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//===- GVN.cpp - Eliminate redundant values and loads ---------------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This pass performs global value numbering to eliminate fully redundant
// instructions. It also performs simple dead load elimination.
// Note that this pass does the value numbering itself; it does not use the
// ValueNumbering analysis passes.
#include "llvm/Transforms/Scalar/GVN.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/PHITransAddr.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/DomTreeUpdater.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.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/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/Transforms/Utils/VNCoercion.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <utility>
#include <vector>
using namespace llvm;
using namespace llvm::gvn;
using namespace llvm::VNCoercion;
using namespace PatternMatch;
#define DEBUG_TYPE "gvn"
STATISTIC(NumGVNInstr, "Number of instructions deleted");
STATISTIC(NumGVNLoad, "Number of loads deleted");
STATISTIC(NumGVNPRE, "Number of instructions PRE'd");
STATISTIC(NumGVNBlocks, "Number of blocks merged");
STATISTIC(NumGVNSimpl, "Number of instructions simplified");
STATISTIC(NumGVNEqProp, "Number of equalities propagated");
STATISTIC(NumPRELoad, "Number of loads PRE'd");
static cl::opt<bool> EnablePRE("enable-pre",
cl::init(true), cl::Hidden);
static cl::opt<bool> EnableLoadPRE("enable-load-pre", cl::init(true));
static cl::opt<bool> EnableMemDep("enable-gvn-memdep", cl::init(true));
// Maximum allowed recursion depth.
static cl::opt<uint32_t>
MaxRecurseDepth("gvn-max-recurse-depth", cl::Hidden, cl::init(1000), cl::ZeroOrMore,
cl::desc("Max recurse depth in GVN (default = 1000)"));
static cl::opt<uint32_t> MaxNumDeps(
"gvn-max-num-deps", cl::Hidden, cl::init(100), cl::ZeroOrMore,
cl::desc("Max number of dependences to attempt Load PRE (default = 100)"));
struct llvm::GVN::Expression {
uint32_t opcode;
Type *type;
bool commutative = false;
SmallVector<uint32_t, 4> varargs;
Expression(uint32_t o = ~2U) : opcode(o) {}
bool operator==(const Expression &other) const {
if (opcode != other.opcode)
return false;
if (opcode == ~0U || opcode == ~1U)
return true;
if (type != other.type)
return false;
if (varargs != other.varargs)
return false;
return true;
friend hash_code hash_value(const Expression &Value) {
return hash_combine(
Value.opcode, Value.type,
hash_combine_range(Value.varargs.begin(), Value.varargs.end()));
namespace llvm {
template <> struct DenseMapInfo<GVN::Expression> {
static inline GVN::Expression getEmptyKey() { return ~0U; }
static inline GVN::Expression getTombstoneKey() { return ~1U; }
static unsigned getHashValue(const GVN::Expression &e) {
using llvm::hash_value;
return static_cast<unsigned>(hash_value(e));
static bool isEqual(const GVN::Expression &LHS, const GVN::Expression &RHS) {
return LHS == RHS;
} // end namespace llvm
/// Represents a particular available value that we know how to materialize.
/// Materialization of an AvailableValue never fails. An AvailableValue is
/// implicitly associated with a rematerialization point which is the
/// location of the instruction from which it was formed.
struct llvm::gvn::AvailableValue {
enum ValType {
SimpleVal, // A simple offsetted value that is accessed.
LoadVal, // A value produced by a load.
MemIntrin, // A memory intrinsic which is loaded from.
UndefVal // A UndefValue representing a value from dead block (which
// is not yet physically removed from the CFG).
/// V - The value that is live out of the block.
PointerIntPair<Value *, 2, ValType> Val;
/// Offset - The byte offset in Val that is interesting for the load query.
unsigned Offset;
static AvailableValue get(Value *V, unsigned Offset = 0) {
AvailableValue Res;
Res.Offset = Offset;
return Res;
static AvailableValue getMI(MemIntrinsic *MI, unsigned Offset = 0) {
AvailableValue Res;
Res.Offset = Offset;
return Res;
static AvailableValue getLoad(LoadInst *LI, unsigned Offset = 0) {
AvailableValue Res;
Res.Offset = Offset;
return Res;
static AvailableValue getUndef() {
AvailableValue Res;
Res.Offset = 0;
return Res;
bool isSimpleValue() const { return Val.getInt() == SimpleVal; }
bool isCoercedLoadValue() const { return Val.getInt() == LoadVal; }
bool isMemIntrinValue() const { return Val.getInt() == MemIntrin; }
bool isUndefValue() const { return Val.getInt() == UndefVal; }
Value *getSimpleValue() const {
assert(isSimpleValue() && "Wrong accessor");
return Val.getPointer();
LoadInst *getCoercedLoadValue() const {
assert(isCoercedLoadValue() && "Wrong accessor");
return cast<LoadInst>(Val.getPointer());
MemIntrinsic *getMemIntrinValue() const {
assert(isMemIntrinValue() && "Wrong accessor");
return cast<MemIntrinsic>(Val.getPointer());
/// Emit code at the specified insertion point to adjust the value defined
/// here to the specified type. This handles various coercion cases.
Value *MaterializeAdjustedValue(LoadInst *LI, Instruction *InsertPt,
GVN &gvn) const;
/// Represents an AvailableValue which can be rematerialized at the end of
/// the associated BasicBlock.
struct llvm::gvn::AvailableValueInBlock {
/// BB - The basic block in question.
BasicBlock *BB;
/// AV - The actual available value
AvailableValue AV;
static AvailableValueInBlock get(BasicBlock *BB, AvailableValue &&AV) {
AvailableValueInBlock Res;
Res.BB = BB;
Res.AV = std::move(AV);
return Res;
static AvailableValueInBlock get(BasicBlock *BB, Value *V,
unsigned Offset = 0) {
return get(BB, AvailableValue::get(V, Offset));
static AvailableValueInBlock getUndef(BasicBlock *BB) {
return get(BB, AvailableValue::getUndef());
/// Emit code at the end of this block to adjust the value defined here to
/// the specified type. This handles various coercion cases.
Value *MaterializeAdjustedValue(LoadInst *LI, GVN &gvn) const {
return AV.MaterializeAdjustedValue(LI, BB->getTerminator(), gvn);
// ValueTable Internal Functions
GVN::Expression GVN::ValueTable::createExpr(Instruction *I) {
Expression e;
e.type = I->getType();
e.opcode = I->getOpcode();
for (Instruction::op_iterator OI = I->op_begin(), OE = I->op_end();
OI != OE; ++OI)
if (I->isCommutative()) {
// Ensure that commutative instructions that only differ by a permutation
// of their operands get the same value number by sorting the operand value
// numbers. Since all commutative instructions have two operands it is more
// efficient to sort by hand rather than using, say, std::sort.
assert(I->getNumOperands() == 2 && "Unsupported commutative instruction!");
if (e.varargs[0] > e.varargs[1])
std::swap(e.varargs[0], e.varargs[1]);
e.commutative = true;
if (CmpInst *C = dyn_cast<CmpInst>(I)) {
// Sort the operand value numbers so x<y and y>x get the same value number.
CmpInst::Predicate Predicate = C->getPredicate();
if (e.varargs[0] > e.varargs[1]) {
std::swap(e.varargs[0], e.varargs[1]);
Predicate = CmpInst::getSwappedPredicate(Predicate);
e.opcode = (C->getOpcode() << 8) | Predicate;
e.commutative = true;
} else if (InsertValueInst *E = dyn_cast<InsertValueInst>(I)) {
for (InsertValueInst::idx_iterator II = E->idx_begin(), IE = E->idx_end();
II != IE; ++II)
return e;
GVN::Expression GVN::ValueTable::createCmpExpr(unsigned Opcode,
CmpInst::Predicate Predicate,
Value *LHS, Value *RHS) {
assert((Opcode == Instruction::ICmp || Opcode == Instruction::FCmp) &&
"Not a comparison!");
Expression e;
e.type = CmpInst::makeCmpResultType(LHS->getType());
// Sort the operand value numbers so x<y and y>x get the same value number.
if (e.varargs[0] > e.varargs[1]) {
std::swap(e.varargs[0], e.varargs[1]);
Predicate = CmpInst::getSwappedPredicate(Predicate);
e.opcode = (Opcode << 8) | Predicate;
e.commutative = true;
return e;
GVN::Expression GVN::ValueTable::createExtractvalueExpr(ExtractValueInst *EI) {
assert(EI && "Not an ExtractValueInst?");
Expression e;
e.type = EI->getType();
e.opcode = 0;
IntrinsicInst *I = dyn_cast<IntrinsicInst>(EI->getAggregateOperand());
if (I != nullptr && EI->getNumIndices() == 1 && *EI->idx_begin() == 0 ) {
// EI might be an extract from one of our recognised intrinsics. If it
// is we'll synthesize a semantically equivalent expression instead on
// an extract value expression.
switch (I->getIntrinsicID()) {
case Intrinsic::sadd_with_overflow:
case Intrinsic::uadd_with_overflow:
e.opcode = Instruction::Add;
case Intrinsic::ssub_with_overflow:
case Intrinsic::usub_with_overflow:
e.opcode = Instruction::Sub;
case Intrinsic::smul_with_overflow:
case Intrinsic::umul_with_overflow:
e.opcode = Instruction::Mul;
if (e.opcode != 0) {
// Intrinsic recognized. Grab its args to finish building the expression.
assert(I->getNumArgOperands() == 2 &&
"Expect two args for recognised intrinsics.");
return e;
// Not a recognised intrinsic. Fall back to producing an extract value
// expression.
e.opcode = EI->getOpcode();
for (Instruction::op_iterator OI = EI->op_begin(), OE = EI->op_end();
OI != OE; ++OI)
for (ExtractValueInst::idx_iterator II = EI->idx_begin(), IE = EI->idx_end();
II != IE; ++II)
return e;
// ValueTable External Functions
GVN::ValueTable::ValueTable() = default;
GVN::ValueTable::ValueTable(const ValueTable &) = default;
GVN::ValueTable::ValueTable(ValueTable &&) = default;
GVN::ValueTable::~ValueTable() = default;
/// add - Insert a value into the table with a specified value number.
