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//===- MemCpyOptimizer.cpp - Optimize use of memcpy and friends -----------===//
// 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 performs various transformations related to eliminating memcpy
// calls, or transforming sets of stores into memset's.
#include "llvm/Transforms/Scalar/MemCpyOptimizer.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <optional>
using namespace llvm;
#define DEBUG_TYPE "memcpyopt"
static cl::opt<bool> EnableMemCpyOptWithoutLibcalls(
"enable-memcpyopt-without-libcalls", cl::Hidden,
cl::desc("Enable memcpyopt even when libcalls are disabled"));
STATISTIC(NumMemCpyInstr, "Number of memcpy instructions deleted");
STATISTIC(NumMemSetInfer, "Number of memsets inferred");
STATISTIC(NumMoveToCpy, "Number of memmoves converted to memcpy");
STATISTIC(NumCpyToSet, "Number of memcpys converted to memset");
STATISTIC(NumCallSlot, "Number of call slot optimizations performed");
namespace {
/// Represents a range of memset'd bytes with the ByteVal value.
/// This allows us to analyze stores like:
/// store 0 -> P+1
/// store 0 -> P+0
/// store 0 -> P+3
/// store 0 -> P+2
/// which sometimes happens with stores to arrays of structs etc. When we see
/// the first store, we make a range [1, 2). The second store extends the range
/// to [0, 2). The third makes a new range [2, 3). The fourth store joins the
/// two ranges into [0, 3) which is memset'able.
struct MemsetRange {
// Start/End - A semi range that describes the span that this range covers.
// The range is closed at the start and open at the end: [Start, End).
int64_t Start, End;
/// StartPtr - The getelementptr instruction that points to the start of the
/// range.
Value *StartPtr;
/// Alignment - The known alignment of the first store.
MaybeAlign Alignment;
/// TheStores - The actual stores that make up this range.
SmallVector<Instruction*, 16> TheStores;
bool isProfitableToUseMemset(const DataLayout &DL) const;
} // end anonymous namespace
bool MemsetRange::isProfitableToUseMemset(const DataLayout &DL) const {
// If we found more than 4 stores to merge or 16 bytes, use memset.
if (TheStores.size() >= 4 || End-Start >= 16) return true;
// If there is nothing to merge, don't do anything.
if (TheStores.size() < 2) return false;
// If any of the stores are a memset, then it is always good to extend the
// memset.
for (Instruction *SI : TheStores)
if (!isa<StoreInst>(SI))
return true;
// Assume that the code generator is capable of merging pairs of stores
// together if it wants to.
if (TheStores.size() == 2) return false;
// If we have fewer than 8 stores, it can still be worthwhile to do this.
// For example, merging 4 i8 stores into an i32 store is useful almost always.
// However, merging 2 32-bit stores isn't useful on a 32-bit architecture (the
// memset will be split into 2 32-bit stores anyway) and doing so can
// pessimize the llvm optimizer.
// Since we don't have perfect knowledge here, make some assumptions: assume
// the maximum GPR width is the same size as the largest legal integer
// size. If so, check to see whether we will end up actually reducing the
// number of stores used.
unsigned Bytes = unsigned(End-Start);
unsigned MaxIntSize = DL.getLargestLegalIntTypeSizeInBits() / 8;
if (MaxIntSize == 0)
MaxIntSize = 1;
unsigned NumPointerStores = Bytes / MaxIntSize;
// Assume the remaining bytes if any are done a byte at a time.
unsigned NumByteStores = Bytes % MaxIntSize;
// If we will reduce the # stores (according to this heuristic), do the
// transformation. This encourages merging 4 x i8 -> i32 and 2 x i16 -> i32
// etc.
return TheStores.size() > NumPointerStores+NumByteStores;
namespace {
class MemsetRanges {
using range_iterator = SmallVectorImpl<MemsetRange>::iterator;
/// A sorted list of the memset ranges.
SmallVector<MemsetRange, 8> Ranges;
const DataLayout &DL;
MemsetRanges(const DataLayout &DL) : DL(DL) {}
using const_iterator = SmallVectorImpl<MemsetRange>::const_iterator;
const_iterator begin() const { return Ranges.begin(); }
const_iterator end() const { return Ranges.end(); }
bool empty() const { return Ranges.empty(); }
void addInst(int64_t OffsetFromFirst, Instruction *Inst) {
if (auto *SI = dyn_cast<StoreInst>(Inst))
addStore(OffsetFromFirst, SI);
addMemSet(OffsetFromFirst, cast<MemSetInst>(Inst));
void addStore(int64_t OffsetFromFirst, StoreInst *SI) {
TypeSize StoreSize = DL.getTypeStoreSize(SI->getOperand(0)->getType());
assert(!StoreSize.isScalable() && "Can't track scalable-typed stores");
addRange(OffsetFromFirst, StoreSize.getFixedValue(),
SI->getPointerOperand(), SI->getAlign(), SI);
void addMemSet(int64_t OffsetFromFirst, MemSetInst *MSI) {
int64_t Size = cast<ConstantInt>(MSI->getLength())->getZExtValue();
addRange(OffsetFromFirst, Size, MSI->getDest(), MSI->getDestAlign(), MSI);
void addRange(int64_t Start, int64_t Size, Value *Ptr, MaybeAlign Alignment,
Instruction *Inst);
} // end anonymous namespace
/// Add a new store to the MemsetRanges data structure. This adds a
/// new range for the specified store at the specified offset, merging into
/// existing ranges as appropriate.
void MemsetRanges::addRange(int64_t Start, int64_t Size, Value *Ptr,
MaybeAlign Alignment, Instruction *Inst) {
int64_t End = Start+Size;
range_iterator I = partition_point(
Ranges, [=](const MemsetRange &O) { return O.End < Start; });
// We now know that I == E, in which case we didn't find anything to merge
// with, or that Start <= I->End. If End < I->Start or I == E, then we need
// to insert a new range. Handle this now.
if (I == Ranges.end() || End < I->Start) {
MemsetRange &R = *Ranges.insert(I, MemsetRange());
R.Start = Start;
R.End = End;
R.StartPtr = Ptr;
R.Alignment = Alignment;
// This store overlaps with I, add it.
// At this point, we may have an interval that completely contains our store.
// If so, just add it to the interval and return.
if (I->Start <= Start && I->End >= End)
// Now we know that Start <= I->End and End >= I->Start so the range overlaps
// but is not entirely contained within the range.
// See if the range extends the start of the range. In this case, it couldn't
// possibly cause it to join the prior range, because otherwise we would have
// stopped on *it*.
if (Start < I->Start) {
I->Start = Start;
I->StartPtr = Ptr;
I->Alignment = Alignment;
// Now we know that Start <= I->End and Start >= I->Start (so the startpoint
// is in or right at the end of I), and that End >= I->Start. Extend I out to
// End.
if (End > I->End) {
I->End = End;
range_iterator NextI = I;
while (++NextI != Ranges.end() && End >= NextI->Start) {
// Merge the range in.
I->TheStores.append(NextI->TheStores.begin(), NextI->TheStores.end());
if (NextI->End > I->End)
I->End = NextI->End;
NextI = I;
// MemCpyOptLegacyPass Pass
namespace {
class MemCpyOptLegacyPass : public FunctionPass {
MemCpyOptPass Impl;
static char ID; // Pass identification, replacement for typeid
MemCpyOptLegacyPass() : FunctionPass(ID) {
bool runOnFunction(Function &F) override;
// This transformation requires dominator postdominator info
void getAnalysisUsage(AnalysisUsage &AU) const override {
} // end anonymous namespace
char MemCpyOptLegacyPass::ID = 0;
/// The public interface to this file...
