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

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Message-ID: <4EF34E1E.6000805@gmail.com>
Date:	Thu, 22 Dec 2011 23:34:54 +0800
From:	"nai.xia" <nai.xia@...il.com>
To:	Brian Gerst <brgerst@...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 2011年12月18日 01:08, Brian Gerst wrote:
> 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.

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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