void GVN::ValueTable::add(Value *V, uint32_t num) {
valueNumbering.insert(std::make_pair(V, num));
if (PHINode *PN = dyn_cast<PHINode>(V))
NumberingPhi[num] = PN;
uint32_t GVN::ValueTable::lookupOrAddCall(CallInst *C) {
if (AA->doesNotAccessMemory(C)) {
Expression exp = createExpr(C);
uint32_t e = assignExpNewValueNum(exp).first;
valueNumbering[C] = e;
return e;
} else if (MD && AA->onlyReadsMemory(C)) {
Expression exp = createExpr(C);
auto ValNum = assignExpNewValueNum(exp);
if (ValNum.second) {
valueNumbering[C] = ValNum.first;
return ValNum.first;
MemDepResult local_dep = MD->getDependency(C);
if (!local_dep.isDef() && !local_dep.isNonLocal()) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
if (local_dep.isDef()) {
CallInst* local_cdep = cast<CallInst>(local_dep.getInst());
if (local_cdep->getNumArgOperands() != C->getNumArgOperands()) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
for (unsigned i = 0, e = C->getNumArgOperands(); i < e; ++i) {
uint32_t c_vn = lookupOrAdd(C->getArgOperand(i));
uint32_t cd_vn = lookupOrAdd(local_cdep->getArgOperand(i));
if (c_vn != cd_vn) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
uint32_t v = lookupOrAdd(local_cdep);
valueNumbering[C] = v;
return v;
// Non-local case.
const MemoryDependenceResults::NonLocalDepInfo &deps =
// FIXME: Move the checking logic to MemDep!
CallInst* cdep = nullptr;
// Check to see if we have a single dominating call instruction that is
// identical to C.
for (unsigned i = 0, e = deps.size(); i != e; ++i) {
const NonLocalDepEntry *I = &deps[i];
if (I->getResult().isNonLocal())
// We don't handle non-definitions. If we already have a call, reject
// instruction dependencies.
if (!I->getResult().isDef() || cdep != nullptr) {
cdep = nullptr;
CallInst *NonLocalDepCall = dyn_cast<CallInst>(I->getResult().getInst());
// FIXME: All duplicated with non-local case.
if (NonLocalDepCall && DT->properlyDominates(I->getBB(), C->getParent())){
cdep = NonLocalDepCall;
cdep = nullptr;
if (!cdep) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
if (cdep->getNumArgOperands() != C->getNumArgOperands()) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
for (unsigned i = 0, e = C->getNumArgOperands(); i < e; ++i) {
uint32_t c_vn = lookupOrAdd(C->getArgOperand(i));
uint32_t cd_vn = lookupOrAdd(cdep->getArgOperand(i));
if (c_vn != cd_vn) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
uint32_t v = lookupOrAdd(cdep);
valueNumbering[C] = v;
return v;
} else {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
/// Returns true if a value number exists for the specified value.
bool GVN::ValueTable::exists(Value *V) const { return valueNumbering.count(V) != 0; }
/// lookup_or_add - Returns the value number for the specified value, assigning
/// it a new number if it did not have one before.
uint32_t GVN::ValueTable::lookupOrAdd(Value *V) {
DenseMap<Value*, uint32_t>::iterator VI = valueNumbering.find(V);
if (VI != valueNumbering.end())
return VI->second;
if (!isa<Instruction>(V)) {
valueNumbering[V] = nextValueNumber;
return nextValueNumber++;
Instruction* I = cast<Instruction>(V);
Expression exp;
switch (I->getOpcode()) {
case Instruction::Call:
return lookupOrAddCall(cast<CallInst>(I));
case Instruction::Add:
case Instruction::FAdd:
case Instruction::Sub:
case Instruction::FSub:
case Instruction::Mul:
case Instruction::FMul:
case Instruction::UDiv:
case Instruction::SDiv:
case Instruction::FDiv:
case Instruction::URem:
case Instruction::SRem:
case Instruction::FRem:
case Instruction::Shl:
case Instruction::LShr:
case Instruction::AShr:
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::ICmp:
case Instruction::FCmp:
case Instruction::Trunc:
case Instruction::ZExt:
case Instruction::SExt:
case Instruction::FPToUI:
case Instruction::FPToSI:
case Instruction::UIToFP:
case Instruction::SIToFP:
case Instruction::FPTrunc:
case Instruction::FPExt:
case Instruction::PtrToInt:
case Instruction::IntToPtr:
case Instruction::BitCast:
case Instruction::Select:
case Instruction::ExtractElement:
case Instruction::InsertElement:
case Instruction::ShuffleVector:
case Instruction::InsertValue:
case Instruction::GetElementPtr:
exp = createExpr(I);
case Instruction::ExtractValue:
exp = createExtractvalueExpr(cast<ExtractValueInst>(I));
case Instruction::PHI:
valueNumbering[V] = nextValueNumber;
NumberingPhi[nextValueNumber] = cast<PHINode>(V);
return nextValueNumber++;
valueNumbering[V] = nextValueNumber;
return nextValueNumber++;
uint32_t e = assignExpNewValueNum(exp).first;
valueNumbering[V] = e;
return e;
/// Returns the value number of the specified value. Fails if
/// the value has not yet been numbered.
uint32_t GVN::ValueTable::lookup(Value *V, bool Verify) const {
DenseMap<Value*, uint32_t>::const_iterator VI = valueNumbering.find(V);
if (Verify) {
assert(VI != valueNumbering.end() && "Value not numbered?");
return VI->second;
return (VI != valueNumbering.end()) ? VI->second : 0;
/// Returns the value number of the given comparison,
/// assigning it a new number if it did not have one before. Useful when
/// we deduced the result of a comparison, but don't immediately have an
/// instruction realizing that comparison to hand.
uint32_t GVN::ValueTable::lookupOrAddCmp(unsigned Opcode,
CmpInst::Predicate Predicate,
Value *LHS, Value *RHS) {
Expression exp = createCmpExpr(Opcode, Predicate, LHS, RHS);
return assignExpNewValueNum(exp).first;
/// Remove all entries from the ValueTable.
void GVN::ValueTable::clear() {
nextValueNumber = 1;
nextExprNumber = 0;
/// Remove a value from the value numbering.
void GVN::ValueTable::erase(Value *V) {
uint32_t Num = valueNumbering.lookup(V);
// If V is PHINode, V <--> value number is an one-to-one mapping.
if (isa<PHINode>(V))
/// verifyRemoved - Verify that the value is removed from all internal data
/// structures.
void GVN::ValueTable::verifyRemoved(const Value *V) const {
for (DenseMap<Value*, uint32_t>::const_iterator
I = valueNumbering.begin(), E = valueNumbering.end(); I != E; ++I) {
assert(I->first != V && "Inst still occurs in value numbering map!");
// GVN Pass
PreservedAnalyses GVN::run(Function &F, FunctionAnalysisManager &AM) {
// FIXME: The order of evaluation of these 'getResult' calls is very
// significant! Re-ordering these variables will cause GVN when run alone to
// be less effective! We should fix memdep and basic-aa to not exhibit this
// behavior, but until then don't change the order here.
auto &AC = AM.getResult<AssumptionAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
auto &AA = AM.getResult<AAManager>(F);
auto &MemDep = AM.getResult<MemoryDependenceAnalysis>(F);
auto *LI = AM.getCachedResult<LoopAnalysis>(F);
auto &ORE = AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
bool Changed = runImpl(F, AC, DT, TLI, AA, &MemDep, LI, &ORE);
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void GVN::dump(DenseMap<uint32_t, Value*>& d) const {
errs() << "{\n";
for (DenseMap<uint32_t, Value*>::iterator I = d.begin(),
E = d.end(); I != E; ++I) {
errs() << I->first << "\n";
errs() << "}\n";
/// Return true if we can prove that the value
/// we're analyzing is fully available in the specified block. As we go, keep
/// track of which blocks we know are fully alive in FullyAvailableBlocks. This
/// map is actually a tri-state map with the following values:
/// 0) we know the block *is not* fully available.
/// 1) we know the block *is* fully available.
/// 2) we do not know whether the block is fully available or not, but we are
/// currently speculating that it will be.
/// 3) we are speculating for this block and have used that to speculate for
/// other blocks.
static bool IsValueFullyAvailableInBlock(BasicBlock *BB,
DenseMap<BasicBlock*, char> &FullyAvailableBlocks,
uint32_t RecurseDepth) {
if (RecurseDepth > MaxRecurseDepth)
return false;
// Optimistically assume that the block is fully available and check to see
// if we already know about this block in one lookup.
std::pair<DenseMap<BasicBlock*, char>::iterator, bool> IV =
FullyAvailableBlocks.insert(std::make_pair(BB, 2));
// If the entry already existed for this block, return the precomputed value.
if (!IV.second) {
// If this is a speculative "available" value, mark it as being used for
// speculation of other blocks.
if (IV.first->second == 2)
IV.first->second = 3;
return IV.first->second != 0;
// Otherwise, see if it is fully available in all predecessors.
pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
// If this block has no predecessors, it isn't live-in here.
if (PI == PE)
goto SpeculationFailure;
for (; PI != PE; ++PI)
// If the value isn't fully available in one of our predecessors, then it
// isn't fully available in this block either. Undo our previous
// optimistic assumption and bail out.
if (!IsValueFullyAvailableInBlock(*PI, FullyAvailableBlocks,RecurseDepth+1))
goto SpeculationFailure;
return true;
// If we get here, we found out that this is not, after
// all, a fully-available block. We have a problem if we speculated on this and
// used the speculation to mark other blocks as available.
char &BBVal = FullyAvailableBlocks[BB];
// If we didn't speculate on this, just return with it set to false.
if (BBVal == 2) {
BBVal = 0;
return false;
// If we did speculate on this value, we could have blocks set to 1 that are
// incorrect. Walk the (transitive) successors of this block and mark them as
// 0 if set to one.
