[PATCH, libstdc++] GLIBCXX_HAVE_INT64_T

[David Edelsohn]

On Fri, Jan 8, 2021 at 1:37 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On 06/01/21 19:41 -0500, David Edelsohn wrote:
> >Thanks for clarifying the issue.
> >
> >As you implicitly point out, GCC knows the type of INT64 and defines
> >the macro __INT64_TYPE__ .  The revised code can use that directly,
> >such as:
> >
> >#if defined(_GLIBCXX_HAVE_INT64_T_LONG) \
> >    || defined(_GLIBCXX_HAVE_INT64_T_LONG_LONG)
> >   typedef __INT64_TYPE__   streamoff;
> > #elif defined(_GLIBCXX_HAVE_INT64_T)
> >   typedef int64_t                     streamoff;
> > #else
> >   typedef long long                 streamoff;
> > #endif
> >
> >Are there any additional issues not addressed by that approach, other
> >than possible further simplification?
>
> That avoids the ABI break that Jakub pointed out. But I think we can
> simplify it further, as in the attached patch.
>
> This uses __INT64_TYPE__ if that's defined, and long long otherwise. I
> think that should be equivalent in all practical cases (I can imagine
> some strange target where __INT64_TYPE__ is defined by the compiler,
> but int64_t isn't defined when the configure checks look for it, and
> so the current code would use long long and with my patch would use
> __INT64_TYPE__ which could be long ... but I think in practice that's
> unlikely. It was probably more likely in older releases where the
> configure test would have been done with -std=gnu++98 and so int64_t
> might not have been declared by libc's <stdint.h>, but if that was the
> case then any ABI break it caused happened years ago.

Hi, Jonathan

Polite ping.

Now that GCC 11.1 has been released, can this patch be applied to
libstdc++?  As I replied at the time to Jakub's concerns, both Clang
(since 3.0.0) and ICC (since at least 16.0.0) have defined
__INT64_TYPE__ .

Thanks, David