FunctionPass *llvm::createMemCpyOptPass() { return new MemCpyOptLegacyPass(); }
INITIALIZE_PASS_BEGIN(MemCpyOptLegacyPass, "memcpyopt", "MemCpy Optimization",
false, false)
INITIALIZE_PASS_END(MemCpyOptLegacyPass, "memcpyopt", "MemCpy Optimization",
false, false)
// Check that V is either not accessible by the caller, or unwinding cannot
// occur between Start and End.
static bool mayBeVisibleThroughUnwinding(Value *V, Instruction *Start,
Instruction *End) {
assert(Start->getParent() == End->getParent() && "Must be in same block");
// Function can't unwind, so it also can't be visible through unwinding.
if (Start->getFunction()->doesNotThrow())
return false;
// Object is not visible on unwind.
// TODO: Support RequiresNoCaptureBeforeUnwind case.
bool RequiresNoCaptureBeforeUnwind;
if (isNotVisibleOnUnwind(getUnderlyingObject(V),
RequiresNoCaptureBeforeUnwind) &&
return false;
// Check whether there are any unwinding instructions in the range.
return any_of(make_range(Start->getIterator(), End->getIterator()),
[](const Instruction &I) { return I.mayThrow(); });
void MemCpyOptPass::eraseInstruction(Instruction *I) {
// Check for mod or ref of Loc between Start and End, excluding both boundaries.
// Start and End must be in the same block.
// If SkippedLifetimeStart is provided, skip over one clobbering lifetime.start
// intrinsic and store it inside SkippedLifetimeStart.
static bool accessedBetween(BatchAAResults &AA, MemoryLocation Loc,
const MemoryUseOrDef *Start,
const MemoryUseOrDef *End,
Instruction **SkippedLifetimeStart = nullptr) {
assert(Start->getBlock() == End->getBlock() && "Only local supported");
for (const MemoryAccess &MA :
make_range(++Start->getIterator(), End->getIterator())) {
Instruction *I = cast<MemoryUseOrDef>(MA).getMemoryInst();
if (isModOrRefSet(AA.getModRefInfo(I, Loc))) {
auto *II = dyn_cast<IntrinsicInst>(I);
if (II && II->getIntrinsicID() == Intrinsic::lifetime_start &&
SkippedLifetimeStart && !*SkippedLifetimeStart) {
*SkippedLifetimeStart = I;
return true;
return false;
// Check for mod of Loc between Start and End, excluding both boundaries.
// Start and End can be in different blocks.
static bool writtenBetween(MemorySSA *MSSA, BatchAAResults &AA,
MemoryLocation Loc, const MemoryUseOrDef *Start,
const MemoryUseOrDef *End) {
if (isa<MemoryUse>(End)) {
// For MemoryUses, getClobberingMemoryAccess may skip non-clobbering writes.
// Manually check read accesses between Start and End, if they are in the
// same block, for clobbers. Otherwise assume Loc is clobbered.
return Start->getBlock() != End->getBlock() ||
make_range(std::next(Start->getIterator()), End->getIterator()),
[&AA, Loc](const MemoryAccess &Acc) {
if (isa<MemoryUse>(&Acc))
return false;
Instruction *AccInst =
return isModSet(AA.getModRefInfo(AccInst, Loc));
// TODO: Only walk until we hit Start.
MemoryAccess *Clobber = MSSA->getWalker()->getClobberingMemoryAccess(
End->getDefiningAccess(), Loc, AA);
return !MSSA->dominates(Clobber, Start);
/// When scanning forward over instructions, we look for some other patterns to
/// fold away. In particular, this looks for stores to neighboring locations of
/// memory. If it sees enough consecutive ones, it attempts to merge them
/// together into a memcpy/memset.
Instruction *MemCpyOptPass::tryMergingIntoMemset(Instruction *StartInst,
Value *StartPtr,
Value *ByteVal) {
const DataLayout &DL = StartInst->getModule()->getDataLayout();
// We can't track scalable types
if (auto *SI = dyn_cast<StoreInst>(StartInst))
if (DL.getTypeStoreSize(SI->getOperand(0)->getType()).isScalable())
return nullptr;
// Okay, so we now have a single store that can be splatable. Scan to find
// all subsequent stores of the same value to offset from the same pointer.
// Join these together into ranges, so we can decide whether contiguous blocks
// are stored.
MemsetRanges Ranges(DL);
BasicBlock::iterator BI(StartInst);
// Keeps track of the last memory use or def before the insertion point for
// the new memset. The new MemoryDef for the inserted memsets will be inserted
// after MemInsertPoint. It points to either LastMemDef or to the last user
// before the insertion point of the memset, if there are any such users.
MemoryUseOrDef *MemInsertPoint = nullptr;
// Keeps track of the last MemoryDef between StartInst and the insertion point
// for the new memset. This will become the defining access of the inserted
// memsets.
MemoryDef *LastMemDef = nullptr;
for (++BI; !BI->isTerminator(); ++BI) {
auto *CurrentAcc = cast_or_null<MemoryUseOrDef>(
if (CurrentAcc) {
MemInsertPoint = CurrentAcc;
if (auto *CurrentDef = dyn_cast<MemoryDef>(CurrentAcc))
LastMemDef = CurrentDef;
// Calls that only access inaccessible memory do not block merging
// accessible stores.
if (auto *CB = dyn_cast<CallBase>(BI)) {
if (CB->onlyAccessesInaccessibleMemory())
if (!isa<StoreInst>(BI) && !isa<MemSetInst>(BI)) {
// If the instruction is readnone, ignore it, otherwise bail out. We
// don't even allow readonly here because we don't want something like:
// A[1] = 2; strlen(A); A[2] = 2; -> memcpy(A, ...); strlen(A).
if (BI->mayWriteToMemory() || BI->mayReadFromMemory())
if (auto *NextStore = dyn_cast<StoreInst>(BI)) {
// If this is a store, see if we can merge it in.
if (!NextStore->isSimple()) break;
Value *StoredVal = NextStore->getValueOperand();
// Don't convert stores of non-integral pointer types to memsets (which
// stores integers).
if (DL.isNonIntegralPointerType(StoredVal->getType()->getScalarType()))
// We can't track ranges involving scalable types.
if (DL.getTypeStoreSize(StoredVal->getType()).isScalable())
// Check to see if this stored value is of the same byte-splattable value.
Value *StoredByte = isBytewiseValue(StoredVal, DL);
if (isa<UndefValue>(ByteVal) && StoredByte)
ByteVal = StoredByte;
if (ByteVal != StoredByte)
// Check to see if this store is to a constant offset from the start ptr.
std::optional<int64_t> Offset =
isPointerOffset(StartPtr, NextStore->getPointerOperand(), DL);
if (!Offset)
Ranges.addStore(*Offset, NextStore);
} else {
auto *MSI = cast<MemSetInst>(BI);
if (MSI->isVolatile() || ByteVal != MSI->getValue() ||
// Check to see if this store is to a constant offset from the start ptr.
std::optional<int64_t> Offset =
isPointerOffset(StartPtr, MSI->getDest(), DL);
if (!Offset)
Ranges.addMemSet(*Offset, MSI);
// If we have no ranges, then we just had a single store with nothing that
// could be merged in. This is a very common case of course.
if (Ranges.empty())
return nullptr;
// If we had at least one store that could be merged in, add the starting
// store as well. We try to avoid this unless there is at least something
// interesting as a small compile-time optimization.
