Finish making AliasAnalysis aware of the fact that most atomic intrinsics only dereference their arguments, and enhance

BasicAA to make use of this fact when computing ModRef info.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63718 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/AliasAnalysis.cpp

@@ -150,6 +150,27 @@ AliasAnalysis::getModRefBehavior(CallSite CS,
 AliasAnalysis::ModRefBehavior
 AliasAnalysis::getModRefBehavior(Function *F,
                                  std::vector<PointerAccessInfo> *Info) {
+  if (F->isIntrinsic()) {
+    switch (F->getIntrinsicID()) {
+      case Intrinsic::atomic_cmp_swap:
+      case Intrinsic::atomic_load_add:
+      case Intrinsic::atomic_load_and:
+      case Intrinsic::atomic_load_max:
+      case Intrinsic::atomic_load_min:
+      case Intrinsic::atomic_load_nand:
+      case Intrinsic::atomic_load_or:
+      case Intrinsic::atomic_load_sub:
+      case Intrinsic::atomic_load_umax:
+      case Intrinsic::atomic_load_umin:
+      case Intrinsic::atomic_load_xor:
+      case Intrinsic::atomic_swap:
+        // CAS and related intrinsics only access their arguments.
+        return AliasAnalysis::AccessesArguments;
+      default:
+        break;
+    }
+  }
+
   if (F->doesNotAccessMemory())
     // Can't do better than this.
     return DoesNotAccessMemory;

lib/Analysis/BasicAliasAnalysis.cpp

@@ -256,6 +256,22 @@ bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
 //
 AliasAnalysis::ModRefResult
 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
+  // If the function only accesses its arguments, it suffices to check that
+  // P does not alias any of those arguments.
+  if (AliasAnalysis::getModRefBehavior(CS, 0) ==
+      AliasAnalysis::AccessesArguments) {
+    bool doesAlias = false;
+    for (CallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
+         AI != AE; ++AI)
+      if (alias(*AI, ~0U, P, Size) != NoAlias) {
+        doesAlias = true;
+        break;
+      }
+    
+    if (!doesAlias)
+      return NoModRef;
+  }
+  
   if (!isa<Constant>(P)) {
     const Value *Object = P->getUnderlyingObject();
     

test/Analysis/BasicAA/cas.ll

@@ -0,0 +1,14 @@
+; RUN: llvm-as < %s | opt -basicaa -gvn | llvm-dis | grep load | count 1
+
+@flag0 = internal global i32 zeroinitializer
+@turn = internal global i32 zeroinitializer
+
+
+define i32 @main() {
+  %a = load i32* @flag0
+	%b = tail call i32 @llvm.atomic.swap.i32.p0i32(i32* @turn, i32 1)
+  %c = load i32* @flag0
+	ret i32 %c
+}
+
+declare i32 @llvm.atomic.swap.i32.p0i32(i32*, i32) nounwind