blob: 2364748efb0579a06596dc3a5e47d24f6c3c4134 [file] [log] [blame]
//===- 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/None.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/GlobalsModRef.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CallSite.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/GetElementPtrTypeIterator.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/Operator.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.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 <algorithm>
#include <cassert>
#include <cstdint>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "memcpyopt"
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");
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.
unsigned 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 (StoreInst *SI = dyn_cast<StoreInst>(Inst))
addStore(OffsetFromFirst, SI);
addMemSet(OffsetFromFirst, cast<MemSetInst>(Inst));
void addStore(int64_t OffsetFromFirst, StoreInst *SI) {
int64_t StoreSize = DL.getTypeStoreSize(SI->getOperand(0)->getType());
addRange(OffsetFromFirst, StoreSize,
SI->getPointerOperand(), SI->getAlignment(), SI);
void addMemSet(int64_t OffsetFromFirst, MemSetInst *MSI) {
int64_t Size = cast<ConstantInt>(MSI->getLength())->getZExtValue();
addRange(OffsetFromFirst, Size, MSI->getDest(), MSI->getDestAlignment(), MSI);
void addRange(int64_t Start, int64_t Size, Value *Ptr,
unsigned 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,
unsigned 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)
/// 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();
// 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);
for (++BI; !BI->isTerminator(); ++BI) {
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 (StoreInst *NextStore = dyn_cast<StoreInst>(BI)) {
// If this is a store, see if we can merge it in.
if (!NextStore->isSimple()) break;
// Check to see if this stored value is of the same byte-splattable value.
Value *StoredByte = isBytewiseValue(NextStore->getOperand(0), 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.
Optional<int64_t> Offset =
isPointerOffset(StartPtr, NextStore->getPointerOperand(), DL);
if (!Offset)
Ranges.addStore(*Offset, NextStore);
} else {
MemSetInst *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.
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;
// Determine alignment
unsigned Alignment = Range.Alignment;
if (Alignment == 0) {
Type *EltType =
Alignment = DL.getABITypeAlignment(EltType);
AMemSet =
Builder.CreateMemSet(StartPtr, ByteVal, Range.End-Range.Start, Alignment);
LLVM_DEBUG(dbgs() << "Replace stores:\n"; for (Instruction *SI
: Range.TheStores) dbgs()
<< *SI << '\n';
dbgs() << "With: " << *AMemSet << '\n');
if (!Range.TheStores.empty())
// Zap all the stores.
for (Instruction *SI : Range.TheStores) {
return AMemSet;
static unsigned findStoreAlignment(const DataLayout &DL, const StoreInst *SI) {
unsigned StoreAlign = SI->getAlignment();
if (!StoreAlign)
StoreAlign = DL.getABITypeAlignment(SI->getOperand(0)->getType());
return StoreAlign;
static unsigned findLoadAlignment(const DataLayout &DL, const LoadInst *LI) {
unsigned LoadAlign = LI->getAlignment();
if (!LoadAlign)
LoadAlign = DL.getABITypeAlignment(LI->getType());
return LoadAlign;
static unsigned findCommonAlignment(const DataLayout &DL, const StoreInst *SI,
const LoadInst *LI) {
unsigned StoreAlign = findStoreAlignment(DL, SI);
unsigned LoadAlign = findLoadAlignment(DL, LI);
return MinAlign(StoreAlign, LoadAlign);
// 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.
static bool moveUp(AliasAnalysis &AA, 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;
if (auto *Ptr = dyn_cast<Instruction>(SI->getPointerOperand()))
if (Ptr->getParent() == SI->getParent())
// Instruction to lift before P.