Ranges.addInst(0, StartInst);
// If we create any memsets, we put it right before the first instruction that
// isn't part of the memset block. This ensure that the memset is dominated
// by any addressing instruction needed by the start of the block.
IRBuilder<> Builder(&*BI);
// Now that we have full information about ranges, loop over the ranges and
// emit memset's for anything big enough to be worthwhile.
Instruction *AMemSet = nullptr;
for (const MemsetRange &Range : Ranges) {
if (Range.TheStores.size() == 1) continue;
// If it is profitable to lower this range to memset, do so now.
if (!Range.isProfitableToUseMemset(DL))
// Otherwise, we do want to transform this! Create a new memset.
// Get the starting pointer of the block.
StartPtr = Range.StartPtr;
AMemSet = Builder.CreateMemSet(StartPtr, ByteVal, Range.End - Range.Start,
LLVM_DEBUG(dbgs() << "Replace stores:\n"; for (Instruction *SI
: Range.TheStores) dbgs()
<< *SI << '\n';
dbgs() << "With: " << *AMemSet << '\n');
if (!Range.TheStores.empty())
assert(LastMemDef && MemInsertPoint &&
"Both LastMemDef and MemInsertPoint need to be set");
auto *NewDef =
cast<MemoryDef>(MemInsertPoint->getMemoryInst() == &*BI
? MSSAU->createMemoryAccessBefore(
AMemSet, LastMemDef, MemInsertPoint)
: MSSAU->createMemoryAccessAfter(
AMemSet, LastMemDef, MemInsertPoint));
MSSAU->insertDef(NewDef, /*RenameUses=*/true);
LastMemDef = NewDef;
MemInsertPoint = NewDef;
// Zap all the stores.
for (Instruction *SI : Range.TheStores)
return AMemSet;
// This method try to lift a store instruction before position P.
// It will lift the store and its argument + that anything that
// may alias with these.
// The method returns true if it was successful.
bool MemCpyOptPass::moveUp(StoreInst *SI, Instruction *P, const LoadInst *LI) {
// If the store alias this position, early bail out.
MemoryLocation StoreLoc = MemoryLocation::get(SI);
if (isModOrRefSet(AA->getModRefInfo(P, StoreLoc)))
return false;
// Keep track of the arguments of all instruction we plan to lift
// so we can make sure to lift them as well if appropriate.
DenseSet<Instruction*> Args;
auto AddArg = [&](Value *Arg) {
auto *I = dyn_cast<Instruction>(Arg);
if (I && I->getParent() == SI->getParent()) {
// Cannot hoist user of P above P
if (I == P) return false;
return true;
if (!AddArg(SI->getPointerOperand()))
return false;
// Instruction to lift before P.
SmallVector<Instruction *, 8> ToLift{SI};
// Memory locations of lifted instructions.
SmallVector<MemoryLocation, 8> MemLocs{StoreLoc};
// Lifted calls.
SmallVector<const CallBase *, 8> Calls;
const MemoryLocation LoadLoc = MemoryLocation::get(LI);
for (auto I = --SI->getIterator(), E = P->getIterator(); I != E; --I) {
auto *C = &*I;
// Make sure hoisting does not perform a store that was not guaranteed to
// happen.
if (!isGuaranteedToTransferExecutionToSuccessor(C))
return false;
bool MayAlias = isModOrRefSet(AA->getModRefInfo(C, std::nullopt));
bool NeedLift = false;
if (Args.erase(C))
NeedLift = true;
else if (MayAlias) {
NeedLift = llvm::any_of(MemLocs, [C, this](const MemoryLocation &ML) {
return isModOrRefSet(AA->getModRefInfo(C, ML));
if (!NeedLift)
NeedLift = llvm::any_of(Calls, [C, this](const CallBase *Call) {
return isModOrRefSet(AA->getModRefInfo(C, Call));
if (!NeedLift)
if (MayAlias) {
// Since LI is implicitly moved downwards past the lifted instructions,
// none of them may modify its source.
if (isModSet(AA->getModRefInfo(C, LoadLoc)))
return false;
else if (const auto *Call = dyn_cast<CallBase>(C)) {
// If we can't lift this before P, it's game over.
if (isModOrRefSet(AA->getModRefInfo(P, Call)))
return false;
} else if (isa<LoadInst>(C) || isa<StoreInst>(C) || isa<VAArgInst>(C)) {
// If we can't lift this before P, it's game over.
auto ML = MemoryLocation::get(C);
if (isModOrRefSet(AA->getModRefInfo(P, ML)))
return false;
} else
// We don't know how to lift this instruction.
return false;
for (Value *Op : C->operands())
if (!AddArg(Op))
return false;
// Find MSSA insertion point. Normally P will always have a corresponding
// memory access before which we can insert. However, with non-standard AA
// pipelines, there may be a mismatch between AA and MSSA, in which case we
// will scan for a memory access before P. In either case, we know for sure
// that at least the load will have a memory access.
// TODO: Simplify this once P will be determined by MSSA, in which case the
// discrepancy can no longer occur.
MemoryUseOrDef *MemInsertPoint = nullptr;
if (MemoryUseOrDef *MA = MSSAU->getMemorySSA()->getMemoryAccess(P)) {
MemInsertPoint = cast<MemoryUseOrDef>(--MA->getIterator());
} else {
const Instruction *ConstP = P;
for (const Instruction &I : make_range(++ConstP->getReverseIterator(),
++LI->getReverseIterator())) {
if (MemoryUseOrDef *MA = MSSAU->getMemorySSA()->getMemoryAccess(&I)) {
MemInsertPoint = MA;
// We made it, we need to lift.
for (auto *I : llvm::reverse(ToLift)) {
LLVM_DEBUG(dbgs() << "Lifting " << *I << " before " << *P << "\n");
assert(MemInsertPoint && "Must have found insert point");
if (MemoryUseOrDef *MA = MSSAU->getMemorySSA()->getMemoryAccess(I)) {
MSSAU->moveAfter(MA, MemInsertPoint);
MemInsertPoint = MA;
return true;
bool MemCpyOptPass::processStoreOfLoad(StoreInst *SI, LoadInst *LI,
const DataLayout &DL,
BasicBlock::iterator &BBI) {
if (!LI->isSimple() || !LI->hasOneUse() ||
LI->getParent() != SI->getParent())
return false;
auto *T = LI->getType();
// Don't introduce calls to memcpy/memmove intrinsics out of thin air if
// the corresponding libcalls are not available.
// TODO: We should really distinguish between libcall availability and
// our ability to introduce intrinsics.
if (T->isAggregateType() &&
(EnableMemCpyOptWithoutLibcalls ||
(TLI->has(LibFunc_memcpy) && TLI->has(LibFunc_memmove)))) {
MemoryLocation LoadLoc = MemoryLocation::get(LI);
// We use alias analysis to check if an instruction may store to
// the memory we load from in between the load and the store. If
// such an instruction is found, we try to promote there instead
// of at the store position.
// TODO: Can use MSSA for this.
Instruction *P = SI;
for (auto &I : make_range(++LI->getIterator(), SI->getIterator())) {
if (isModSet(AA->getModRefInfo(&I, LoadLoc))) {
P = &I;
// We found an instruction that may write to the loaded memory.
