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llvm/llvm-project/refs/heads/users/void/array-bounds-accessed/./llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
blob: fc72917017d78d5aea23daecb069eae8bc508537 [file] [edit]
//===- MemCpyOptimizer.cpp - Optimize use of memcpy and friends -----------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// 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/ScopeExit.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/CFG.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/PostDominators.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/ProfDataUtils.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/Local.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <optional>
using namespace llvm;
#define DEBUG_TYPE "memcpyopt"
STATISTIC(NumMemCpyInstr, "Number of memcpy instructions deleted");
STATISTIC(NumMemMoveInstr, "Number of memmove 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");
STATISTIC(NumStackMove, "Number of stack-move 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
static bool overreadUndefContents(MemorySSA *MSSA, MemCpyInst *MemCpy,
MemIntrinsic *MemSrc, BatchAAResults &BAA);
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;
public:
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);
else
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;
R.TheStores.push_back(Inst);
return;
}
// This store overlaps with I, add it.
I->TheStores.push_back(Inst);
// 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)
return;
// 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;
Ranges.erase(NextI);
NextI = I;
}
}
}
//===----------------------------------------------------------------------===//
// MemCpyOptLegacyPass Pass
//===----------------------------------------------------------------------===//
// 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) &&
!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) {
MSSAU->removeMemoryAccess(I);
EEA->removeInstruction(I);
I->eraseFromParent();
}
// 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;
continue;
}
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() ||
any_of(
make_range(std::next(Start->getIterator()), End->getIterator()),
[&AA, Loc](const MemoryAccess &Acc) {
if (isa<MemoryUse>(&Acc))
return false;
Instruction *AccInst =
cast<MemoryUseOrDef>(&Acc)->getMemoryInst();
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->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.
MemoryUseOrDef *MemInsertPoint = nullptr;
for (++BI; !BI->isTerminator(); ++BI) {
auto *CurrentAcc =
cast_or_null<MemoryUseOrDef>(MSSA->getMemoryAccess(&*BI));
if (CurrentAcc)
MemInsertPoint = CurrentAcc;
// Calls that only access inaccessible memory do not block merging
// accessible stores.
if (auto *CB = dyn_cast<CallBase>(BI)) {
if (CB->onlyAccessesInaccessibleMemory())
continue;
}
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())
break;
continue;
}
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()))
break;
// We can't track ranges involving scalable types.
if (DL.getTypeStoreSize(StoredVal->getType()).isScalable())
break;
// Check to see if this stored value is of the same byte-splattable value.
Value *StoredByte = isBytewiseValue(StoredVal, DL);
// We can blindly merge this store into `StartInst` if it's being filled
// with an undef value but we don't because:
// 1. `StartInst` can be removed since it's storing an `undef`.
// 2. The resulting memset will be much larger than it needs to be.
if (ByteVal != StoredByte)
break;
// Check to see if this store is to a constant offset from the start ptr.
std::optional<int64_t> Offset =
NextStore->getPointerOperand()->getPointerOffsetFrom(StartPtr, DL);
if (!Offset)
break;
Ranges.addStore(*Offset, NextStore);
} else {
auto *MSI = cast<MemSetInst>(BI);
if (MSI->isVolatile() || ByteVal != MSI->getValue() ||
!isa<ConstantInt>(MSI->getLength()))
break;
// Check to see if this store is to a constant offset from the start ptr.
std::optional<int64_t> Offset =
MSI->getDest()->getPointerOffsetFrom(StartPtr, DL);
if (!Offset)
break;
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))
continue;
// 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,
Range.Alignment);
AMemSet->mergeDIAssignID(Range.TheStores);
LLVM_DEBUG(dbgs() << "Replace stores:\n"; for (Instruction *SI
: Range.TheStores) dbgs()
<< *SI << '\n';
dbgs() << "With: " << *AMemSet << '\n');
if (!Range.TheStores.empty())
AMemSet->setDebugLoc(Range.TheStores[0]->getDebugLoc());
auto *NewDef = cast<MemoryDef>(
MemInsertPoint->getMemoryInst() == &*BI
? MSSAU->createMemoryAccessBefore(AMemSet, nullptr, MemInsertPoint)
: MSSAU->createMemoryAccessAfter(AMemSet, nullptr, MemInsertPoint));
MSSAU->insertDef(NewDef, /*RenameUses=*/true);
MemInsertPoint = NewDef;
// Zap all the stores.
for (Instruction *SI : Range.TheStores)
eraseInstruction(SI);
++NumMemSetInfer;
}
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;
Args.insert(I);
}
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)
continue;
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;
Calls.push_back(Call);
} 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;
MemLocs.push_back(ML);
} else
// We don't know how to lift this instruction.
return false;
}
ToLift.push_back(C);
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 = MSSA->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 = MSSA->getMemoryAccess(&I)) {
MemInsertPoint = MA;
break;
}
}
}
// We made it, we need to lift.
for (auto *I : llvm::reverse(ToLift)) {
LLVM_DEBUG(dbgs() << "Lifting " << *I << " before " << *P << "\n");
I->moveBefore(P->getIterator());
assert(MemInsertPoint && "Must have found insert point");
if (MemoryUseOrDef *MA = MSSA->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;
BatchAAResults BAA(*AA, EEA);
auto *T = LI->getType();
if (T->isAggregateType()) {
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(BAA.getModRefInfo(&I, LoadLoc))) {
P = &I;
break;
}
}
// If 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 == SI || moveUp(SI, P, LI)) {
// 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;
IRBuilder<> Builder(P);
Value *Size =
Builder.CreateTypeSize(Builder.getInt64Ty(), DL.getTypeStoreSize(T));
Instruction *M;
if (UseMemMove)
M = Builder.CreateMemMove(SI->getPointerOperand(), SI->getAlign(),
LI->getPointerOperand(), LI->getAlign(),
Size);
else
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 = cast<MemoryDef>(MSSA->getMemoryAccess(SI));
auto *NewAccess = MSSAU->createMemoryAccessAfter(M, nullptr, LastDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
eraseInstruction(SI);
eraseInstruction(LI);
++NumMemCpyInstr;
// 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.
