Re: [libc-coord] Add new ABI '__memcmpeq()' to libc

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Fri, Sep 17, 2021 at 3:32 AM Richard Biener via Gcc <gcc@gcc.gnu.org>
wrote:

> 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.
>

Where would the jmp be and under what conditions? Is there no way to achieve
this with zero overhead?


>
> > 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
> >
>