Re: Fix typo 586/686 in atomics/arch-x86.h

Hi,

On 2026-03-11 17:18:19 +0100, Tomas Vondra wrote:
> On 3/11/26 16:51, Andres Freund wrote:
> > On 2025-11-28 10:00:21 +0100, Daniel Gustafsson wrote:
> >>> On 28 Nov 2025, at 09:44, Jakub Wartak <jakub(dot)wartak(at)enterprisedb(dot)com> wrote:
> >>>
> >>> That's a typo in src/include/port/atomics/arch-x86.h, isn't it ?:
> >>> if defined(__i568__) || defined(__i668__) || /* gcc i586+ */
> >>> If yes, then a patch is attached. Not that it harms something or
> >>> somebody has such old hardware, but I've just spotted it while looking
> >>> for something else.
> >>
> >> That indeed looks like a clear typo, but if noone has complained since 2017
> >> then maybe removing the checks is the right course of action?
> >
> > Tomas also just found these typos, which made me find this thread.
> >
> > These typos are obviously mine. Ugh.
> >
> >
> > I do think we should drop the 32bit support, rather than fixing the typos.
> >
> >
> > While architecturally correct, the 586 indeed can do tear free 8 byte reads /
> > writes, some quick experiments show that it's actually not entirely trivial to
> > get the compiler to generate the right code, at least with gcc.
> >
> > A volatile 8 byte read / store with gcc only generates correct code when
> > building with a newer -march= (it's using movq when correct, but it doesn't
> > start using it with just -mmmx, which added the instruction). For 586 one
> > needs to get the compiler to use fildq/fistpq, which I could only make happen
> > when using the atomic builtins / C11 atomics.
> >
> > I also just can't get excited about expending any work on performance for
> > 32bit builds.
> >
>
> True. Are you suggesting we "drop" the support even in backbranches?
> AFAIK those never actually supported this due to the typos, so it's not
> really a change in behavior.

I can see either just not touching the backbranches or applying the explicit
removal there too. I don't really have a preference.

Greetings,

Andres Freund