[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, 3 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/AliasAnalysis.cpp b/llvm/lib/Analysis/AliasAnalysis.cpp
index ab5f814fb39..f931b6fc652 100644
--- a/llvm/lib/Analysis/AliasAnalysis.cpp
+++ b/llvm/lib/Analysis/AliasAnalysis.cpp
@@ -111,6 +111,9 @@ ModRefInfo AAResults::getModRefInfo(Instruction *I, ImmutableCallSite Call) {
   if (auto CS = ImmutableCallSite(I)) {
     // Check if the two calls modify the same memory
     return getModRefInfo(CS, Call);
+  } else if (I->isFenceLike()) {
+    // If this is a fence, just return MRI_ModRef.
+    return MRI_ModRef;
   } else {
     // Otherwise, check if the call modifies or references the
     // location this memory access defines.  The best we can say