// We can try to promote at this position instead of the store
// position if nothing aliases the store memory after this and the store
// destination is not in the range.
if (P && P != SI) {
if (!moveUp(SI, P, LI))
P = nullptr;
// If a valid insertion position is found, then we can promote
// the load/store pair to a memcpy.
if (P) {
// If we load from memory that may alias the memory we store to,
// memmove must be used to preserve semantic. If not, memcpy can
// be used. Also, if we load from constant memory, memcpy can be used
// as the constant memory won't be modified.
bool UseMemMove = false;
if (isModSet(AA->getModRefInfo(SI, LoadLoc)))
UseMemMove = true;
uint64_t Size = DL.getTypeStoreSize(T);
IRBuilder<> Builder(P);
Instruction *M;
if (UseMemMove)
M = Builder.CreateMemMove(
SI->getPointerOperand(), SI->getAlign(),
LI->getPointerOperand(), LI->getAlign(), Size);
M = Builder.CreateMemCpy(
SI->getPointerOperand(), SI->getAlign(),
LI->getPointerOperand(), LI->getAlign(), Size);
M->copyMetadata(*SI, LLVMContext::MD_DIAssignID);
LLVM_DEBUG(dbgs() << "Promoting " << *LI << " to " << *SI << " => "
<< *M << "\n");
auto *LastDef =
auto *NewAccess = MSSAU->createMemoryAccessAfter(M, LastDef, LastDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
// Make sure we do not invalidate the iterator.
BBI = M->getIterator();
return true;
// Detect cases where we're performing call slot forwarding, but
// happen to be using a load-store pair to implement it, rather than
// a memcpy.
BatchAAResults BAA(*AA);
auto GetCall = [&]() -> CallInst * {
// We defer this expensive clobber walk until the cheap checks
// have been done on the source inside performCallSlotOptzn.
if (auto *LoadClobber = dyn_cast<MemoryUseOrDef>(
MSSA->getWalker()->getClobberingMemoryAccess(LI, BAA)))
return dyn_cast_or_null<CallInst>(LoadClobber->getMemoryInst());
return nullptr;
bool Changed = performCallSlotOptzn(
LI, SI, SI->getPointerOperand()->stripPointerCasts(),
std::min(SI->getAlign(), LI->getAlign()), BAA, GetCall);
if (Changed) {
return true;
return false;
bool MemCpyOptPass::processStore(StoreInst *SI, BasicBlock::iterator &BBI) {
if (!SI->isSimple()) return false;
// Avoid merging nontemporal stores since the resulting
// memcpy/memset would not be able to preserve the nontemporal hint.
// In theory we could teach how to propagate the !nontemporal metadata to
// memset calls. However, that change would force the backend to
// conservatively expand !nontemporal memset calls back to sequences of
// store instructions (effectively undoing the merging).
if (SI->getMetadata(LLVMContext::MD_nontemporal))
return false;
const DataLayout &DL = SI->getModule()->getDataLayout();
Value *StoredVal = SI->getValueOperand();
// Not all the transforms below are correct for non-integral pointers, bail
// until we've audited the individual pieces.
if (DL.isNonIntegralPointerType(StoredVal->getType()->getScalarType()))
return false;
// Load to store forwarding can be interpreted as memcpy.
if (auto *LI = dyn_cast<LoadInst>(StoredVal))
return processStoreOfLoad(SI, LI, DL, BBI);
// The following code creates memset intrinsics out of thin air. Don't do
// this if the corresponding libfunc is not available.
// TODO: We should really distinguish between libcall availability and
// our ability to introduce intrinsics.
if (!(TLI->has(LibFunc_memset) || EnableMemCpyOptWithoutLibcalls))
return false;
// There are two cases that are interesting for this code to handle: memcpy
// and memset. Right now we only handle memset.
// Ensure that the value being stored is something that can be memset'able a
// byte at a time like "0" or "-1" or any width, as well as things like
// 0xA0A0A0A0 and 0.0.
auto *V = SI->getOperand(0);
if (Value *ByteVal = isBytewiseValue(V, DL)) {
if (Instruction *I = tryMergingIntoMemset(SI, SI->getPointerOperand(),
ByteVal)) {
BBI = I->getIterator(); // Don't invalidate iterator.
return true;
// If we have an aggregate, we try to promote it to memset regardless
// of opportunity for merging as it can expose optimization opportunities
// in subsequent passes.
auto *T = V->getType();
if (T->isAggregateType()) {
uint64_t Size = DL.getTypeStoreSize(T);
IRBuilder<> Builder(SI);
auto *M = Builder.CreateMemSet(SI->getPointerOperand(), ByteVal, Size,
M->copyMetadata(*SI, LLVMContext::MD_DIAssignID);
LLVM_DEBUG(dbgs() << "Promoting " << *SI << " to " << *M << "\n");
// The newly inserted memset is immediately overwritten by the original
// store, so we do not need to rename uses.
auto *StoreDef = cast<MemoryDef>(MSSA->getMemoryAccess(SI));
auto *NewAccess = MSSAU->createMemoryAccessBefore(
M, StoreDef->getDefiningAccess(), StoreDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/false);
// Make sure we do not invalidate the iterator.
BBI = M->getIterator();
return true;
return false;
bool MemCpyOptPass::processMemSet(MemSetInst *MSI, BasicBlock::iterator &BBI) {
// See if there is another memset or store neighboring this memset which
// allows us to widen out the memset to do a single larger store.
if (isa<ConstantInt>(MSI->getLength()) && !MSI->isVolatile())
if (Instruction *I = tryMergingIntoMemset(MSI, MSI->getDest(),
MSI->getValue())) {
BBI = I->getIterator(); // Don't invalidate iterator.
return true;
return false;
/// Takes a memcpy and a call that it depends on,
/// and checks for the possibility of a call slot optimization by having
/// the call write its result directly into the destination of the memcpy.
bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
Instruction *cpyStore, Value *cpyDest,
Value *cpySrc, TypeSize cpySize,
Align cpyDestAlign, BatchAAResults &BAA,
std::function<CallInst *()> GetC) {
// The general transformation to keep in mind is
// call @func(..., src, ...)
// memcpy(dest, src, ...)
// ->
// memcpy(dest, src, ...)
// call @func(..., dest, ...)
// Since moving the memcpy is technically awkward, we additionally check that
// src only holds uninitialized values at the moment of the call, meaning that
// the memcpy can be discarded rather than moved.
// We can't optimize scalable types.
if (cpySize.isScalable())
return false;
// Require that src be an alloca. This simplifies the reasoning considerably.
auto *srcAlloca = dyn_cast<AllocaInst>(cpySrc);
if (!srcAlloca)
return false;
ConstantInt *srcArraySize = dyn_cast<ConstantInt>(srcAlloca->getArraySize());
if (!srcArraySize)
return false;
const DataLayout &DL = cpyLoad->getModule()->getDataLayout();
uint64_t srcSize = DL.getTypeAllocSize(srcAlloca->getAllocatedType()) *
if (cpySize < srcSize)
return false;
CallInst *C = GetC();
if (!C)
return false;
// Lifetime marks shouldn't be operated on.
if (Function *F = C->getCalledFunction())
if (F->isIntrinsic() && F->getIntrinsicID() == Intrinsic::lifetime_start)
return false;
if (C->getParent() != cpyStore->getParent()) {
LLVM_DEBUG(dbgs() << "Call Slot: block local restriction\n");
return false;
MemoryLocation DestLoc = isa<StoreInst>(cpyStore) ?
MemoryLocation::get(cpyStore) :
// Check that nothing touches the dest of the copy between
// the call and the store/memcpy.
