On Wed, 14 Jun 2023, Richard Sandiford wrote: > Richard Biener <rguent...@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 <rguent...@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)
