[llvm] [NewGVN] Fix lifetime coercion (PR #141477)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: None (ManuelJBrito)

<details>
<summary>Changes</summary>

Before commit 14dee0a, NewGVN would not miscompile the function foo, because `isMustAlias` would return false for non-pointers, particularly for `lifetime.start`. We need to check whether the loaded pointer is defined by`lifetime.start` in order to safely simplify the load to uninitialized memory.

For `getInitialValueOfAllocation`, the behavior depends on the allocation function. Therefore, we take a conservative approach: we check whether it's a pointer type. If it is, then—based on the earlier check—we know that the allocation function defines the loaded pointer.



---
Full diff: https://github.com/llvm/llvm-project/pull/141477.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/NewGVN.cpp (+11-3) 
- (modified) llvm/test/Transforms/NewGVN/coercion-different-ptr.ll (+2-1) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/NewGVN.cpp b/llvm/lib/Transforms/Scalar/NewGVN.cpp
index 3eb118908959f..8f53b511ba2d2 100644
--- a/llvm/lib/Transforms/Scalar/NewGVN.cpp
+++ b/llvm/lib/Transforms/Scalar/NewGVN.cpp
@@ -1531,6 +1531,8 @@ NewGVN::performSymbolicLoadCoercion(Type *LoadType, Value *LoadPtr,
 
   // All of the below are only true if the loaded pointer is produced
   // by the dependent instruction.
+  // The following only checks the cases where DepInst returns a pointer.
+  // For lifetime begin and similar instructions we need to check the operand.
   if (LoadPtr != lookupOperandLeader(DepInst) &&
       DepInst->getType()->isPointerTy() && !AA->isMustAlias(LoadPtr, DepInst))
     return nullptr;
@@ -1543,11 +1545,17 @@ NewGVN::performSymbolicLoadCoercion(Type *LoadType, Value *LoadPtr,
   }
   // If this load occurs either right after a lifetime begin,
   // then the loaded value is undefined.
+  // Make sure the loaded pointer is defined by the lifetime begin.
   else if (auto *II = dyn_cast<IntrinsicInst>(DepInst)) {
-    if (II->getIntrinsicID() == Intrinsic::lifetime_start)
+    auto *LifetimePtr = II->getOperand(1);
+    if (II->getIntrinsicID() == Intrinsic::lifetime_start &&
+        (LoadPtr == lookupOperandLeader(LifetimePtr) ||
+         AA->isMustAlias(LoadPtr, LifetimePtr)))
       return createConstantExpression(UndefValue::get(LoadType));
-  } else if (auto *InitVal =
-                 getInitialValueOfAllocation(DepInst, TLI, LoadType))
+  } else if (DepInst->getType()->isPointerTy())
+    // Only use the allocation value if we known for sure that the loaded ptr is
+    // defined by the allocation function.
+    if (auto *InitVal = getInitialValueOfAllocation(DepInst, TLI, LoadType))
       return createConstantExpression(InitVal);
 
   return nullptr;
diff --git a/llvm/test/Transforms/NewGVN/coercion-different-ptr.ll b/llvm/test/Transforms/NewGVN/coercion-different-ptr.ll
index 61a6a633788e1..847f0868cfa61 100644
--- a/llvm/test/Transforms/NewGVN/coercion-different-ptr.ll
+++ b/llvm/test/Transforms/NewGVN/coercion-different-ptr.ll
@@ -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:

``````````

</details>


https://github.com/llvm/llvm-project/pull/141477