[BasicAA] Treat llvm.assume as not accessing memory in getModRefBehavior(Function)

Reviewers: dberlin, chandlerc, hfinkel, reames, sanjoy

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19730

llvm-svn: 268068

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -575,6 +575,12 @@ FunctionModRefBehavior BasicAAResult::getModRefBehavior(const Function *F) {
   if (F->doesNotAccessMemory())
     return FMRB_DoesNotAccessMemory;
 
+  // While the assume intrinsic is marked as arbitrarily writing so that
+  // proper control dependencies will be maintained, it never aliases any
+  // particular memory location.
+  if (F->getIntrinsicID() == Intrinsic::assume)
+    return FMRB_DoesNotAccessMemory;
+
   FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
 
   // If the function declares it only reads memory, go with that.

llvm/test/Transforms/Util/MemorySSA/assume.ll

@@ -0,0 +1,17 @@
+; RUN: opt -basicaa -print-memoryssa -verify-memoryssa -analyze < %s 2>&1 | FileCheck %s
+;
+; Ensures that assumes are treated as not reading or writing memory.
+
+declare void @llvm.assume(i1)
+
+define i32 @foo(i32* %a, i32* %b, i1 %c) {
+; CHECK: 1 = MemoryDef(liveOnEntry)
+; CHECK-NEXT: store i32 4
+  store i32 4, i32* %a, align 4
+; CHECK: call void @llvm.assume
+  call void @llvm.assume(i1 %c)
+; CHECK: MemoryUse(1)
+; CHECK-NEXT: %1 = load i32
+  %1 = load i32, i32* %a, align 4
+  ret i32 %1
+}