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(),
LI->getPointerOperand()->stripPointerCasts(),
DL.getTypeStoreSize(SI->getOperand(0)->getType()),
std::min(SI->getAlign(), LI->getAlign()), BAA, GetCall);
if (Changed) {
eraseInstruction(SI);
eraseInstruction(LI);
++NumMemCpyInstr;
return true;
}
// If this is a load-store pair from a stack slot to a stack slot, we
// might be able to perform the stack-move optimization just as we do for
// memcpys from an alloca to an alloca.
if (performStackMoveOptzn(LI, SI, SI->getPointerOperand(),
LI->getPointerOperand(), DL.getTypeStoreSize(T),
BAA)) {
// Avoid invalidating the iterator.
BBI = SI->getNextNode()->getIterator();
eraseInstruction(SI);
eraseInstruction(LI);
++NumMemCpyInstr;
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->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);
// 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.
Value *V = SI->getOperand(0);
Value *ByteVal = isBytewiseValue(V, DL);
if (!ByteVal)
return false;
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())
return false;
TypeSize Size = DL.getTypeStoreSize(T);
if (Size.isScalable())
return false;
IRBuilder<> Builder(SI);
auto *M = Builder.CreateMemSet(SI->getPointerOperand(), ByteVal, Size,
SI->getAlign());
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, nullptr, StoreDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/false);
eraseInstruction(SI);
NumMemSetInfer++;
// Make sure we do not invalidate the iterator.
BBI = M->getIterator();
return true;
}
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;
const DataLayout &DL = cpyLoad->getDataLayout();
// We can't optimize scalable types or variable-length allocas.
std::optional<TypeSize> SrcAllocaSize = srcAlloca->getAllocationSize(DL);
if (!SrcAllocaSize || SrcAllocaSize->isScalable())
return false;
uint64_t srcSize = SrcAllocaSize->getFixedValue();
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)
: MemoryLocation::getForDest(cast<MemCpyInst>(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 =
dyn_cast<Instruction>(SkippedLifetimeStart->getOperand(0));
if (LifetimeArg && LifetimeArg->getParent() == C->getParent() &&
C->comesBefore(LifetimeArg))
return false;
}
// Check that storing to the first srcSize bytes of dest will not cause a
// trap or data race.
bool ExplicitlyDereferenceableOnly;
if (!isWritableObject(getUnderlyingObject(cpyDest),
ExplicitlyDereferenceableOnly) ||
!isDereferenceablePointer(cpyDest, APInt(64, cpySize),
SimplifyQuery(DL, DT, AC, C))) {
// If the call is guaranteed to return normally (willreturn + nounwind),
// and there are no instructions between the call and the store that might
// trap or throw, execution will reach the store. Since the store would
// trap anyway if the pointer was not dereferenceable, we can forward the
// pointer to the call. Perform optimization only for non-memcpy/memset
// calls, as those are special cased later.
if (!isGuaranteedToTransferExecutionToSuccessor(C->getIterator(),
cpyStore->getIterator())) {
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<AddrSpaceCastInst>(U)) {
append_range(srcUseList, U->users());
continue;
}
if (isa<LifetimeIntrinsic>(U))
continue;
if (U != C && U != cpyLoad) {
LLVM_DEBUG(dbgs() << "Call slot: Source accessed by " << *U << "\n");
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 &&
!C->doesNotCapture(C->getArgOperandNo(&U));
});
// 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, 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(0) == srcAlloca)
break;
}
// Lifetime of srcAlloca ends at return.
if (isa<ReturnInst>(&I))
break;
// Ignore the direct read of src in the load.
if (&I == cpyLoad)
continue;
// 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.
bool NeedMoveGEP = false;
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))
NeedMoveGEP = true;
else
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() != cpyDest->getType())
return false;
for (unsigned ArgI = 0; ArgI < C->arg_size(); ++ArgI)
if (C->getArgOperand(ArgI)->stripPointerCasts() == cpySrc &&
cpySrc->getType() != C->getArgOperand(ArgI)->getType())
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) {
changedArgument = true;
C->setArgOperand(ArgI, cpyDest);
}
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!");
AllocaInst *DestAlloca = cast<AllocaInst>(cpyDest);
DestAlloca->setAlignment(std::max(DestAlloca->getAlign(), srcAlign));
}
if (NeedMoveGEP) {
auto *GEP = dyn_cast<GetElementPtrInst>(cpyDest);
GEP->moveBefore(C->getIterator());
}
if (SkippedLifetimeStart) {
SkippedLifetimeStart->moveBefore(C->getIterator());
MSSAU->moveBefore(MSSA->getMemoryAccess(SkippedLifetimeStart),
MSSA->getMemoryAccess(C));
}
combineAAMetadata(C, cpyLoad);
if (cpyLoad != cpyStore)
combineAAMetadata(C, cpyStore);
++NumCallSlot;
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 optimize non-volatile memcpy's.
if (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)
// This also avoids infinite loops.
if (BAA.isMustAlias(MDep->getDest(), MDep->getSource()))
return false;
int64_t MForwardOffset = 0;
const DataLayout &DL = M->getDataLayout();
// We can only transforms memcpy's where the dest of one is the source of the
// other, or they have an offset in a range.
if (M->getSource() != MDep->getDest()) {
std::optional<int64_t> Offset =
M->getSource()->getPointerOffsetFrom(MDep->getDest(), DL);
if (!Offset || *Offset < 0)
return false;
MForwardOffset = *Offset;
}
Value *CopyLength = M->getLength();
// The length of the memcpy's must be the same, or the preceding one must be
// larger than the following one, or the contents of the overread must be
// undefined bytes of a defined size.
