https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110751
--- Comment #20 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org>
---
(In reply to Richard Biener from comment #19)
> Sure, I can kind of see the usefulness elsewhere. Just for this particular
> issue it doesn't seem necessary to sit down and design this when we can
> represent it like we do for MASK_LOAD (omit the 'else' value).
Yeah, that's fair.
For the ifn->optab interface, I think it'd be natural to use an actual rtx
rather than a null pointer, since e.g. predicates are not set up to handle
nulls. So perhaps we should start the process there. We could add an UNDEF
rtl code that is initially only used for the ifn->optab interface, and expand
it as we find new use cases. We can grow the semantics based on those use
cases and based on LLVM's experience.
> In other context we discussed specifying zero for MASK_LOAD masked elements
> so we can for example CSE better. CSE with UNDEF might be possible as well,
> but I'm not sure what LLVM's undef would allow and whether it's defined
> rigidly enough.
One of the main optimisations I wanted from that was:
a = IFN_MASK_LOAD (…, mask)
b = VEC_COND_EXPR <mask, a, {0,0,…}>
→
a = IFN_MASK_LOAD (…, mask)
b = a
which wouldn't be valid for undef.