Instruction *SkippedLifetimeStart = nullptr;
if (accessedBetween(BAA, DestLoc, MSSA->getMemoryAccess(C),
MSSA->getMemoryAccess(cpyStore), &SkippedLifetimeStart)) {
LLVM_DEBUG(dbgs() << "Call Slot: Dest pointer modified after call\n");
return false;
// If we need to move a lifetime.start above the call, make sure that we can
// actually do so. If the argument is bitcasted for example, we would have to
// move the bitcast as well, which we don't handle.
if (SkippedLifetimeStart) {
auto *LifetimeArg =
if (LifetimeArg && LifetimeArg->getParent() == C->getParent() &&
return false;
// Check that accessing the first srcSize bytes of dest will not cause a
// trap. Otherwise the transform is invalid since it might cause a trap
// to occur earlier than it otherwise would.
if (!isDereferenceableAndAlignedPointer(cpyDest, Align(1), APInt(64, cpySize),
DL, C, AC, DT)) {
LLVM_DEBUG(dbgs() << "Call Slot: Dest pointer not dereferenceable\n");
return false;
// Make sure that nothing can observe cpyDest being written early. There are
// a number of cases to consider:
// 1. cpyDest cannot be accessed between C and cpyStore as a precondition of
// the transform.
// 2. C itself may not access cpyDest (prior to the transform). This is
// checked further below.
// 3. If cpyDest is accessible to the caller of this function (potentially
// captured and not based on an alloca), we need to ensure that we cannot
// unwind between C and cpyStore. This is checked here.
// 4. If cpyDest is potentially captured, there may be accesses to it from
// another thread. In this case, we need to check that cpyStore is
// guaranteed to be executed if C is. As it is a non-atomic access, it
// renders accesses from other threads undefined.
// TODO: This is currently not checked.
if (mayBeVisibleThroughUnwinding(cpyDest, C, cpyStore)) {
LLVM_DEBUG(dbgs() << "Call Slot: Dest may be visible through unwinding\n");
return false;
// Check that dest points to memory that is at least as aligned as src.
Align srcAlign = srcAlloca->getAlign();
bool isDestSufficientlyAligned = srcAlign <= cpyDestAlign;
// If dest is not aligned enough and we can't increase its alignment then
// bail out.
if (!isDestSufficientlyAligned && !isa<AllocaInst>(cpyDest)) {
LLVM_DEBUG(dbgs() << "Call Slot: Dest not sufficiently aligned\n");
return false;
// Check that src is not accessed except via the call and the memcpy. This
// guarantees that it holds only undefined values when passed in (so the final
// memcpy can be dropped), that it is not read or written between the call and
// the memcpy, and that writing beyond the end of it is undefined.
SmallVector<User *, 8> srcUseList(srcAlloca->users());
while (!srcUseList.empty()) {
User *U = srcUseList.pop_back_val();
if (isa<BitCastInst>(U) || isa<AddrSpaceCastInst>(U)) {
append_range(srcUseList, U->users());
if (const auto *G = dyn_cast<GetElementPtrInst>(U)) {
if (!G->hasAllZeroIndices())
return false;
append_range(srcUseList, U->users());
if (const auto *IT = dyn_cast<IntrinsicInst>(U))
if (IT->isLifetimeStartOrEnd())
if (U != C && U != cpyLoad)
return false;
// Check whether src is captured by the called function, in which case there
// may be further indirect uses of src.
bool SrcIsCaptured = any_of(C->args(), [&](Use &U) {
return U->stripPointerCasts() == cpySrc &&
// If src is captured, then check whether there are any potential uses of
// src through the captured pointer before the lifetime of src ends, either
// due to a lifetime.end or a return from the function.
if (SrcIsCaptured) {
// Check that dest is not captured before/at the call. We have already
// checked that src is not captured before it. If either had been captured,
// then the call might be comparing the argument against the captured dest
// or src pointer.
Value *DestObj = getUnderlyingObject(cpyDest);
if (!isIdentifiedFunctionLocal(DestObj) ||
PointerMayBeCapturedBefore(DestObj, /* ReturnCaptures */ true,
/* StoreCaptures */ true, C, DT,
/* IncludeI */ true))
return false;
MemoryLocation SrcLoc =
MemoryLocation(srcAlloca, LocationSize::precise(srcSize));
for (Instruction &I :
make_range(++C->getIterator(), C->getParent()->end())) {
// Lifetime of srcAlloca ends at lifetime.end.
if (auto *II = dyn_cast<IntrinsicInst>(&I)) {
if (II->getIntrinsicID() == Intrinsic::lifetime_end &&
II->getArgOperand(1)->stripPointerCasts() == srcAlloca &&
// Lifetime of srcAlloca ends at return.
if (isa<ReturnInst>(&I))
// Ignore the direct read of src in the load.
if (&I == cpyLoad)
// Check whether this instruction may mod/ref src through the captured
// pointer (we have already any direct mod/refs in the loop above).
// Also bail if we hit a terminator, as we don't want to scan into other
// blocks.
if (isModOrRefSet(BAA.getModRefInfo(&I, SrcLoc)) || I.isTerminator())
return false;
// Since we're changing the parameter to the callsite, we need to make sure
// that what would be the new parameter dominates the callsite.
if (!DT->dominates(cpyDest, C)) {
// Support moving a constant index GEP before the call.
auto *GEP = dyn_cast<GetElementPtrInst>(cpyDest);
if (GEP && GEP->hasAllConstantIndices() &&
DT->dominates(GEP->getPointerOperand(), C))
return false;
// In addition to knowing that the call does not access src in some
// unexpected manner, for example via a global, which we deduce from
// the use analysis, we also need to know that it does not sneakily
// access dest. We rely on AA to figure this out for us.
MemoryLocation DestWithSrcSize(cpyDest, LocationSize::precise(srcSize));
ModRefInfo MR = BAA.getModRefInfo(C, DestWithSrcSize);
// If necessary, perform additional analysis.
if (isModOrRefSet(MR))
MR = BAA.callCapturesBefore(C, DestWithSrcSize, DT);
if (isModOrRefSet(MR))
return false;
// We can't create address space casts here because we don't know if they're
// safe for the target.
if (cpySrc->getType()->getPointerAddressSpace() !=
return false;
for (unsigned ArgI = 0; ArgI < C->arg_size(); ++ArgI)
if (C->getArgOperand(ArgI)->stripPointerCasts() == cpySrc &&
cpySrc->getType()->getPointerAddressSpace() !=
return false;
// All the checks have passed, so do the transformation.
bool changedArgument = false;
for (unsigned ArgI = 0; ArgI < C->arg_size(); ++ArgI)
if (C->getArgOperand(ArgI)->stripPointerCasts() == cpySrc) {
Value *Dest = cpySrc->getType() == cpyDest->getType() ? cpyDest
: CastInst::CreatePointerCast(cpyDest, cpySrc->getType(),
cpyDest->getName(), C);
changedArgument = true;
if (C->getArgOperand(ArgI)->getType() == Dest->getType())
C->setArgOperand(ArgI, Dest);
C->setArgOperand(ArgI, CastInst::CreatePointerCast(
Dest, C->getArgOperand(ArgI)->getType(),
Dest->getName(), C));
if (!changedArgument)
return false;
// If the destination wasn't sufficiently aligned then increase its alignment.
