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

["Christoph Müllner" <cmuellner@gcc.gnu.org>]

Would it make sense to extend this proposal to include __strcmpeq()
and __strncmpeq()?

Both are already available internally in GCC in form of
BUILT_IN_STRCMP_EQ and BUILT_IN_STRNCMP_EQ
(tree-ssa-strlen.c detects them in handle_builtin_string_cmp() and
builtins.c tries to inline them in expand_builtin_memcmp()).
However, they are currently restricted to cases where the length of
the string or the size of the array (of both arguments) is known.

A use case for strcmpeq() would be the comparison of std::type_info
objects (equality and inequality operator) in libstdc++.

On Tue, Sep 21, 2021 at 9:54 PM Noah Goldstein via Gcc <gcc@gcc.gnu.org> wrote:
>
> On Fri, Sep 17, 2021 at 9:27 AM Florian Weimer via Libc-alpha <
> libc-alpha@sourceware.org> wrote:
>
> > * Joseph Myers:
> >
> > > I was supposing a build-time decision (using
> > GCC_GLIBC_VERSION_GTE_IFELSE
> > > to know if the glibc version on the target definitely has this
> > function).
> > > But if we add a header declaration, you could check for __memcmpeq being
> > > declared (and so cover arbitrary C libraries, not just glibc, and avoid
> > > issues of needing to disable this logic for freestanding compilations,
> > > which would otherwise be an issue if a glibc-target toolchain is used
> > for
> > > a freestanding kernel compilation).  The case of people calling
> > > __builtin_memcmp (or declaring memcmp themselves) without string.h
> > > included probably isn't one it's important to optimize.
> >
> > The header-less case looks relevant to C++ and other language front
> > ends, though.  So a GCC_GLIBC_VERSION_GTE_IFELSE check could still make
> > sense for them.
> >
> > (Dropping libc-coord.)
> >
> > Thanks,
> > Florian
> >
> >
> What are we going with?
>
> Should I go forward with the proposal in GLIBC?
>
> If I should go forward with it should I include a def in string.h?