LICM: Don't sink stores out of loops that may throw.

Summary:
This hasn't been caught before because it requires noalias or similarly
strong alias analysis to actually reproduce.

Fixes http://llvm.org/PR27952 .

Reviewers: hfinkel, sanjoy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20944

llvm-svn: 271858

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index 3b197e4ac2c..13b5c141fcf 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -928,6 +928,16 @@ bool llvm::promoteLoopAccessesToScalars(
 
   const DataLayout &MDL = Preheader->getModule()->getDataLayout();
 
+  if (SafetyInfo->MayThrow) {
+    // If a loop can throw, we have to insert a store along each unwind edge.
+    // That said, we can't actually make the unwind edge explicit. Therefore,
+    // we have to prove that the store is dead along the unwind edge.
+    //
+    // Currently, this code just special-cases alloca instructions.
+    if (!isa<AllocaInst>(GetUnderlyingObject(SomePtr, MDL)))
+      return false;
+  }
+
   // Check that all of the pointers in the alias set have the same type.  We
   // cannot (yet) promote a memory location that is loaded and stored in
   // different sizes.  While we are at it, collect alignment and AA info.