[PATCH] D60047: [CaptureTracking] Don't let comparisons against null escape inbounds pointers

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert added a comment.

Should we interpret your comment as "the reasoning seems sound", or did you only want to point out the special case which I discuss below?

> @sanjoy wrote:
>  However, this example is kind of iffy because in general the `deref_or_null` attribute is not valid since `p` in `main` could have been something other than `-4`.  But *if* the execution has `p` == `-4` then the attribute is well defined.

I thought about this case, though I didn't spell it out explicitly in my earlier comment (the part: //as the addition "with infinitely precise signed arithmetic" would not overflow//).

Let me start by stating that I interpret pointers as unsigned values which, under certain conditions, are allowed to overflow. (Maybe that is in itself a problem in my thinking.)
So for me `p` is not `-4` but always some positive value, probably `2^N - 1 - 4` where `N` is the pointer bit-width. If that is acceptable, then the inbounds wording in the lang ref
prevents the problem you describe. Any addition with a positive value, e.g., the one you choose `(2^N - 1 - 4) + 4`, cannot be `null` without an overflow and inbounds prohibits this overflow.


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