SmallVector<BasicBlock*, 32> BBWorklist;
do {
BasicBlock *Entry = BBWorklist.pop_back_val();
// Note that this sets blocks to 0 (unavailable) if they happen to not
// already be in FullyAvailableBlocks. This is safe.
char &EntryVal = FullyAvailableBlocks[Entry];
if (EntryVal == 0) continue; // Already unavailable.
// Mark as unavailable.
EntryVal = 0;
BBWorklist.append(succ_begin(Entry), succ_end(Entry));
} while (!BBWorklist.empty());
return false;
/// Given a set of loads specified by ValuesPerBlock,
/// construct SSA form, allowing us to eliminate LI. This returns the value
/// that should be used at LI's definition site.
static Value *ConstructSSAForLoadSet(LoadInst *LI,
SmallVectorImpl<AvailableValueInBlock> &ValuesPerBlock,
GVN &gvn) {
// Check for the fully redundant, dominating load case. In this case, we can
// just use the dominating value directly.
if (ValuesPerBlock.size() == 1 &&
LI->getParent())) {
assert(!ValuesPerBlock[0].AV.isUndefValue() &&
"Dead BB dominate this block");
return ValuesPerBlock[0].MaterializeAdjustedValue(LI, gvn);
// Otherwise, we have to construct SSA form.
SmallVector<PHINode*, 8> NewPHIs;
SSAUpdater SSAUpdate(&NewPHIs);
SSAUpdate.Initialize(LI->getType(), LI->getName());
for (const AvailableValueInBlock &AV : ValuesPerBlock) {
BasicBlock *BB = AV.BB;
if (SSAUpdate.HasValueForBlock(BB))
// If the value is the load that we will be eliminating, and the block it's
// available in is the block that the load is in, then don't add it as
// SSAUpdater will resolve the value to the relevant phi which may let it
// avoid phi construction entirely if there's actually only one value.
if (BB == LI->getParent() &&
((AV.AV.isSimpleValue() && AV.AV.getSimpleValue() == LI) ||
(AV.AV.isCoercedLoadValue() && AV.AV.getCoercedLoadValue() == LI)))
SSAUpdate.AddAvailableValue(BB, AV.MaterializeAdjustedValue(LI, gvn));
// Perform PHI construction.
return SSAUpdate.GetValueInMiddleOfBlock(LI->getParent());
Value *AvailableValue::MaterializeAdjustedValue(LoadInst *LI,
Instruction *InsertPt,
GVN &gvn) const {
Value *Res;
Type *LoadTy = LI->getType();
const DataLayout &DL = LI->getModule()->getDataLayout();
if (isSimpleValue()) {
Res = getSimpleValue();
if (Res->getType() != LoadTy) {
Res = getStoreValueForLoad(Res, Offset, LoadTy, InsertPt, DL);
LLVM_DEBUG(dbgs() << "GVN COERCED NONLOCAL VAL:\nOffset: " << Offset
<< " " << *getSimpleValue() << '\n'
<< *Res << '\n'
<< "\n\n\n");
} else if (isCoercedLoadValue()) {
LoadInst *Load = getCoercedLoadValue();
if (Load->getType() == LoadTy && Offset == 0) {
Res = Load;
} else {
Res = getLoadValueForLoad(Load, Offset, LoadTy, InsertPt, DL);
// We would like to use gvn.markInstructionForDeletion here, but we can't
// because the load is already memoized into the leader map table that GVN
// tracks. It is potentially possible to remove the load from the table,
// but then there all of the operations based on it would need to be
// rehashed. Just leave the dead load around.
LLVM_DEBUG(dbgs() << "GVN COERCED NONLOCAL LOAD:\nOffset: " << Offset
<< " " << *getCoercedLoadValue() << '\n'
<< *Res << '\n'
<< "\n\n\n");
} else if (isMemIntrinValue()) {
Res = getMemInstValueForLoad(getMemIntrinValue(), Offset, LoadTy,
InsertPt, DL);
<< " " << *getMemIntrinValue() << '\n'
<< *Res << '\n'
<< "\n\n\n");
} else {
assert(isUndefValue() && "Should be UndefVal");
return UndefValue::get(LoadTy);
assert(Res && "failed to materialize?");
return Res;
static bool isLifetimeStart(const Instruction *Inst) {
if (const IntrinsicInst* II = dyn_cast<IntrinsicInst>(Inst))
return II->getIntrinsicID() == Intrinsic::lifetime_start;
return false;
/// Try to locate the three instruction involved in a missed
/// load-elimination case that is due to an intervening store.
static void reportMayClobberedLoad(LoadInst *LI, MemDepResult DepInfo,
DominatorTree *DT,
OptimizationRemarkEmitter *ORE) {
using namespace ore;
User *OtherAccess = nullptr;
OptimizationRemarkMissed R(DEBUG_TYPE, "LoadClobbered", LI);
R << "load of type " << NV("Type", LI->getType()) << " not eliminated"
<< setExtraArgs();
for (auto *U : LI->getPointerOperand()->users())
if (U != LI && (isa<LoadInst>(U) || isa<StoreInst>(U)) &&
DT->dominates(cast<Instruction>(U), LI)) {
// FIXME: for now give up if there are multiple memory accesses that
// dominate the load. We need further analysis to decide which one is
// that we're forwarding from.
if (OtherAccess)
OtherAccess = nullptr;
OtherAccess = U;
if (OtherAccess)
R << " in favor of " << NV("OtherAccess", OtherAccess);
R << " because it is clobbered by " << NV("ClobberedBy", DepInfo.getInst());
bool GVN::AnalyzeLoadAvailability(LoadInst *LI, MemDepResult DepInfo,
Value *Address, AvailableValue &Res) {
assert((DepInfo.isDef() || DepInfo.isClobber()) &&
"expected a local dependence");
assert(LI->isUnordered() && "rules below are incorrect for ordered access");
const DataLayout &DL = LI->getModule()->getDataLayout();
if (DepInfo.isClobber()) {
// If the dependence is to a store that writes to a superset of the bits
// read by the load, we can extract the bits we need for the load from the
// stored value.
if (StoreInst *DepSI = dyn_cast<StoreInst>(DepInfo.getInst())) {
// Can't forward from non-atomic to atomic without violating memory model.
if (Address && LI->isAtomic() <= DepSI->isAtomic()) {
int Offset =
analyzeLoadFromClobberingStore(LI->getType(), Address, DepSI, DL);
if (Offset != -1) {
Res = AvailableValue::get(DepSI->getValueOperand(), Offset);
return true;
// Check to see if we have something like this:
// load i32* P
// load i8* (P+1)
// if we have this, replace the later with an extraction from the former.
if (LoadInst *DepLI = dyn_cast<LoadInst>(DepInfo.getInst())) {
// If this is a clobber and L is the first instruction in its block, then
// we have the first instruction in the entry block.
// Can't forward from non-atomic to atomic without violating memory model.
if (DepLI != LI && Address && LI->isAtomic() <= DepLI->isAtomic()) {
int Offset =
analyzeLoadFromClobberingLoad(LI->getType(), Address, DepLI, DL);
if (Offset != -1) {
Res = AvailableValue::getLoad(DepLI, Offset);
return true;
// If the clobbering value is a memset/memcpy/memmove, see if we can
// forward a value on from it.
if (MemIntrinsic *DepMI = dyn_cast<MemIntrinsic>(DepInfo.getInst())) {
if (Address && !LI->isAtomic()) {
int Offset = analyzeLoadFromClobberingMemInst(LI->getType(), Address,
DepMI, DL);
if (Offset != -1) {
Res = AvailableValue::getMI(DepMI, Offset);
return true;
// Nothing known about this clobber, have to be conservative
// fast print dep, using operator<< on instruction is too slow.
dbgs() << "GVN: load "; LI->printAsOperand(dbgs());
Instruction *I = DepInfo.getInst();
dbgs() << " is clobbered by " << *I << '\n';);
if (ORE->allowExtraAnalysis(DEBUG_TYPE))
reportMayClobberedLoad(LI, DepInfo, DT, ORE);
return false;
assert(DepInfo.isDef() && "follows from above");
Instruction *DepInst = DepInfo.getInst();
// Loading the allocation -> undef.
if (isa<AllocaInst>(DepInst) || isMallocLikeFn(DepInst, TLI) ||
// Loading immediately after lifetime begin -> undef.
isLifetimeStart(DepInst)) {
Res = AvailableValue::get(UndefValue::get(LI->getType()));
return true;
// Loading from calloc (which zero initializes memory) -> zero
if (isCallocLikeFn(DepInst, TLI)) {
Res = AvailableValue::get(Constant::getNullValue(LI->getType()));
return true;
if (StoreInst *S = dyn_cast<StoreInst>(DepInst)) {
// Reject loads and stores that are to the same address but are of
// different types if we have to. If the stored value is larger or equal to
// the loaded value, we can reuse it.
if (S->getValueOperand()->getType() != LI->getType() &&
LI->getType(), DL))
return false;
// Can't forward from non-atomic to atomic without violating memory model.
if (S->isAtomic() < LI->isAtomic())
return false;
Res = AvailableValue::get(S->getValueOperand());
return true;
if (LoadInst *LD = dyn_cast<LoadInst>(DepInst)) {
// If the types mismatch and we can't handle it, reject reuse of the load.