SmallVector<Instruction*, 8> ToLift;
// 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;
bool MayAlias = isModOrRefSet(AA.getModRefInfo(C, None));
bool NeedLift = false;
if (Args.erase(C))
NeedLift = true;
else if (MayAlias) {
NeedLift = llvm::any_of(MemLocs, [C, &AA](const MemoryLocation &ML) {
return isModOrRefSet(AA.getModRefInfo(C, ML));
if (!NeedLift)
NeedLift = llvm::any_of(Calls, [C, &AA](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 (unsigned k = 0, e = C->getNumOperands(); k != e; ++k)
if (auto *A = dyn_cast<Instruction>(C->getOperand(k))) {
if (A->getParent() == SI->getParent()) {
// Cannot hoist user of P above P
if(A == P) return false;
// We made it, we need to lift
for (auto *I : llvm::reverse(ToLift)) {
LLVM_DEBUG(dbgs() << "Lifting " << *I << " before " << *P << "\n");
return true;
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();
// Load to store forwarding can be interpreted as memcpy.
if (LoadInst *LI = dyn_cast<LoadInst>(SI->getOperand(0))) {
if (LI->isSimple() && LI->hasOneUse() &&
LI->getParent() == SI->getParent()) {
auto *T = LI->getType();
if (T->isAggregateType()) {
AliasAnalysis &AA = LookupAliasAnalysis();
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.
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 alias the store memory after this and the store
// destination is not in the range.
if (P && P != SI) {
if (!moveUp(AA, 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.
bool UseMemMove = false;
if (!AA.isNoAlias(MemoryLocation::get(SI), LoadLoc))
UseMemMove = true;
uint64_t Size = DL.getTypeStoreSize(T);
IRBuilder<> Builder(P);
Instruction *M;
if (UseMemMove)
M = Builder.CreateMemMove(
SI->getPointerOperand(), findStoreAlignment(DL, SI),
LI->getPointerOperand(), findLoadAlignment(DL, LI), Size);
M = Builder.CreateMemCpy(
SI->getPointerOperand(), findStoreAlignment(DL, SI),
LI->getPointerOperand(), findLoadAlignment(DL, LI), Size);
LLVM_DEBUG(dbgs() << "Promoting " << *LI << " to " << *SI << " => "
<< *M << "\n");
// 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.
MemDepResult ldep = MD->getDependency(LI);
CallInst *C = nullptr;
if (ldep.isClobber() && !isa<MemCpyInst>(ldep.getInst()))
C = dyn_cast<CallInst>(ldep.getInst());
if (C) {
// Check that nothing touches the dest of the "copy" between
// the call and the store.
Value *CpyDest = SI->getPointerOperand()->stripPointerCasts();
bool CpyDestIsLocal = isa<AllocaInst>(CpyDest);
AliasAnalysis &AA = LookupAliasAnalysis();
MemoryLocation StoreLoc = MemoryLocation::get(SI);
for (BasicBlock::iterator I = --SI->getIterator(), E = C->getIterator();
I != E; --I) {
if (isModOrRefSet(AA.getModRefInfo(&*I, StoreLoc))) {
C = nullptr;
// The store to dest may never happen if an exception can be thrown
// between the load and the store.
if (I->mayThrow() && !CpyDestIsLocal) {
C = nullptr;
if (C) {
bool changed = performCallSlotOptzn(
LI, SI->getPointerOperand()->stripPointerCasts(),
findCommonAlignment(DL, SI, LI), C);
if (changed) {
return true;
// 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);
unsigned Align = SI->getAlignment();
if (!Align)
Align = DL.getABITypeAlignment(T);
IRBuilder<> Builder(SI);
auto *M =
Builder.CreateMemSet(SI->getPointerOperand(), ByteVal, Size, Align);
LLVM_DEBUG(dbgs() << "Promoting " << *SI << " to " << *M << "\n");
// 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 *cpy, Value *cpyDest,
Value *cpySrc, uint64_t cpyLen,
unsigned cpyAlign, CallInst *C) {
// 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.
// Lifetime marks shouldn't be operated on.
if (Function *F = C->getCalledFunction())
if (F->isIntrinsic() && F->getIntrinsicID() == Intrinsic::lifetime_start)
return false;
// Deliberately get the source and destination with bitcasts stripped away,
// because we'll need to do type comparisons based on the underlying type.
CallSite CS(C);
// Require that src be an alloca. This simplifies the reasoning considerably.
AllocaInst *srcAlloca = dyn_cast<AllocaInst>(cpySrc);
if (!srcAlloca)
return false;
ConstantInt *srcArraySize = dyn_cast<ConstantInt>(srcAlloca->getArraySize());
if (!srcArraySize)
return false;
const DataLayout &DL = cpy->getModule()->getDataLayout();
uint64_t srcSize = DL.getTypeAllocSize(srcAlloca->getAllocatedType()) *
if (cpyLen < srcSize)
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 (AllocaInst *A = dyn_cast<AllocaInst>(cpyDest)) {
// The destination is an alloca. Check it is larger than srcSize.
