From: "nai.xia" <nai.xia@gmail.com>
To: Brian Gerst <brgerst@gmail.com>
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>
Subject: Re: Question about missing "cld" in x86 string assembly code
Date: Thu, 22 Dec 2011 23:34:54 +0800 [thread overview]
Message-ID: <4EF34E1E.6000805@gmail.com> (raw)
In-Reply-To: <CAMzpN2inHxaSnaYaYXZ4Ya3rK+MWXqR6dN5NVNZy3=OvP04uQA@mail.gmail.com>
On 2011a1'12ae??18ae?JPY 01:08, Brian Gerst wrote:
> 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.
>>
>> =======
>> 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
> a??a??forwarda??a?? (that is, zero) direction before entry and upon exit from
> a function." Therefore it can be assumed to be clear, unless
> explicitly set.
Hmm, I get those lines now. Thanks for reply.
Seems gcc started to strictly conform to this ABI since 4.3 . But glibc
seems still have some leading "cld" string code, maybe just prepared for
non-conforming kernels other than Linux.
>
> --
> Brian Gerst
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next prev parent reply other threads:[~2011-12-22 15:35 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-17 14:58 Question about missing "cld" in x86 string assembly code Nai Xia
2011-12-17 17:08 ` Brian Gerst
2011-12-17 18:39 ` Linus Torvalds
2011-12-22 15:52 ` nai.xia
2011-12-22 15:34 ` nai.xia [this message]
-- strict thread matches above, loose matches on Subject: below --
2011-12-17 14:52 Nai Xia
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