Visited) {
if (!Visited.insert(BB).second) {
// Already visited. Without more analysis, this could indicate an infinite
// loop.
return false;
if (!L->contains(BB)) {
// Otherwise, this is a loop exit, this is fine so long as this is the
// first exit.
if (ExitBB) return false;
ExitBB = BB;
return true;
// Otherwise, this is an unvisited intra-loop node. Check all successors.
for (BasicBlock *Succ : successors(BB)) {
// Check to see if the successor is a trivial loop exit.
if (!isTrivialLoopExitBlockHelper(L, Succ, ExitBB, Visited))
return false;
// Okay, everything after this looks good, check to make sure that this block
// doesn't include any side effects.
for (Instruction &I : *BB)
if (I.mayHaveSideEffects())
return false;
return true;
/// Return true if the specified block unconditionally leads to an exit from
/// the specified loop, and has no side-effects in the process. If so, return
/// the block that is exited to, otherwise return null.
static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
std::set<BasicBlock*> Visited;
Visited.insert(L->getHeader()); // Branches to header make infinite loops.
BasicBlock *ExitBB = nullptr;
if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
return ExitBB;
return nullptr;
/// We have found that we can unswitch CurrentLoop when LoopCond == Val to
/// simplify the loop. If we decide that this is profitable,
/// unswitch the loop, reprocess the pieces, then return true.
bool LoopUnswitch::unswitchIfProfitable(Value *LoopCond, Constant *Val,
Instruction *TI,
ArrayRef<Instruction *> ToDuplicate) {
// Check to see if it would be profitable to unswitch current loop.
if (!BranchesInfo.costAllowsUnswitching()) {
LLVM_DEBUG(dbgs() << "NOT unswitching loop %"
<< CurrentLoop->getHeader()->getName()
<< " at non-trivial condition '" << *Val
<< "' == " << *LoopCond << "\n"
<< ". Cost too high.\n");
return false;
if (HasBranchDivergence &&
getAnalysis<LegacyDivergenceAnalysis>().isDivergent(LoopCond)) {
LLVM_DEBUG(dbgs() << "NOT unswitching loop %"
<< CurrentLoop->getHeader()->getName()
<< " at non-trivial condition '" << *Val
<< "' == " << *LoopCond << "\n"
<< ". Condition is divergent.\n");
return false;
unswitchNontrivialCondition(LoopCond, Val, CurrentLoop, TI, ToDuplicate);
return true;
/// Emit a conditional branch on two values if LIC == Val, branch to TrueDst,
/// otherwise branch to FalseDest. Insert the code immediately before OldBranch
/// and remove (but not erase!) it from the function.
void LoopUnswitch::emitPreheaderBranchOnCondition(
Value *LIC, Constant *Val, BasicBlock *TrueDest, BasicBlock *FalseDest,
BranchInst *OldBranch, Instruction *TI,
ArrayRef<Instruction *> ToDuplicate) {
assert(OldBranch->isUnconditional() && "Preheader is not split correctly");
assert(TrueDest != FalseDest && "Branch targets should be different");
// Insert a conditional branch on LIC to the two preheaders. The original
// code is the true version and the new code is the false version.
Value *BranchVal = LIC;
bool Swapped = false;
if (!ToDuplicate.empty()) {
ValueToValueMapTy Old2New;
for (Instruction *I : reverse(ToDuplicate)) {
auto *New = I->clone();
RemapInstruction(New, Old2New,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
Old2New[I] = New;
if (MSSAU) {
MemorySSA *MSSA = MSSAU->getMemorySSA();
auto *MemA = dyn_cast_or_null<MemoryUse>(MSSA->getMemoryAccess(I));
if (!MemA)
Loop *L = LI->getLoopFor(I->getParent());
auto *DefiningAccess = MemA->getDefiningAccess();
// Get the first defining access before the loop.
while (L->contains(DefiningAccess->getBlock())) {
// If the defining access is a MemoryPhi, get the incoming
// value for the pre-header as defining access.
if (auto *MemPhi = dyn_cast<MemoryPhi>(DefiningAccess)) {
DefiningAccess =
} else {
DefiningAccess =
MSSAU->createMemoryAccessInBB(New, DefiningAccess, New->getParent(),
BranchVal = Old2New[ToDuplicate[0]];
} else {
if (!isa<ConstantInt>(Val) ||
Val->getType() != Type::getInt1Ty(LIC->getContext()))
BranchVal = new ICmpInst(OldBranch, ICmpInst::ICMP_EQ, LIC, Val);
else if (Val != ConstantInt::getTrue(Val->getContext())) {
// We want to enter the new loop when the condition is true.
std::swap(TrueDest, FalseDest);
Swapped = true;
// Old branch will be removed, so save its parent and successor to update the
// DomTree.
auto *OldBranchSucc = OldBranch->getSuccessor(0);
auto *OldBranchParent = OldBranch->getParent();
// Insert the new branch.
