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:
> > 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.
> 
> 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?  It feels like
the stack_ties are a delicate hack preventing enough but not too
much optimization ...

> In contrast, a set of memory with an unspec source is in dataflow terms
> the same as a set of memory with a specified source.  (some unspecs
> actually have well-defined values, it's just that only the target code
> knows what those well-defined value are.)
> 
> So a set of memory could only be removed if DSE proves that there are no
> reads of the set bytes before the next set(s) to the same bytes of memory.
> And memory is always live.
> 
> Thanks,
> Richard
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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