Re: [PATCH] gimple-match: Do not try UNCOND optimization with COND_LEN.

[Richard Sandiford <richard.sandiford@arm.com>]

Robin Dapp <rdapp.gcc@gmail.com> writes:
> Thank you for the explanation.
>
> So, assuming I added an IFN_VCOND_MASK and IFN_VCOND_MASK_LEN along
> with the respective helper and expand functions, what would be the
> way forward?

IMO it'd be worth starting with the _LEN form only.

> Generate an IFN_VCOND_MASK(_LEN) here instead of a VEC_COND_EXPR?
> How would I make sure all of match.pd's vec_cond optimizations
> applied to it as well?

I think the most important ones are:

/* Simplify:

     a = a1 op a2
     r = c ? a : b;

   to:

     r = c ? a1 op a2 : b;

   if the target can do it in one go.  This makes the operation conditional
   on c, so could drop potentially-trapping arithmetic, but that's a valid
   simplification if the result of the operation isn't needed.

   Avoid speculatively generating a stand-alone vector comparison
   on targets that might not support them.  Any target implementing
   conditional internal functions must support the same comparisons
   inside and outside a VEC_COND_EXPR.  */

It would be nice if there was some match.pd syntax that automatically
extended these rules to IFN_VCOND_MASK_LEN, but I don't know how easy
that would be, due to the extra two parameters.

Perhaps that itself could be done in gimple-match-exports.cc, in a similar
way to the current conditional stuff.  That is:

- for IFN_VCOND_MASK_LEN, try folding as a VEC_COND_EXPR and then "adding
  the length back"

- for IFN_COND_LEN_FOO, try folding as an IFN_COND_FOO and then
  "add the length back"

Not sure how important the second one is.

Thanks,
Richard

> Right now AFAIK IFN_VCOND_MASK only gets created in isel and
> everything is just a VEC_COND before.  But that does not provide
> length masking so is not the way to go?
>
> Thanks.
>
> Regards
>  Robin