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

[Richard Sandiford <richard.sandiford@arm.com>]

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.

Thanks,
Richard