if (!isDestSufficientlyAligned) {
assert(isa<AllocaInst>(cpyDest) && "Can only increase alloca alignment!");
if (SkippedLifetimeStart) {
// Update AA metadata
// FIXME: MD_tbaa_struct and MD_mem_parallel_loop_access should also be
// handled here, but combineMetadata doesn't support them yet
unsigned KnownIDs[] = {LLVMContext::MD_tbaa, LLVMContext::MD_alias_scope,
combineMetadata(C, cpyLoad, KnownIDs, true);
if (cpyLoad != cpyStore)
combineMetadata(C, cpyStore, KnownIDs, true);
return true;
/// We've found that the (upward scanning) memory dependence of memcpy 'M' is
/// the memcpy 'MDep'. Try to simplify M to copy from MDep's input if we can.
bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
MemCpyInst *MDep,
BatchAAResults &BAA) {
// We can only transforms memcpy's where the dest of one is the source of the
// other.
if (M->getSource() != MDep->getDest() || MDep->isVolatile())
return false;
// If dep instruction is reading from our current input, then it is a noop
// transfer and substituting the input won't change this instruction. Just
// ignore the input and let someone else zap MDep. This handles cases like:
// memcpy(a <- a)
// memcpy(b <- a)
if (M->getSource() == MDep->getSource())
return false;
// Second, the length of the memcpy's must be the same, or the preceding one
// must be larger than the following one.
if (MDep->getLength() != M->getLength()) {
auto *MDepLen = dyn_cast<ConstantInt>(MDep->getLength());
auto *MLen = dyn_cast<ConstantInt>(M->getLength());
if (!MDepLen || !MLen || MDepLen->getZExtValue() < MLen->getZExtValue())
return false;
// Verify that the copied-from memory doesn't change in between the two
// transfers. For example, in:
// memcpy(a <- b)
// *b = 42;
// memcpy(c <- a)
// It would be invalid to transform the second memcpy into memcpy(c <- b).
// TODO: If the code between M and MDep is transparent to the destination "c",
// then we could still perform the xform by moving M up to the first memcpy.
// TODO: It would be sufficient to check the MDep source up to the memcpy
// size of M, rather than MDep.
if (writtenBetween(MSSA, BAA, MemoryLocation::getForSource(MDep),
MSSA->getMemoryAccess(MDep), MSSA->getMemoryAccess(M)))
return false;
// If the dest of the second might alias the source of the first, then the
// source and dest might overlap. In addition, if the source of the first
// points to constant memory, they won't overlap by definition. Otherwise, we
// still want to eliminate the intermediate value, but we have to generate a
// memmove instead of memcpy.
bool UseMemMove = false;
if (isModSet(BAA.getModRefInfo(M, MemoryLocation::getForSource(MDep))))
UseMemMove = true;
// If all checks passed, then we can transform M.
LLVM_DEBUG(dbgs() << "MemCpyOptPass: Forwarding memcpy->memcpy src:\n"
<< *MDep << '\n' << *M << '\n');
// TODO: Is this worth it if we're creating a less aligned memcpy? For
// example we could be moving from movaps -> movq on x86.
IRBuilder<> Builder(M);
Instruction *NewM;
if (UseMemMove)
NewM = Builder.CreateMemMove(M->getRawDest(), M->getDestAlign(),
MDep->getRawSource(), MDep->getSourceAlign(),
M->getLength(), M->isVolatile());
else if (isa<MemCpyInlineInst>(M)) {
// llvm.memcpy may be promoted to llvm.memcpy.inline, but the converse is
// never allowed since that would allow the latter to be lowered as a call
// to an external function.
NewM = Builder.CreateMemCpyInline(
M->getRawDest(), M->getDestAlign(), MDep->getRawSource(),
MDep->getSourceAlign(), M->getLength(), M->isVolatile());
} else
NewM = Builder.CreateMemCpy(M->getRawDest(), M->getDestAlign(),
MDep->getRawSource(), MDep->getSourceAlign(),
M->getLength(), M->isVolatile());
NewM->copyMetadata(*M, LLVMContext::MD_DIAssignID);
auto *LastDef = cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(M));
auto *NewAccess = MSSAU->createMemoryAccessAfter(NewM, LastDef, LastDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
// Remove the instruction we're replacing.
return true;
/// We've found that the (upward scanning) memory dependence of \p MemCpy is
/// \p MemSet. Try to simplify \p MemSet to only set the trailing bytes that
/// weren't copied over by \p MemCpy.
/// In other words, transform:
/// \code
/// memset(dst, c, dst_size);
/// memcpy(dst, src, src_size);
/// \endcode
/// into:
/// \code
/// memcpy(dst, src, src_size);
/// memset(dst + src_size, c, dst_size <= src_size ? 0 : dst_size - src_size);
/// \endcode
bool MemCpyOptPass::processMemSetMemCpyDependence(MemCpyInst *MemCpy,
MemSetInst *MemSet,
BatchAAResults &BAA) {
// We can only transform memset/memcpy with the same destination.
if (!BAA.isMustAlias(MemSet->getDest(), MemCpy->getDest()))
return false;
// Check that src and dst of the memcpy aren't the same. While memcpy
// operands cannot partially overlap, exact equality is allowed.
if (isModSet(BAA.getModRefInfo(MemCpy, MemoryLocation::getForSource(MemCpy))))
return false;
// We know that dst up to src_size is not written. We now need to make sure
// that dst up to dst_size is not accessed. (If we did not move the memset,
// checking for reads would be sufficient.)
if (accessedBetween(BAA, MemoryLocation::getForDest(MemSet),
return false;
// Use the same i8* dest as the memcpy, killing the memset dest if different.
Value *Dest = MemCpy->getRawDest();
Value *DestSize = MemSet->getLength();
Value *SrcSize = MemCpy->getLength();
if (mayBeVisibleThroughUnwinding(Dest, MemSet, MemCpy))
return false;
// If the sizes are the same, simply drop the memset instead of generating
// a replacement with zero size.
if (DestSize == SrcSize) {
return true;
// By default, create an unaligned memset.
Align Alignment = Align(1);
// If Dest is aligned, and SrcSize is constant, use the minimum alignment
// of the sum.
const Align DestAlign = std::max(MemSet->getDestAlign().valueOrOne(),
if (DestAlign > 1)
if (auto *SrcSizeC = dyn_cast<ConstantInt>(SrcSize))
Alignment = commonAlignment(DestAlign, SrcSizeC->getZExtValue());
IRBuilder<> Builder(MemCpy);
// If the sizes have different types, zext the smaller one.
if (DestSize->getType() != SrcSize->getType()) {
if (DestSize->getType()->getIntegerBitWidth() >
SrcSize = Builder.CreateZExt(SrcSize, DestSize->getType());
DestSize = Builder.CreateZExt(DestSize, SrcSize->getType());
Value *Ule = Builder.CreateICmpULE(DestSize, SrcSize);
Value *SizeDiff = Builder.CreateSub(DestSize, SrcSize);
Value *MemsetLen = Builder.CreateSelect(
Ule, ConstantInt::getNullValue(DestSize->getType()), SizeDiff);
unsigned DestAS = Dest->getType()->getPointerAddressSpace();
Instruction *NewMemSet = Builder.CreateMemSet(
Builder.CreatePointerCast(Dest, Builder.getInt8PtrTy(DestAS)),
MemSet->getOperand(1), MemsetLen, Alignment);
assert(isa<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(MemCpy)) &&
"MemCpy must be a MemoryDef");
// The new memset is inserted after the memcpy, but it is known that its
// defining access is the memset about to be removed which immediately
// precedes the memcpy.
auto *LastDef =
auto *NewAccess = MSSAU->createMemoryAccessBefore(
NewMemSet, LastDef->getDefiningAccess(), LastDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
return true;
/// Determine whether the instruction has undefined content for the given Size,
/// either because it was freshly alloca'd or started its lifetime.
static bool hasUndefContents(MemorySSA *MSSA, BatchAAResults &AA, Value *V,
MemoryDef *Def, Value *Size) {
if (MSSA->isLiveOnEntryDef(Def))
return isa<AllocaInst>(getUnderlyingObject(V));
if (auto *II = dyn_cast_or_null<IntrinsicInst>(Def->getMemoryInst())) {
if (II->getIntrinsicID() == Intrinsic::lifetime_start) {
auto *LTSize = cast<ConstantInt>(II->getArgOperand(0));
if (auto *CSize = dyn_cast<ConstantInt>(Size)) {
if (AA.isMustAlias(V, II->getArgOperand(1)) &&
LTSize->getZExtValue() >= CSize->getZExtValue())
return true;
// If the lifetime.start covers a whole alloca (as it almost always
// does) and we're querying a pointer based on that alloca, then we know
// the memory is definitely undef, regardless of how exactly we alias.
