[MemorySSA] Create query after checking if instruction is a fence.

The alternative is checking if I is a fence in the Query constructor, so
as to not attempt to get a non-existent MemoryLocation.

llvm-svn: 346798

1 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/MemorySSA.cpp b/llvm/lib/Analysis/MemorySSA.cpp
index 1e45cc4bdf8..3d98fca823a 100644
--- a/llvm/lib/Analysis/MemorySSA.cpp
+++ b/llvm/lib/Analysis/MemorySSA.cpp
@@ -2199,13 +2199,14 @@ MemorySSA::CachingWalker::getClobberingMemoryAccess(MemoryAccess *MA) {
     return StartingAccess->getOptimized();
 
   const Instruction *I = StartingAccess->getMemoryInst();
-  UpwardsMemoryQuery Q(I, StartingAccess);
   // We can't sanely do anything with a fence, since they conservatively clobber
   // all memory, and have no locations to get pointers from to try to
   // disambiguate.
-  if (!Q.IsCall && I->isFenceLike())
+  if (!ImmutableCallSite(I) && I->isFenceLike())
     return StartingAccess;
 
+  UpwardsMemoryQuery Q(I, StartingAccess);
+
   if (isUseTriviallyOptimizableToLiveOnEntry(*MSSA->AA, I)) {
     MemoryAccess *LiveOnEntry = MSSA->getLiveOnEntryDef();
     StartingAccess->setOptimized(LiveOnEntry);