linux-kernel - Re: Linux doesn't follow x86/x86-64 ABI wrt direction flag

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Message-ID: <47CF0627.2070200@zytor.com>
Date:	Wed, 05 Mar 2008 12:44:23 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Michael Matz <matz@...e.de>
CC:	Aurelien Jarno <aurelien@...el32.net>,
	Joe Buck <Joe.Buck@...opsys.COM>, linux-kernel@...r.kernel.org,
	gcc@....gnu.org
Subject: Re: Linux doesn't follow x86/x86-64 ABI wrt direction flag

Michael Matz wrote:
> Hi,
> 
> On Wed, 5 Mar 2008, Aurelien Jarno wrote:
> 
>>> So I think gcc at least needs an *option* to revert to the old behavior,
>>> and there's a good argument to make it the default for now, at least for
>>> x86/x86-64 on Linux.
>> And for other kernels. I tested OpenBSD 4.1, FreeBSD 6.3, NetBSD 4.0, 
>> they have the same behaviour as Linux, that is they don't clear DF 
>> before calling the signal handler.
> 
> Sigh.  We could perhaps insert a cld for all functions which can be 
> recognized as possible signal handlers and call other unknown or string 
> functions.  But it's probably even faster to emit cld in front of the 
> inline copies of mem functions again :-(
> 

Well, there is a (slight) difference: you know that a called function 
will not clobber your DF state; it's only the entry condition which is 
imprecise.

The best would be if this could be controlled by a flag, which we can 
flip once kernel fixes has been around for long enough.

	-hpa
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