This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: x86 D flag: what's its status thru the program?
On Mon, Jul 05, 2004 at 07:19:06PM +0200, Lukasz Michal Rak wrote:
> > > The convention is that the D flag in IA32 is cleared. So if you call a
> > > subroutine you have to clear D flag before making call.
> >
> > Do you mean that 'I have to clear the D flag if I had set it somewhere
> > before?' Else what is the sense of the convention?
> Yes. Other code may assume that D is cleared and for example after calling
> your code will not check state of D flag, so if you will left D flag set
> this may cause errors.
>
> > > The subroutine can change the state of D flag, but before returning
> > > the routine has to clear the D flag.
> >
> > If the subroutine can change the flag, that it will violate the
> > convention, won't it? And what is the strength of the convention,
> > again?
> >
> Subroutine can change the D flag, but before exiting must clear it.
>
> The convention should make you free from checking the state of flag
> and clearing at the beginning of your code.
>
> > > Hope I helped.
> >
> > Thanks, but I still don't understand something. Again: if every
> > procedure keeps the flag clear at exit, then why the same every
> > procedure clears it at entry? In a suspicion, that caller has changed
> > the flag against the rules?
> Just to be sure that the D flag is cleared. The convetion is one thing and
> reality other.
Ok. Thanks! It's the thing that I didn't understand. BTW, where is
this convention documented, if either?