ConstantInt *destArraySize = dyn_cast<ConstantInt>(A->getArraySize());
if (!destArraySize)
return false;
uint64_t destSize = DL.getTypeAllocSize(A->getAllocatedType()) *
if (destSize < srcSize)
return false;
} else if (Argument *A = dyn_cast<Argument>(cpyDest)) {
// The store to dest may never happen if the call can throw.
if (C->mayThrow())
return false;
if (A->getDereferenceableBytes() < srcSize) {
// If the destination is an sret parameter then only accesses that are
// outside of the returned struct type can trap.
if (!A->hasStructRetAttr())
return false;
Type *StructTy = cast<PointerType>(A->getType())->getElementType();
if (!StructTy->isSized()) {
// The call may never return and hence the copy-instruction may never
// be executed, and therefore it's not safe to say "the destination
// has at least <cpyLen> bytes, as implied by the copy-instruction",
return false;
uint64_t destSize = DL.getTypeAllocSize(StructTy);
if (destSize < srcSize)
return false;
} else {
return false;
// Check that dest points to memory that is at least as aligned as src.
unsigned srcAlign = srcAlloca->getAlignment();
if (!srcAlign)
srcAlign = DL.getABITypeAlignment(srcAlloca->getAllocatedType());
bool isDestSufficientlyAligned = srcAlign <= cpyAlign;
// If dest is not aligned enough and we can't increase its alignment then
// bail out.
if (!isDestSufficientlyAligned && !isa<AllocaInst>(cpyDest))
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->user_begin(),
while (!srcUseList.empty()) {
User *U = srcUseList.pop_back_val();
if (isa<BitCastInst>(U) || isa<AddrSpaceCastInst>(U)) {
for (User *UU : U->users())
if (GetElementPtrInst *G = dyn_cast<GetElementPtrInst>(U)) {
if (!G->hasAllZeroIndices())
return false;
for (User *UU : U->users())
if (const IntrinsicInst *IT = dyn_cast<IntrinsicInst>(U))
if (IT->isLifetimeStartOrEnd())
if (U != C && U != cpy)
return false;
// Check that src isn't captured by the called function since the
// transformation can cause aliasing issues in that case.
for (unsigned i = 0, e = CS.arg_size(); i != e; ++i)
if (CS.getArgument(i) == cpySrc && !CS.doesNotCapture(i))
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.
DominatorTree &DT = LookupDomTree();
if (Instruction *cpyDestInst = dyn_cast<Instruction>(cpyDest))
if (!DT.dominates(cpyDestInst, 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.
AliasAnalysis &AA = LookupAliasAnalysis();
ModRefInfo MR = AA.getModRefInfo(C, cpyDest, LocationSize::precise(srcSize));
// If necessary, perform additional analysis.
if (isModOrRefSet(MR))
MR = AA.callCapturesBefore(C, cpyDest, LocationSize::precise(srcSize), &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 i = 0; i < CS.arg_size(); ++i)
if (CS.getArgument(i)->stripPointerCasts() == cpySrc &&
cpySrc->getType()->getPointerAddressSpace() !=
return false;
// All the checks have passed, so do the transformation.
bool changedArgument = false;
for (unsigned i = 0; i < CS.arg_size(); ++i)
if (CS.getArgument(i)->stripPointerCasts() == cpySrc) {
Value *Dest = cpySrc->getType() == cpyDest->getType() ? cpyDest
: CastInst::CreatePointerCast(cpyDest, cpySrc->getType(),
cpyDest->getName(), C);
changedArgument = true;
if (CS.getArgument(i)->getType() == Dest->getType())
CS.setArgument(i, Dest);
CS.setArgument(i, CastInst::CreatePointerCast(Dest,
CS.getArgument(i)->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!");
// Drop any cached information about the call, because we may have changed
// its dependence information by changing its parameter.
// 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, cpy, KnownIDs, true);
// Remove the memcpy.
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) {
// 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.