// If the stored value is larger or equal to the loaded value, we can reuse
// it.
if (LD->getType() != LI->getType() &&
!canCoerceMustAliasedValueToLoad(LD, LI->getType(), DL))
return false;
// Can't forward from non-atomic to atomic without violating memory model.
if (LD->isAtomic() < LI->isAtomic())
return false;
Res = AvailableValue::getLoad(LD);
return true;
// Unknown def - must be conservative
// fast print dep, using operator<< on instruction is too slow.
dbgs() << "GVN: load "; LI->printAsOperand(dbgs());
dbgs() << " has unknown def " << *DepInst << '\n';);
return false;
void GVN::AnalyzeLoadAvailability(LoadInst *LI, LoadDepVect &Deps,
AvailValInBlkVect &ValuesPerBlock,
UnavailBlkVect &UnavailableBlocks) {
// Filter out useless results (non-locals, etc). Keep track of the blocks
// where we have a value available in repl, also keep track of whether we see
// dependencies that produce an unknown value for the load (such as a call
// that could potentially clobber the load).
unsigned NumDeps = Deps.size();
for (unsigned i = 0, e = NumDeps; i != e; ++i) {
BasicBlock *DepBB = Deps[i].getBB();
MemDepResult DepInfo = Deps[i].getResult();
if (DeadBlocks.count(DepBB)) {
// Dead dependent mem-op disguise as a load evaluating the same value
// as the load in question.
if (!DepInfo.isDef() && !DepInfo.isClobber()) {
// The address being loaded in this non-local block may not be the same as
// the pointer operand of the load if PHI translation occurs. Make sure
// to consider the right address.
Value *Address = Deps[i].getAddress();
AvailableValue AV;
if (AnalyzeLoadAvailability(LI, DepInfo, Address, AV)) {
// subtlety: because we know this was a non-local dependency, we know
// it's safe to materialize anywhere between the instruction within
// DepInfo and the end of it's block.
} else {
assert(NumDeps == ValuesPerBlock.size() + UnavailableBlocks.size() &&
"post condition violation");
bool GVN::PerformLoadPRE(LoadInst *LI, AvailValInBlkVect &ValuesPerBlock,
UnavailBlkVect &UnavailableBlocks) {
// Okay, we have *some* definitions of the value. This means that the value
// is available in some of our (transitive) predecessors. Lets think about
// doing PRE of this load. This will involve inserting a new load into the
// predecessor when it's not available. We could do this in general, but
// prefer to not increase code size. As such, we only do this when we know
// that we only have to insert *one* load (which means we're basically moving
// the load, not inserting a new one).
SmallPtrSet<BasicBlock *, 4> Blockers(UnavailableBlocks.begin(),
// Let's find the first basic block with more than one predecessor. Walk
// backwards through predecessors if needed.
BasicBlock *LoadBB = LI->getParent();
BasicBlock *TmpBB = LoadBB;
bool IsSafeToSpeculativelyExecute = isSafeToSpeculativelyExecute(LI);
// Check that there is no implicit control flow instructions above our load in
// its block. If there is an instruction that doesn't always pass the
// execution to the following instruction, then moving through it may become
// invalid. For example:
// int arr[LEN];
// int index = ???;
// ...
// guard(0 <= index && index < LEN);
// use(arr[index]);
// It is illegal to move the array access to any point above the guard,
// because if the index is out of bounds we should deoptimize rather than
// access the array.
// Check that there is no guard in this block above our instruction.
if (!IsSafeToSpeculativelyExecute && ICF->isDominatedByICFIFromSameBlock(LI))
return false;
while (TmpBB->getSinglePredecessor()) {
TmpBB = TmpBB->getSinglePredecessor();
if (TmpBB == LoadBB) // Infinite (unreachable) loop.
return false;
if (Blockers.count(TmpBB))
return false;
// If any of these blocks has more than one successor (i.e. if the edge we
// just traversed was critical), then there are other paths through this
// block along which the load may not be anticipated. Hoisting the load
// above this block would be adding the load to execution paths along
// which it was not previously executed.
if (TmpBB->getTerminator()->getNumSuccessors() != 1)
return false;
// Check that there is no implicit control flow in a block above.
if (!IsSafeToSpeculativelyExecute && ICF->hasICF(TmpBB))
return false;
LoadBB = TmpBB;
// Check to see how many predecessors have the loaded value fully
// available.
MapVector<BasicBlock *, Value *> PredLoads;
DenseMap<BasicBlock*, char> FullyAvailableBlocks;
for (const AvailableValueInBlock &AV : ValuesPerBlock)
FullyAvailableBlocks[AV.BB] = true;
for (BasicBlock *UnavailableBB : UnavailableBlocks)
FullyAvailableBlocks[UnavailableBB] = false;
SmallVector<BasicBlock *, 4> CriticalEdgePred;
for (BasicBlock *Pred : predecessors(LoadBB)) {
// If any predecessor block is an EH pad that does not allow non-PHI
// instructions before the terminator, we can't PRE the load.
if (Pred->getTerminator()->isEHPad()) {
<< Pred->getName() << "': " << *LI << '\n');
return false;
if (IsValueFullyAvailableInBlock(Pred, FullyAvailableBlocks, 0)) {
if (Pred->getTerminator()->getNumSuccessors() != 1) {
if (isa<IndirectBrInst>(Pred->getTerminator())) {
<< Pred->getName() << "': " << *LI << '\n');
return false;
if (LoadBB->isEHPad()) {
<< Pred->getName() << "': " << *LI << '\n');
return false;
} else {
// Only add the predecessors that will not be split for now.
PredLoads[Pred] = nullptr;
// Decide whether PRE is profitable for this load.
unsigned NumUnavailablePreds = PredLoads.size() + CriticalEdgePred.size();
assert(NumUnavailablePreds != 0 &&
"Fully available value should already be eliminated!");
// If this load is unavailable in multiple predecessors, reject it.
// FIXME: If we could restructure the CFG, we could make a common pred with
// all the preds that don't have an available LI and insert a new load into
// that one block.
if (NumUnavailablePreds != 1)
return false;
// Split critical edges, and update the unavailable predecessors accordingly.
for (BasicBlock *OrigPred : CriticalEdgePred) {
BasicBlock *NewPred = splitCriticalEdges(OrigPred, LoadBB);
assert(!PredLoads.count(OrigPred) && "Split edges shouldn't be in map!");
PredLoads[NewPred] = nullptr;
LLVM_DEBUG(dbgs() << "Split critical edge " << OrigPred->getName() << "->"
<< LoadBB->getName() << '\n');
// Check if the load can safely be moved to all the unavailable predecessors.
bool CanDoPRE = true;
const DataLayout &DL = LI->getModule()->getDataLayout();
SmallVector<Instruction*, 8> NewInsts;
for (auto &PredLoad : PredLoads) {
BasicBlock *UnavailablePred = PredLoad.first;
// Do PHI translation to get its value in the predecessor if necessary. The
// returned pointer (if non-null) is guaranteed to dominate UnavailablePred.
// If all preds have a single successor, then we know it is safe to insert
// the load on the pred (?!?), so we can insert code to materialize the
// pointer if it is not available.
PHITransAddr Address(LI->getPointerOperand(), DL, AC);
Value *LoadPtr = nullptr;
LoadPtr = Address.PHITranslateWithInsertion(LoadBB, UnavailablePred,
*DT, NewInsts);
// If we couldn't find or insert a computation of this phi translated value,
// we fail PRE.
if (!LoadPtr) {
<< *LI->getPointerOperand() << "\n");
CanDoPRE = false;
PredLoad.second = LoadPtr;
if (!CanDoPRE) {
while (!NewInsts.empty()) {
Instruction *I = NewInsts.pop_back_val();
// HINT: Don't revert the edge-splitting as following transformation may
// also need to split these critical edges.
return !CriticalEdgePred.empty();
// Okay, we can eliminate this load by inserting a reload in the predecessor
// and using PHI construction to get the value in the other predecessors, do
// it.
LLVM_DEBUG(dbgs() << "GVN REMOVING PRE LOAD: " << *LI << '\n');
LLVM_DEBUG(if (!NewInsts.empty()) dbgs()
<< "INSERTED " << NewInsts.size() << " INSTS: " << *NewInsts.back()
<< '\n');
// Assign value numbers to the new instructions.
for (Instruction *I : NewInsts) {
// Instructions that have been inserted in predecessor(s) to materialize
// the load address do not retain their original debug locations. Doing
// so could lead to confusing (but correct) source attributions.
// FIXME: How do we retain source locations without causing poor debugging
// behavior?
// FIXME: We really _ought_ to insert these value numbers into their
// parent's availability map. However, in doing so, we risk getting into
// ordering issues. If a block hasn't been processed yet, we would be
// marking a value as AVAIL-IN, which isn't what we intend.
for (const auto &PredLoad : PredLoads) {
BasicBlock *UnavailablePred = PredLoad.first;
Value *LoadPtr = PredLoad.second;
auto *NewLoad = new LoadInst(LoadPtr, LI->getName()+".pre",
LI->isVolatile(), LI->getAlignment(),
LI->getOrdering(), LI->getSyncScopeID(),
// Transfer the old load's AA tags to the new load.
AAMDNodes Tags;
if (Tags)
if (auto *MD = LI->getMetadata(LLVMContext::MD_invariant_load))
NewLoad->setMetadata(LLVMContext::MD_invariant_load, MD);
if (auto *InvGroupMD = LI->getMetadata(LLVMContext::MD_invariant_group))
NewLoad->setMetadata(LLVMContext::MD_invariant_group, InvGroupMD);
if (auto *RangeMD = LI->getMetadata(LLVMContext::MD_range))
NewLoad->setMetadata(LLVMContext::MD_range, RangeMD);
// We do not propagate the old load's debug location, because the new
// load now lives in a different BB, and we want to avoid a jumpy line
// table.
// FIXME: How do we retain source locations without causing poor debugging
// behavior?
// Add the newly created load.
LLVM_DEBUG(dbgs() << "GVN INSERTED " << *NewLoad << '\n');
// Perform PHI construction.
Value *V = ConstructSSAForLoadSet(LI, ValuesPerBlock, *this);
if (isa<PHINode>(V))
if (Instruction *I = dyn_cast<Instruction>(V))
if (V->getType()->isPtrOrPtrVectorTy())
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "LoadPRE", LI)
<< "load eliminated by PRE";
return true;
static void reportLoadElim(LoadInst *LI, Value *AvailableValue,
OptimizationRemarkEmitter *ORE) {
using namespace ore;
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "LoadElim", LI)
<< "load of type " << NV("Type", LI->getType()) << " eliminated"
<< setExtraArgs() << " in favor of "
<< NV("InfavorOfValue", AvailableValue);
/// Attempt to eliminate a load whose dependencies are
/// non-local by performing PHI construction.
bool GVN::processNonLocalLoad(LoadInst *LI) {
// non-local speculations are not allowed under asan.
if (LI->getParent()->getParent()->hasFnAttribute(
Attribute::SanitizeAddress) ||
return false;
// Step 1: Find the non-local dependencies of the load.