// The size also doesn't matter, as an out-of-bounds access would be UB.
if (auto *Alloca = dyn_cast<AllocaInst>(getUnderlyingObject(V))) {
if (getUnderlyingObject(II->getArgOperand(1)) == Alloca) {
const DataLayout &DL = Alloca->getModule()->getDataLayout();
if (std::optional<TypeSize> AllocaSize =
if (*AllocaSize == LTSize->getValue())
return true;
return false;
/// Transform memcpy to memset when its source was just memset.
/// In other words, turn:
/// \code
/// memset(dst1, c, dst1_size);
/// memcpy(dst2, dst1, dst2_size);
/// \endcode
/// into:
/// \code
/// memset(dst1, c, dst1_size);
/// memset(dst2, c, dst2_size);
/// \endcode
/// When dst2_size <= dst1_size.
bool MemCpyOptPass::performMemCpyToMemSetOptzn(MemCpyInst *MemCpy,
MemSetInst *MemSet,
BatchAAResults &BAA) {
// Make sure that memcpy(..., memset(...), ...), that is we are memsetting and
// memcpying from the same address. Otherwise it is hard to reason about.
if (!BAA.isMustAlias(MemSet->getRawDest(), MemCpy->getRawSource()))
return false;
Value *MemSetSize = MemSet->getLength();
Value *CopySize = MemCpy->getLength();
if (MemSetSize != CopySize) {
// Make sure the memcpy doesn't read any more than what the memset wrote.
// Don't worry about sizes larger than i64.
// A known memset size is required.
auto *CMemSetSize = dyn_cast<ConstantInt>(MemSetSize);
if (!CMemSetSize)
return false;
// A known memcpy size is also required.
auto *CCopySize = dyn_cast<ConstantInt>(CopySize);
if (!CCopySize)
return false;
if (CCopySize->getZExtValue() > CMemSetSize->getZExtValue()) {
// If the memcpy is larger than the memset, but the memory was undef prior
// to the memset, we can just ignore the tail. Technically we're only
// interested in the bytes from MemSetSize..CopySize here, but as we can't
// easily represent this location, we use the full 0..CopySize range.
MemoryLocation MemCpyLoc = MemoryLocation::getForSource(MemCpy);
bool CanReduceSize = false;
MemoryUseOrDef *MemSetAccess = MSSA->getMemoryAccess(MemSet);
MemoryAccess *Clobber = MSSA->getWalker()->getClobberingMemoryAccess(
MemSetAccess->getDefiningAccess(), MemCpyLoc, BAA);
if (auto *MD = dyn_cast<MemoryDef>(Clobber))
if (hasUndefContents(MSSA, BAA, MemCpy->getSource(), MD, CopySize))
CanReduceSize = true;
if (!CanReduceSize)
return false;
CopySize = MemSetSize;
IRBuilder<> Builder(MemCpy);
Instruction *NewM =
Builder.CreateMemSet(MemCpy->getRawDest(), MemSet->getOperand(1),
CopySize, MemCpy->getDestAlign());
auto *LastDef =
auto *NewAccess = MSSAU->createMemoryAccessAfter(NewM, LastDef, LastDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
return true;
/// Perform simplification of memcpy's. If we have memcpy A
/// which copies X to Y, and memcpy B which copies Y to Z, then we can rewrite
/// B to be a memcpy from X to Z (or potentially a memmove, depending on
/// circumstances). This allows later passes to remove the first memcpy
/// altogether.
bool MemCpyOptPass::processMemCpy(MemCpyInst *M, BasicBlock::iterator &BBI) {
// We can only optimize non-volatile memcpy's.
if (M->isVolatile()) return false;
// If the source and destination of the memcpy are the same, then zap it.
if (M->getSource() == M->getDest()) {
return true;
// If copying from a constant, try to turn the memcpy into a memset.
if (auto *GV = dyn_cast<GlobalVariable>(M->getSource()))
if (GV->isConstant() && GV->hasDefinitiveInitializer())
if (Value *ByteVal = isBytewiseValue(GV->getInitializer(),
M->getModule()->getDataLayout())) {
IRBuilder<> Builder(M);
Instruction *NewM = Builder.CreateMemSet(
M->getRawDest(), ByteVal, M->getLength(), M->getDestAlign(), false);
auto *LastDef =
auto *NewAccess =
MSSAU->createMemoryAccessAfter(NewM, LastDef, LastDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
return true;
BatchAAResults BAA(*AA);
MemoryUseOrDef *MA = MSSA->getMemoryAccess(M);
// FIXME: Not using getClobberingMemoryAccess() here due to PR54682.
MemoryAccess *AnyClobber = MA->getDefiningAccess();
MemoryLocation DestLoc = MemoryLocation::getForDest(M);
const MemoryAccess *DestClobber =
MSSA->getWalker()->getClobberingMemoryAccess(AnyClobber, DestLoc, BAA);
// Try to turn a partially redundant memset + memcpy into
// memcpy + smaller memset. We don't need the memcpy size for this.
// The memcpy most post-dom the memset, so limit this to the same basic
// block. A non-local generalization is likely not worthwhile.
if (auto *MD = dyn_cast<MemoryDef>(DestClobber))
if (auto *MDep = dyn_cast_or_null<MemSetInst>(MD->getMemoryInst()))
if (DestClobber->getBlock() == M->getParent())
if (processMemSetMemCpyDependence(M, MDep, BAA))
return true;
MemoryAccess *SrcClobber = MSSA->getWalker()->getClobberingMemoryAccess(
AnyClobber, MemoryLocation::getForSource(M), BAA);
// There are four possible optimizations we can do for memcpy:
// a) memcpy-memcpy xform which exposes redundance for DSE.
// b) call-memcpy xform for return slot optimization.
// c) memcpy from freshly alloca'd space or space that has just started
// its lifetime copies undefined data, and we can therefore eliminate
// the memcpy in favor of the data that was already at the destination.
