[AliasAnalysis] Fences do not modify constant memory location

Summary:
Fence instructions are currently marked as `ModRef` for all memory locations.

We can improve this for constant memory locations (such as constant globals),
since fence instructions cannot modify these locations.

This helps us to forward constant loads across fences (added test case in GVN).
There were no changes in behaviour for similar test cases in early-cse and licm.

Reviewers: dberlin, sanjoy, reames

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28914

llvm-svn: 292546

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/AliasAnalysis.cpp b/llvm/lib/Analysis/AliasAnalysis.cpp
index 84da76be98b..fbc030b26aa 100644
--- a/llvm/lib/Analysis/AliasAnalysis.cpp
+++ b/llvm/lib/Analysis/AliasAnalysis.cpp
@@ -367,6 +367,14 @@ ModRefInfo AAResults::getModRefInfo(const StoreInst *S,
   return MRI_Mod;
 }
 
+ModRefInfo AAResults::getModRefInfo(const FenceInst *S, const MemoryLocation &Loc) {
+  // If we know that the location is a constant memory location, the fence
+  // cannot modify this location.
+  if (Loc.Ptr && pointsToConstantMemory(Loc))
+    return MRI_Ref;
+  return MRI_ModRef;
+}
+
 ModRefInfo AAResults::getModRefInfo(const VAArgInst *V,
                                     const MemoryLocation &Loc) {