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[MemCpyOpt] memset->memcpy forwarding with undef tail

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Currently memcpyopt optimizes cases like

    memset(a, byte, N);
    memcpy(b, a, M);

to

    memset(a, byte, N);
    memset(b, byte, M);

if M <= N. Often this allows further simplifications down the line,
which drop the first memset entirely.

This patch extends this optimization for the case where M > N, but we
know that the bytes a[N..M] are undef due to alloca/lifetime.start.

This situation arises relatively often for Rust code, because Rust does
not initialize trailing structure padding and loves to insert redundant
memcpys. This also fixes https://bugs.llvm.org/show_bug.cgi?id=39844.

For the implementation, I'm reusing a bit of code for a similar existing
optimization (direct memcpy of undef). I've also added memset support to
MemDepAnalysis GetLocation -- Instead, getPointerDependencyFrom could be
used, but it seems to make more sense to add this to GetLocation and thus
make the computation cachable.

Differential Revision: https://reviews.llvm.org/D55120

llvm-svn: 348645
This commit is contained in:
Nikita Popov 2018-12-07 21:16:58 +00:00
parent 6fc1e6bf13
commit 31fa5fa3a2
3 changed files with 40 additions and 20 deletions
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6
lib/Analysis/MemoryDependenceAnalysis.cpp
View File

@ -154,6 +154,12 @@ static ModRefInfo GetLocation(const Instruction *Inst, MemoryLocation &Loc,
return ModRefInfo::Mod;
}
if (const MemSetInst *MI = dyn_cast<MemSetInst>(Inst)) {
Loc = MemoryLocation::getForDest(MI);
// Conversatively assume ModRef for volatile memset.
return MI->isVolatile() ? ModRefInfo::ModRef : ModRefInfo::Mod;
}
if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst)) {
switch (II->getIntrinsicID()) {
case Intrinsic::lifetime_start:

46
lib/Transforms/Scalar/MemCpyOptimizer.cpp
View File

@ -1144,6 +1144,21 @@ bool MemCpyOptPass::processMemSetMemCpyDependence(MemCpyInst *MemCpy,
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
@ -1167,12 +1182,23 @@ bool MemCpyOptPass::performMemCpyToMemSetOptzn(MemCpyInst *MemCpy,
if (!AA.isMustAlias(MemSet->getRawDest(), MemCpy->getRawSource()))
return false;
ConstantInt *CopySize = cast<ConstantInt>(MemCpy->getLength());
// 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.
if (!MemSetSize || CopySize->getZExtValue() > MemSetSize->getZExtValue())
return false;
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.
MemDepResult DepInfo = MD->getDependency(MemSet);
if (DepInfo.isDef() && hasUndefContents(DepInfo.getInst(), CopySize))
CopySize = MemSetSize;
else
return false;
}
IRBuilder<> Builder(MemCpy);
Builder.CreateMemSet(MemCpy->getRawDest(), MemSet->getOperand(1),
@ -1252,19 +1278,7 @@ bool MemCpyOptPass::processMemCpy(MemCpyInst *M) {
if (MemCpyInst *MDep = dyn_cast<MemCpyInst>(SrcDepInfo.getInst()))
return processMemCpyMemCpyDependence(M, MDep);
} else if (SrcDepInfo.isDef()) {
Instruction *I = SrcDepInfo.getInst();
bool hasUndefContents = false;
if (isa<AllocaInst>(I)) {
hasUndefContents = true;
} else 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() >= CopySize->getZExtValue())
hasUndefContents = true;
}
if (hasUndefContents) {
if (hasUndefContents(SrcDepInfo.getInst(), CopySize)) {
MD->removeInstruction(M);
M->eraseFromParent();
++NumMemCpyInstr;

8
test/Transforms/MemCpyOpt/memset-memcpy-oversized.ll
View File

@ -12,7 +12,7 @@ define void @test_alloca(i8* %result) {
; CHECK-NEXT: [[A:%.*]] = alloca [[T:%.*]], align 8
; CHECK-NEXT: [[B:%.*]] = bitcast %T* [[A]] to i8*
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* align 8 [[B]], i8 0, i64 12, i1 false)
; CHECK-NEXT: call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[RESULT:%.*]], i8* align 8 [[B]], i64 16, i1 false)
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* [[RESULT:%.*]], i8 0, i64 12, i1 false)
; CHECK-NEXT: ret void
;
%a = alloca %T, align 8
@ -28,7 +28,7 @@ define void @test_alloca_with_lifetimes(i8* %result) {
; CHECK-NEXT: [[B:%.*]] = bitcast %T* [[A]] to i8*
; CHECK-NEXT: call void @llvm.lifetime.start.p0i8(i64 16, i8* [[B]])
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* align 8 [[B]], i8 0, i64 12, i1 false)
; CHECK-NEXT: call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[RESULT:%.*]], i8* align 8 [[B]], i64 16, i1 false)
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* [[RESULT:%.*]], i8 0, i64 12, i1 false)
; CHECK-NEXT: call void @llvm.lifetime.end.p0i8(i64 16, i8* [[B]])
; CHECK-NEXT: ret void
;
@ -46,7 +46,7 @@ define void @test_malloc_with_lifetimes(i8* %result) {
; CHECK-NEXT: [[A:%.*]] = call i8* @malloc(i64 16)
; CHECK-NEXT: call void @llvm.lifetime.start.p0i8(i64 16, i8* [[A]])
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* align 8 [[A]], i8 0, i64 12, i1 false)
; CHECK-NEXT: call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[RESULT:%.*]], i8* align 8 [[A]], i64 16, i1 false)
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* [[RESULT:%.*]], i8 0, i64 12, i1 false)
; CHECK-NEXT: call void @llvm.lifetime.end.p0i8(i64 16, i8* [[A]])
; CHECK-NEXT: call void @free(i8* [[A]])
; CHECK-NEXT: ret void
@ -98,7 +98,7 @@ define void @test_volatile_memset(i8* %result) {
; CHECK-NEXT: [[A:%.*]] = alloca [[T:%.*]], align 8
; CHECK-NEXT: [[B:%.*]] = bitcast %T* [[A]] to i8*
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* align 8 [[B]], i8 0, i64 12, i1 true)
; CHECK-NEXT: call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[RESULT:%.*]], i8* align 8 [[B]], i64 16, i1 false)
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* [[RESULT:%.*]], i8 0, i64 12, i1 false)
; CHECK-NEXT: ret void
;
%a = alloca %T, align 8