[PATCH] D59065: Add ptrmask intrinsic

Tue May 14 08:57:56 PDT 2019

jdoerfert added a comment.

In D59065#1501459 <https://reviews.llvm.org/D59065#1501459>, @fhahn wrote:

> > - What happened to the poison result sentence. I think we need that to ensure the "based on the same ..." part. Otherwise, we can butcher the pointer with the mask and end up with a "valid pointer" into a different object. I might be wrong here, just want to make sure we have thought of it.
>
> My understanding from Hal's comments was that the behavior should be clear from the pre-existing semantics, but I might have missed something?

I don't see why but I might be wrong (@hfinkel). What I thought is detailed below.

> The case where we fail to clear bits that must be zero should be clearly handled by the existing semantics.

Sure.

> The case where we might clear meaningful bits

I'm not sure about this. At least I think this is not what you intended to do initially but maybe this is OK for you.
With this definition, I don't see why clearing lower bits would result in poison as long as the result points into the
same object. Even if not, the current definition doesn't really state what would happen if the resulting pointer would point into any valid object.

> If this creates invalid pointers, the existing semantics handle that case too.

I don't know, see above.

================
Comment at: llvm/docs/LangRef.rst:16456
+
+      declare ptrty llvm.ptrmask(ptrty %ptr, intty %mask) readnone
+
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`speculatable`

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https://reviews.llvm.org/D59065