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

[Florian Weimer]

* 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.

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.

> (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?

Thanks,
Florian