LoadDepVect Deps;
MD->getNonLocalPointerDependency(LI, Deps);
// If we had to process more than one hundred blocks to find the
// dependencies, this load isn't worth worrying about. Optimizing
// it will be too expensive.
unsigned NumDeps = Deps.size();
if (NumDeps > MaxNumDeps)
return false;
// If we had a phi translation failure, we'll have a single entry which is a
// clobber in the current block. Reject this early.
if (NumDeps == 1 &&
!Deps[0].getResult().isDef() && !Deps[0].getResult().isClobber()) {
LLVM_DEBUG(dbgs() << "GVN: non-local load "; LI->printAsOperand(dbgs());
dbgs() << " has unknown dependencies\n";);
return false;
// If this load follows a GEP, see if we can PRE the indices before analyzing.
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(LI->getOperand(0))) {
for (GetElementPtrInst::op_iterator OI = GEP->idx_begin(),
OE = GEP->idx_end();
OI != OE; ++OI)
if (Instruction *I = dyn_cast<Instruction>(OI->get()))
// Step 2: Analyze the availability of the load
AvailValInBlkVect ValuesPerBlock;
UnavailBlkVect UnavailableBlocks;
AnalyzeLoadAvailability(LI, Deps, ValuesPerBlock, UnavailableBlocks);
// If we have no predecessors that produce a known value for this load, exit
// early.
if (ValuesPerBlock.empty())
return false;
// Step 3: Eliminate fully redundancy.
// If all of the instructions we depend on produce a known value for this
// load, then it is fully redundant and we can use PHI insertion to compute
// its value. Insert PHIs and remove the fully redundant value now.
if (UnavailableBlocks.empty()) {
LLVM_DEBUG(dbgs() << "GVN REMOVING NONLOCAL LOAD: " << *LI << '\n');
// Perform PHI construction.
Value *V = ConstructSSAForLoadSet(LI, ValuesPerBlock, *this);
if (isa<PHINode>(V))
if (Instruction *I = dyn_cast<Instruction>(V))
// If instruction I has debug info, then we should not update it.
// Also, if I has a null DebugLoc, then it is still potentially incorrect
// to propagate LI's DebugLoc because LI may not post-dominate I.
if (LI->getDebugLoc() && LI->getParent() == I->getParent())
if (V->getType()->isPtrOrPtrVectorTy())
reportLoadElim(LI, V, ORE);
return true;
// Step 4: Eliminate partial redundancy.
if (!EnablePRE || !EnableLoadPRE)
return false;
return PerformLoadPRE(LI, ValuesPerBlock, UnavailableBlocks);
bool GVN::processAssumeIntrinsic(IntrinsicInst *IntrinsicI) {
assert(IntrinsicI->getIntrinsicID() == Intrinsic::assume &&
"This function can only be called with llvm.assume intrinsic");
Value *V = IntrinsicI->getArgOperand(0);
if (ConstantInt *Cond = dyn_cast<ConstantInt>(V)) {
if (Cond->isZero()) {
Type *Int8Ty = Type::getInt8Ty(V->getContext());
// Insert a new store to null instruction before the load to indicate that
// this code is not reachable. FIXME: We could insert unreachable
// instruction directly because we can modify the CFG.
new StoreInst(UndefValue::get(Int8Ty),
return false;
} else if (isa<Constant>(V)) {
// If it's not false, and constant, it must evaluate to true. This means our
// assume is assume(true), and thus, pointless, and we don't want to do
// anything more here.
return false;
Constant *True = ConstantInt::getTrue(V->getContext());
bool Changed = false;
for (BasicBlock *Successor : successors(IntrinsicI->getParent())) {
BasicBlockEdge Edge(IntrinsicI->getParent(), Successor);
// This property is only true in dominated successors, propagateEquality
// will check dominance for us.
Changed |= propagateEquality(V, True, Edge, false);
// We can replace assume value with true, which covers cases like this:
// call void @llvm.assume(i1 %cmp)
// br i1 %cmp, label %bb1, label %bb2 ; will change %cmp to true
ReplaceWithConstMap[V] = True;
// If one of *cmp *eq operand is const, adding it to map will cover this:
// %cmp = fcmp oeq float 3.000000e+00, %0 ; const on lhs could happen
// call void @llvm.assume(i1 %cmp)
// ret float %0 ; will change it to ret float 3.000000e+00
if (auto *CmpI = dyn_cast<CmpInst>(V)) {
if (CmpI->getPredicate() == CmpInst::Predicate::ICMP_EQ ||
CmpI->getPredicate() == CmpInst::Predicate::FCMP_OEQ ||
(CmpI->getPredicate() == CmpInst::Predicate::FCMP_UEQ &&
CmpI->getFastMathFlags().noNaNs())) {
Value *CmpLHS = CmpI->getOperand(0);
Value *CmpRHS = CmpI->getOperand(1);
if (isa<Constant>(CmpLHS))
std::swap(CmpLHS, CmpRHS);
auto *RHSConst = dyn_cast<Constant>(CmpRHS);
// If only one operand is constant.
if (RHSConst != nullptr && !isa<Constant>(CmpLHS))
ReplaceWithConstMap[CmpLHS] = RHSConst;
return Changed;
static void patchAndReplaceAllUsesWith(Instruction *I, Value *Repl) {
patchReplacementInstruction(I, Repl);
/// Attempt to eliminate a load, first by eliminating it
/// locally, and then attempting non-local elimination if that fails.
bool GVN::processLoad(LoadInst *L) {
if (!MD)
return false;
// This code hasn't been audited for ordered or volatile memory access
if (!L->isUnordered())
return false;
if (L->use_empty()) {
return true;
// ... to a pointer that has been loaded from before...
MemDepResult Dep = MD->getDependency(L);
// If it is defined in another block, try harder.
if (Dep.isNonLocal())
return processNonLocalLoad(L);
// Only handle the local case below
if (!Dep.isDef() && !Dep.isClobber()) {
// This might be a NonFuncLocal or an Unknown
// fast print dep, using operator<< on instruction is too slow.
dbgs() << "GVN: load "; L->printAsOperand(dbgs());
dbgs() << " has unknown dependence\n";);
return false;
AvailableValue AV;
if (AnalyzeLoadAvailability(L, Dep, L->getPointerOperand(), AV)) {
Value *AvailableValue = AV.MaterializeAdjustedValue(L, L, *this);
// Replace the load!
patchAndReplaceAllUsesWith(L, AvailableValue);
reportLoadElim(L, AvailableValue, ORE);
// Tell MDA to rexamine the reused pointer since we might have more
// information after forwarding it.
if (MD && AvailableValue->getType()->isPtrOrPtrVectorTy())
return true;
return false;
/// Return a pair the first field showing the value number of \p Exp and the
/// second field showing whether it is a value number newly created.
std::pair<uint32_t, bool>
GVN::ValueTable::assignExpNewValueNum(Expression &Exp) {
uint32_t &e = expressionNumbering[Exp];
bool CreateNewValNum = !e;
if (CreateNewValNum) {
if (ExprIdx.size() < nextValueNumber + 1)
ExprIdx.resize(nextValueNumber * 2);
e = nextValueNumber;
ExprIdx[nextValueNumber++] = nextExprNumber++;
return {e, CreateNewValNum};
/// Return whether all the values related with the same \p num are
/// defined in \p BB.
bool GVN::ValueTable::areAllValsInBB(uint32_t Num, const BasicBlock *BB,
GVN &Gvn) {
LeaderTableEntry *Vals = &Gvn.LeaderTable[Num];
while (Vals && Vals->BB == BB)
Vals = Vals->Next;
return !Vals;
/// Wrap phiTranslateImpl to provide caching functionality.
uint32_t GVN::ValueTable::phiTranslate(const BasicBlock *Pred,
const BasicBlock *PhiBlock, uint32_t Num,
GVN &Gvn) {
auto FindRes = PhiTranslateTable.find({Num, Pred});
if (FindRes != PhiTranslateTable.end())
return FindRes->second;
uint32_t NewNum = phiTranslateImpl(Pred, PhiBlock, Num, Gvn);
PhiTranslateTable.insert({{Num, Pred}, NewNum});
return NewNum;
/// Translate value number \p Num using phis, so that it has the values of
/// the phis in BB.
uint32_t GVN::ValueTable::phiTranslateImpl(const BasicBlock *Pred,
const BasicBlock *PhiBlock,
uint32_t Num, GVN &Gvn) {
if (PHINode *PN = NumberingPhi[Num]) {
for (unsigned i = 0; i != PN->getNumIncomingValues(); ++i) {
if (PN->getParent() == PhiBlock && PN->getIncomingBlock(i) == Pred)
if (uint32_t TransVal = lookup(PN->getIncomingValue(i), false))
return TransVal;
return Num;
// If there is any value related with Num is defined in a BB other than
// PhiBlock, it cannot depend on a phi in PhiBlock without going through
// a backedge. We can do an early exit in that case to save compile time.
if (!areAllValsInBB(Num, PhiBlock, Gvn))
return Num;
if (Num >= ExprIdx.size() || ExprIdx[Num] == 0)
return Num;
Expression Exp = Expressions[ExprIdx[Num]];
for (unsigned i = 0; i < Exp.varargs.size(); i++) {
// For InsertValue and ExtractValue, some varargs are index numbers
// instead of value numbers. Those index numbers should not be
// translated.
if ((i > 1 && Exp.opcode == Instruction::InsertValue) ||
(i > 0 && Exp.opcode == Instruction::ExtractValue))
Exp.varargs[i] = phiTranslate(Pred, PhiBlock, Exp.varargs[i], Gvn);
if (Exp.commutative) {
assert(Exp.varargs.size() == 2 && "Unsupported commutative expression!");
if (Exp.varargs[0] > Exp.varargs[1]) {
std::swap(Exp.varargs[0], Exp.varargs[1]);
uint32_t Opcode = Exp.opcode >> 8;
if (Opcode == Instruction::ICmp || Opcode == Instruction::FCmp)
Exp.opcode = (Opcode << 8) |
static_cast<CmpInst::Predicate>(Exp.opcode & 255));
if (uint32_t NewNum = expressionNumbering[Exp])
return NewNum;
return Num;
/// Erase stale entry from phiTranslate cache so phiTranslate can be computed
/// again.
void GVN::ValueTable::eraseTranslateCacheEntry(uint32_t Num,
const BasicBlock &CurrBlock) {
for (const BasicBlock *Pred : predecessors(&CurrBlock)) {
auto FindRes = PhiTranslateTable.find({Num, Pred});
if (FindRes != PhiTranslateTable.end())
// In order to find a leader for a given value number at a
// specific basic block, we first obtain the list of all Values for that number,
// and then scan the list to find one whose block dominates the block in
// question. This is fast because dominator tree queries consist of only
// a few comparisons of DFS numbers.