BranchInst *BI =
IRBuilder<>(OldBranch).CreateCondBr(BranchVal, TrueDest, FalseDest, TI);
if (Swapped)
// Remove the old branch so there is only one branch at the end. This is
// needed to perform DomTree's internal DFS walk on the function's CFG.
// Inform the DT about the new branch.
if (DT) {
// First, add both successors.
SmallVector<DominatorTree::UpdateType, 3> Updates;
if (TrueDest != OldBranchSucc)
Updates.push_back({DominatorTree::Insert, OldBranchParent, TrueDest});
if (FalseDest != OldBranchSucc)
Updates.push_back({DominatorTree::Insert, OldBranchParent, FalseDest});
// If both of the new successors are different from the old one, inform the
// DT that the edge was deleted.
if (OldBranchSucc != TrueDest && OldBranchSucc != FalseDest) {
Updates.push_back({DominatorTree::Delete, OldBranchParent, OldBranchSucc});
if (MSSAU)
MSSAU->applyUpdates(Updates, *DT, /*UpdateDT=*/true);
// If either edge is critical, split it. This helps preserve LoopSimplify
// form for enclosing loops.
auto Options =
CriticalEdgeSplittingOptions(DT, LI, MSSAU.get()).setPreserveLCSSA();
SplitCriticalEdge(BI, 0, Options);
SplitCriticalEdge(BI, 1, Options);
/// Given a loop that has a trivial unswitchable condition in it (a cond branch
/// from its header block to its latch block, where the path through the loop
/// that doesn't execute its body has no side-effects), unswitch it. This
/// doesn't involve any code duplication, just moving the conditional branch
/// outside of the loop and updating loop info.
void LoopUnswitch::unswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
BasicBlock *ExitBlock,
Instruction *TI) {
LLVM_DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
<< LoopHeader->getName() << " [" << L->getBlocks().size()
<< " blocks] in Function "
<< L->getHeader()->getParent()->getName()
<< " on cond: " << *Val << " == " << *Cond << "\n");
// We are going to make essential changes to CFG. This may invalidate cached
// information for L or one of its parent loops in SCEV.
if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>())
// First step, split the preheader, so that we know that there is a safe place
// to insert the conditional branch. We will change LoopPreheader to have a
// conditional branch on Cond.
BasicBlock *NewPH = SplitEdge(LoopPreheader, LoopHeader, DT, LI, MSSAU.get());
// Now that we have a place to insert the conditional branch, create a place
// to branch to: this is the exit block out of the loop that we should
// short-circuit to.
// Split this block now, so that the loop maintains its exit block, and so
// that the jump from the preheader can execute the contents of the exit block
// without actually branching to it (the exit block should be dominated by the
// loop header, not the preheader).
assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
BasicBlock *NewExit =
SplitBlock(ExitBlock, &ExitBlock->front(), DT, LI, MSSAU.get());
// Okay, now we have a position to branch from and a position to branch to,
// insert the new conditional branch.
auto *OldBranch = dyn_cast<BranchInst>(LoopPreheader->getTerminator());
assert(OldBranch && "Failed to split the preheader");
emitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH, OldBranch, TI);
// emitPreheaderBranchOnCondition removed the OldBranch from the function.
// Delete it, as it is no longer needed.
delete OldBranch;
// We need to reprocess this loop, it could be unswitched again.
