[PATCH] Avoid reading errno in syscall implementations

[H.J. Lu]

On Mon, Oct 19, 2015 at 10:00 AM, Joseph Myers <joseph@codesourcery.com> wrote:
> On Mon, 19 Oct 2015, H.J. Lu wrote:
>
>> Reading errno is expensive for x86 PIC.  With INTERNAL_SYSCALL,
>> INTERNAL_SYSCALL_ERROR_P, INTERNAL_SYSCALL_ERRNO and
>> INLINE_SYSCALL_ERROR_RETURN_VALUE, we can avoid reading errno.
>
> I don't follow how this patch works.  How do you ensure that in the cases
> where there is an error that is not ENOSYS, errno does get set as it would
> have been before?
>
>> -  int res = INLINE_SYSCALL (eventfd2, 2, count, flags);
>>  # ifndef __ASSUME_EVENTFD2
>> -  if (res != -1 || errno != ENOSYS)
>> -# endif
>> +  INTERNAL_SYSCALL_DECL (err);
>> +  int res = INTERNAL_SYSCALL (eventfd2, err, 2, count, flags);
>> +  if (!__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (res, err))
>> +      || INTERNAL_SYSCALL_ERRNO (res, err) != ENOSYS)
>>      return res;
>
> E.g. this appears to be semantically different from the previous code -
> the previous code would have set errno here from a non-ENOSYS error, and
> the new code wouldn't.
>
> (__ASSUME_EVENTFD2 is always defined except on alpha.  Presumably your "no
> code changes" was because code paths used on x86_64 didn't get changed by
> the patch.)

Here is the updated patch.  Only getrlimit64.os and setrlimit64.os
are changed on i686.  OK for master?

Thanks.


-- 
H.J.
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