if (MForwardOffset != 0 || MDep->getLength() != CopyLength) {
auto *MDepLen = dyn_cast<ConstantInt>(MDep->getLength());
auto *MLen = dyn_cast<ConstantInt>(CopyLength);
// This could be converted to a runtime test (%CopyLength =
// min(max(0, MDepLen - MForwardOffset), MLen)), but it is
// unclear if that is useful
if (!MDepLen || !MLen)
return false;
if (MDepLen->getZExtValue() < MLen->getZExtValue() + MForwardOffset) {
if (!overreadUndefContents(MSSA, M, MDep, BAA))
return false;
if (MDepLen->getZExtValue() <= (uint64_t)MForwardOffset)
return false; // Should not reach here (there is obviously no aliasing
// with MDep), so just bail in case it had incomplete info
// somehow
CopyLength = ConstantInt::get(CopyLength->getType(),
MDepLen->getZExtValue() - MForwardOffset);
}
}
IRBuilder<> Builder(M);
auto *CopySource = MDep->getSource();
Instruction *NewCopySource = nullptr;
llvm::scope_exit CleanupOnRet([&] {
if (NewCopySource && NewCopySource->use_empty())
// Safety: It's safe here because we will only allocate more instructions
// after finishing all BatchAA queries, but we have to be careful if we
// want to do something like this in another place. Then we'd probably
// have to delay instruction removal until all transforms on an
// instruction finished.
eraseInstruction(NewCopySource);
});
MaybeAlign CopySourceAlign = MDep->getSourceAlign();
auto MCopyLoc = MemoryLocation::getForSource(MDep);
// Truncate the size of the MDep access to just the bytes read
if (MDep->getLength() != CopyLength) {
auto *ConstLength = cast<ConstantInt>(CopyLength);
MCopyLoc = MCopyLoc.getWithNewSize(
LocationSize::precise(ConstLength->getZExtValue()));
}
// When the forwarding offset is greater than 0, we transform
// memcpy(d1 <- s1)
// memcpy(d2 <- d1+o)
// to
// memcpy(d2 <- s1+o)
if (MForwardOffset > 0) {
// The copy destination of `M` maybe can serve as the source of copying.
std::optional<int64_t> MDestOffset =
M->getRawDest()->getPointerOffsetFrom(MDep->getRawSource(), DL);
if (MDestOffset == MForwardOffset)
CopySource = M->getDest();
else {
CopySource = Builder.CreateInBoundsPtrAdd(
CopySource, Builder.getInt64(MForwardOffset));
NewCopySource = dyn_cast<Instruction>(CopySource);
}
// We need to update `MCopyLoc` if an offset exists.
MCopyLoc = MCopyLoc.getWithNewPtr(CopySource);
if (CopySourceAlign)
CopySourceAlign = commonAlignment(*CopySourceAlign, MForwardOffset);
}
// 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.
if (writtenBetween(MSSA, BAA, MCopyLoc, MSSA->getMemoryAccess(MDep),
MSSA->getMemoryAccess(M)))
return false;
// No need to create `memcpy(a <- a)`.
if (BAA.isMustAlias(M->getDest(), CopySource)) {
// Remove the instruction we're replacing.
eraseInstruction(M);
++NumMemCpyInstr;
return true;
}
// 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)))) {
// Don't convert llvm.memcpy.inline into memmove because memmove can be
// lowered as a call, and that is not allowed for llvm.memcpy.inline (and
// there is no inline version of llvm.memmove)
if (M->isForceInlined())
return false;
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.
Instruction *NewM;
if (UseMemMove)
NewM = Builder.CreateMemMove(M->getDest(), M->getDestAlign(), CopySource,
CopySourceAlign, CopyLength, M->isVolatile());
else if (M->isForceInlined())
// 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->getDest(), M->getDestAlign(),
CopySource, CopySourceAlign, CopyLength,
M->isVolatile());
else
NewM = Builder.CreateMemCpy(M->getDest(), M->getDestAlign(), CopySource,
CopySourceAlign, CopyLength, M->isVolatile());
NewM->copyMetadata(*M, LLVMContext::MD_DIAssignID);
assert(isa<MemoryDef>(MSSA->getMemoryAccess(M)));
auto *LastDef = cast<MemoryDef>(MSSA->getMemoryAccess(M));
auto *NewAccess = MSSAU->createMemoryAccessAfter(NewM, nullptr, LastDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
// Remove the instruction we're replacing.
eraseInstruction(M);
++NumMemCpyInstr;
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
/// ...
/// memset(dst + src_size, c, dst_size <= src_size ? 0 : dst_size - src_size);
/// memcpy(dst, src, src_size);
/// \endcode
///
/// The memset is sunk to just before the memcpy to ensure that src_size is
/// present when emitting the simplified memset.
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;
if (MemSet->isVolatile())
return false;
// Don't perform the transform if src_size may be zero. In that case, the
// transform is essentially a complex no-op and may lead to an infinite
// loop if BasicAA is smart enough to understand that dst and dst + src_size
// are still MustAlias after the transform.
Value *SrcSize = MemCpy->getLength();
if (!isKnownNonZero(SrcSize,
SimplifyQuery(MemCpy->getDataLayout(), DT, AC, MemCpy)))
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),
MSSA->getMemoryAccess(MemSet),
MSSA->getMemoryAccess(MemCpy)))
return false;
// Use the same i8* dest as the memcpy, killing the memset dest if different.
