linux-kernel - Re: Question about missing "cld" in x86 string assembly code

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Message-ID: <CAMzpN2inHxaSnaYaYXZ4Ya3rK+MWXqR6dN5NVNZy3=OvP04uQA@mail.gmail.com>
Date:	Sat, 17 Dec 2011 12:08:36 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	nai.xia@...il.com
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Question about missing "cld" in x86 string assembly code

On Sat, Dec 17, 2011 at 9:58 AM, Nai Xia <nai.xia@...il.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.
>
> =======
> 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
‘‘forward’’ (that is, zero) direction before entry and upon exit from
a function."  Therefore it can be assumed to be clear, unless
explicitly set.

--
Brian Gerst
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