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Date: Sat, 17 Dec 2011 12:08:36 -0500
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Subject: Re: Question about missing "cld" in x86 string assembly code
From: Brian Gerst <brgerst@gmail.com>
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Cc: Andi Kleen <ak@linux.intel.com>, Linus Torvalds <torvalds@linux-foundation.org>, Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>

On Sat, Dec 17, 2011 at 9:58 AM, Nai Xia <nai.xia@gmail.com> wrote:
> Hi Andi,
>
> Seems I used a stale email address of you from a related git commit log,
> so this is a resend, sorry.
>
> =3D=3D=3D=3D=3D=3D=3D
> Hi,
>
> I notice that all x86 assembly code for string operations containing
> "scasb, lodsb", etc does not have "cld" at the beginning.
> Is this 100% safe?
> Or in other words, how could we be sure that
> there is no "std" generated by compiler somewhere just before
> the string operations?
>
>
> Thanks,
>
> Nai
> --
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The i386 ELF ABI states "The direction flag must be set to the
=E2=80=98=E2=80=98forward=E2=80=99=E2=80=99 (that is, zero) direction befor=
e entry and upon exit from
a function."  Therefore it can be assumed to be clear, unless
explicitly set.

--
Brian Gerst

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