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:
> 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?

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