Value *Dest = MemCpy->getRawDest();
Value *DestSize = MemSet->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) {
eraseInstruction(MemSet);
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(),
MemCpy->getDestAlign().valueOrOne());
if (DestAlign > 1)
if (auto *SrcSizeC = dyn_cast<ConstantInt>(SrcSize))
Alignment = commonAlignment(DestAlign, SrcSizeC->getZExtValue());
IRBuilder<> Builder(MemCpy);
// Preserve the debug location of the old memset for the code emitted here
// related to the new memset. This is correct according to the rules in
// https://llvm.org/docs/HowToUpdateDebugInfo.html about "when to preserve an
// instruction location", given that we move the memset within the basic
// block.
assert(MemSet->getParent() == MemCpy->getParent() &&
"Preserving debug location based on moving memset within BB.");
Builder.SetCurrentDebugLocation(MemSet->getDebugLoc());
// If the sizes have different types, zext the smaller one.
if (DestSize->getType() != SrcSize->getType()) {
if (DestSize->getType()->getIntegerBitWidth() >
SrcSize->getType()->getIntegerBitWidth())
SrcSize = Builder.CreateZExt(SrcSize, DestSize->getType());
else
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);
// FIXME (#167968): we could explore estimating the branch_weights based on
// value profiling data about the 2 sizes.
if (auto *SI = dyn_cast<SelectInst>(MemsetLen))
setExplicitlyUnknownBranchWeightsIfProfiled(*SI, DEBUG_TYPE);
Instruction *NewMemSet =
Builder.CreateMemSet(Builder.CreatePtrAdd(Dest, SrcSize),
MemSet->getOperand(1), MemsetLen, Alignment);
assert(isa<MemoryDef>(MSSA->getMemoryAccess(MemCpy)) &&
"MemCpy must be a MemoryDef");
// The new memset is inserted before the memcpy, and it is known that the
// memcpy's defining access is the memset about to be removed.
auto *LastDef = cast<MemoryDef>(MSSA->getMemoryAccess(MemCpy));
auto *NewAccess =
MSSAU->createMemoryAccessBefore(NewMemSet, nullptr, LastDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
eraseInstruction(MemSet);
return true;
}
/// Determine whether the pointer V had only undefined content (due to Def),
/// either because it was freshly alloca'd or started its lifetime.
static bool hasUndefContents(MemorySSA *MSSA, BatchAAResults &AA, Value *V,
MemoryDef *Def) {
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)
if (auto *Alloca = dyn_cast<AllocaInst>(getUnderlyingObject(V)))
return II->getArgOperand(0) == Alloca;
return false;
}
// If the memcpy is larger than the previous, but the memory was undef prior to
// that, we can just ignore the tail. Technically we're only interested in the
// bytes from 0..MemSrcOffset and MemSrcLength+MemSrcOffset..CopySize here, but
// as we can't easily represent this location (hasUndefContents uses mustAlias
// which cannot deal with offsets), we use the full 0..CopySize range.
static bool overreadUndefContents(MemorySSA *MSSA, MemCpyInst *MemCpy,
MemIntrinsic *MemSrc, BatchAAResults &BAA) {
MemoryLocation MemCpyLoc = MemoryLocation::getForSource(MemCpy);
MemoryUseOrDef *MemSrcAccess = MSSA->getMemoryAccess(MemSrc);
MemoryAccess *Clobber = MSSA->getWalker()->getClobberingMemoryAccess(
MemSrcAccess->getDefiningAccess(), MemCpyLoc, BAA);
if (auto *MD = dyn_cast<MemoryDef>(Clobber))
if (hasUndefContents(MSSA, BAA, MemCpy->getSource(), MD))
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
bool MemCpyOptPass::performMemCpyToMemSetOptzn(MemCpyInst *MemCpy,
MemSetInst *MemSet,
BatchAAResults &BAA) {
Value *MemSetSize = MemSet->getLength();
Value *CopySize = MemCpy->getLength();
int64_t MOffset = 0;
const DataLayout &DL = MemCpy->getDataLayout();
// We can only transforms memcpy's where the dest of one is the source of the
// other, or they have a known offset.
if (MemCpy->getSource() != MemSet->getDest()) {
std::optional<int64_t> Offset =
MemCpy->getSource()->getPointerOffsetFrom(MemSet->getDest(), DL);
if (!Offset)
return false;
// On positive offsets, the memcpy source is at a offset into the memset'd
// region. On negative offsets, the copy starts at a offset prior to the
// previously memset'd area, namely, we memcpy from a partially initialized
// region.
MOffset = *Offset;
}
if (MOffset != 0 || MemSetSize != CopySize) {
// Make sure the memcpy doesn't read any more than what the memset wrote,
// other than undef. Likewise, the memcpy should not read from an area not
// covered by the memset unless undef bytes. Don't worry about sizes larger
// than i64.
auto *CMemSetSize = dyn_cast<ConstantInt>(MemSetSize);
auto *CCopySize = dyn_cast<ConstantInt>(CopySize);
if (!CMemSetSize || !CCopySize || MOffset < 0 ||
CCopySize->getZExtValue() + MOffset > CMemSetSize->getZExtValue()) {
if (!overreadUndefContents(MSSA, MemCpy, MemSet, BAA))
return false;
if (CMemSetSize && CCopySize) {
uint64_t MemSetSizeVal = CMemSetSize->getZExtValue();
uint64_t MemCpySizeVal = CCopySize->getZExtValue();
uint64_t NewSize;
if (MOffset < 0) {
// Offset from beginning of the initialized region.
uint64_t Offset = -MOffset;
NewSize = MemCpySizeVal <= Offset ? 0 : MemCpySizeVal - Offset;
} else if (MOffset == 0) {
NewSize = MemSetSizeVal;
} else {
NewSize =
MemSetSizeVal <= (uint64_t)MOffset ? 0 : MemSetSizeVal - MOffset;
}
CopySize = ConstantInt::get(CopySize->getType(), NewSize);
} else {
if (MOffset < 0)
return false;
}
}
}
IRBuilder<> Builder(MemCpy);
Value *DestPtr = MemCpy->getRawDest();
MaybeAlign Align = MemCpy->getDestAlign();
if (MOffset < 0) {
DestPtr = Builder.CreatePtrAdd(DestPtr, Builder.getInt64(-MOffset));
if (Align)
Align = commonAlignment(*Align, -MOffset);
}
Instruction *NewM =
Builder.CreateMemSet(DestPtr, MemSet->getOperand(1), CopySize, Align);
auto *LastDef = cast<MemoryDef>(MSSA->getMemoryAccess(MemCpy));
auto *NewAccess = MSSAU->createMemoryAccessAfter(NewM, nullptr, LastDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
return true;
}
// Attempts to optimize the pattern whereby memory is copied from an alloca to
// another alloca, where the two allocas don't have conflicting mod/ref. If
// successful, the two allocas can be merged into one and the transfer can be
// deleted. This pattern is generated frequently in Rust, due to the ubiquity of
// move operations in that language.
