Re: [PATCH] Add emulated gather capability to the vectorizer

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

Richard Biener <rguenther@suse.de> writes:
> On Mon, 2 Aug 2021, Richard Sandiford wrote:
>
>> Richard Biener <rguenther@suse.de> writes:
>> > On Fri, 30 Jul 2021, Richard Sandiford wrote:
>> >> > @@ -9456,6 +9499,51 @@ vectorizable_load (vec_info *vinfo,
>> >> >  			data_ref = NULL_TREE;
>> >> >  			break;
>> >> >  		      }
>> >> > +		    else if (memory_access_type == VMAT_GATHER_SCATTER)
>> >> > +		      {
>> >> > +			/* Emulated gather-scatter.  */
>> >> > +			gcc_assert (!final_mask);
>> >> 
>> >> For this to be safe, we need to make sure that
>> >> check_load_store_for_partial_vectors clears
>> >> LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P.  Is that already guaranteed?
>> >> (Maybe yes, I didn't look closely. :-))
>> >
>> > I've made sure to fail earlier:
>> >
>> > @@ -8692,6 +8725,15 @@ vectorizable_load (vec_info *vinfo,
>> >                              "unsupported access type for masked 
>> > load.\n");
>> >           return false;
>> >         }
>> > +      else if (memory_access_type == VMAT_GATHER_SCATTER
>> > +              && gs_info.ifn == IFN_LAST
>> > +              && !gs_info.decl)
>> > +       {
>> > +         if (dump_enabled_p ())
>> > +           dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
>> > +                            "unsupported masked emulated gather.\n");
>> > +         return false;
>> > +       }
>> >      }
>> >
>> > at least I don't see a way to emulate masked gathers ;)
>> 
>> Yeah, that handles masked loads created by ifcvt.  But we also need to
>> protect against an unconditional load being converted into a predicated
>> load for partial loop vectorisation.  I think we still need a check in
>> check_load_store_for_partial_vectors to handle that.  The check wouldn't
>> prevent vectorisation, it would just prevent using fully-predicated loops.
>
> But that seems to check for availability of the masked gather IFN which
> of course isn't available and then sets 
> LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P to false:

Ah, yeah, OK.  I wrongly thought that the relaxed check for gather/scatter
support might have affected that too.

Should be OK then. :-)

Richard