RedoLoop = true;
// Now that we know that the loop is never entered when this condition is a
// particular value, rewrite the loop with this info. We know that this will
// at least eliminate the old branch.
rewriteLoopBodyWithConditionConstant(L, Cond, Val, /*IsEqual=*/false);
/// Check if the first non-constant condition starting from the loop header is
/// a trivial unswitch condition: that is, a condition controls whether or not
/// the loop does anything at all. If it is a trivial condition, unswitching
/// produces no code duplications (equivalently, it produces a simpler loop and
/// a new empty loop, which gets deleted). Therefore always unswitch trivial
/// condition.
bool LoopUnswitch::tryTrivialLoopUnswitch(bool &Changed) {
BasicBlock *CurrentBB = CurrentLoop->getHeader();
Instruction *CurrentTerm = CurrentBB->getTerminator();
LLVMContext &Context = CurrentBB->getContext();
// If loop header has only one reachable successor (currently via an
// unconditional branch or constant foldable conditional branch, but
// should also consider adding constant foldable switch instruction in
// future), we should keep looking for trivial condition candidates in
// the successor as well. An alternative is to constant fold conditions
// and merge successors into loop header (then we only need to check header's
// terminator). The reason for not doing this in LoopUnswitch pass is that
// it could potentially break LoopPassManager's invariants. Folding dead
// branches could either eliminate the current loop or make other loops
// unreachable. LCSSA form might also not be preserved after deleting
// branches. The following code keeps traversing loop header's successors
// until it finds the trivial condition candidate (condition that is not a
// constant). Since unswitching generates branches with constant conditions,
// this scenario could be very common in practice.
SmallPtrSet<BasicBlock*, 8> Visited;
while (true) {
// If we exit loop or reach a previous visited block, then
// we can not reach any trivial condition candidates (unfoldable
// branch instructions or switch instructions) and no unswitch
// can happen. Exit and return false.
if (!CurrentLoop->contains(CurrentBB) || !Visited.insert(CurrentBB).second)
return false;
// Check if this loop will execute any side-effecting instructions (e.g.
// stores, calls, volatile loads) in the part of the loop that the code
// *would* execute. Check the header first.
for (Instruction &I : *CurrentBB)
if (I.mayHaveSideEffects())
return false;
if (BranchInst *BI = dyn_cast<BranchInst>(CurrentTerm)) {
if (BI->isUnconditional()) {
CurrentBB = BI->getSuccessor(0);
} else if (BI->getCondition() == ConstantInt::getTrue(Context)) {
CurrentBB = BI->getSuccessor(0);
} else if (BI->getCondition() == ConstantInt::getFalse(Context)) {
CurrentBB = BI->getSuccessor(1);
} else {
// Found a trivial condition candidate: non-foldable conditional branch.
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(CurrentTerm)) {
// At this point, any constant-foldable instructions should have probably
// been folded.
ConstantInt *Cond = dyn_cast<ConstantInt>(SI->getCondition());
if (!Cond)
// Find the target block we are definitely going to.
CurrentBB = SI->findCaseValue(Cond)->getCaseSuccessor();
} else {
// We do not understand these terminator instructions.
CurrentTerm = CurrentBB->getTerminator();
// CondVal is the condition that controls the trivial condition.
// LoopExitBB is the BasicBlock that loop exits when meets trivial condition.
Constant *CondVal = nullptr;
BasicBlock *LoopExitBB = nullptr;
if (BranchInst *BI = dyn_cast<BranchInst>(CurrentTerm)) {
// If this isn't branching on an invariant condition, we can't unswitch it.
if (!BI->isConditional())
return false;
Value *LoopCond = findLIVLoopCondition(BI->getCondition(), CurrentLoop,
Changed, MSSAU.get())
// Unswitch only if the trivial condition itself is an LIV (not
// partial LIV which could occur in and/or)
if (!LoopCond || LoopCond != BI->getCondition())
return false;
// Check to see if a successor of the branch is guaranteed to
// exit through a unique exit block without having any
// side-effects. If so, determine the value of Cond that causes
// it to do this.
if ((LoopExitBB =
isTrivialLoopExitBlock(CurrentLoop, BI->getSuccessor(0)))) {
CondVal = ConstantInt::getTrue(Context);
} else if ((LoopExitBB =
isTrivialLoopExitBlock(CurrentLoop, BI->getSuccessor(1)))) {
CondVal = ConstantInt::getFalse(Context);
// If we didn't find a single unique LoopExit block, or if the loop exit
// block contains phi nodes, this isn't trivial.
if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
return false; // Can't handle this.
if (equalityPropUnSafe(*LoopCond))
return false;
unswitchTrivialCondition(CurrentLoop, LoopCond, CondVal, LoopExitBB,
return true;
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(CurrentTerm)) {
// If this isn't switching on an invariant condition, we can't unswitch it.
