[PATCH, libstdc++] GLIBCXX_HAVE_INT64_T

Jonathan Wakely jwakely@redhat.com
Fri Jan 8 18:37:03 GMT 2021


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.



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