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Re: [PATCH 01/10] Add vectorized getenv for glibc use
- From: OndÅej BÃlka <neleai at seznam dot cz>
- To: Andi Kleen <andi at firstfloor dot org>
- Cc: libc-alpha at sourceware dot org, Andi Kleen <ak at linux dot intel dot com>
- Date: Fri, 17 May 2013 18:16:32 +0200
- Subject: Re: [PATCH 01/10] Add vectorized getenv for glibc use
- References: <1368756775-23245-1-git-send-email-andi at firstfloor dot org> <1368756775-23245-2-git-send-email-andi at firstfloor dot org> <20130517073751 dot GA24334 at domone dot kolej dot mff dot cuni dot cz> <20130517154854 dot GB24555 at two dot firstfloor dot org>
On Fri, May 17, 2013 at 05:48:54PM +0200, Andi Kleen wrote:
> On Fri, May 17, 2013 at 09:37:51AM +0200, OndÅej BÃlka wrote:
> > On Thu, May 16, 2013 at 07:12:46PM -0700, Andi Kleen wrote:
> > > From: Andi Kleen <ak@linux.intel.com>
> > >
> > > This adds a general "vectorized getenv" for glibc internal use.
> > > The motivation is to allow subsystems to access environment variables
> > > cheaply without having to rescan the environment completely.
> > >
> >
> > > + if (__builtin_expect ((*current)[0] == 'G', 0)
> > > + && (*current)[1] == 'L' && (*current)[2] == 'I'
> > > + && (*current)[3] == 'B' && (*current)[4] == 'C'
> > > + && (*current)[5] == '_')
> > This pattern repeats few times. A memcmp(current,"GLIBC",5) is shorter.
>
> Just copied this from the dynamic linker.
>
> Don't really care about "short" just that the first character
> comparison is fast. In principle this could be all vectorized
> too at least on x86 to compare 32 or 64bits, but I was slightly
> worried about overrunning the end.
>
> If you want to fix it I think it should be a indepdendent followup all over
> the tree. Most likely it would be a candidate for coccinelle script.
>
I learned about nice tool, thanks.
> So I'll just leave it as is for now.
>
I will leave it and post followup.
Ondra.