// d) memcpy from a just-memset'd source can be turned into memset.
if (auto *MD = dyn_cast<MemoryDef>(SrcClobber)) {
if (Instruction *MI = MD->getMemoryInst()) {
if (auto *CopySize = dyn_cast<ConstantInt>(M->getLength())) {
if (auto *C = dyn_cast<CallInst>(MI)) {
if (performCallSlotOptzn(M, M, M->getDest(), M->getSource(),
M->getDestAlign().valueOrOne(), BAA,
[C]() -> CallInst * { return C; })) {
LLVM_DEBUG(dbgs() << "Performed call slot optimization:\n"
<< " call: " << *C << "\n"
<< " memcpy: " << *M << "\n");
return true;
if (auto *MDep = dyn_cast<MemCpyInst>(MI))
return processMemCpyMemCpyDependence(M, MDep, BAA);
if (auto *MDep = dyn_cast<MemSetInst>(MI)) {
if (performMemCpyToMemSetOptzn(M, MDep, BAA)) {
LLVM_DEBUG(dbgs() << "Converted memcpy to memset\n");
return true;
if (hasUndefContents(MSSA, BAA, M->getSource(), MD, M->getLength())) {
LLVM_DEBUG(dbgs() << "Removed memcpy from undef\n");
return true;
return false;
/// Transforms memmove calls to memcpy calls when the src/dst are guaranteed
/// not to alias.
bool MemCpyOptPass::processMemMove(MemMoveInst *M) {
// See if the source could be modified by this memmove potentially.
if (isModSet(AA->getModRefInfo(M, MemoryLocation::getForSource(M))))
return false;
LLVM_DEBUG(dbgs() << "MemCpyOptPass: Optimizing memmove -> memcpy: " << *M
<< "\n");
// If not, then we know we can transform this.
Type *ArgTys[3] = { M->getRawDest()->getType(),
M->getLength()->getType() };
Intrinsic::memcpy, ArgTys));
// For MemorySSA nothing really changes (except that memcpy may imply stricter
// aliasing guarantees).
return true;
/// This is called on every byval argument in call sites.
bool MemCpyOptPass::processByValArgument(CallBase &CB, unsigned ArgNo) {
const DataLayout &DL = CB.getCaller()->getParent()->getDataLayout();
// Find out what feeds this byval argument.
Value *ByValArg = CB.getArgOperand(ArgNo);
Type *ByValTy = CB.getParamByValType(ArgNo);
TypeSize ByValSize = DL.getTypeAllocSize(ByValTy);
MemoryLocation Loc(ByValArg, LocationSize::precise(ByValSize));
MemoryUseOrDef *CallAccess = MSSA->getMemoryAccess(&CB);
if (!CallAccess)
return false;
MemCpyInst *MDep = nullptr;
BatchAAResults BAA(*AA);
MemoryAccess *Clobber = MSSA->getWalker()->getClobberingMemoryAccess(
CallAccess->getDefiningAccess(), Loc, BAA);
if (auto *MD = dyn_cast<MemoryDef>(Clobber))
MDep = dyn_cast_or_null<MemCpyInst>(MD->getMemoryInst());
// If the byval argument isn't fed by a memcpy, ignore it. If it is fed by
// a memcpy, see if we can byval from the source of the memcpy instead of the
// result.
if (!MDep || MDep->isVolatile() ||
ByValArg->stripPointerCasts() != MDep->getDest())
return false;
// The length of the memcpy must be larger or equal to the size of the byval.
auto *C1 = dyn_cast<ConstantInt>(MDep->getLength());
if (!C1 || !TypeSize::isKnownGE(
TypeSize::getFixed(C1->getValue().getZExtValue()), ByValSize))
return false;
// Get the alignment of the byval. If the call doesn't specify the alignment,
// then it is some target specific value that we can't know.
MaybeAlign ByValAlign = CB.getParamAlign(ArgNo);
if (!ByValAlign) return false;
// If it is greater than the memcpy, then we check to see if we can force the
// source of the memcpy to the alignment we need. If we fail, we bail out.
MaybeAlign MemDepAlign = MDep->getSourceAlign();
if ((!MemDepAlign || *MemDepAlign < *ByValAlign) &&
getOrEnforceKnownAlignment(MDep->getSource(), ByValAlign, DL, &CB, AC,
DT) < *ByValAlign)
return false;
// The address space of the memcpy source must match the byval argument
if (MDep->getSource()->getType()->getPointerAddressSpace() !=
return false;
// Verify that the copied-from memory doesn't change in between the memcpy and
// the byval call.
// memcpy(a <- b)
// *b = 42;
// foo(*a)
// It would be invalid to transform the second memcpy into foo(*b).
if (writtenBetween(MSSA, BAA, MemoryLocation::getForSource(MDep),
MSSA->getMemoryAccess(MDep), MSSA->getMemoryAccess(&CB)))
return false;
Value *TmpCast = MDep->getSource();
if (MDep->getSource()->getType() != ByValArg->getType()) {
BitCastInst *TmpBitCast = new BitCastInst(MDep->getSource(), ByValArg->getType(),
"tmpcast", &CB);
// Set the tmpcast's DebugLoc to MDep's
TmpCast = TmpBitCast;
LLVM_DEBUG(dbgs() << "MemCpyOptPass: Forwarding memcpy to byval:\n"
<< " " << *MDep << "\n"
<< " " << CB << "\n");
// Otherwise we're good! Update the byval argument.
CB.setArgOperand(ArgNo, TmpCast);
return true;
/// Executes one iteration of MemCpyOptPass.
bool MemCpyOptPass::iterateOnFunction(Function &F) {
bool MadeChange = false;
// Walk all instruction in the function.
for (BasicBlock &BB : F) {
// Skip unreachable blocks. For example processStore assumes that an
// instruction in a BB can't be dominated by a later instruction in the
// same BB (which is a scenario that can happen for an unreachable BB that
// has itself as a predecessor).
if (!DT->isReachableFromEntry(&BB))
for (BasicBlock::iterator BI = BB.begin(), BE = BB.end(); BI != BE;) {
// Avoid invalidating the iterator.
Instruction *I = &*BI++;
bool RepeatInstruction = false;
if (auto *SI = dyn_cast<StoreInst>(I))
MadeChange |= processStore(SI, BI);
else if (auto *M = dyn_cast<MemSetInst>(I))
RepeatInstruction = processMemSet(M, BI);
else if (auto *M = dyn_cast<MemCpyInst>(I))
RepeatInstruction = processMemCpy(M, BI);
else if (auto *M = dyn_cast<MemMoveInst>(I))
RepeatInstruction = processMemMove(M);
else if (auto *CB = dyn_cast<CallBase>(I)) {
for (unsigned i = 0, e = CB->arg_size(); i != e; ++i)
if (CB->isByValArgument(i))
MadeChange |= processByValArgument(*CB, i);
// Reprocess the instruction if desired.
if (RepeatInstruction) {
if (BI != BB.begin())
MadeChange = true;
return MadeChange;
PreservedAnalyses MemCpyOptPass::run(Function &F, FunctionAnalysisManager &AM) {
auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
auto *AA = &AM.getResult<AAManager>(F);
auto *AC = &AM.getResult<AssumptionAnalysis>(F);
auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
auto *MSSA = &AM.getResult<MemorySSAAnalysis>(F);
bool MadeChange = runImpl(F, &TLI, AA, AC, DT, &MSSA->getMSSA());
if (!MadeChange)
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
bool MemCpyOptPass::runImpl(Function &F, TargetLibraryInfo *TLI_,
AliasAnalysis *AA_, AssumptionCache *AC_,
DominatorTree *DT_, MemorySSA *MSSA_) {
bool MadeChange = false;
AA = AA_;
AC = AC_;
DT = DT_;
MemorySSAUpdater MSSAU_(MSSA_);
while (true) {
if (!iterateOnFunction(F))
MadeChange = true;
if (VerifyMemorySSA)
return MadeChange;
/// This is the main transformation entry point for a function.
bool MemCpyOptLegacyPass::runOnFunction(Function &F) {
if (skipFunction(F))
return false;
auto *TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
auto *AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto *MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA();
return Impl.runImpl(F, TLI, AA, AC, DT, MSSA);