Value *LoopCond = findLIVLoopCondition(SI->getCondition(), CurrentLoop,
Changed, MSSAU.get())
// Unswitch only if the trivial condition itself is an LIV (not
// partial LIV which could occur in and/or)
if (!LoopCond || LoopCond != SI->getCondition())
return false;
// Check to see if a successor of the switch is guaranteed to go to the
// latch block or exit through a one exit block without having any
// side-effects. If so, determine the value of Cond that causes it to do
// this.
// Note that we can't trivially unswitch on the default case or
// on already unswitched cases.
for (auto Case : SI->cases()) {
BasicBlock *LoopExitCandidate;
if ((LoopExitCandidate =
isTrivialLoopExitBlock(CurrentLoop, Case.getCaseSuccessor()))) {
// Okay, we found a trivial case, remember the value that is trivial.
ConstantInt *CaseVal = Case.getCaseValue();
// Check that it was not unswitched before, since already unswitched
// trivial vals are looks trivial too.
if (BranchesInfo.isUnswitched(SI, CaseVal))
LoopExitBB = LoopExitCandidate;
CondVal = CaseVal;
// If we didn't find a single unique LoopExit block, or if the loop exit
// block contains phi nodes, this isn't trivial.
if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
return false; // Can't handle this.
unswitchTrivialCondition(CurrentLoop, LoopCond, CondVal, LoopExitBB,
// We are only unswitching full LIV.
BranchesInfo.setUnswitched(SI, CondVal);
return true;
return false;
/// Split all of the edges from inside the loop to their exit blocks.
/// Update the appropriate Phi nodes as we do so.
void LoopUnswitch::splitExitEdges(
Loop *L, const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
for (unsigned I = 0, E = ExitBlocks.size(); I != E; ++I) {
BasicBlock *ExitBlock = ExitBlocks[I];
SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
// Although SplitBlockPredecessors doesn't preserve loop-simplify in
// general, if we call it on all predecessors of all exits then it does.
SplitBlockPredecessors(ExitBlock, Preds, ".us-lcssa", DT, LI, MSSAU.get(),
/*PreserveLCSSA*/ true);
/// We determined that the loop is profitable to unswitch when LIC equal Val.
/// Split it into loop versions and test the condition outside of either loop.
/// Return the loops created as Out1/Out2.
void LoopUnswitch::unswitchNontrivialCondition(
Value *LIC, Constant *Val, Loop *L, Instruction *TI,
ArrayRef<Instruction *> ToDuplicate) {
Function *F = LoopHeader->getParent();
LLVM_DEBUG(dbgs() << "loop-unswitch: Unswitching loop %"
<< LoopHeader->getName() << " [" << L->getBlocks().size()
<< " blocks] in Function " << F->getName() << " when '"
<< *Val << "' == " << *LIC << "\n");
// We are going to make essential changes to CFG. This may invalidate cached
// information for L or one of its parent loops in SCEV.
if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>())
if (MSSAU && VerifyMemorySSA)
// First step, split the preheader and exit blocks, and add these blocks to
// the LoopBlocks list.
BasicBlock *NewPreheader =
SplitEdge(LoopPreheader, LoopHeader, DT, LI, MSSAU.get());
// We want the loop to come after the preheader, but before the exit blocks.
llvm::append_range(LoopBlocks, L->blocks());
SmallVector<BasicBlock*, 8> ExitBlocks;
// Split all of the edges from inside the loop to their exit blocks. Update
// the appropriate Phi nodes as we do so.
splitExitEdges(L, ExitBlocks);
// The exit blocks may have been changed due to edge splitting, recompute.
// Add exit blocks to the loop blocks.
llvm::append_range(LoopBlocks, ExitBlocks);
// Next step, clone all of the basic blocks that make up the loop (including
// the loop preheader and exit blocks), keeping track of the mapping between
// the instructions and blocks.
ValueToValueMapTy VMap;
for (unsigned I = 0, E = LoopBlocks.size(); I != E; ++I) {
BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[I], VMap, ".us", F);
VMap[LoopBlocks[I]] = NewBB; // Keep the BB mapping.
