From: Jeremy Fitzhardinge <jeremy@goop.org>
To: David Newall <davidn@davidnewall.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Screwy arch/x86/include/asm/string_32.h
Date: Sat, 14 Mar 2009 15:53:02 -0700 [thread overview]
Message-ID: <49BC354E.4070809@goop.org> (raw)
In-Reply-To: <49BBD619.6050809@davidnewall.com>
David Newall wrote:
> There's a lot of weirdness in __constant_memcpy in
> arch/x86/include/asm/string_32.h (2.6.29-rc7).
>
> 1. The first switch is missing a case for n equal 7.
>
That's because there's no way to copy 7 bytes in two operations of 1/2/4
bytes; it would need to be 3 (4+2+1), which I guess the author deemed
overly complex.
> 2. When n < 20, the four tests (n > 16, n > 12, n > 8 & n > 4)
> execute identical statements; always 'asm volatile("movsl" :
> "=&D"(edi), "=&S"(esi) : "0"(edi), "1"(esi) : "memory")'.
>
movs is a string instruction which has the side-effect of incrementing
esi and edi, so each if() falls through to copy the next 4 bytes.
However, the tail switch should probably use plain "mov" rather than
"movs" because nobody uses esi/edi afterwards.
J
prev parent reply other threads:[~2009-03-14 22:53 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-03-14 16:06 Screwy arch/x86/include/asm/string_32.h David Newall
2009-03-14 22:53 ` Jeremy Fitzhardinge [this message]
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