[NewGVN] Fix lifetime coercion (llvm#141477)

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.

Author: ManuelJBrito

llvm/lib/Transforms/Scalar/NewGVN.cpp

@@ -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);

llvm/test/Transforms/NewGVN/coercion-different-ptr.ll

@@ -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.
@@ -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: