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Reapply "[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.

The previous version of this patch did not perform dependency checking
properly: While the dependency is checked at the position of the memset,
the used size must be that of the memcpy. Previously the size of the
memset was used, which missed modification in the region
MemSetSize..CopySize, resulting in miscompiles. The added tests cover
variations of this issue.

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

llvm-svn: 349078
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@nikic
nikic committed Dec 13, 2018
1 parent c2e2dd4 commit dc73a6e
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Showing 2 changed files with 99 additions and 20 deletions.
50 changes: 34 additions & 16 deletions llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -1167,12 +1182,27 @@ 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. 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;
else
return false;
}

IRBuilder<> Builder(MemCpy);
Builder.CreateMemSet(MemCpy->getRawDest(), MemSet->getOperand(1),
Expand Down Expand Up @@ -1252,19 +1282,7 @@ 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;
Expand Down
69 changes: 65 additions & 4 deletions llvm/test/Transforms/MemCpyOpt/memset-memcpy-oversized.ll
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -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
;
Expand All @@ -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
Expand Down Expand Up @@ -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
Expand Down Expand Up @@ -142,6 +142,67 @@ define void @test_write_between(i8* %result) {
ret void
}

; A write prior to the memset, which is part of the memset region.
; We could optimize this, but currently don't, because the used memory location is imprecise.
define void @test_write_before_memset_in_memset_region(i8* %result) {
; CHECK-LABEL: @test_write_before_memset_in_memset_region(
; CHECK-NEXT: [[A:%.*]] = alloca [[T:%.*]], align 8
; CHECK-NEXT: [[B:%.*]] = bitcast %T* [[A]] to i8*
; CHECK-NEXT: store i8 -1, i8* [[B]]
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* align 8 [[B]], i8 0, i64 8, i1 false)
; CHECK-NEXT: call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[RESULT:%.*]], i8* align 8 [[B]], i64 16, i1 false)
; CHECK-NEXT: ret void
;
%a = alloca %T, align 8
%b = bitcast %T* %a to i8*
store i8 -1, i8* %b
call void @llvm.memset.p0i8.i64(i8* align 8 %b, i8 0, i64 8, i1 false)
call void @llvm.memcpy.p0i8.p0i8.i64(i8* %result, i8* align 8 %b, i64 16, i1 false)
ret void
}

; A write prior to the memset, which is part of the memcpy (but not memset) region.
; This cannot be optimized.
define void @test_write_before_memset_in_memcpy_region(i8* %result) {
; CHECK-LABEL: @test_write_before_memset_in_memcpy_region(
; CHECK-NEXT: [[A:%.*]] = alloca [[T:%.*]], align 8
; CHECK-NEXT: [[B:%.*]] = bitcast %T* [[A]] to i8*
; CHECK-NEXT: [[C:%.*]] = getelementptr inbounds [[T]], %T* [[A]], i64 0, i32 2
; CHECK-NEXT: store i32 -1, i32* [[C]]
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* align 8 [[B]], i8 0, i64 8, i1 false)
; CHECK-NEXT: call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[RESULT:%.*]], i8* align 8 [[B]], i64 16, i1 false)
; CHECK-NEXT: ret void
;
%a = alloca %T, align 8
%b = bitcast %T* %a to i8*
%c = getelementptr inbounds %T, %T* %a, i64 0, i32 2
store i32 -1, i32* %c
call void @llvm.memset.p0i8.i64(i8* align 8 %b, i8 0, i64 8, i1 false)
call void @llvm.memcpy.p0i8.p0i8.i64(i8* %result, i8* align 8 %b, i64 16, i1 false)
ret void
}

; A write prior to the memset, which is part of both the memset and memcpy regions.
; This cannot be optimized.
define void @test_write_before_memset_in_both_regions(i8* %result) {
; CHECK-LABEL: @test_write_before_memset_in_both_regions(
; CHECK-NEXT: [[A:%.*]] = alloca [[T:%.*]], align 8
; CHECK-NEXT: [[B:%.*]] = bitcast %T* [[A]] to i8*
; CHECK-NEXT: [[C:%.*]] = getelementptr inbounds [[T]], %T* [[A]], i64 0, i32 1
; CHECK-NEXT: store i32 -1, i32* [[C]]
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* align 8 [[B]], i8 0, i64 10, i1 false)
; CHECK-NEXT: call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[RESULT:%.*]], i8* align 8 [[B]], i64 16, i1 false)
; CHECK-NEXT: ret void
;
%a = alloca %T, align 8
%b = bitcast %T* %a to i8*
%c = getelementptr inbounds %T, %T* %a, i64 0, i32 1
store i32 -1, i32* %c
call void @llvm.memset.p0i8.i64(i8* align 8 %b, i8 0, i64 10, i1 false)
call void @llvm.memcpy.p0i8.p0i8.i64(i8* %result, i8* align 8 %b, i64 16, i1 false)
ret void
}

declare i8* @malloc(i64)
declare void @free(i8*)

Expand Down

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