Re: Patch ping (Re: [PATCH] middle-end, i386: Pattern recognize add/subtract with carry [PR79173])

[Uros Bizjak <ubizjak@gmail.com>]

On Tue, Jun 13, 2023 at 9:06 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> Hi!
>
> On Tue, Jun 06, 2023 at 11:42:07PM +0200, Jakub Jelinek via Gcc-patches wrote:
> > The following patch introduces {add,sub}c5_optab and pattern recognizes
> > various forms of add with carry and subtract with carry/borrow, see
> > pr79173-{1,2,3,4,5,6}.c tests on what is matched.
> > Primarily forms with 2 __builtin_add_overflow or __builtin_sub_overflow
> > calls per limb (with just one for the least significant one), for
> > add with carry even when it is hand written in C (for subtraction
> > reassoc seems to change it too much so that the pattern recognition
> > doesn't work).  __builtin_{add,sub}_overflow are standardized in C23
> > under ckd_{add,sub} names, so it isn't any longer a GNU only extension.
> >
> > Note, clang has for these has (IMHO badly designed)
> > __builtin_{add,sub}c{b,s,,l,ll} builtins which don't add/subtract just
> > a single bit of carry, but basically add 3 unsigned values or
> > subtract 2 unsigned values from one, and result in carry out of 0, 1, or 2
> > because of that.  If we wanted to introduce those for clang compatibility,
> > we could and lower them early to just two __builtin_{add,sub}_overflow
> > calls and let the pattern matching in this patch recognize it later.
> >
> > I've added expanders for this on ix86 and in addition to that
> > added various peephole2s to make sure we get nice (and small) code
> > for the common cases.  I think there are other PRs which request that
> > e.g. for the _{addcarry,subborrow}_u{32,64} intrinsics, which the patch
> > also improves.
>
> I'd like to ping this patch.

I briefly went over the x86 part (LGTM), but please get a middle-end
approval first.

Thanks,
Uros.

>
> Thanks.
>
>         Jakub
>