[TBAA] Model call accessing immutable type as readnone

Accesses to constant memory are not observable and should be
reported as readnone, not readonly. This is consistent with what
we do for normal (non-call) instructions: For those, the TBAA
metadata will result in pointsToConstantMemory() returning true,
which will then result in a NoModRef result, not a Ref result.

Differential Revision: https://reviews.llvm.org/D135864

diff --git a/llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp b/llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
index f5a451b..d2d7044 100644 (file)
--- a/llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
@@ -410,12 +410,11 @@ TypeBasedAAResult::getModRefBehavior(const CallBase *Call,
   if (!EnableTBAA)
     return AAResultBase::getModRefBehavior(Call, AAQI);
 
-  // If this is an "immutable" type, we can assume the call doesn't write
-  // to memory.
+  // If this is an "immutable" type, the access is not observable.
   if (const MDNode *M = Call->getMetadata(LLVMContext::MD_tbaa))
     if ((!isStructPathTBAA(M) && TBAANode(M).isTypeImmutable()) ||
         (isStructPathTBAA(M) && TBAAStructTagNode(M).isTypeImmutable()))
-      return FunctionModRefBehavior::readOnly();
+      return FunctionModRefBehavior::none();
 
   return AAResultBase::getModRefBehavior(Call, AAQI);
 }

diff --git a/llvm/test/Analysis/TypeBasedAliasAnalysis/functionattrs.ll b/llvm/test/Analysis/TypeBasedAliasAnalysis/functionattrs.ll
index b34a1a5..6134f57 100644 (file)
--- a/llvm/test/Analysis/TypeBasedAliasAnalysis/functionattrs.ll
+++ b/llvm/test/Analysis/TypeBasedAliasAnalysis/functionattrs.ll
@@ -21,8 +21,8 @@ define void @test0_no(i32* %p) nounwind {
   ret void
 }
 
-; Add the readonly attribute, since there's just a call to a function which 
-; TBAA says doesn't modify any memory.
+; Add the readnone attribute, since there's just a call to a function which
+; TBAA says only accesses constant memory.
 
 ; CHECK: define void @test1_yes(i32* nocapture %p) #2 {
 define void @test1_yes(i32* %p) nounwind {
@@ -74,7 +74,7 @@ declare void @llvm.memcpy.p0i8.p0i8.i64(i8*, i8*, i64, i1) nounwind
 
 ; CHECK: attributes #0 = { mustprogress nofree norecurse nosync nounwind readnone willreturn }
 ; CHECK: attributes #1 = { argmemonly mustprogress nofree norecurse nosync nounwind willreturn writeonly }
-; CHECK: attributes #2 = { nofree nounwind readonly }
+; CHECK: attributes #2 = { nofree nosync nounwind readnone }
 ; CHECK: attributes #3 = { nounwind }
 ; CHECK: attributes #4 = { mustprogress nofree nosync nounwind readnone willreturn }
 ; CHECK: attributes #5 = { argmemonly mustprogress nofree nosync nounwind willreturn }