Index: llvm/trunk/include/llvm/Analysis/AliasAnalysis.h
===================================================================
--- llvm/trunk/include/llvm/Analysis/AliasAnalysis.h
+++ llvm/trunk/include/llvm/Analysis/AliasAnalysis.h
@@ -443,11 +443,7 @@
 
   /// getModRefInfo (for fences) - Return information about whether
   /// a particular store modifies or reads the specified memory location.
-  ModRefInfo getModRefInfo(const FenceInst *S, const MemoryLocation &Loc) {
-    // Conservatively correct.  (We could possibly be a bit smarter if
-    // Loc is a alloca that doesn't escape.)
-    return MRI_ModRef;
-  }
+  ModRefInfo getModRefInfo(const FenceInst *S, const MemoryLocation &Loc);
 
   /// getModRefInfo (for fences) - A convenience wrapper.
   ModRefInfo getModRefInfo(const FenceInst *S, const Value *P, uint64_t Size) {
Index: llvm/trunk/lib/Analysis/AliasAnalysis.cpp
===================================================================
--- llvm/trunk/lib/Analysis/AliasAnalysis.cpp
+++ llvm/trunk/lib/Analysis/AliasAnalysis.cpp
@@ -367,6 +367,14 @@
   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) {
 
Index: llvm/trunk/test/Transforms/GVN/fence.ll
===================================================================
--- llvm/trunk/test/Transforms/GVN/fence.ll
+++ llvm/trunk/test/Transforms/GVN/fence.ll
@@ -1,5 +1,6 @@
 ; RUN: opt -S -basicaa -gvn < %s | FileCheck %s
 
+@a = external constant i32
 ; We can value forward across the fence since we can (semantically) 
 ; reorder the following load before the fence.
 define i32 @test(i32* %addr.i) {
@@ -52,6 +53,25 @@
   ret i32 %res
 }
 
+; We can forward the value forward the load
+; across both the fences, because the load is from
+; a constant memory location.
+define i32 @test4(i32* %addr) {
+; CHECK-LABEL: @test4
+; CHECK-NOT: load
+; CHECK: fence release
+; CHECK: store
+; CHECK: fence seq_cst
+; CHECK: ret i32 0
+  %var = load i32, i32* @a
+  fence release
+  store i32 42, i32* %addr, align 8
+  fence seq_cst
+  %var2 = load i32, i32* @a
+  %var3 = sub i32 %var, %var2
+  ret i32 %var3
+}
+
 ; Another example of why forwarding across an acquire fence is problematic
 ; can be seen in a normal locking operation.  Say we had:
 ; *p = 5; unlock(l); lock(l); use(p);
Index: llvm/trunk/test/Transforms/NewGVN/fence.ll
===================================================================
--- llvm/trunk/test/Transforms/NewGVN/fence.ll
+++ llvm/trunk/test/Transforms/NewGVN/fence.ll
@@ -1,6 +1,7 @@
 ; XFAIL: *
 ; RUN: opt -S -basicaa -newgvn < %s | FileCheck %s
 
+@a = external constant i32
 ; We can value forward across the fence since we can (semantically) 
 ; reorder the following load before the fence.
 define i32 @test(i32* %addr.i) {
@@ -53,6 +54,25 @@
   ret i32 %res
 }
 
+; We can forward the value forward the load
+; across both the fences, because the load is from
+; a constant memory location.
+define i32 @test4(i32* %addr) {
+; CHECK-LABEL: @test4
+; CHECK-NOT: load
+; CHECK: fence release
+; CHECK: store
+; CHECK: fence seq_cst
+; CHECK: ret i32 0
+  %var = load i32, i32* @a
+  fence release
+  store i32 42, i32* %addr, align 8
+  fence seq_cst
+  %var2 = load i32, i32* @a
+  %var3 = sub i32 %var, %var2
+  ret i32 %var3
+}
+
 ; Another example of why forwarding across an acquire fence is problematic
 ; can be seen in a normal locking operation.  Say we had:
 ; *p = 5; unlock(l); lock(l); use(p);