//
// Once we determine that the optimization is safe to perform, we replace all
// uses of the destination alloca with the source alloca. We also "shrink wrap"
// the lifetime markers of the single merged alloca to before the first use
// and after the last use. Note that the "shrink wrapping" procedure is a safe
// transformation only because we restrict the scope of this optimization to
// allocas that aren't captured.
bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
Value *DestPtr, Value *SrcPtr,
TypeSize Size, BatchAAResults &BAA) {
LLVM_DEBUG(dbgs() << "Stack Move: Attempting to optimize:\n"
<< *Store << "\n");
AllocaInst *DestAlloca = dyn_cast<AllocaInst>(getUnderlyingObject(DestPtr));
if (!DestAlloca)
return false;
AllocaInst *SrcAlloca = dyn_cast<AllocaInst>(getUnderlyingObject(SrcPtr));
if (!SrcAlloca)
return false;
// Explicitly don't handle degenerate case of a partial copy within one
// alloca. It would always fail the dominator check later anyways, and
// possibly the modref checks also.
if (SrcAlloca == DestAlloca)
return false;
// Make sure the two allocas are in the same address space.
if (SrcAlloca->getAddressSpace() != DestAlloca->getAddressSpace()) {
LLVM_DEBUG(dbgs() << "Stack Move: Address space mismatch\n");
return false;
}
if (!SrcAlloca->isStaticAlloca() || !DestAlloca->isStaticAlloca())
return false;
// Check that copy is full with static size.
const DataLayout &DL = DestAlloca->getDataLayout();
auto DestOffset = DestPtr->getPointerOffsetFrom(DestAlloca, DL);
if (!DestOffset)
return false;
auto SrcOffset = SrcPtr->getPointerOffsetFrom(SrcAlloca, DL);
if (!SrcOffset || *SrcOffset < *DestOffset || *SrcOffset < 0)
return false;
// Offset difference must preserve dest alloca's alignment.
if ((*SrcOffset - *DestOffset) % DestAlloca->getAlign().value() != 0)
return false;
std::optional<TypeSize> SrcSize = SrcAlloca->getAllocationSize(DL);
std::optional<TypeSize> DestSize = DestAlloca->getAllocationSize(DL);
if (!SrcSize || !DestSize)
return false;
if (*SrcSize != *DestSize)
if (!SrcSize->isFixed() || !DestSize->isFixed())
return false;
// Check that copy covers entirety of dest alloca.
if (Size != *DestSize || *DestOffset != 0) {
LLVM_DEBUG(dbgs() << "Stack Move: Destination alloca size mismatch\n");
return false;
}
// Make sure that the copied offset is actually part of the alloca. There
// might be an out-of-bounds copy in dead code.
if (Size.isFixed()) {
if (*SrcOffset + Size > *SrcSize)
return false;
} else if (*SrcOffset != 0) {
// Cannot compute an in-bounds offset on scalable sizes.
return false;
}
// Check if it will be legal to combine allocas without breaking dominator.
bool MoveSrc = !DT->dominates(SrcAlloca, DestAlloca);
if (MoveSrc) {
if (!DT->dominates(DestAlloca, SrcAlloca))
return false;
}
// Check that src and dest are never captured, unescaped allocas. Also
// find the nearest common dominator and postdominator for all users in
// order to shrink wrap the lifetimes, and instructions with noalias metadata
// to remove them.
SmallVector<Instruction *, 4> LifetimeMarkers;
SmallPtrSet<Instruction *, 4> AAMetadataInstrs;
auto CaptureTrackingWithModRef =
[&](Instruction *AI, function_ref<bool(Instruction *)> ModRefCallback,
bool &AddressCaptured) -> bool {
SmallVector<Instruction *, 8> Worklist;
Worklist.push_back(AI);
unsigned MaxUsesToExplore = getDefaultMaxUsesToExploreForCaptureTracking();
Worklist.reserve(MaxUsesToExplore);
SmallPtrSet<const Use *, 20> Visited;
while (!Worklist.empty()) {
Instruction *I = Worklist.pop_back_val();
for (const Use &U : I->uses()) {
auto *UI = cast<Instruction>(U.getUser());
if (Visited.size() >= MaxUsesToExplore) {
LLVM_DEBUG(
dbgs()
<< "Stack Move: Exceeded max uses to see ModRef, bailing\n");
return false;
}
if (!Visited.insert(&U).second)
continue;
UseCaptureInfo CI = DetermineUseCaptureKind(U, AI);
if (capturesAnyProvenance(CI.UseCC))
return false;
AddressCaptured |= capturesAddress(CI.UseCC);
if (UI->mayReadOrWriteMemory()) {
if (UI->isLifetimeStartOrEnd()) {
// We note the locations of these intrinsic calls so that we can
// delete them later if the optimization succeeds, this is safe
// since both llvm.lifetime.start and llvm.lifetime.end intrinsics
// practically fill all the bytes of the alloca with an undefined
// value, although conceptually marked as alive/dead.
LifetimeMarkers.push_back(UI);
continue;
}
AAMetadataInstrs.insert(UI);
if (!ModRefCallback(UI))
return false;
}
if (capturesAnything(CI.ResultCC)) {
Worklist.push_back(UI);
continue;
}
}
}
return true;
};
// Check that dest alloca has no Mod/Ref, from the alloca to the Store. And
// collect modref inst for the reachability check.
ModRefInfo DestModRef = ModRefInfo::NoModRef;
MemoryLocation DestLoc(DestAlloca, LocationSize::precise(*DestSize));
SmallVector<BasicBlock *, 8> ReachabilityWorklist;
auto DestModRefCallback = [&](Instruction *UI) -> bool {
// We don't care about the store itself.
if (UI == Store)
return true;
ModRefInfo Res = BAA.getModRefInfo(UI, DestLoc);
DestModRef |= Res;
if (isModOrRefSet(Res)) {
// Instructions reachability checks.
