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Date:      Thu, 9 May 2013 15:36:59 +0300
From:      Konstantin Belousov <kostikbel@gmail.com>
To:        Jilles Tjoelker <jilles@stack.nl>
Cc:        src-committers@freebsd.org, Andrey Chernov <ache@freebsd.org>, svn-src-stable@freebsd.org, svn-src-all@freebsd.org, svn-src-stable-9@freebsd.org, Sergey Kandaurov <pluknet@freebsd.org>
Subject:   Re: svn commit: r250215 - stable/9/lib/libc/locale
Message-ID:  <20130509123659.GU3047@kib.kiev.ua>
In-Reply-To: <20130509122001.GB48322@stack.nl>
References:  <201305031552.r43FqiPN024580@svn.freebsd.org> <5183E899.4000503@freebsd.org> <CAE-mSO%2BB_p_HCbKwSO-rJ%2BdforcPEfThmOxy%2BKi_1e9zPn3q_w@mail.gmail.com> <20130503195540.GA52657@stack.nl> <CAE-mSOLT6EdaYQheNka%2B%2BNPZRbUFM=kXv6i9k=uRiyQTy1JuuA@mail.gmail.com> <5184ED7E.3040703@freebsd.org> <CAE-mSO%2BJOTcfx1vDbiux8LpikZV0J1ti2HJ0ypCsotfeJ4qKzg@mail.gmail.com> <51851969.6020802@freebsd.org> <CAE-mSOKPjx-tF5gtXXtNUHYraPL-Rd1FPxq5ECw8Nbup=jahng@mail.gmail.com> <20130509122001.GB48322@stack.nl>

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On Thu, May 09, 2013 at 02:20:01PM +0200, Jilles Tjoelker wrote:
> I think architecture-specific memcmp() for i386 and amd64 can still be
> beneficial because of the fast unaligned access offered by these CPUs,
> which allows comparison of 4 or 8 bytes at a time. SSE2 allows
> comparison of 16 bytes at a time but is somewhat harder: not all i386
> CPUs support SSE2, unaligned access is slow on some older CPUs and it
> requires assembly so it only uses %xmm8-%xmm15 so rtld does not trash
> function parameters (or rtld needs to use non-SSE2 code).

FWIW, rtld is not allowed to modify any registers in the bind code
called from the PLT trampoline. The C ABI is not mandated for the
functions resolved through the PLT, so our rtld care to not destroy even
caller-save or scratch registers, at least on x86*.

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