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[NewGVN] Fix lifetime coercion #141477

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Merged
merged 1 commit into from
May 26, 2025
Merged

[NewGVN] Fix lifetime coercion #141477

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19 changes: 11 additions & 8 deletions llvm/lib/Transforms/Scalar/NewGVN.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1529,23 +1529,26 @@ NewGVN::performSymbolicLoadCoercion(Type *LoadType, Value *LoadPtr,
}
}

if (auto *II = dyn_cast<IntrinsicInst>(DepInst)) {
auto *LifetimePtr = II->getOperand(1);
if (II->getIntrinsicID() == Intrinsic::lifetime_start &&
(LoadPtr == lookupOperandLeader(LifetimePtr) ||
AA->isMustAlias(LoadPtr, LifetimePtr)))
return createConstantExpression(UndefValue::get(LoadType));
}

// All of the below are only true if the loaded pointer is produced
// by the dependent instruction.
if (LoadPtr != lookupOperandLeader(DepInst) &&
DepInst->getType()->isPointerTy() && !AA->isMustAlias(LoadPtr, DepInst))
if (!DepInst->getType()->isPointerTy() ||
(LoadPtr != lookupOperandLeader(DepInst) &&
!AA->isMustAlias(LoadPtr, DepInst)))
return nullptr;
// If this load really doesn't depend on anything, then we must be loading an
// undef value. This can happen when loading for a fresh allocation with no
// intervening stores, for example. Note that this is only true in the case
// that the result of the allocation is pointer equal to the load ptr.
if (isa<AllocaInst>(DepInst)) {
return createConstantExpression(UndefValue::get(LoadType));
}
// If this load occurs either right after a lifetime begin,
// then the loaded value is undefined.
else if (auto *II = dyn_cast<IntrinsicInst>(DepInst)) {
if (II->getIntrinsicID() == Intrinsic::lifetime_start)
return createConstantExpression(UndefValue::get(LoadType));
} else if (auto *InitVal =
getInitialValueOfAllocation(DepInst, TLI, LoadType))
return createConstantExpression(InitVal);
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5 changes: 3 additions & 2 deletions llvm/test/Transforms/NewGVN/coercion-different-ptr.ll
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Expand Up @@ -2,7 +2,7 @@
; RUN: opt -S -passes=newgvn < %s | FileCheck %s


; FIXME: MemorySSA says that load1 depends on the lifetime start.
; MemorySSA says that load1 depends on the lifetime start.
; That's OK since MemorySSA is may-alias; however, NewGVN should
; check whether the lifetime start *actually* defines the loaded pointer
; before simplifying to uninitialized memory.
Expand All @@ -13,7 +13,8 @@ define void @foo(ptr %arg) {
; CHECK-NEXT: [[ALLOCA:%.*]] = alloca i8, align 16
; CHECK-NEXT: call void @llvm.lifetime.start.p0(i64 1, ptr [[ALLOCA]])
; CHECK-NEXT: [[LOAD:%.*]] = load ptr, ptr [[ARG]], align 8
; CHECK-NEXT: [[CALL:%.*]] = call ptr undef(ptr [[ALLOCA]])
; CHECK-NEXT: [[LOAD1:%.*]] = load ptr, ptr [[LOAD]], align 8
; CHECK-NEXT: [[CALL:%.*]] = call ptr [[LOAD1]](ptr [[ALLOCA]])
; CHECK-NEXT: ret void
;
bb:
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