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[InstSimplify] Correctly handle comparison with zero-size allocs #115728

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Merged
merged 1 commit into from
Nov 14, 2024
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[InstSimplify] Correctly handle comparison with zero-size allocs #115728

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4 changes: 2 additions & 2 deletions llvm/lib/Analysis/InstructionSimplify.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2774,8 +2774,8 @@ static Constant *computePointerICmp(CmpInst::Predicate Pred, Value *LHS,
return nullptr;
}(LHS);
Opts.NullIsUnknownSize = F ? NullPointerIsDefined(F) : true;
if (getObjectSize(LHS, LHSSize, DL, TLI, Opts) &&
getObjectSize(RHS, RHSSize, DL, TLI, Opts)) {
if (getObjectSize(LHS, LHSSize, DL, TLI, Opts) && LHSSize != 0 &&
getObjectSize(RHS, RHSSize, DL, TLI, Opts) && RHSSize != 0) {
APInt Dist = LHSOffset - RHSOffset;
if (Dist.isNonNegative() ? Dist.ult(LHSSize) : (-Dist).ult(RHSSize))
return ConstantInt::get(getCompareTy(LHS),
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14 changes: 10 additions & 4 deletions llvm/test/Transforms/InstSimplify/cmp-alloca-offsets.ll
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Expand Up @@ -234,8 +234,9 @@ define i1 @zst_alloca_start() {
; CHECK-LABEL: @zst_alloca_start(
; CHECK-NEXT: [[A:%.*]] = alloca i64, align 8
; CHECK-NEXT: [[A2:%.*]] = alloca {}, align 1
; CHECK-NEXT: [[CMP:%.*]] = icmp eq ptr [[A]], [[A2]]
; CHECK-NEXT: call void @escape(ptr [[A]], ptr [[A2]])
; CHECK-NEXT: ret i1 false
; CHECK-NEXT: ret i1 [[CMP]]
;
%a = alloca i64
%a2 = alloca {}, align 1
Expand All @@ -249,8 +250,10 @@ define i1 @zst_alloca_middle() {
; CHECK-LABEL: @zst_alloca_middle(
; CHECK-NEXT: [[A:%.*]] = alloca i64, align 8
; CHECK-NEXT: [[A2:%.*]] = alloca {}, align 1
; CHECK-NEXT: [[GEP:%.*]] = getelementptr i8, ptr [[A]], i64 4
; CHECK-NEXT: [[CMP:%.*]] = icmp eq ptr [[GEP]], [[A2]]
; CHECK-NEXT: call void @escape(ptr [[A]], ptr [[A2]])
; CHECK-NEXT: ret i1 false
; CHECK-NEXT: ret i1 [[CMP]]
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Can you add an InstCombine test to demonstrate that this case can be folded using KnownBits (??000 != ??100)?

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Oops, this was actually a mistake in the test. The zero-size allocation was supposed to have align 1. It's weird that LLVM infers align 8 for it by default. I adjusted the test.

InstCombine actually fails to handle this (see https://llvm.godbolt.org/z/WMPKdnrMd for variant without allocas) due to a weakness in foldICmpUsingKnownBits. I'll work on a fix.

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Here's the fix: #115874

;
%a = alloca i64
%a2 = alloca {}, align 1
Expand Down Expand Up @@ -282,8 +285,9 @@ define i1 @zst_alloca_end() {
define i1 @zst_global_start() {
; CHECK-LABEL: @zst_global_start(
; CHECK-NEXT: [[A:%.*]] = alloca i64, align 8
; CHECK-NEXT: [[CMP:%.*]] = icmp eq ptr [[A]], @gz
; CHECK-NEXT: call void @escape(ptr [[A]], ptr @gz)
; CHECK-NEXT: ret i1 false
; CHECK-NEXT: ret i1 [[CMP]]
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This case (alloca != global) should be folded.

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Currently LLVM doesn't make assumptions about how different allocations types are positioned relative to each other. We'd have to add LangRef wording for this, to require that allocas and globals allocations cannot be adjacent.

;
%a = alloca i64
%gep = getelementptr i8, ptr %a, i64 0
Expand All @@ -295,8 +299,10 @@ define i1 @zst_global_start() {
define i1 @zst_global_middle() {
; CHECK-LABEL: @zst_global_middle(
; CHECK-NEXT: [[A:%.*]] = alloca i64, align 8
; CHECK-NEXT: [[GEP:%.*]] = getelementptr i8, ptr [[A]], i64 4
; CHECK-NEXT: [[CMP:%.*]] = icmp eq ptr [[GEP]], @gz
; CHECK-NEXT: call void @escape(ptr [[A]], ptr @gz)
; CHECK-NEXT: ret i1 false
; CHECK-NEXT: ret i1 [[CMP]]
;
%a = alloca i64
%gep = getelementptr i8, ptr %a, i64 4
Expand Down
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