// Splice the newly inserted blocks into the function right before the
// original preheader.
NewBlocks[0]->getIterator(), F->end());
// Now we create the new Loop object for the versioned loop.
Loop *NewLoop = cloneLoop(L, L->getParentLoop(), VMap, LI, LPM);
// Recalculate unswitching quota, inherit simplified switches info for NewBB,
// Probably clone more loop-unswitch related loop properties.
BranchesInfo.cloneData(NewLoop, L, VMap);
Loop *ParentLoop = L->getParentLoop();
if (ParentLoop) {
// Make sure to add the cloned preheader and exit blocks to the parent loop
// as well.
ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI);
for (unsigned EBI = 0, EBE = ExitBlocks.size(); EBI != EBE; ++EBI) {
BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[EBI]]);
// The new exit block should be in the same loop as the old one.
if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[EBI]))
ExitBBLoop->addBasicBlockToLoop(NewExit, *LI);
assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
"Exit block should have been split to have one successor!");
BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
// If the successor of the exit block had PHI nodes, add an entry for
// NewExit.
for (PHINode &PN : ExitSucc->phis()) {
Value *V = PN.getIncomingValueForBlock(ExitBlocks[EBI]);
ValueToValueMapTy::iterator It = VMap.find(V);
if (It != VMap.end()) V = It->second;
PN.addIncoming(V, NewExit);
if (LandingPadInst *LPad = NewExit->getLandingPadInst()) {
PHINode *PN = PHINode::Create(LPad->getType(), 0, "",
for (BasicBlock *BB : predecessors(ExitSucc)) {
LandingPadInst *LPI = BB->getLandingPadInst();
PN->addIncoming(LPI, BB);
// Rewrite the code to refer to itself.
for (unsigned NBI = 0, NBE = NewBlocks.size(); NBI != NBE; ++NBI) {
for (Instruction &I : *NewBlocks[NBI]) {
RemapInstruction(&I, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
if (auto *II = dyn_cast<IntrinsicInst>(&I))
if (II->getIntrinsicID() == Intrinsic::assume)
// Rewrite the original preheader to select between versions of the loop.
BranchInst *OldBR = cast<BranchInst>(LoopPreheader->getTerminator());
assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
"Preheader splitting did not work correctly!");
if (MSSAU) {
// Update MemorySSA after cloning, and before splitting to unreachables,
// since that invalidates the 1:1 mapping of clones in VMap.
LoopBlocksRPO LBRPO(L);
MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, VMap);
// Emit the new branch that selects between the two versions of this loop.
emitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR,
TI, ToDuplicate);
if (MSSAU) {
// Update MemoryPhis in Exit blocks.
MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMap, *DT);
if (VerifyMemorySSA)
// The OldBr was replaced by a new one and removed (but not erased) by
// emitPreheaderBranchOnCondition. It is no longer needed, so delete it.
delete OldBR;
RedoLoop = true;
// Keep a WeakTrackingVH holding onto LIC. If the first call to
// RewriteLoopBody
// deletes the instruction (for example by simplifying a PHI that feeds into
// the condition that we're unswitching on), we don't rewrite the second
// iteration.
WeakTrackingVH LICHandle(LIC);
if (ToDuplicate.empty()) {
// Now we rewrite the original code to know that the condition is true and
// the new code to know that the condition is false.
rewriteLoopBodyWithConditionConstant(L, LIC, Val, /*IsEqual=*/false);
// It's possible that simplifying one loop could cause the other to be
// changed to another value or a constant. If its a constant, don't
// simplify it.
if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
LICHandle && !isa<Constant>(LICHandle))
rewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val,
} else {
// Partial unswitching. Update the condition in the right loop with the
// constant.
auto *CC = cast<ConstantInt>(Val);
if (CC->isOneValue()) {
rewriteLoopBodyWithConditionConstant(NewLoop, VMap[LIC], Val,
} else
rewriteLoopBodyWithConditionConstant(L, LIC, Val, /*IsEqual=*/true);
// Mark the new loop as partially unswitched, to avoid unswitching on the
// same condition again.
auto &Context = NewLoop->getHeader()->getContext();
MDNode *DisableUnswitchMD = MDNode::get(
Context, MDString::get(Context, "llvm.loop.unswitch.partial.disable"));
MDNode *NewLoopID = makePostTransformationMetadata(
Context, L->getLoopID(), {"llvm.loop.unswitch.partial"},
if (MSSA && VerifyMemorySSA)
/// Remove all instances of I from the worklist vector specified.
