From: Richard Biener <richard.guenther@gmail.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: Richard Biener via Gcc <gcc@gcc.gnu.org>,
Joseph Myers <joseph@codesourcery.com>,
GNU C Library <libc-alpha@sourceware.org>,
libc-coord@lists.openwall.com
Subject: Re: [libc-coord] Add new ABI '__memcmpeq()' to libc
Date: Fri, 17 Sep 2021 10:31:54 +0200 [thread overview]
Message-ID: <CAFiYyc3m8VM37Y=9EidVO7pT1GitLKRFPA8=c7FXv2QaFqs9ow@mail.gmail.com> (raw)
In-Reply-To: <87y27vhbel.fsf@oldenburg.str.redhat.com>
On Fri, Sep 17, 2021 at 10:08 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Richard Biener via Gcc:
>
> > On Thu, Sep 16, 2021 at 10:36 PM Joseph Myers <joseph@codesourcery.com> wrote:
> >>
> >> On Thu, 16 Sep 2021, Chris Kennelly wrote:
> >>
> >> > In terms of relying on the feature: If __memcmpeq is ever exposed as an a
> >> > simple alias for memcmp (since the notes mention that it's a valid
> >> > implementation), does that open up the possibility of depending on the
> >> > bcmp-like behavior that we were trying to escape?
> >>
> >> The proposal is as an ABI only (compilers would generate calls to
> >> __memcmpeq from boolean uses of memcmp, users wouldn't write calls to
> >> __memcmpeq directly, __memcmpeq wouldn't be declared in installed libc
> >> headers). If such dependence arises, that would suggest a compiler bug
> >> wrongly generating such calls for non-boolean memcmp uses.
> >
> > So the compiler would emit a call to __memcmpeq and at the same time
> > emit a weak alias of __memcmpeq to memcmp so the program links
> > when the libc version targeted does not provide __memcmpeq? Or would
> > glibc through <string.h> magically communicate the availability of the new ABI
> > without actually declaring the function?
>
> I do not think ELF provides that capability.
I guess a weak forwarder should do the trick at the cost of a jmp.
> We can add a declaration to <string.h> to communicate the availability.
> I think this is how glibc (and other libcs) communicate the availability
> of non-standard interfaces to GCC.
OK, I guess that's fine.
> > (I'm not sure whether a GCC build-time decision via configure is the
> > very best idea)
>
> If libstdc++ or libgcc_s have a symbol dependency on glibc 2.35 for
> other (unrelated) reasons, would the build-time dependency be less of a
> concern? Because another such dependency exists?
Not sure, I was thinking that we'd need to re-compile GCC when we
upgrade glibc to make use of the feature.
But then being able to run an executable on a system that does not
provide the ABI but a compatible one (memcmp) might be a nice
thing.
Richard.
> Thanks,
> Florian
>
next prev parent reply other threads:[~2021-09-17 8:32 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-16 17:02 Noah Goldstein
2021-09-16 17:55 ` [libc-coord] " Chris Kennelly
2021-09-16 18:31 ` Noah Goldstein
2021-09-16 20:32 ` Chris Kennelly
2021-09-16 20:35 ` Joseph Myers
2021-09-16 20:55 ` enh
2021-09-17 7:43 ` Richard Biener
2021-09-17 8:08 ` Florian Weimer
2021-09-17 8:31 ` Richard Biener [this message]
2021-09-17 8:37 ` Florian Weimer
2021-09-17 9:30 ` Richard Biener
2021-09-17 17:40 ` Noah Goldstein
2021-09-17 9:12 ` Jakub Jelinek
2021-09-17 16:55 ` Martin Sebor
2021-09-17 14:19 ` Joseph Myers
2021-09-17 14:26 ` Florian Weimer
2021-09-21 19:53 ` Noah Goldstein
2021-09-22 17:46 ` Christoph Müllner
2021-09-22 18:15 ` Noah Goldstein
2021-09-16 21:27 ` James Y Knight
2021-09-16 21:42 ` Joseph Myers
2021-09-16 21:50 ` enh
2021-09-16 21:59 ` Noah Goldstein
2021-09-16 22:17 ` Chris Kennelly
2021-09-16 22:36 ` Joseph Myers
2021-09-16 23:24 ` Noah Goldstein
2021-09-18 1:36 ` James Y Knight
2021-10-26 22:47 ` Noah Goldstein
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