// FIXME: adding the Instruction version isPotentiallyReachableFromMany on
// lib/Analysis/CFG.cpp (currently only for BasicBlocks) might be helpful.
if (UI->getParent() == Store->getParent()) {
// The same block case is special because it's the only time we're
// looking within a single block to see which instruction comes first.
// Once we start looking at multiple blocks, the first instruction of
// the block is reachable, so we only need to determine reachability
// between whole blocks.
BasicBlock *BB = UI->getParent();
// If A comes before B, then B is definitively reachable from A.
if (UI->comesBefore(Store))
return false;
// If the user's parent block is entry, no predecessor exists.
if (BB->isEntryBlock())
return true;
// Otherwise, continue doing the normal per-BB CFG walk.
ReachabilityWorklist.append(succ_begin(BB), succ_end(BB));
} else {
ReachabilityWorklist.push_back(UI->getParent());
}
}
return true;
};
bool DestAddressCaptured = false;
if (!CaptureTrackingWithModRef(DestAlloca, DestModRefCallback,
DestAddressCaptured))
return false;
// Bailout if Dest may have any ModRef before Store.
if (!ReachabilityWorklist.empty() &&
isPotentiallyReachableFromMany(ReachabilityWorklist, Store->getParent(),
nullptr, DT, nullptr))
return false;
// Check that, from after the Load to the end of the BB,
// - if the dest has any Mod, src has no Ref, and
// - if the dest has any Ref, src has no Mod except full-sized lifetimes
// Where:
// - src is defined as the memory from max(SrcAlloca, SrcPtr minus
// dest_offset) to min(dest_size, SrcSize minus SrcOffset)
// - dest_offset and dest_size could be computed by DestModRefCallback
// to be the bounds of the first and last mod region, and which is at
// least as large as DestOffset to DestSize, and at most as large as
// SrcAlloca to SrcSize.
// - Currently DestOffset==0 and DestSize==Size, so this math is simplified.
MemoryLocation SrcLoc(SrcPtr, LocationSize::precise(Size));
auto SrcModRefCallback = [&](Instruction *UI) -> bool {
// Any ModRef post-dominated by Load doesn't matter, also Load and Store
// themselves can be ignored.
if (PDT->dominates(Load, UI) || UI == Load || UI == Store)
return true;
ModRefInfo Res = BAA.getModRefInfo(UI, SrcLoc);
if ((isModSet(DestModRef) && isRefSet(Res)) ||
(isRefSet(DestModRef) && isModSet(Res)))
return false;
return true;
};
bool SrcAddressCaptured = false;
if (!CaptureTrackingWithModRef(SrcAlloca, SrcModRefCallback,
SrcAddressCaptured))
return false;
// If both the source and destination address are captured, the fact that they
// are no longer two separate allocations may be observed.
if (DestAddressCaptured && SrcAddressCaptured)
return false;
// We can now do the transformation. First move the Src if it was after Dest.
if (MoveSrc)
SrcAlloca->moveBefore(DestAlloca->getIterator());
// Align the allocas appropriately.
SrcAlloca->setAlignment(
std::max(SrcAlloca->getAlign(), DestAlloca->getAlign()));
// Size the allocas appropriately.
if (*SrcSize != *DestSize) {
// Only possible if both sizes are fixed (due to earlier check)
// Set Src to the type and array size of Dest if Dest was larger
if (DestSize->getFixedValue() > SrcSize->getFixedValue()) {
SrcAlloca->setAllocatedType(DestAlloca->getAllocatedType());
SrcAlloca->setOperand(0, DestAlloca->getArraySize());
}
}
// Merge the two allocas.
Value *NewDestPtr = SrcAlloca;
if (*SrcOffset != *DestOffset) {
IRBuilder<> Builder(DestAlloca);
NewDestPtr = Builder.CreateInBoundsPtrAdd(
SrcAlloca, Builder.getInt64(*SrcOffset - *DestOffset));
}
DestAlloca->replaceAllUsesWith(NewDestPtr);
eraseInstruction(DestAlloca);
// Drop metadata on the source alloca.
SrcAlloca->dropUnknownNonDebugMetadata();
// TODO: Reconstruct merged lifetime markers.
// Remove all other lifetime markers. if the original lifetime intrinsics
// exists.
if (!LifetimeMarkers.empty()) {
for (Instruction *I : LifetimeMarkers)
eraseInstruction(I);
}
// As this transformation can cause memory accesses that didn't previously
// alias to begin to alias one another, we remove !alias.scope, !noalias,
// !tbaa and !tbaa_struct metadata from any uses of either alloca.
// This is conservative, but more precision doesn't seem worthwhile
// right now.
for (Instruction *I : AAMetadataInstrs) {
I->setMetadata(LLVMContext::MD_alias_scope, nullptr);
I->setMetadata(LLVMContext::MD_noalias, nullptr);
I->setMetadata(LLVMContext::MD_tbaa, nullptr);
I->setMetadata(LLVMContext::MD_tbaa_struct, nullptr);
}
LLVM_DEBUG(dbgs() << "Stack Move: Performed stack-move optimization\n");
NumStackMove++;
return true;
}
static bool isZeroSize(Value *Size) {
if (auto *I = dyn_cast<Instruction>(Size))
if (auto *Res = simplifyInstruction(I, I->getDataLayout()))
Size = Res;
// Treat undef/poison size like zero.
if (auto *C = dyn_cast<Constant>(Size))
return isa<UndefValue>(C) || C->isNullValue();
return false;
}
/// 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()) {
++BBI;
eraseInstruction(M);
return true;
}
// If the size is zero, remove the memcpy.
if (isZeroSize(M->getLength())) {
++BBI;
eraseInstruction(M);
return true;
}
MemoryUseOrDef *MA = MSSA->getMemoryAccess(M);
if (!MA)
// Degenerate case: memcpy marked as not accessing memory.
return false;
// If copying from a constant, try to turn the memcpy into a memset.
if (auto *GV = dyn_cast<GlobalVariable>(getUnderlyingObject(M->getSource())))
if (GV->isConstant() && GV->hasDefinitiveInitializer())
if (Value *ByteVal = isBytewiseValue(GV->getInitializer(),
M->getDataLayout())) {
IRBuilder<> Builder(M);
Instruction *NewM = Builder.CreateMemSet(
M->getRawDest(), ByteVal, M->getLength(), M->getDestAlign(), false);
auto *LastDef = cast<MemoryDef>(MA);
auto *NewAccess =
MSSAU->createMemoryAccessAfter(NewM, nullptr, LastDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
eraseInstruction(M);
++NumCpyToSet;
return true;
}
BatchAAResults BAA(*AA, EEA);
// 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
// smaller memset + memcpy. We don't need the memcpy size for this.