Value *GVN::findLeader(const BasicBlock *BB, uint32_t num) {
LeaderTableEntry Vals = LeaderTable[num];
if (!Vals.Val) return nullptr;
Value *Val = nullptr;
if (DT->dominates(Vals.BB, BB)) {
Val = Vals.Val;
if (isa<Constant>(Val)) return Val;
LeaderTableEntry* Next = Vals.Next;
while (Next) {
if (DT->dominates(Next->BB, BB)) {
if (isa<Constant>(Next->Val)) return Next->Val;
if (!Val) Val = Next->Val;
Next = Next->Next;
return Val;
/// There is an edge from 'Src' to 'Dst'. Return
/// true if every path from the entry block to 'Dst' passes via this edge. In
/// particular 'Dst' must not be reachable via another edge from 'Src'.
static bool isOnlyReachableViaThisEdge(const BasicBlockEdge &E,
DominatorTree *DT) {
// While in theory it is interesting to consider the case in which Dst has
// more than one predecessor, because Dst might be part of a loop which is
// only reachable from Src, in practice it is pointless since at the time
// GVN runs all such loops have preheaders, which means that Dst will have
// been changed to have only one predecessor, namely Src.
const BasicBlock *Pred = E.getEnd()->getSinglePredecessor();
assert((!Pred || Pred == E.getStart()) &&
"No edge between these basic blocks!");
return Pred != nullptr;
void GVN::assignBlockRPONumber(Function &F) {
uint32_t NextBlockNumber = 1;
ReversePostOrderTraversal<Function *> RPOT(&F);
for (BasicBlock *BB : RPOT)
BlockRPONumber[BB] = NextBlockNumber++;
InvalidBlockRPONumbers = false;
// Tries to replace instruction with const, using information from
// ReplaceWithConstMap.
bool GVN::replaceOperandsWithConsts(Instruction *Instr) const {
bool Changed = false;
for (unsigned OpNum = 0; OpNum < Instr->getNumOperands(); ++OpNum) {
Value *Operand = Instr->getOperand(OpNum);
auto it = ReplaceWithConstMap.find(Operand);
if (it != ReplaceWithConstMap.end()) {
assert(!isa<Constant>(Operand) &&
"Replacing constants with constants is invalid");
LLVM_DEBUG(dbgs() << "GVN replacing: " << *Operand << " with "
<< *it->second << " in instruction " << *Instr << '\n');
Instr->setOperand(OpNum, it->second);
Changed = true;
return Changed;
/// The given values are known to be equal in every block
/// dominated by 'Root'. Exploit this, for example by replacing 'LHS' with
/// 'RHS' everywhere in the scope. Returns whether a change was made.
/// If DominatesByEdge is false, then it means that we will propagate the RHS
/// value starting from the end of Root.Start.
bool GVN::propagateEquality(Value *LHS, Value *RHS, const BasicBlockEdge &Root,
bool DominatesByEdge) {
SmallVector<std::pair<Value*, Value*>, 4> Worklist;
Worklist.push_back(std::make_pair(LHS, RHS));
bool Changed = false;
// For speed, compute a conservative fast approximation to
// DT->dominates(Root, Root.getEnd());
const bool RootDominatesEnd = isOnlyReachableViaThisEdge(Root, DT);
while (!Worklist.empty()) {
std::pair<Value*, Value*> Item = Worklist.pop_back_val();
LHS = Item.first; RHS = Item.second;
if (LHS == RHS)
assert(LHS->getType() == RHS->getType() && "Equality but unequal types!");
// Don't try to propagate equalities between constants.
if (isa<Constant>(LHS) && isa<Constant>(RHS))
// Prefer a constant on the right-hand side, or an Argument if no constants.
if (isa<Constant>(LHS) || (isa<Argument>(LHS) && !isa<Constant>(RHS)))
std::swap(LHS, RHS);
assert((isa<Argument>(LHS) || isa<Instruction>(LHS)) && "Unexpected value!");
// If there is no obvious reason to prefer the left-hand side over the
// right-hand side, ensure the longest lived term is on the right-hand side,
// so the shortest lived term will be replaced by the longest lived.
// This tends to expose more simplifications.
uint32_t LVN = VN.lookupOrAdd(LHS);
if ((isa<Argument>(LHS) && isa<Argument>(RHS)) ||
(isa<Instruction>(LHS) && isa<Instruction>(RHS))) {
// Move the 'oldest' value to the right-hand side, using the value number
// as a proxy for age.
uint32_t RVN = VN.lookupOrAdd(RHS);
if (LVN < RVN) {
std::swap(LHS, RHS);
// If value numbering later sees that an instruction in the scope is equal
// to 'LHS' then ensure it will be turned into 'RHS'. In order to preserve
// the invariant that instructions only occur in the leader table for their
// own value number (this is used by removeFromLeaderTable), do not do this
// if RHS is an instruction (if an instruction in the scope is morphed into
// LHS then it will be turned into RHS by the next GVN iteration anyway, so
// using the leader table is about compiling faster, not optimizing better).
// The leader table only tracks basic blocks, not edges. Only add to if we
// have the simple case where the edge dominates the end.
if (RootDominatesEnd && !isa<Instruction>(RHS))
addToLeaderTable(LVN, RHS, Root.getEnd());
// Replace all occurrences of 'LHS' with 'RHS' everywhere in the scope. As
// LHS always has at least one use that is not dominated by Root, this will
// never do anything if LHS has only one use.
if (!LHS->hasOneUse()) {
unsigned NumReplacements =
? replaceDominatedUsesWith(LHS, RHS, *DT, Root)
: replaceDominatedUsesWith(LHS, RHS, *DT, Root.getStart());
Changed |= NumReplacements > 0;
NumGVNEqProp += NumReplacements;
// Cached information for anything that uses LHS will be invalid.
if (MD)
// Now try to deduce additional equalities from this one. For example, if
// the known equality was "(A != B)" == "false" then it follows that A and B
// are equal in the scope. Only boolean equalities with an explicit true or
// false RHS are currently supported.
if (!RHS->getType()->isIntegerTy(1))
// Not a boolean equality - bail out.
ConstantInt *CI = dyn_cast<ConstantInt>(RHS);
if (!CI)
// RHS neither 'true' nor 'false' - bail out.
// Whether RHS equals 'true'. Otherwise it equals 'false'.
bool isKnownTrue = CI->isMinusOne();
bool isKnownFalse = !isKnownTrue;
// If "A && B" is known true then both A and B are known true. If "A || B"
// is known false then both A and B are known false.
Value *A, *B;
if ((isKnownTrue && match(LHS, m_And(m_Value(A), m_Value(B)))) ||
(isKnownFalse && match(LHS, m_Or(m_Value(A), m_Value(B))))) {
Worklist.push_back(std::make_pair(A, RHS));
Worklist.push_back(std::make_pair(B, RHS));
// If we are propagating an equality like "(A == B)" == "true" then also
// propagate the equality A == B. When propagating a comparison such as
// "(A >= B)" == "true", replace all instances of "A < B" with "false".
if (CmpInst *Cmp = dyn_cast<CmpInst>(LHS)) {
Value *Op0 = Cmp->getOperand(0), *Op1 = Cmp->getOperand(1);
// If "A == B" is known true, or "A != B" is known false, then replace
// A with B everywhere in the scope.
if ((isKnownTrue && Cmp->getPredicate() == CmpInst::ICMP_EQ) ||
(isKnownFalse && Cmp->getPredicate() == CmpInst::ICMP_NE))
Worklist.push_back(std::make_pair(Op0, Op1));
// Handle the floating point versions of equality comparisons too.
if ((isKnownTrue && Cmp->getPredicate() == CmpInst::FCMP_OEQ) ||
(isKnownFalse && Cmp->getPredicate() == CmpInst::FCMP_UNE)) {
// Floating point -0.0 and 0.0 compare equal, so we can only
// propagate values if we know that we have a constant and that
// its value is non-zero.
// FIXME: We should do this optimization if 'no signed zeros' is
// applicable via an instruction-level fast-math-flag or some other
// indicator that relaxed FP semantics are being used.
if (isa<ConstantFP>(Op1) && !cast<ConstantFP>(Op1)->isZero())
Worklist.push_back(std::make_pair(Op0, Op1));
// If "A >= B" is known true, replace "A < B" with false everywhere.
CmpInst::Predicate NotPred = Cmp->getInversePredicate();
Constant *NotVal = ConstantInt::get(Cmp->getType(), isKnownFalse);
// Since we don't have the instruction "A < B" immediately to hand, work
// out the value number that it would have and use that to find an
// appropriate instruction (if any).
uint32_t NextNum = VN.getNextUnusedValueNumber();
uint32_t Num = VN.lookupOrAddCmp(Cmp->getOpcode(), NotPred, Op0, Op1);
// If the number we were assigned was brand new then there is no point in
// looking for an instruction realizing it: there cannot be one!
if (Num < NextNum) {
Value *NotCmp = findLeader(Root.getEnd(), Num);
if (NotCmp && isa<Instruction>(NotCmp)) {
unsigned NumReplacements =
? replaceDominatedUsesWith(NotCmp, NotVal, *DT, Root)
: replaceDominatedUsesWith(NotCmp, NotVal, *DT,
Changed |= NumReplacements > 0;
NumGVNEqProp += NumReplacements;
// Cached information for anything that uses NotCmp will be invalid.
if (MD)
// Ensure that any instruction in scope that gets the "A < B" value number
// is replaced with false.
