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.
Thanks.
Jakub
