Re: Use pg_neg_s*_overflow() for open-coded negation overflow checks

On Wed, Sep 2, 2026 at 7:16 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> On Tue, Sep 01, 2026 at 02:26:38PM +0800, Ewan Young wrote:
> > I see this went in as 8e483af5515 in the meantime -- thanks a lot for
> > the commit and the credit!
> >
> > FWIW, the two um/div sites left out, int2um() and int2div(), turn out
> > to be mixed rather than clean: clang 15 compiles the int16
> > __builtin_sub_overflow() one instruction worse on the hot path, while
> > gcc 12 does one better (neg + jo).
>
> Please note that I'd be OK to look at this stuff again if we have
> other similar opportunities. I am not really convinced by your
> v2-0002. The other opportunities in v2-0001 could be reconsidered if
> rewritten in a different way, but it's also tricky to evaluate due to
> the with/without builtin and compiler requirements.
>
> I would accept something that proves to be a net benefit in rather new
> versions of gcc and clang, as well as a net benefit with/without the
> builtin. 8e483af5515 was exactly that, with less or the same amount of
> instructions (the cash change was super nice) for all setups I have
> tested (Linux and macos).

Fair enough on 0002 -- let's drop it. Thanks again for taking the good batch!

> --
> Michael

--
Regards,
Ewan Young