[AA] A conservative fix for atomic instruciton.

Author: Jin Huang

llvm/lib/Analysis/AliasAnalysis.cpp

@@ -421,9 +421,18 @@ ModRefInfo AAResults::getModRefInfo(const LoadInst *L,
                                     const MemoryLocation &Loc,
                                     AAQueryInfo &AAQI) {
   // Be conservative in the face of atomic.
-  if (isStrongerThan(L->getOrdering(), AtomicOrdering::Unordered))
+  if (isStrongerThan(L->getOrdering(), AtomicOrdering::Monotonic))
     return ModRefInfo::ModRef;
 
+  // For Monotonic and unordered atomic loads, if the locations are not NoAlias,
+  // we must be conservative and return ModRef to prevent unsafe reordering of
+  // accesses to the same memory.
+  if (L->isAtomic()){
+    if (Loc.Ptr &&
+        alias(MemoryLocation::get(L), Loc, AAQI, L) != AliasResult::NoAlias)
+      return ModRefInfo::ModRef;
+  }
+
   // If the load address doesn't alias the given address, it doesn't read
   // or write the specified memory.
   if (Loc.Ptr) {
@@ -439,7 +448,7 @@ ModRefInfo AAResults::getModRefInfo(const StoreInst *S,
                                     const MemoryLocation &Loc,
                                     AAQueryInfo &AAQI) {
   // Be conservative in the face of atomic.
-  if (isStrongerThan(S->getOrdering(), AtomicOrdering::Unordered))
+  if (isStrongerThan(S->getOrdering(), AtomicOrdering::Monotonic))
     return ModRefInfo::ModRef;
 
   if (Loc.Ptr) {
@@ -458,7 +467,7 @@ ModRefInfo AAResults::getModRefInfo(const StoreInst *S,
   }
 
   // Otherwise, a store just writes.
-  return ModRefInfo::Mod;
+  return ModRefInfo::ModRef;
 }
 
 ModRefInfo AAResults::getModRefInfo(const FenceInst *S,

llvm/test/Transforms/DeadStoreElimination/atomic-todo.ll

@@ -1,4 +1,3 @@
-; XFAIL: *
 ; RUN: opt -passes=dse -S < %s | FileCheck %s
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
@@ -11,7 +10,7 @@ target triple = "x86_64-apple-macosx10.7.0"
 @x = common global i32 0, align 4
 @y = common global i32 0, align 4
 
-; DSE across monotonic load (allowed as long as the eliminated store isUnordered)
+; DSE across monotonic load (allowed if the monotonic load's address is NoAlias)
 define i32 @test9() {
 ; CHECK-LABEL: test9
 ; CHECK-NOT: store i32 0
@@ -21,3 +20,11 @@ define i32 @test9() {
   store i32 1, ptr @x
   ret i32 %x
 }
+
+; DSE across monotonic load (blocked if the atomic load's address isn't NoAlias)
+define i32 @test9a() {
+  store i32 0, ptr @x
+  %x = load atomic i32, ptr @ptr monotonic, align 4
+  store i32 1, ptr @x
+  ret i32 %x
+}