ConstantInt *MDepLen = dyn_cast<ConstantInt>(MDep->getLength());
ConstantInt *MLen = dyn_cast<ConstantInt>(M->getLength());
if (!MDepLen || !MLen || MDepLen->getZExtValue() < MLen->getZExtValue())
return false;
AliasAnalysis &AA = LookupAliasAnalysis();
// 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.
// NOTE: This is conservative, it will stop on any read from the source loc,
// not just the defining memcpy.
MemDepResult SourceDep =
MD->getPointerDependencyFrom(MemoryLocation::getForSource(MDep), false,
M->getIterator(), M->getParent());
if (!SourceDep.isClobber() || SourceDep.getInst() != MDep)
return false;
// If the dest of the second might alias the source of the first, then the
// source and dest might overlap. We still want to eliminate the intermediate
// value, but we have to generate a memmove instead of memcpy.
bool UseMemMove = false;
if (!AA.isNoAlias(MemoryLocation::getForDest(M),
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);
if (UseMemMove)
Builder.CreateMemMove(M->getRawDest(), M->getDestAlignment(),
MDep->getRawSource(), MDep->getSourceAlignment(),
M->getLength(), M->isVolatile());
Builder.CreateMemCpy(M->getRawDest(), M->getDestAlignment(),
MDep->getRawSource(), MDep->getSourceAlignment(),
M->getLength(), M->isVolatile());
// 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) {
// We can only transform memset/memcpy with the same destination.
if (MemSet->getDest() != MemCpy->getDest())
return false;
// Check that there are no other dependencies on the memset destination.
MemDepResult DstDepInfo =
MD->getPointerDependencyFrom(MemoryLocation::getForDest(MemSet), false,
MemCpy->getIterator(), MemCpy->getParent());
if (DstDepInfo.getInst() != 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();
// By default, create an unaligned memset.
unsigned Align = 1;
// If Dest is aligned, and SrcSize is constant, use the minimum alignment
// of the sum.
const unsigned DestAlign =
std::max(MemSet->getDestAlignment(), MemCpy->getDestAlignment());
if (DestAlign > 1)
if (ConstantInt *SrcSizeC = dyn_cast<ConstantInt>(SrcSize))
Align = MinAlign(SrcSizeC->getZExtValue(), DestAlign);
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);
Builder.CreateGEP(Dest->getType()->getPointerElementType(), Dest,
MemSet->getOperand(1), MemsetLen, Align);
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(Instruction *I, ConstantInt *Size) {
if (isa<AllocaInst>(I))
return true;
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
if (II->getIntrinsicID() == Intrinsic::lifetime_start)
if (ConstantInt *LTSize = dyn_cast<ConstantInt>(II->getArgOperand(0)))
if (LTSize->getZExtValue() >= Size->getZExtValue())
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.
/// The \p MemCpy must have a Constant length.
bool MemCpyOptPass::performMemCpyToMemSetOptzn(MemCpyInst *MemCpy,
MemSetInst *MemSet) {
AliasAnalysis &AA = LookupAliasAnalysis();
// Make sure that memcpy(..., memset(...), ...), that is we are memsetting and
// memcpying from the same address. Otherwise it is hard to reason about.
if (!AA.isMustAlias(MemSet->getRawDest(), MemCpy->getRawSource()))
return false;
// A known memset size is required.
ConstantInt *MemSetSize = dyn_cast<ConstantInt>(MemSet->getLength());
if (!MemSetSize)
return false;
// Make sure the memcpy doesn't read any more than what the memset wrote.
// Don't worry about sizes larger than i64.
ConstantInt *CopySize = cast<ConstantInt>(MemCpy->getLength());
if (CopySize->getZExtValue() > MemSetSize->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);
MemDepResult DepInfo = MD->getPointerDependencyFrom(
MemCpyLoc, true, MemSet->getIterator(), MemSet->getParent());
if (DepInfo.isDef() && hasUndefContents(DepInfo.getInst(), CopySize))
CopySize = MemSetSize;
return false;
IRBuilder<> Builder(MemCpy);
Builder.CreateMemSet(MemCpy->getRawDest(), MemSet->getOperand(1),
CopySize, MemCpy->getDestAlignment());
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) {
// 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 false;
// If copying from a constant, try to turn the memcpy into a memset.
