Richard Biener <rguent...@suse.de> writes: > On Wed, 14 Jun 2023, Richard Sandiford wrote: > >> Richard Biener <rguent...@suse.de> writes: >> > 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. >> >> Clobbers are for side-effects. They don't start a def-use chain. >> E.g. any use after a full clobber is an uninitialised read rather >> than a read of the clobber ?result?. > > I see. So > > (parallel > (unspec stack_tie) > (clobber (mem:BLK ...))) > > then? I suppose it needs to be an unspec_volatile?
Yeah, it would need to be unspec_volatile, at which point it becomes quite a big hammer. > It feels like the stack_ties are a delicate hack preventing enough but > not too much optimization ... Yup. I think the only non-hacky way would be to have dedicated RTL for memory becoming valid and becoming invalid. Anything else is a compromise. But TBH, I still think the (set (mem:BLK …) (unspec:BLK …)) strikes the right balance, unless there's a specific argument otherwise. The effect on memory isn't a side effect (contrary to what clobber implies) but instead is the main purpose of allocating and deallocating stack memory. Thanks, Richard