// The leader table only tracks basic blocks, not edges. Only add to if we
// have the simple case where the edge dominates the end.
if (RootDominatesEnd)
addToLeaderTable(Num, NotVal, Root.getEnd());
return Changed;
/// When calculating availability, handle an instruction
/// by inserting it into the appropriate sets
bool GVN::processInstruction(Instruction *I) {
// Ignore dbg info intrinsics.
if (isa<DbgInfoIntrinsic>(I))
return false;
// If the instruction can be easily simplified then do so now in preference
// to value numbering it. Value numbering often exposes redundancies, for
// example if it determines that %y is equal to %x then the instruction
// "%z = and i32 %x, %y" becomes "%z = and i32 %x, %x" which we now simplify.
const DataLayout &DL = I->getModule()->getDataLayout();
if (Value *V = SimplifyInstruction(I, {DL, TLI, DT, AC})) {
bool Changed = false;
if (!I->use_empty()) {
Changed = true;
if (isInstructionTriviallyDead(I, TLI)) {
Changed = true;
if (Changed) {
if (MD && V->getType()->isPtrOrPtrVectorTy())
return true;
if (IntrinsicInst *IntrinsicI = dyn_cast<IntrinsicInst>(I))
if (IntrinsicI->getIntrinsicID() == Intrinsic::assume)
return processAssumeIntrinsic(IntrinsicI);
if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
if (processLoad(LI))
return true;
unsigned Num = VN.lookupOrAdd(LI);
addToLeaderTable(Num, LI, LI->getParent());
return false;
// For conditional branches, we can perform simple conditional propagation on
// the condition value itself.
if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
if (!BI->isConditional())
return false;
if (isa<Constant>(BI->getCondition()))
return processFoldableCondBr(BI);
Value *BranchCond = BI->getCondition();
BasicBlock *TrueSucc = BI->getSuccessor(0);
BasicBlock *FalseSucc = BI->getSuccessor(1);
// Avoid multiple edges early.
if (TrueSucc == FalseSucc)
return false;
BasicBlock *Parent = BI->getParent();
bool Changed = false;
Value *TrueVal = ConstantInt::getTrue(TrueSucc->getContext());
BasicBlockEdge TrueE(Parent, TrueSucc);
Changed |= propagateEquality(BranchCond, TrueVal, TrueE, true);
Value *FalseVal = ConstantInt::getFalse(FalseSucc->getContext());
BasicBlockEdge FalseE(Parent, FalseSucc);
Changed |= propagateEquality(BranchCond, FalseVal, FalseE, true);
return Changed;
// For switches, propagate the case values into the case destinations.
if (SwitchInst *SI = dyn_cast<SwitchInst>(I)) {
Value *SwitchCond = SI->getCondition();
BasicBlock *Parent = SI->getParent();
bool Changed = false;
// Remember how many outgoing edges there are to every successor.
SmallDenseMap<BasicBlock *, unsigned, 16> SwitchEdges;
for (unsigned i = 0, n = SI->getNumSuccessors(); i != n; ++i)
for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
i != e; ++i) {
BasicBlock *Dst = i->getCaseSuccessor();
// If there is only a single edge, propagate the case value into it.
if (SwitchEdges.lookup(Dst) == 1) {
BasicBlockEdge E(Parent, Dst);
Changed |= propagateEquality(SwitchCond, i->getCaseValue(), E, true);
return Changed;
// Instructions with void type don't return a value, so there's
// no point in trying to find redundancies in them.
if (I->getType()->isVoidTy())
return false;
uint32_t NextNum = VN.getNextUnusedValueNumber();
unsigned Num = VN.lookupOrAdd(I);
// Allocations are always uniquely numbered, so we can save time and memory
// by fast failing them.
if (isa<AllocaInst>(I) || I->isTerminator() || isa<PHINode>(I)) {
addToLeaderTable(Num, I, I->getParent());
return false;
// If the number we were assigned was a brand new VN, then we don't
// need to do a lookup to see if the number already exists
// somewhere in the domtree: it can't!
if (Num >= NextNum) {
addToLeaderTable(Num, I, I->getParent());
return false;
// Perform fast-path value-number based elimination of values inherited from
// dominators.
Value *Repl = findLeader(I->getParent(), Num);
if (!Repl) {
// Failure, just remember this instance for future use.
addToLeaderTable(Num, I, I->getParent());
return false;
} else if (Repl == I) {
// If I was the result of a shortcut PRE, it might already be in the table
// and the best replacement for itself. Nothing to do.
return false;
// Remove it!
patchAndReplaceAllUsesWith(I, Repl);
if (MD && Repl->getType()->isPtrOrPtrVectorTy())
return true;
/// runOnFunction - This is the main transformation entry point for a function.
bool GVN::runImpl(Function &F, AssumptionCache &RunAC, DominatorTree &RunDT,
const TargetLibraryInfo &RunTLI, AAResults &RunAA,
MemoryDependenceResults *RunMD, LoopInfo *LI,
OptimizationRemarkEmitter *RunORE) {
AC = &RunAC;
DT = &RunDT;
TLI = &RunTLI;
MD = RunMD;
ImplicitControlFlowTracking ImplicitCFT(DT);
ICF = &ImplicitCFT;
InvalidBlockRPONumbers = true;
bool Changed = false;
bool ShouldContinue = true;
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
// Merge unconditional branches, allowing PRE to catch more
// optimization opportunities.
for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ) {
BasicBlock *BB = &*FI++;
bool removedBlock = MergeBlockIntoPredecessor(BB, &DTU, LI, nullptr, MD);
if (removedBlock)
Changed |= removedBlock;
unsigned Iteration = 0;
while (ShouldContinue) {
LLVM_DEBUG(dbgs() << "GVN iteration: " << Iteration << "\n");
ShouldContinue = iterateOnFunction(F);
Changed |= ShouldContinue;
if (EnablePRE) {
// Fabricate val-num for dead-code in order to suppress assertion in
// performPRE().
bool PREChanged = true;
while (PREChanged) {
PREChanged = performPRE(F);
Changed |= PREChanged;
// FIXME: Should perform GVN again after PRE does something. PRE can move
// computations into blocks where they become fully redundant. Note that
// we can't do this until PRE's critical edge splitting updates memdep.
// Actually, when this happens, we should just fully integrate PRE into GVN.
// Do not cleanup DeadBlocks in cleanupGlobalSets() as it's called for each
// iteration.
return Changed;
bool GVN::processBlock(BasicBlock *BB) {
// FIXME: Kill off InstrsToErase by doing erasing eagerly in a helper function
// (and incrementing BI before processing an instruction).
assert(InstrsToErase.empty() &&
"We expect InstrsToErase to be empty across iterations");
if (DeadBlocks.count(BB))
return false;
// Clearing map before every BB because it can be used only for single BB.
bool ChangedFunction = false;
for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
BI != BE;) {
if (!ReplaceWithConstMap.empty())
ChangedFunction |= replaceOperandsWithConsts(&*BI);
ChangedFunction |= processInstruction(&*BI);
if (InstrsToErase.empty()) {
// If we need some instructions deleted, do it now.
NumGVNInstr += InstrsToErase.size();
// Avoid iterator invalidation.
bool AtStart = BI == BB->begin();
if (!AtStart)
for (auto *I : InstrsToErase) {
assert(I->getParent() == BB && "Removing instruction from wrong block?");
LLVM_DEBUG(dbgs() << "GVN removed: " << *I << '\n');
if (MD) MD->removeInstruction(I);
if (AtStart)
BI = BB->begin();
return ChangedFunction;
// Instantiate an expression in a predecessor that lacked it.
bool GVN::performScalarPREInsertion(Instruction *Instr, BasicBlock *Pred,
BasicBlock *Curr, unsigned int ValNo) {
// Because we are going top-down through the block, all value numbers
// will be available in the predecessor by the time we need them. Any
// that weren't originally present will have been instantiated earlier
// in this loop.
bool success = true;
for (unsigned i = 0, e = Instr->getNumOperands(); i != e; ++i) {
Value *Op = Instr->getOperand(i);
if (isa<Argument>(Op) || isa<Constant>(Op) || isa<GlobalValue>(Op))
// This could be a newly inserted instruction, in which case, we won't
// find a value number, and should give up before we hurt ourselves.
// FIXME: Rewrite the infrastructure to let it easier to value number
// and process newly inserted instructions.
if (!VN.exists(Op)) {
success = false;
uint32_t TValNo =
VN.phiTranslate(Pred, Curr, VN.lookup(Op), *this);
if (Value *V = findLeader(Pred, TValNo)) {
Instr->setOperand(i, V);
} else {
success = false;
// Fail out if we encounter an operand that is not available in
// the PRE predecessor. This is typically because of loads which
// are not value numbered precisely.
if (!success)
return false;
Instr->setName(Instr->getName() + ".pre");
unsigned Num = VN.lookupOrAdd(Instr);
VN.add(Instr, Num);
// Update the availability map to include the new instruction.
addToLeaderTable(Num, Instr, Pred);
return true;
bool GVN::performScalarPRE(Instruction *CurInst) {
if (isa<AllocaInst>(CurInst) || CurInst->isTerminator() ||
isa<PHINode>(CurInst) || CurInst->getType()->isVoidTy() ||
CurInst->mayReadFromMemory() || CurInst->mayHaveSideEffects() ||
return false;
// Don't do PRE on compares. The PHI would prevent CodeGenPrepare from
// sinking the compare again, and it would force the code generator to
// move the i1 from processor flags or predicate registers into a general
// purpose register.
if (isa<CmpInst>(CurInst))
return false;
// Don't do PRE on GEPs. The inserted PHI would prevent CodeGenPrepare from
// sinking the addressing mode computation back to its uses. Extending the
// GEP's live range increases the register pressure, and therefore it can
// introduce unnecessary spills.
