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

H. Peter Anvin
2008-03-05T20:48:33Z

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