static void removeFromWorklist(Instruction *I,
std::vector<Instruction *> &Worklist) {
llvm::erase_value(Worklist, I);
/// When we find that I really equals V, remove I from the
/// program, replacing all uses with V and update the worklist.
static void replaceUsesOfWith(Instruction *I, Value *V,
std::vector<Instruction *> &Worklist, Loop *L,
LPPassManager *LPM, MemorySSAUpdater *MSSAU) {
LLVM_DEBUG(dbgs() << "Replace with '" << *V << "': " << *I << "\n");
// Add uses to the worklist, which may be dead now.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
// Add users to the worklist which may be simplified now.
for (User *U : I->users())
removeFromWorklist(I, Worklist);
if (!I->mayHaveSideEffects()) {
if (MSSAU)
/// We know either that the value LIC has the value specified by Val in the
/// specified loop, or we know it does NOT have that value.
/// Rewrite any uses of LIC or of properties correlated to it.
void LoopUnswitch::rewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
Constant *Val,
bool IsEqual) {
assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
// FIXME: Support correlated properties, like:
// for (...)
// if (li1 < li2)
// ...
// if (li1 > li2)
// ...
// FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
// selects, switches.
std::vector<Instruction*> Worklist;
LLVMContext &Context = Val->getContext();
// If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
// in the loop with the appropriate one directly.
if (IsEqual || (isa<ConstantInt>(Val) &&
Val->getType()->isIntegerTy(1))) {
Value *Replacement;
if (IsEqual)
Replacement = Val;
Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
for (User *U : LIC->users()) {
Instruction *UI = dyn_cast<Instruction>(U);
if (!UI || !L->contains(UI))
for (Instruction *UI : Worklist)
UI->replaceUsesOfWith(LIC, Replacement);
simplifyCode(Worklist, L);
// Otherwise, we don't know the precise value of LIC, but we do know that it
// is certainly NOT "Val". As such, simplify any uses in the loop that we
// can. This case occurs when we unswitch switch statements.
for (User *U : LIC->users()) {
Instruction *UI = dyn_cast<Instruction>(U);
if (!UI || !L->contains(UI))
// At this point, we know LIC is definitely not Val. Try to use some simple
// logic to simplify the user w.r.t. to the context.
if (Value *Replacement = simplifyInstructionWithNotEqual(UI, LIC, Val)) {
if (LI->replacementPreservesLCSSAForm(UI, Replacement)) {
// This in-loop instruction has been simplified w.r.t. its context,
// i.e. LIC != Val, make sure we propagate its replacement value to
// all its users.
// We can not yet delete UI, the LIC user, yet, because that would invalidate
// the LIC->users() iterator !. However, we can make this instruction
// dead by replacing all its users and push it onto the worklist so that
// it can be properly deleted and its operands simplified.
// This is a LIC user, push it into the worklist so that simplifyCode can
// attempt to simplify it.
// If we know that LIC is not Val, use this info to simplify code.
SwitchInst *SI = dyn_cast<SwitchInst>(UI);
if (!SI || !isa<ConstantInt>(Val)) continue;
// NOTE: if a case value for the switch is unswitched out, we record it
// after the unswitch finishes. We can not record it here as the switch
// is not a direct user of the partial LIV.
SwitchInst::CaseHandle DeadCase =
// Default case is live for multiple values.
if (DeadCase == *SI->case_default())
// Found a dead case value. Don't remove PHI nodes in the
// successor if they become single-entry, those PHI nodes may
// be in the Users list.
BasicBlock *Switch = SI->getParent();
BasicBlock *SISucc = DeadCase.getCaseSuccessor();
BasicBlock *Latch = L->getLoopLatch();
if (!SI->findCaseDest(SISucc)) continue; // Edge is critical.
// If the DeadCase successor dominates the loop latch, then the
// transformation isn't safe since it will delete the sole predecessor edge
// to the latch.
if (Latch && DT->dominates(SISucc, Latch))
// FIXME: This is a hack. We need to keep the successor around
// and hooked up so as to preserve the loop structure, because
// trying to update it is complicated. So instead we preserve the
// loop structure and put the block on a dead code path.