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(M->getSource()))
if (GV->isConstant() && GV->hasDefinitiveInitializer())
if (Value *ByteVal = isBytewiseValue(GV->getInitializer(),
M->getModule()->getDataLayout())) {
IRBuilder<> Builder(M);
Builder.CreateMemSet(M->getRawDest(), ByteVal, M->getLength(),
M->getDestAlignment(), false);
return true;
MemDepResult DepInfo = MD->getDependency(M);
// Try to turn a partially redundant memset + memcpy into
// memcpy + smaller memset. We don't need the memcpy size for this.
if (DepInfo.isClobber())
if (MemSetInst *MDep = dyn_cast<MemSetInst>(DepInfo.getInst()))
if (processMemSetMemCpyDependence(M, MDep))
return true;
// The optimizations after this point require the memcpy size.
ConstantInt *CopySize = dyn_cast<ConstantInt>(M->getLength());
if (!CopySize) return false;
// 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 (DepInfo.isClobber()) {
if (CallInst *C = dyn_cast<CallInst>(DepInfo.getInst())) {
// FIXME: Can we pass in either of dest/src alignment here instead
// of conservatively taking the minimum?
unsigned Align = MinAlign(M->getDestAlignment(), M->getSourceAlignment());
if (performCallSlotOptzn(M, M->getDest(), M->getSource(),
CopySize->getZExtValue(), Align,
C)) {
return true;
MemoryLocation SrcLoc = MemoryLocation::getForSource(M);
MemDepResult SrcDepInfo = MD->getPointerDependencyFrom(
SrcLoc, true, M->getIterator(), M->getParent());
if (SrcDepInfo.isClobber()) {
if (MemCpyInst *MDep = dyn_cast<MemCpyInst>(SrcDepInfo.getInst()))
return processMemCpyMemCpyDependence(M, MDep);
} else if (SrcDepInfo.isDef()) {
if (hasUndefContents(SrcDepInfo.getInst(), CopySize)) {
return true;
if (SrcDepInfo.isClobber())
if (MemSetInst *MDep = dyn_cast<MemSetInst>(SrcDepInfo.getInst()))
if (performMemCpyToMemSetOptzn(M, MDep)) {
return true;
return false;
/// Transforms memmove calls to memcpy calls when the src/dst are guaranteed
/// not to alias.
bool MemCpyOptPass::processMemMove(MemMoveInst *M) {
AliasAnalysis &AA = LookupAliasAnalysis();
if (!TLI->has(LibFunc_memmove))
return false;
// See if the pointers alias.
if (!AA.isNoAlias(MemoryLocation::getForDest(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));
// MemDep may have over conservative information about this instruction, just
// conservatively flush it from the cache.
return true;
/// This is called on every byval argument in call sites.
bool MemCpyOptPass::processByValArgument(CallSite CS, unsigned ArgNo) {
const DataLayout &DL = CS.getCaller()->getParent()->getDataLayout();
// Find out what feeds this byval argument.
Value *ByValArg = CS.getArgument(ArgNo);
Type *ByValTy = cast<PointerType>(ByValArg->getType())->getElementType();
uint64_t ByValSize = DL.getTypeAllocSize(ByValTy);
MemDepResult DepInfo = MD->getPointerDependencyFrom(
MemoryLocation(ByValArg, LocationSize::precise(ByValSize)), true,
CS.getInstruction()->getIterator(), CS.getInstruction()->getParent());
if (!DepInfo.isClobber())
return false;
// 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.
MemCpyInst *MDep = dyn_cast<MemCpyInst>(DepInfo.getInst());
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.
ConstantInt *C1 = dyn_cast<ConstantInt>(MDep->getLength());
if (!C1 || 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.
unsigned ByValAlign = CS.getParamAlignment(ArgNo);
if (ByValAlign == 0) 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.
AssumptionCache &AC = LookupAssumptionCache();
DominatorTree &DT = LookupDomTree();
if (MDep->getSourceAlignment() < ByValAlign &&
getOrEnforceKnownAlignment(MDep->getSource(), ByValAlign, DL,
CS.getInstruction(), &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).
// NOTE: This is conservative, it will stop on any read from the source loc,
// not just the defining memcpy.
