[LLVMdev] The nsw story

[David A. Greene]

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.

                               -Dave