[MemCpy] Check for alias in performMemCpyToMemSetOptzn, instead of the identity of two operands

Summary:
This fixes pr29105. The reason is that lifetime marks creates new
aliasing pointers the original ones, but before this patch aliases
were not checked in performMemCpyToMemSetOptzn.

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23846

llvm-svn: 279769

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index e77a8936bb7..27c37cd5289 100644
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -1108,8 +1108,10 @@ bool MemCpyOptPass::processMemSetMemCpyDependence(MemCpyInst *MemCpy,
 /// The \p MemCpy must have a Constant length.
 bool MemCpyOptPass::performMemCpyToMemSetOptzn(MemCpyInst *MemCpy,
                                                MemSetInst *MemSet) {
+  AliasAnalysis &AA = LookupAliasAnalysis();
+
   // This only makes sense on memcpy(..., memset(...), ...).
-  if (MemSet->getRawDest() != MemCpy->getRawSource())
+  if (!AA.isMustAlias(MemSet->getRawDest(), MemCpy->getRawSource()))
     return false;
 
   ConstantInt *CopySize = cast<ConstantInt>(MemCpy->getLength());