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[Mem2Reg] Generate non-terminator unreachable for !noundef undef #96639

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Merged
merged 1 commit into from
Jun 25, 2024
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[Mem2Reg] Generate non-terminator unreachable for !noundef undef #96639

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[Mem2Reg] Generate non-terminator unreachable for !noundef undef 
When performing a load from uninitialized memory using !noundef,
insert a non-terminator unreachable instruction, which will be
converted to a proper unreachable by SimplifyCFG. This way we
retain the fact that UB occurred on this code path.
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@nikic
nikic committed Jun 25, 2024
commit 25b7dec4d24d85e27b4e68f8da907834daf7a552
9 changes: 9 additions & 0 deletions llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -453,6 +453,15 @@ static void addAssumeNonNull(AssumptionCache *AC, LoadInst *LI) {
static void convertMetadataToAssumes(LoadInst *LI, Value *Val,
const DataLayout &DL, AssumptionCache *AC,
const DominatorTree *DT) {
if (isa<UndefValue>(Val) && LI->hasMetadata(LLVMContext::MD_noundef)) {
// Insert non-terminator unreachable.
LLVMContext &Ctx = LI->getContext();
new StoreInst(ConstantInt::getTrue(Ctx),
PoisonValue::get(PointerType::getUnqual(Ctx)),
/*isVolatile=*/false, Align(1), LI);
return;
}

// If the load was marked as nonnull we don't want to lose that information
// when we erase this Load. So we preserve it with an assume. As !nonnull
// returns poison while assume violations are immediate undefined behavior,
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12 changes: 6 additions & 6 deletions llvm/test/Transforms/Mem2Reg/preserve-nonnull-load-metadata.ll
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Expand Up @@ -143,6 +143,7 @@ fin:
define ptr @no_store_single_load_noundef() {
; CHECK-LABEL: @no_store_single_load_noundef(
; CHECK-NEXT: entry:
; CHECK-NEXT: store i1 true, ptr poison, align 1
; CHECK-NEXT: ret ptr undef
;
entry:
Expand All @@ -156,8 +157,10 @@ define ptr @no_store_multiple_loads_noundef(i1 %c) {
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 [[C:%.*]], label [[IF:%.*]], label [[ELSE:%.*]]
; CHECK: if:
; CHECK-NEXT: store i1 true, ptr poison, align 1
; CHECK-NEXT: ret ptr undef
; CHECK: else:
; CHECK-NEXT: store i1 true, ptr poison, align 1
; CHECK-NEXT: ret ptr undef
;
entry:
Expand All @@ -176,8 +179,7 @@ if:
define ptr @no_store_single_load_nonnull_noundef() {
; CHECK-LABEL: @no_store_single_load_nonnull_noundef(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[TMP0:%.*]] = icmp ne ptr undef, null
; CHECK-NEXT: call void @llvm.assume(i1 [[TMP0]])
; CHECK-NEXT: store i1 true, ptr poison, align 1
; CHECK-NEXT: ret ptr undef
;
entry:
Expand All @@ -191,12 +193,10 @@ define ptr @no_store_multiple_loads_nonnull_noundef(i1 %c) {
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 [[C:%.*]], label [[IF:%.*]], label [[ELSE:%.*]]
; CHECK: if:
; CHECK-NEXT: [[TMP0:%.*]] = icmp ne ptr undef, null
; CHECK-NEXT: call void @llvm.assume(i1 [[TMP0]])
; CHECK-NEXT: store i1 true, ptr poison, align 1
; CHECK-NEXT: ret ptr undef
; CHECK: else:
; CHECK-NEXT: [[TMP1:%.*]] = icmp ne ptr undef, null
; CHECK-NEXT: call void @llvm.assume(i1 [[TMP1]])
; CHECK-NEXT: store i1 true, ptr poison, align 1
; CHECK-NEXT: ret ptr undef
;
entry:
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