SplitEdge(Switch, SISucc, DT, LI, MSSAU.get());
// Compute the successors instead of relying on the return value
// of SplitEdge, since it may have split the switch successor
// after PHI nodes.
BasicBlock *NewSISucc = DeadCase.getCaseSuccessor();
BasicBlock *OldSISucc = *succ_begin(NewSISucc);
// Create an "unreachable" destination.
BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
new UnreachableInst(Context, Abort);
// Force the new case destination to branch to the "unreachable"
// block while maintaining a (dead) CFG edge to the old block.
BranchInst::Create(Abort, OldSISucc,
ConstantInt::getTrue(Context), NewSISucc);
// Release the PHI operands for this edge.
for (PHINode &PN : NewSISucc->phis())
PN.setIncomingValueForBlock(Switch, UndefValue::get(PN.getType()));
// Tell the domtree about the new block. We don't fully update the
// domtree here -- instead we force it to do a full recomputation
// after the pass is complete -- but we do need to inform it of
// new blocks.
DT->addNewBlock(Abort, NewSISucc);
simplifyCode(Worklist, L);
/// Now that we have simplified some instructions in the loop, walk over it and
/// constant prop, dce, and fold control flow where possible. Note that this is
/// effectively a very simple loop-structure-aware optimizer. During processing
/// of this loop, L could very well be deleted, so it must not be used.
/// FIXME: When the loop optimizer is more mature, separate this out to a new
/// pass.
void LoopUnswitch::simplifyCode(std::vector<Instruction *> &Worklist, Loop *L) {
const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
while (!Worklist.empty()) {
Instruction *I = Worklist.back();
// Simple DCE.
if (isInstructionTriviallyDead(I)) {
LLVM_DEBUG(dbgs() << "Remove dead instruction '" << *I << "\n");
// Add uses to the worklist, which may be dead now.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
removeFromWorklist(I, Worklist);
if (MSSAU)
// See if instruction simplification can hack this up. This is common for
// things like "select false, X, Y" after unswitching made the condition be
// 'false'. TODO: update the domtree properly so we can pass it here.
if (Value *V = SimplifyInstruction(I, DL))
if (LI->replacementPreservesLCSSAForm(I, V)) {
replaceUsesOfWith(I, V, Worklist, L, LPM, MSSAU.get());
// Special case hacks that appear commonly in unswitched code.
if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
if (BI->isUnconditional()) {
// If BI's parent is the only pred of the successor, fold the two blocks
// together.
BasicBlock *Pred = BI->getParent();
BasicBlock *Succ = BI->getSuccessor(0);
BasicBlock *SinglePred = Succ->getSinglePredecessor();
if (!SinglePred) continue; // Nothing to do.
assert(SinglePred == Pred && "CFG broken");
// Make the LPM and Worklist updates specific to LoopUnswitch.
removeFromWorklist(BI, Worklist);
auto SuccIt = Succ->begin();
while (PHINode *PN = dyn_cast<PHINode>(SuccIt++)) {
for (unsigned It = 0, E = PN->getNumOperands(); It != E; ++It)
if (Instruction *Use = dyn_cast<Instruction>(PN->getOperand(It)))
for (User *U : PN->users())
removeFromWorklist(PN, Worklist);
// Merge the block and make the remaining analyses updates.
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
MergeBlockIntoPredecessor(Succ, &DTU, LI, MSSAU.get());
/// Simple simplifications we can do given the information that Cond is
/// definitely not equal to Val.
Value *LoopUnswitch::simplifyInstructionWithNotEqual(Instruction *Inst,
Value *Invariant,
Constant *Val) {
// icmp eq cond, val -> false
ICmpInst *CI = dyn_cast<ICmpInst>(Inst);
if (CI && CI->isEquality()) {
Value *Op0 = CI->getOperand(0);
Value *Op1 = CI->getOperand(1);
if ((Op0 == Invariant && Op1 == Val) || (Op0 == Val && Op1 == Invariant)) {
LLVMContext &Ctx = Inst->getContext();
if (CI->getPredicate() == CmpInst::ICMP_EQ)
return ConstantInt::getFalse(Ctx);
return ConstantInt::getTrue(Ctx);
// FIXME: there may be other opportunities, e.g. comparison with floating
// point, or Invariant - Val != 0, etc.
return nullptr;