// The memcpy must 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 five 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.
// e) elimination of memcpy via stack-move optimization.
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(),
TypeSize::getFixed(CopySize->getZExtValue()),
M->getDestAlign().valueOrOne(), BAA,
[C]() -> CallInst * { return C; })) {
LLVM_DEBUG(dbgs() << "Performed call slot optimization:\n"
<< " call: " << *C << "\n"
<< " memcpy: " << *M << "\n");
eraseInstruction(M);
++NumMemCpyInstr;
return true;
}
}
}
if (auto *MDep = dyn_cast<MemCpyInst>(MI))
if (processMemCpyMemCpyDependence(M, MDep, BAA))
return true;
if (auto *MDep = dyn_cast<MemSetInst>(MI)) {
if (performMemCpyToMemSetOptzn(M, MDep, BAA)) {
LLVM_DEBUG(dbgs() << "Converted memcpy to memset\n");
eraseInstruction(M);
++NumCpyToSet;
return true;
}
}
}
if (hasUndefContents(MSSA, BAA, M->getSource(), MD)) {
LLVM_DEBUG(dbgs() << "Removed memcpy from undef\n");
eraseInstruction(M);
++NumMemCpyInstr;
return true;
}
}
// If the transfer is from a stack slot to a stack slot, then we may be able
// to perform the stack-move optimization. See the comments in
// performStackMoveOptzn() for more details.
ConstantInt *Len = dyn_cast<ConstantInt>(M->getLength());
if (Len == nullptr)
return false;
if (performStackMoveOptzn(M, M, M->getDest(), M->getSource(),
TypeSize::getFixed(Len->getZExtValue()), BAA)) {
// Avoid invalidating the iterator.
BBI = M->getNextNode()->getIterator();
eraseInstruction(M);
++NumMemCpyInstr;
return true;
}
return false;
}
/// Memmove calls with overlapping src/dest buffers that come after a memset may
/// be removed.
bool MemCpyOptPass::isMemMoveMemSetDependency(MemMoveInst *M) {
const auto &DL = M->getDataLayout();
MemoryUseOrDef *MemMoveAccess = MSSA->getMemoryAccess(M);
if (!MemMoveAccess)
return false;
// The memmove is of form memmove(x, x + A, B).
MemoryLocation SourceLoc = MemoryLocation::getForSource(M);
auto *MemMoveSourceOp = M->getSource();
auto *Source = dyn_cast<GEPOperator>(MemMoveSourceOp);
if (!Source)
return false;
APInt Offset(DL.getIndexTypeSizeInBits(Source->getType()), 0);
LocationSize MemMoveLocSize = SourceLoc.Size;
if (Source->getPointerOperand() != M->getDest() ||
!MemMoveLocSize.hasValue() ||
!Source->accumulateConstantOffset(DL, Offset) || Offset.isNegative()) {
return false;
}
uint64_t MemMoveSize = MemMoveLocSize.getValue();
LocationSize TotalSize =
LocationSize::precise(Offset.getZExtValue() + MemMoveSize);
MemoryLocation CombinedLoc(M->getDest(), TotalSize);
// The first dominating clobbering MemoryAccess for the combined location
// needs to be a memset.
BatchAAResults BAA(*AA);
MemoryAccess *FirstDef = MemMoveAccess->getDefiningAccess();
auto *DestClobber = dyn_cast<MemoryDef>(
MSSA->getWalker()->getClobberingMemoryAccess(FirstDef, CombinedLoc, BAA));
if (!DestClobber)
return false;
auto *MS = dyn_cast_or_null<MemSetInst>(DestClobber->getMemoryInst());
if (!MS)
return false;
// Memset length must be sufficiently large.
auto *MemSetLength = dyn_cast<ConstantInt>(MS->getLength());
if (!MemSetLength ||
MemSetLength->getZExtValue() < Offset.getZExtValue() + MemMoveSize)
return false;
// The destination buffer must have been memset'd.
if (!BAA.isMustAlias(MS->getDest(), M->getDest()))
return false;
return true;
}
/// Transforms memmove calls to memcpy calls when the src/dst are guaranteed
/// not to alias.
bool MemCpyOptPass::processMemMove(MemMoveInst *M, BasicBlock::iterator &BBI) {
// See if the source could be modified by this memmove potentially.
if (isModSet(AA->getModRefInfo(M, MemoryLocation::getForSource(M)))) {
// On the off-chance the memmove clobbers src with previously memset'd
// bytes, the memmove may be redundant.
if (!M->isVolatile() && isMemMoveMemSetDependency(M)) {
LLVM_DEBUG(dbgs() << "Removed redundant memmove.\n");
++BBI;
eraseInstruction(M);
++NumMemMoveInstr;
return true;
}
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->getRawSource()->getType(),
M->getLength()->getType()};
M->setCalledFunction(Intrinsic::getOrInsertDeclaration(
M->getModule(), Intrinsic::memcpy, ArgTys));
// For MemorySSA nothing really changes (except that memcpy may imply stricter
// aliasing guarantees).