// This doesn't prevent Load PRE. PHI translation will make the GEP available
// to the load by moving it to the predecessor block if necessary.
if (isa<GetElementPtrInst>(CurInst))
return false;
// We don't currently value number ANY inline asm calls.
if (CallInst *CallI = dyn_cast<CallInst>(CurInst))
if (CallI->isInlineAsm())
return false;
uint32_t ValNo = VN.lookup(CurInst);
// Look for the predecessors for PRE opportunities. We're
// only trying to solve the basic diamond case, where
// a value is computed in the successor and one predecessor,
// but not the other. We also explicitly disallow cases
// where the successor is its own predecessor, because they're
// more complicated to get right.
unsigned NumWith = 0;
unsigned NumWithout = 0;
BasicBlock *PREPred = nullptr;
BasicBlock *CurrentBlock = CurInst->getParent();
// Update the RPO numbers for this function.
if (InvalidBlockRPONumbers)
SmallVector<std::pair<Value *, BasicBlock *>, 8> predMap;
for (BasicBlock *P : predecessors(CurrentBlock)) {
// We're not interested in PRE where blocks with predecessors that are
// not reachable.
if (!DT->isReachableFromEntry(P)) {
NumWithout = 2;
// It is not safe to do PRE when P->CurrentBlock is a loop backedge, and
// when CurInst has operand defined in CurrentBlock (so it may be defined
// by phi in the loop header).
assert(BlockRPONumber.count(P) && BlockRPONumber.count(CurrentBlock) &&
"Invalid BlockRPONumber map.");
if (BlockRPONumber[P] >= BlockRPONumber[CurrentBlock] &&
llvm::any_of(CurInst->operands(), [&](const Use &U) {
if (auto *Inst = dyn_cast<Instruction>(U.get()))
return Inst->getParent() == CurrentBlock;
return false;
})) {
NumWithout = 2;
uint32_t TValNo = VN.phiTranslate(P, CurrentBlock, ValNo, *this);
Value *predV = findLeader(P, TValNo);
if (!predV) {
predMap.push_back(std::make_pair(static_cast<Value *>(nullptr), P));
PREPred = P;
} else if (predV == CurInst) {
/* CurInst dominates this predecessor. */
NumWithout = 2;
} else {
predMap.push_back(std::make_pair(predV, P));
// Don't do PRE when it might increase code size, i.e. when
// we would need to insert instructions in more than one pred.
if (NumWithout > 1 || NumWith == 0)
return false;
// We may have a case where all predecessors have the instruction,
// and we just need to insert a phi node. Otherwise, perform
// insertion.
Instruction *PREInstr = nullptr;
if (NumWithout != 0) {
if (!isSafeToSpeculativelyExecute(CurInst)) {
// It is only valid to insert a new instruction if the current instruction
// is always executed. An instruction with implicit control flow could
// prevent us from doing it. If we cannot speculate the execution, then
// PRE should be prohibited.
if (ICF->isDominatedByICFIFromSameBlock(CurInst))
return false;
// Don't do PRE across indirect branch.
if (isa<IndirectBrInst>(PREPred->getTerminator()))
return false;
// We can't do PRE safely on a critical edge, so instead we schedule
// the edge to be split and perform the PRE the next time we iterate
// on the function.
unsigned SuccNum = GetSuccessorNumber(PREPred, CurrentBlock);
if (isCriticalEdge(PREPred->getTerminator(), SuccNum)) {
toSplit.push_back(std::make_pair(PREPred->getTerminator(), SuccNum));
return false;
// We need to insert somewhere, so let's give it a shot
PREInstr = CurInst->clone();
if (!performScalarPREInsertion(PREInstr, PREPred, CurrentBlock, ValNo)) {
// If we failed insertion, make sure we remove the instruction.
return false;
// Either we should have filled in the PRE instruction, or we should
// not have needed insertions.
assert(PREInstr != nullptr || NumWithout == 0);
// Create a PHI to make the value available in this block.
PHINode *Phi =
PHINode::Create(CurInst->getType(), predMap.size(),
CurInst->getName() + ".pre-phi", &CurrentBlock->front());
for (unsigned i = 0, e = predMap.size(); i != e; ++i) {
if (Value *V = predMap[i].first) {
// If we use an existing value in this phi, we have to patch the original
// value because the phi will be used to replace a later value.
patchReplacementInstruction(CurInst, V);
Phi->addIncoming(V, predMap[i].second);
} else
Phi->addIncoming(PREInstr, PREPred);
VN.add(Phi, ValNo);
// After creating a new PHI for ValNo, the phi translate result for ValNo will
// be changed, so erase the related stale entries in phi translate cache.
VN.eraseTranslateCacheEntry(ValNo, *CurrentBlock);
addToLeaderTable(ValNo, Phi, CurrentBlock);
if (MD && Phi->getType()->isPtrOrPtrVectorTy())
removeFromLeaderTable(ValNo, CurInst, CurrentBlock);
LLVM_DEBUG(dbgs() << "GVN PRE removed: " << *CurInst << '\n');
if (MD)
// FIXME: Intended to be markInstructionForDeletion(CurInst), but it causes
// some assertion failures.
return true;
/// Perform a purely local form of PRE that looks for diamond
/// control flow patterns and attempts to perform simple PRE at the join point.
bool GVN::performPRE(Function &F) {
bool Changed = false;
for (BasicBlock *CurrentBlock : depth_first(&F.getEntryBlock())) {
// Nothing to PRE in the entry block.
if (CurrentBlock == &F.getEntryBlock())
// Don't perform PRE on an EH pad.
if (CurrentBlock->isEHPad())
for (BasicBlock::iterator BI = CurrentBlock->begin(),
BE = CurrentBlock->end();
BI != BE;) {
Instruction *CurInst = &*BI++;
Changed |= performScalarPRE(CurInst);
if (splitCriticalEdges())
Changed = true;
return Changed;
/// Split the critical edge connecting the given two blocks, and return
/// the block inserted to the critical edge.
BasicBlock *GVN::splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ) {
BasicBlock *BB =
SplitCriticalEdge(Pred, Succ, CriticalEdgeSplittingOptions(DT));
if (MD)
InvalidBlockRPONumbers = true;
return BB;
/// Split critical edges found during the previous
/// iteration that may enable further optimization.
bool GVN::splitCriticalEdges() {
if (toSplit.empty())
return false;
do {
std::pair<Instruction *, unsigned> Edge = toSplit.pop_back_val();
SplitCriticalEdge(Edge.first, Edge.second,
} while (!toSplit.empty());
if (MD) MD->invalidateCachedPredecessors();
InvalidBlockRPONumbers = true;
return true;
/// Executes one iteration of GVN
bool GVN::iterateOnFunction(Function &F) {
// Top-down walk of the dominator tree
bool Changed = false;
// Needed for value numbering with phi construction to work.
// RPOT walks the graph in its constructor and will not be invalidated during
// processBlock.
ReversePostOrderTraversal<Function *> RPOT(&F);
for (BasicBlock *BB : RPOT)
Changed |= processBlock(BB);
return Changed;
void GVN::cleanupGlobalSets() {
InvalidBlockRPONumbers = true;
/// Verify that the specified instruction does not occur in our
/// internal data structures.
void GVN::verifyRemoved(const Instruction *Inst) const {
// Walk through the value number scope to make sure the instruction isn't
// ferreted away in it.
for (DenseMap<uint32_t, LeaderTableEntry>::const_iterator
I = LeaderTable.begin(), E = LeaderTable.end(); I != E; ++I) {
const LeaderTableEntry *Node = &I->second;
assert(Node->Val != Inst && "Inst still in value numbering scope!");
while (Node->Next) {
Node = Node->Next;
assert(Node->Val != Inst && "Inst still in value numbering scope!");
/// BB is declared dead, which implied other blocks become dead as well. This
/// function is to add all these blocks to "DeadBlocks". For the dead blocks'
/// live successors, update their phi nodes by replacing the operands
/// corresponding to dead blocks with UndefVal.
void GVN::addDeadBlock(BasicBlock *BB) {
SmallVector<BasicBlock *, 4> NewDead;
SmallSetVector<BasicBlock *, 4> DF;
while (!NewDead.empty()) {
BasicBlock *D = NewDead.pop_back_val();
if (DeadBlocks.count(D))
// All blocks dominated by D are dead.
SmallVector<BasicBlock *, 8> Dom;
DT->getDescendants(D, Dom);
DeadBlocks.insert(Dom.begin(), Dom.end());
// Figure out the dominance-frontier(D).
for (BasicBlock *B : Dom) {
for (BasicBlock *S : successors(B)) {
if (DeadBlocks.count(S))
bool AllPredDead = true;
for (BasicBlock *P : predecessors(S))
if (!DeadBlocks.count(P)) {
AllPredDead = false;
if (!AllPredDead) {
// S could be proved dead later on. That is why we don't update phi
// operands at this moment.
} else {
// While S is not dominated by D, it is dead by now. This could take
// place if S already have a dead predecessor before D is declared
// dead.
// For the dead blocks' live successors, update their phi nodes by replacing
// the operands corresponding to dead blocks with UndefVal.
for(SmallSetVector<BasicBlock *, 4>::iterator I = DF.begin(), E = DF.end();
I != E; I++) {
BasicBlock *B = *I;
if (DeadBlocks.count(B))
SmallVector<BasicBlock *, 4> Preds(pred_begin(B), pred_end(B));
for (BasicBlock *P : Preds) {
if (!DeadBlocks.count(P))
if (isCriticalEdge(P->getTerminator(), GetSuccessorNumber(P, B))) {
if (BasicBlock *S = splitCriticalEdges(P, B))
DeadBlocks.insert(P = S);
for (BasicBlock::iterator II = B->begin(); isa<PHINode>(II); ++II) {
PHINode &Phi = cast<PHINode>(*II);
if (MD)