Re: [PATCH] linux: Use 'long int' for timespec tv_nsec on x32 (BZ #16437)

[Florian Weimer <fweimer@redhat.com>]

* Adhemerval Zanella via Libc-alpha:

> diff --git a/time/bits/types/struct_timespec.h b/time/bits/types/struct_timespec.h
> index 489e81136d..b962d3f95f 100644
> --- a/time/bits/types/struct_timespec.h
> +++ b/time/bits/types/struct_timespec.h
> @@ -15,19 +15,9 @@ struct timespec
>  #else
>    __time_t tv_sec;		/* Seconds.  */
>  #endif
> -#if __WORDSIZE == 64 \
> -  || (defined __SYSCALL_WORDSIZE && __SYSCALL_WORDSIZE == 64) \
> -  || (__TIMESIZE == 32 && !defined __USE_TIME_BITS64)
> -  __syscall_slong_t tv_nsec;	/* Nanoseconds.  */
> -#else
> -# if __BYTE_ORDER == __BIG_ENDIAN
> -  int: 32;           /* Padding.  */
> -  long int tv_nsec;  /* Nanoseconds.  */
> -# else
> -  long int tv_nsec;  /* Nanoseconds.  */
> -  int: 32;           /* Padding.  */
> -# endif
> -#endif
> +  int : 8 * (sizeof (time_t) - sizeof (long)) * (__BYTE_ORDER == __BIG_ENDIAN);
> +  long int tv_nsec;
> +  int : 8 * (sizeof (time_t) - sizeof (long)) * (__BYTE_ORDER == __LITTLE_ENDIAN);
>  };

I think zero-width bitfields are problematic from an ABI perspective:

  zero width bitfields and ABIs
  <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102024>

The change is also not entirely ABI-compatible because existing code on
x32 depends on the implementation clearing the upper 32 bits, and if new
code copies around structs, it won't necessary copy the zero bits.

Thanks,
Florian