Re: [PATCH] Add -fextra-libc-function=memcmpeq for __memcmpeq

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Jun 20, 2022 at 2:39 AM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Thu, Jun 16, 2022 at 1:38 AM Fangrui Song <maskray@google.com> wrote:
> >
> > On Wed, Jun 15, 2022 at 2:44 PM H.J. Lu via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> > >
> > > On Mon, Jun 13, 2022 at 9:01 AM Richard Biener
> > > <richard.guenther@gmail.com> wrote:
> > > >
> > > >
> > > >
> > > > > Am 13.06.2022 um 16:36 schrieb H.J. Lu <hjl.tools@gmail.com>:
> > > > >
> > > > > On Mon, Jun 13, 2022 at 3:11 AM Richard Biener
> > > > > <richard.guenther@gmail.com> wrote:
> > > > >>
> > > > >>> On Tue, Jun 7, 2022 at 9:02 PM H.J. Lu via Gcc-patches
> > > > >>> <gcc-patches@gcc.gnu.org> wrote:
> > > > >>>
> > > > >>> Add -fextra-libc-function=memcmpeq to map
> > > > >>>
> > > > >>> extern int __memcmpeq (const void *, const void *, size_t);
> > > > >>>
> > > > >>> which was added to GLIBC 2.35, to __builtin_memcmp_eq.
> > > > >>
> > > > >> Humm.  Can't we instead use the presence of a declaration
> > > > >> of __memcmpeq with a GNU standard dialect as this instead of
> > > > >> adding a weird -fextra-libc-function= option?  Maybe that's even
> > > > >> reasonable with a non-GNU dialect standard in effect since
> > > > >> __ prefixed names are in the implementation namespace?
> > > > >
> > > > > But not all source codes include <string.h> and GCC may generate
> > > > > memcmp directly.  How should we handle these cases?
> > > >
> > > > Not.  Similar as to vectorized math functions.
> > > > I think it’s not worth optimizing for this case.
> > >
> > > Another question.  Should we consider any __memcmpeq prototype
> > > or just the one in the system header file?
>
> Any.

Here is the v2 patch:

https://gcc.gnu.org/pipermail/gcc-patches/2022-June/596881.html

> > An idea from https://reviews.llvm.org/D56593#3586673: -fbuiltin-__memcmpeq
> >
> > This requires making -fbuiltin-function available, see
> > https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html
> > ("There is no corresponding -fbuiltin-function option")
> >
> > I prefer an option over a magic behavior about whether a declaration exists.
>
> But we already have this behavior for multiple cases.  It's also the only
> way that in practice __memcmpeq will be used - _nobody_ (but maybe
> special crafted SPEC peak runs) will add explicit -fbuiltin-__memcmpeq.
>
> Richard.

Thanks.

-- 
H.J.