++NumMoveToCpy;
return true;
}
/// This is called on every byval argument in call sites.
bool MemCpyOptPass::processByValArgument(CallBase &CB, unsigned ArgNo) {
const DataLayout &DL = CB.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, EEA);
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 type of the memcpy source must match the byval argument
if (MDep->getSource()->getType() != ByValArg->getType())
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), CallAccess))
return false;
LLVM_DEBUG(dbgs() << "MemCpyOptPass: Forwarding memcpy to byval:\n"
<< " " << *MDep << "\n"
<< " " << CB << "\n");
// Otherwise we're good! Update the byval argument.
combineAAMetadata(&CB, MDep);
CB.setArgOperand(ArgNo, MDep->getSource());
++NumMemCpyInstr;
return true;
}
/// This is called on memcpy dest pointer arguments attributed as immutable
/// during call. Try to use memcpy source directly if all of the following
/// conditions are satisfied.
/// 1. The memcpy dst is neither modified during the call nor captured by the
/// call.
/// 2. The memcpy dst is an alloca with known alignment & size.
/// 2-1. The memcpy length == the alloca size which ensures that the new
/// pointer is dereferenceable for the required range
/// 2-2. The src pointer has alignment >= the alloca alignment or can be
/// enforced so.
/// 3. The memcpy dst and src is not modified between the memcpy and the call.
/// (if MSSA clobber check is safe.)
/// 4. The memcpy src is not modified during the call. (ModRef check shows no
/// Mod.)
bool MemCpyOptPass::processImmutArgument(CallBase &CB, unsigned ArgNo) {
BatchAAResults BAA(*AA, EEA);
Value *ImmutArg = CB.getArgOperand(ArgNo);
// 1. Ensure passed argument is immutable during call.
if (!CB.doesNotCapture(ArgNo))
return false;
// We know that the argument is readonly at this point, but the function
// might still modify the same memory through a different pointer. Exclude
// this either via noalias, or alias analysis.
if (!CB.paramHasAttr(ArgNo, Attribute::NoAlias) &&
isModSet(
BAA.getModRefInfo(&CB, MemoryLocation::getBeforeOrAfter(ImmutArg))))
return false;
const DataLayout &DL = CB.getDataLayout();
// 2. Check that arg is alloca
// TODO: Even if the arg gets back to branches, we can remove memcpy if all
// the alloca alignments can be enforced to source alignment.
auto *AI = dyn_cast<AllocaInst>(ImmutArg->stripPointerCasts());
if (!AI)
return false;
std::optional<TypeSize> AllocaSize = AI->getAllocationSize(DL);
// Can't handle unknown size alloca.
// (e.g. Variable Length Array, Scalable Vector)
if (!AllocaSize || AllocaSize->isScalable())
return false;
MemoryLocation Loc(ImmutArg, LocationSize::precise(*AllocaSize));
MemoryUseOrDef *CallAccess = MSSA->getMemoryAccess(&CB);
if (!CallAccess)
return false;
MemCpyInst *MDep = nullptr;
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 immut argument isn't fed by a memcpy, ignore it. If it is fed by
// a memcpy, check that the arg equals the memcpy dest.
if (!MDep || MDep->isVolatile() || AI != MDep->getDest())
return false;
// The type of the memcpy source must match the immut argument
if (MDep->getSource()->getType() != ImmutArg->getType())
return false;
// 2-1. The length of the memcpy must be equal to the size of the alloca.
auto *MDepLen = dyn_cast<ConstantInt>(MDep->getLength());
if (!MDepLen || AllocaSize != MDepLen->getValue())
return false;
// 2-2. the memcpy source align must be larger than or equal the alloca's
// align. If not so, we check to see if we can force the source of the memcpy
// to the alignment we need. If we fail, we bail out.
Align MemDepAlign = MDep->getSourceAlign().valueOrOne();
Align AllocaAlign = AI->getAlign();
if (MemDepAlign < AllocaAlign &&
getOrEnforceKnownAlignment(MDep->getSource(), AllocaAlign, DL, &CB, AC,
DT) < AllocaAlign)
return false;
// 3. Verify that the source doesn't change in between the memcpy and
// the 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), CallAccess))
return false;
// 4. The memcpy src must not be modified during the call.
if (isModSet(BAA.getModRefInfo(&CB, MemoryLocation::getForSource(MDep))))
return false;
LLVM_DEBUG(dbgs() << "MemCpyOptPass: Forwarding memcpy to Immut src:\n"
<< " " << *MDep << "\n"
<< " " << CB << "\n");
// Otherwise we're good! Update the immut argument.
combineAAMetadata(&CB, MDep);
CB.setArgOperand(ArgNo, MDep->getSource());
++NumMemCpyInstr;
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))
continue;
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, BI);
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);
else if (CB->onlyReadsMemory(i))
MadeChange |= processImmutArgument(*CB, i);
}
}
// Reprocess the instruction if desired.
if (RepeatInstruction) {
if (BI != BB.begin())
--BI;
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 *PDT = &AM.getResult<PostDominatorTreeAnalysis>(F);
auto *MSSA = &AM.getResult<MemorySSAAnalysis>(F);
bool MadeChange = runImpl(F, &TLI, AA, AC, DT, PDT, &MSSA->getMSSA());
if (!MadeChange)
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserveSet<CFGAnalyses>();
PA.preserve<MemorySSAAnalysis>();
return PA;
}
bool MemCpyOptPass::runImpl(Function &F, TargetLibraryInfo *TLI_,
AliasAnalysis *AA_, AssumptionCache *AC_,
DominatorTree *DT_, PostDominatorTree *PDT_,
MemorySSA *MSSA_) {
bool MadeChange = false;
TLI = TLI_;
AA = AA_;
AC = AC_;
DT = DT_;
PDT = PDT_;
MSSA = MSSA_;
MemorySSAUpdater MSSAU_(MSSA_);
MSSAU = &MSSAU_;
EarliestEscapeAnalysis EEA_(*DT);
EEA = &EEA_;
while (true) {
if (!iterateOnFunction(F))
break;
MadeChange = true;
}
if (VerifyMemorySSA)
MSSA_->verifyMemorySSA();
return MadeChange;
}