[MemCpyOpt] Do move the memset, but look at its dest's dependencies.

In effect a partial revert of r237858, which was a dumb shortcut.
Looking at the dependencies of the destination should be the proper
fix: if the new memset would depend on anything other than itself,
the transformation isn't correct.

llvm-svn: 237874

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index a90204d26bd..66d6ac6f3a0 100644
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -859,6 +859,13 @@ bool MemCpyOpt::processMemSetMemCpyDependence(MemCpyInst *MemCpy,
   if (MemSet->getDest() != MemCpy->getDest())
     return false;
 
+  // Check that there are no other dependencies on the memset destination.
+  MemDepResult DstDepInfo =
+      MD->getPointerDependencyFrom(AliasAnalysis::getLocationForDest(MemSet),
+                                   false, MemCpy, MemCpy->getParent());
+  if (DstDepInfo.getInst() != MemSet)
+    return false;
+
   // Use the same i8* dest as the memcpy, killing the memset dest if different.
   Value *Dest = MemCpy->getRawDest();
   Value *DestSize = MemSet->getLength();
@@ -874,7 +881,7 @@ bool MemCpyOpt::processMemSetMemCpyDependence(MemCpyInst *MemCpy,
     if (ConstantInt *SrcSizeC = dyn_cast<ConstantInt>(SrcSize))
       Align = MinAlign(SrcSizeC->getZExtValue(), DestAlign);
 
-  IRBuilder<> Builder(MemSet);
+  IRBuilder<> Builder(MemCpy);
 
   // If the sizes have different types, zext the smaller one.
   if (DestSize->getType() != SrcSize->getType()) {