[LLVMdev] RFC: Proposal for Poison Semantics (original) (raw)

Chandler Carruth chandlerc at google.com
Thu Jan 29 22:49:28 PST 2015

On Thu, Jan 29, 2015 at 9:43 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:

One way around this is to say that there are some special instructions, icmp, sext and zext which produce a value solely composed of poison bits if any of their input bits is poison. So <poison> icmp X is poison for any value of X, including INTMAX. This is one way poison could be fundamentally different from undef.

So far, this is the model I like the best, but I do still have some concern.

The primary concern I have is that with this model, zext is no longer 100% equivalent to anyext + mask. Much like you say, you could implement zext that way, but once you convert them, the poison is lost.

Maybe that's OK though. I'm curious what others think. I haven't really had enough time to fully explore this in my head. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150129/318772ed/attachment.html>

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