Re: [PATCH] rs6000: replace '(const_int 0)' to 'unspec:BLK [(const_int 0)]' for stack_tie

[Richard Biener <rguenther@suse.de>]

On Wed, 14 Jun 2023, Richard Sandiford wrote:

> Richard Biener <rguenther@suse.de> writes:
> > AFAIU this special instruction is only supposed to prevent
> > code motion (of stack memory accesses?) across this instruction?
> > I'd say a
> >
> >   (may_clobber (mem:BLK (reg:DI 1 1)))
> >
> > might be more to the point?  I've used "may_clobber" which doesn't
> > exist since I'm not sure whether a clobber is considered a kill.
> > The docs say "Represents the storing or possible storing of an 
> > unpredictable..." - what is it? Storing or possible storing?
> 
> I'd also understood it to be either.  As in, it is a may-clobber
> that can be used for must-clobber.  Alternatively: the value stored
> is unpredictable, and can therefore be the same as the current value.
> 
> I think the main difference between:
> 
>   (clobber (mem:BLK ?))
> 
> and
> 
>   (set (mem:BLK ?) (unspec:BLK ?))
> 
> is that the latter must happen for correctness (unless something
> that understands the unspec proves otherwise) whereas a clobber
> can validly be dropped.  So for something like stack_tie, a set
> seems more correct than a clobber.

How can a clobber be validly dropped?  For the case of stack
memory if there's no stack use after it it could be elided
and I suppose the clobber itself can be moved.  But then
the function return is a stack use as well.

Btw, with the same reason the (set (mem:...)) could be removed, no?
Or is the (unspec:) SET_SRC having implicit side-effects that
prevents the removal (so rs6000 could have its stack_tie removed)?

That said, I fail to see how a clobber is special here.

Richard.

> Thanks,
> Richard
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)