Re: [PATCH 3/3] Add i386 memset and memcpy assembly functions

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Thu, Aug 27, 2015 at 4:46 AM, OndÅej BÃlka <neleai@seznam.cz> wrote:
> On Wed, Aug 26, 2015 at 08:31:30AM -0700, H.J. Lu wrote:
>> On Wed, Aug 26, 2015 at 8:15 AM, OndÅej BÃlka <neleai@seznam.cz> wrote:
>> > On Wed, Aug 26, 2015 at 07:49:51AM -0700, H.J. Lu wrote:
>> >> On Wed, Aug 26, 2015 at 7:29 AM, OndÅej BÃlka <neleai@seznam.cz> wrote:
>> >> > On Wed, Aug 26, 2015 at 06:46:31AM -0700, H.J. Lu wrote:
>> >> >> Add i386 memset and memcpy assembly functions with REP MOVSB/STOSB
>> >> >> instructions.  They will be used to implement i386 multi-arch memcpy.
>> >> >>
>> >> >> OK for master?
>> >> >>
>> >> > No, as rep stosb has terrible performance on most of machines, on ivy
>> >> > bridge its around six times slower than rep stosq. I wouldn't be
>> >> >
>> >>
>> >> I added them for i386 memcpy family multiarch functions.  We have
>> >> memcpy for i586 and i686:
>> >>
>> >> sysdeps/i386/i586/memcpy.S
>> >> sysdeps/i386/i686/memcpy.S
>> >>
>> >> But we don't have it for i486.  I add them so that I can implement
>> >> i386 memset and memcpy family multiarch functions for i486,
>> >> i586 and i686 targets.  i386 memset and memcpy are used only
>> >> when
>> >>
>> >> 1. Building glibc for i486 with --disable-multi-arch.  Or
>> >> 2. Processor doesn't support i686 nor SSE2.
>> >>
>> >> I believe these are a very rare cases.
>> >>
>> > While true a existing implementation looked better. So if you need use
>> > assembly could you pick these files compiled with gcc -S or something
>> > like that?
>>
>> We don't know if they are better than REP MOVSB/STOSB in cases of
>>
>> 1. Building glibc for i486 with --disable-multi-arch.  Or
>> 2. Processor doesn't support i686 nor SSE2.
>>
>> and on Haswell/Skylake, REP MOVSB/STOSB aren't too bad.
>>
> I could accept that if we decide that we dont care about performance in
> these cases. As 1. user already doesn't care as we need to use very slow
> implementations with --disable-multi-arch.
>
> As for 2 its about if we care about performance of old machines or not.
> I would be for not optimizing for machines without sse2 as we don't have
> these to test that.
>
> With these arguments a change would be acceptable but of course I would
> prefer one that is better on sandy bridge with disable-multiarch.

If people want better performance on Sandy Bridge, they shouldn't
configure glibc as i486 with --disable-multi-arch :-(.

-- 
H.J.