[PATCH] D135864: [TBAA] Model call accessing immutable type as readnone

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: jdoerfert, jeroen.dobbelaere, fhahn.
Herald added subscribers: kosarev, hiraditya.
Herald added a project: All.
nikic requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

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.


https://reviews.llvm.org/D135864

Files:
  llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
  llvm/test/Analysis/TypeBasedAliasAnalysis/functionattrs.ll


Index: llvm/test/Analysis/TypeBasedAliasAnalysis/functionattrs.ll
===================================================================
--- llvm/test/Analysis/TypeBasedAliasAnalysis/functionattrs.ll
+++ llvm/test/Analysis/TypeBasedAliasAnalysis/functionattrs.ll
@@ -21,8 +21,8 @@
   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 @@
 
 ; 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 }
Index: llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
===================================================================
--- llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
+++ llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
@@ -410,12 +410,11 @@
   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);
 }


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D135864.467426.patch
Type: text/x-patch
Size: 2034 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20221013/44c07154/attachment.bin>