[LLVMdev] The nsw story

David A. Greene greened at obbligato.org
Thu Dec 1 09:37:17 PST 2011

Dan Gohman <gohman at apple.com> writes:

> Prohibiting poison values from propogating through memory would mean
> that the reg2mem pass would no longer be a semantics-preserving pass.

Or it means you couldn't demote those values.

> Prohibiting it from propogating through control flow would mean that a
> select instruction wouldn't be equivalent to a conditional branch and
> a phi. These are problems that aren't really important for hardware
> ISAs, but are important for LLVM IR.

We could consider select an observation point and consider its use of
poisoned values undefined behavior.


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