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

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Message-ID: <CA+55aFzyEgDUSaNVQ1Nw5SBNd36Cvb-KrVdc1MYj+oRJt8xWgg@mail.gmail.com>
Date:	Sat, 17 Dec 2011 10:39:03 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	nai.xia@...il.com, Andi Kleen <ak@...ux.intel.com>,
	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 9:08 AM, Brian Gerst <brgerst@...il.com> wrote:
>
> 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.

The exception, of course, being bootup, fault and interrupt handlers,
and after we've called out to foreign code (ie BIOS).

So there *are* a few cld's sprinkled around, they are just fairly rare.

                    Linus
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