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

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

> 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.

Okay, I just realized that this statement is not quite true. In fact, we seem to be very inconsistent about how we handle this. When calculating function attributes / FMRB, we ignore constant memory entirely, resulting in NoModRef / readnone. The same is true for the getModRef() API on some instruction kinds, like VAArgInst, CatchPadInst or CatchReturnInst, where we downgrade ModRef to NoModRef.

But then there are other places where we preserve Ref effects on pointsToConstantMemory(). We do this for calls in https://github.com/llvm/llvm-project/blob/98eedd406aa0a9360a8ef3b8bbe85d3d30367fdc/llvm/lib/Analysis/AliasAnalysis.cpp#L257-L260 and for fences in https://github.com/llvm/llvm-project/blob/98eedd406aa0a9360a8ef3b8bbe85d3d30367fdc/llvm/lib/Analysis/AliasAnalysis.cpp#L534-L537.

I believe that the NoModRef / readnone modelling is the correct one, given the current pointsToConstantMemory() semantics of only considering globally constant memory.


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