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Re: [PATCH 3/3] Add i386 memset and memcpy assembly functions
- From: OndÅej BÃlka <neleai at seznam dot cz>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: GNU C Library <libc-alpha at sourceware dot org>
- Date: Thu, 27 Aug 2015 13:46:31 +0200
- Subject: Re: [PATCH 3/3] Add i386 memset and memcpy assembly functions
- Authentication-results: sourceware.org; auth=none
- References: <20150826134631 dot GC19484 at gmail dot com> <20150826142924 dot GC13215 at domone> <CAMe9rOrceRyZnYA2qdhni=nNUPswXCNVgVMhtaqVf74XMgyL5g at mail dot gmail dot com> <20150826151556 dot GA21092 at domone> <CAMe9rOrGJtSp3FPntkyYH_gg-s4eR_8PdkKtBQi5MazzcBD3yg at mail dot gmail dot com>
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.