Re: [PATCH, i386]: Emit cld instruction when stringops are used

[Chris Lattner <clattner at apple dot com>]

> > > > You forget that there is a ton of code in the world that does:
> > > >
> > > > void foo() {
> > > >  asm ( " rep; movsb", ...)
> > > > }
> > > >
> > > > Knowing that the direction flag is clear on entry to the function.
> > > > You propose auditing all the *existing* inline asm code active in  
> > > > the
> > > > world?  I thought the issue here was that *user* space can't trust
> > > > that the direction flag is clear because of the kernel bug: if so,
> > > > this isn't just about inline asm in the kernel.
> > >
> > > Such code was broken in the past anyway, no need to try to fix that
> > > now.
> >
> > How so?
>
> Because it relies on the direction flag clear which is neither  
> guaranteed
> by the ABI, nor was ensured by GCC in previous releases.

You seem confused.  The i386 psABI explicitly states:

"EFLAGS:

The flags register contains the system ïags, such as the direction
flag and the carry flag. The direction flag must be set to the
ââforwardââ (that is, zero) direction before entry and upon exit
from a function. Other user flags have no specified role in the
standard calling sequence and are not preserved. "

That is pretty unambiguous.  GCC always preserves this property as  
well, in absence of the kernel bug.

-Chris