MemDepResult SourceDep = MD->getPointerDependencyFrom(
MemoryLocation::getForSource(MDep), false,
CS.getInstruction()->getIterator(), MDep->getParent());
if (!SourceDep.isClobber() || SourceDep.getInst() != MDep)
return false;
Value *TmpCast = MDep->getSource();
if (MDep->getSource()->getType() != ByValArg->getType())
TmpCast = new BitCastInst(MDep->getSource(), ByValArg->getType(),
"tmpcast", CS.getInstruction());
LLVM_DEBUG(dbgs() << "MemCpyOptPass: Forwarding memcpy to byval:\n"
<< " " << *MDep << "\n"
<< " " << *CS.getInstruction() << "\n");
// Otherwise we're good! Update the byval argument.
CS.setArgument(ArgNo, TmpCast);
return true;
/// Executes one iteration of MemCpyOptPass.
bool MemCpyOptPass::iterateOnFunction(Function &F) {
bool MadeChange = false;
DominatorTree &DT = LookupDomTree();
// 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 (StoreInst *SI = dyn_cast<StoreInst>(I))
MadeChange |= processStore(SI, BI);
else if (MemSetInst *M = dyn_cast<MemSetInst>(I))
RepeatInstruction = processMemSet(M, BI);
else if (MemCpyInst *M = dyn_cast<MemCpyInst>(I))
RepeatInstruction = processMemCpy(M);
else if (MemMoveInst *M = dyn_cast<MemMoveInst>(I))
RepeatInstruction = processMemMove(M);
else if (auto CS = CallSite(I)) {
for (unsigned i = 0, e = CS.arg_size(); i != e; ++i)
if (CS.isByValArgument(i))
MadeChange |= processByValArgument(CS, i);
// Reprocess the instruction if desired.
if (RepeatInstruction) {
if (BI != BB.begin())
MadeChange = true;
return MadeChange;
PreservedAnalyses MemCpyOptPass::run(Function &F, FunctionAnalysisManager &AM) {
auto &MD = AM.getResult<MemoryDependenceAnalysis>(F);
auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
auto LookupAliasAnalysis = [&]() -> AliasAnalysis & {
return AM.getResult<AAManager>(F);
auto LookupAssumptionCache = [&]() -> AssumptionCache & {
return AM.getResult<AssumptionAnalysis>(F);
auto LookupDomTree = [&]() -> DominatorTree & {
return AM.getResult<DominatorTreeAnalysis>(F);
bool MadeChange = runImpl(F, &MD, &TLI, LookupAliasAnalysis,
LookupAssumptionCache, LookupDomTree);
if (!MadeChange)
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
bool MemCpyOptPass::runImpl(
Function &F, MemoryDependenceResults *MD_, TargetLibraryInfo *TLI_,
std::function<AliasAnalysis &()> LookupAliasAnalysis_,
std::function<AssumptionCache &()> LookupAssumptionCache_,
std::function<DominatorTree &()> LookupDomTree_) {
bool MadeChange = false;
MD = MD_;
LookupAliasAnalysis = std::move(LookupAliasAnalysis_);
LookupAssumptionCache = std::move(LookupAssumptionCache_);
LookupDomTree = std::move(LookupDomTree_);
// If we don't have at least memset and memcpy, there is little point of doing
// anything here. These are required by a freestanding implementation, so if
// even they are disabled, there is no point in trying hard.
if (!TLI->has(LibFunc_memset) || !TLI->has(LibFunc_memcpy))
return false;
while (true) {
if (!iterateOnFunction(F))
MadeChange = true;
MD = nullptr;
return MadeChange;
/// This is the main transformation entry point for a function.
bool MemCpyOptLegacyPass::runOnFunction(Function &F) {
if (skipFunction(F))
return false;
auto *MD = &getAnalysis<MemoryDependenceWrapperPass>().getMemDep();
auto *TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
auto LookupAliasAnalysis = [this]() -> AliasAnalysis & {
return getAnalysis<AAResultsWrapperPass>().getAAResults();
auto LookupAssumptionCache = [this, &F]() -> AssumptionCache & {
return getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
auto LookupDomTree = [this]() -> DominatorTree & {
return getAnalysis<DominatorTreeWrapperPass>().getDomTree();
return Impl.runImpl(F, MD, TLI, LookupAliasAnalysis, LookupAssumptionCache,