From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx169.postini.com [74.125.245.169]) by kanga.kvack.org (Postfix) with SMTP id DCE736B004D for ; Thu, 22 Dec 2011 10:35:01 -0500 (EST) Received: by iacb35 with SMTP id b35so15464912iac.14 for ; Thu, 22 Dec 2011 07:35:01 -0800 (PST) Message-ID: <4EF34E1E.6000805@gmail.com> Date: Thu, 22 Dec 2011 23:34:54 +0800 From: "nai.xia" MIME-Version: 1.0 Subject: Re: Question about missing "cld" in x86 string assembly code References: <201112172258.24221.nai.xia@gmail.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Brian Gerst Cc: Andi Kleen , Linus Torvalds , Linux-MM , LKML On 2011a1'12ae??18ae?JPY 01:08, Brian Gerst wrote: > On Sat, Dec 17, 2011 at 9:58 AM, Nai Xia 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 >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> Please read the FAQ at http://www.tux.org/lkml/ > > 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org