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Re: [PATCH 2/2][BZ #16640] Remove strtok assembly implementation.
- From: OndÅej BÃlka <neleai at seznam dot cz>
- To: Carlos O'Donell <carlos at redhat dot com>
- Cc: libc-alpha at sourceware dot org
- Date: Fri, 14 Mar 2014 11:26:27 +0100
- Subject: Re: [PATCH 2/2][BZ #16640] Remove strtok assembly implementation.
- Authentication-results: sourceware.org; auth=none
- References: <20140227123238 dot GA26291 at domone dot podge> <20140227124206 dot GA26474 at domone dot podge> <5318A03D dot 3000705 at redhat dot com> <20140306163241 dot GA11843 at domone dot podge> <5318B58B dot 5040704 at redhat dot com> <20140306205212 dot GB11843 at domone dot podge> <53192422 dot 2050101 at redhat dot com> <20140310212923 dot GG6407 at domone dot podge> <53214E1B dot 6040601 at redhat dot com>
On Thu, Mar 13, 2014 at 02:20:11AM -0400, Carlos O'Donell wrote:
> On 03/10/2014 05:29 PM, OndÅej BÃlka wrote:
> > On Thu, Mar 06, 2014 at 08:42:58PM -0500, Carlos O'Donell wrote:
> >> On 03/06/2014 03:52 PM, OndÅej BÃlka wrote:
> >>> On Thu, Mar 06, 2014 at 12:51:07PM -0500, Carlos O'Donell wrote:
> >>>> On 03/06/2014 11:32 AM, OndÅej BÃlka wrote:
> >>>>> On Thu, Mar 06, 2014 at 11:20:13AM -0500, Carlos O'Donell wrote:
> >>>>>> On 02/27/2014 07:42 AM, OndÅej BÃlka wrote:
> >>>>>>> As followup this patch removes strtok assembly implementation as it is
> >>>>>>> around 2-4 times slower on sse4_2 capable machines which is a majority,
> >>>>>>> as previous benchtest demonstrated.
> >>>>>>>
> >>>>>>> You could try to gain some cycles on older machines by using ifuncs but
> >>>>>>> is a strtok important enough to complicate matters?
> >>>>>>>
> >>>>>>> OK to commit?
> >>>>>>>
> >>>>>>> * sysdeps/i386/i686/strtok.S: Remove.
> >>>>>>> * sysdeps/i386/i686/strtok_r.S: Likewise.
> >>>>>>> * sysdeps/i386/strtok.S: Likewise.
> >>>>>>> * sysdeps/i386/strtok_r.S: Likewise.
> >>>>>>> * sysdeps/x86_64/strtok.S: Likewise.
> >>>>>>> * sysdeps/x86_64/strtok_r.S: Likewise.
> >>>>>>
> >>>>>> Is this still the case with the oldest compiler we support?
> >>>>>>
> >>>>> I do not understand what are you reffering to.
> >>>>
> >>>> If you remove the strtok assembly implemetnation, the build will
> >>>> fall back on use the C implementation e.g. string/strtok.c.
> >>>> That C implementation will have been compiled by whichever
> >>>> compiler you're using for your build. In glibc we support a wide
> >>>> range of compilers from gcc 4.4 and newer. Do the older compilers,
> >>>> specifically gcc 4.4, still generate good code for strtok.c which
> >>>> is faster than strtok.S?
> >>>>
> >>> There is nothing to optimize, strtok code consist from two function
> >>> calls and some ifs and I do not see optimization oppurtinities here.
> >>>
> >>
> >> The problem I'm seeing is that we are removing assembly versions
> >> which might be faster than the code generated by the oldest and
> >> possible worst performing compiler that we support (assuming that
> >> gcc gets better with each revision).
> >>
> >> Could you please test with gcc 4.4 and see if it still generates
> >> 2-4x faster strtok or if the assembly version is faster?
> >>
> > When I tested this I found no difference in compilers but found that
> > speedup was caused by inlining. There is little difference on
> > implementations when I do not inline it.
>
> Are you saying then that the resulting code generated with gcc 4.4
> is approximately the same performance as that generated with a
> newer gcc, both of which are 2-4x faster than the assembly
> implementation?
>
> If that's the case then please commit your patch.
>
Not exactly. when I tested compilers strtok was slower because plt
avoidance called default not sse4_2 one. It needs additional patch to
remove hidden_def.