[AliasAnalysis] Give back AA results for fence instructions

Calling getModRefInfo with a fence resulted in crashes because fences
don't have a memory location.  Add a new predicate to Instruction
called isFenceLike which indicates that the instruction mutates memory
but not any single memory location in particular. In practice, it is a
proxy for the set of instructions which "mayWriteToMemory" but cannot be
used with MemoryLocation::get.

This fixes PR28570.

llvm-svn: 275581

1 files changed, 7 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Utils/MemorySSA.cpp b/llvm/lib/Transforms/Utils/MemorySSA.cpp
index 0aed1334571..f93917f9861 100644
--- a/llvm/lib/Transforms/Utils/MemorySSA.cpp
+++ b/llvm/lib/Transforms/Utils/MemorySSA.cpp
@@ -1252,7 +1252,7 @@ MemoryAccess *MemorySSA::CachingWalker::getClobberingMemoryAccess(
 
   // Conservatively, fences are always clobbers, so don't perform the walk if we
   // hit a fence.
-  if (isa<FenceInst>(I))
+  if (!ImmutableCallSite(I) && I->isFenceLike())
     return StartingUseOrDef;
 
   UpwardsMemoryQuery Q;
@@ -1287,15 +1287,17 @@ MemorySSA::CachingWalker::getClobberingMemoryAccess(const Instruction *I) {
   // access, since we only map BB's to PHI's. So, this must be a use or def.
   auto *StartingAccess = cast<MemoryUseOrDef>(MSSA->getMemoryAccess(I));
 
-  // We can't sanely do anything with a FenceInst, they conservatively
+  bool IsCall = bool(ImmutableCallSite(I));
+
+  // We can't sanely do anything with a fences, they conservatively
   // clobber all memory, and have no locations to get pointers from to
-  // try to disambiguate
-  if (isa<FenceInst>(I))
+  // try to disambiguate.
+  if (!IsCall && I->isFenceLike())
     return StartingAccess;
 
   UpwardsMemoryQuery Q;
   Q.OriginalAccess = StartingAccess;
-  Q.IsCall = bool(ImmutableCallSite(I));
+  Q.IsCall = IsCall;
   if (!Q.IsCall)
     Q.StartingLoc = MemoryLocation::get(I);
   Q.Inst = I;