[PATCH] D72562: [Attributor][Fix] AAHeapToStack and AAIsDead connection

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert added inline comments.


================
Comment at: llvm/lib/Transforms/IPO/Attributor.cpp:2915
+      if (isMallocOrCallocLikeFn(I, TLI) || isFreeCall(I, TLI))
+        return ChangeStatus::CHANGED;
+
----------------
sstefan1 wrote:
> jdoerfert wrote:
> > sstefan1 wrote:
> > > jdoerfert wrote:
> > > > Would it work the other way around, as well? Deleting them here not in H2S. If so it would remove the special case here and some code in H2S, right?
> > > I had this in mind, but I don't think it would work because `isAssumedSideEffectFree()` will be false.
> > I see. In that case, can you move the special logic into `isAssumedSideEffectFree` to cut down on the duplication and allow reuse later on?
> I've given this some more thought. While it would indeed be nicer to delete H2S stuff here as well, I think that can't work. Consider this example from the tests:
> 
>   define void @nofree_arg_only(i8* %p1, i8* %p2) {
>     tail call void @free(i8* %p2)
>     tail call void @nofree_func(i8* %p1)
>     ret void
>   }
> 
> Here `free()` is neither nounwind nor readonly but would be considered side effect free because we made `isAssumedSideEffectFree` consider it that way. As a result, since it is void value, it is deleted. Although it shouldn't have been.
> 
> If you still think we can/should do it, let me know.
I did confuse you by putting the comment here, sorry. The code I want to move into `isAssumedSideEffectFree` is the code below, not the code here. By moving the logic from `updateImpl` into `isAssumedSideEffectFree` we should be able to remove the code here, that is what I should have said.


> Here free() is neither nounwind nor readonly but would be considered side effect free because we made isAssumedSideEffectFree consider it that way. 

I don't think this is what should happen. `isAssumedSideEffectFree` will ask H2S and H2S::isAssumedToBeDeleted() will return `false` which will cause the normal logic in `isAssumedSideEffectFree` to continue and conclude we don't know anything about `free` and we need to assume it has side effects.



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Comment at: llvm/lib/Transforms/IPO/Attributor.cpp:3048
+        IsAssumedSideEffectFree = true;
+        return indicateOptimisticFixpoint();
+      }
----------------
sstefan1 wrote:
> jdoerfert wrote:
> > This is assumed information, correct? If so, we cannot indicate a fixpoint and we need to record a dependence if we use the information.
> Correct. Interestingly, though, it doesn't work without indicating fixpoint.
"Doesn't work" is a symptom not an explanation ;)

We cannot indicate an optimistic fixpoint based on assumed information. If the tests fail it needs to be investigated why. See my other comment though.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D72562/new/

https://reviews.llvm.org/D72562