Re: gcc feature request / RFC: extra clobbered regs

[Jeff Law]

On 06/30/2015 04:02 PM, H. Peter Anvin wrote:
> On 06/30/2015 02:55 PM, Andy Lutomirski wrote:
> > On Tue, Jun 30, 2015 at 2:52 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> > > On 06/30/2015 02:48 PM, Andy Lutomirski wrote:
> > > > On Tue, Jun 30, 2015 at 2:41 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> > > > > On 06/30/2015 02:37 PM, Jakub Jelinek wrote:
> > > > > > I'd say the most natural API for this would be to allow
> > > > > > f{fixed,call-{used,saved}}-REG in target attribute.
> > > > >
> > > > > Either that or
> > > > >
> > > > >          __attribute__((fixed(rbp,rcx),used(rax,rbx),saved(r11)))
> > > > >
> > > > > ... just to be shorter.  Either way, I would consider this to be
> > > > > desirable -- I have myself used this to good effect in a past life
> > > > > (*cough* Transmeta *cough*) -- but not a high priority feature.
> > > >
> > > > I think I mean the per-function equivalent of -fcall-used-reg, so
> > > > hpa's "used" suggestion would do the trick.
> > > >
> > > > I guess that clobbering the frame pointer is a non-starter, but five
> > > > out of six isn't so bad.  It would be nice to error out instead of
> > > > producing "disastrous results", though, if another bad reg is chosen.
> > > > (Presumably the PIC register on PIC builds would be an example of
> > > > that.)
> > >
> > > Clobbering the frame pointer is perfectly fine, as is the PIC register.
> > >   However, gcc might need to handle them as "fixed" rather than "clobbered".
> >
> > Hmm.  True, I guess, although I wouldn't necessarily expect gcc to be
> > able to generate code to call a function like that.
>
> No, but you need to be able to call other functions, or you just push
> the issue down one level.
For ia32, the PIC register really isn't special anymore.  I'd be 
surprised if you couldn't clobber it.

jeff
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