On Tue, 17 Feb 2026, Robin Dapp wrote:
> > I wonder how much if-then-else processing code would trip on the
> > unexpected mode...
>
> Yeah, it should rather be like
> (if_then_else:V4SI
> (vec_predicate:V4BI
>
> >
> > What I do not like is that this if-then-else gets us a third way of
> > selecting lanes, apart from vec_merge and (vec_select (vec_concat ..)).
> > I was always hoping we can get rid of vec_merge and/or
> > (vec_select (vec_concat ..)) in favor of a single way to do permutes.
> > Your if-then-else can only do blends like vec_merge. Maybe that's
> > a fine thing, but then can we get rid of vec_merge?
>
> My mental model was always that a masked operation directly maps to an
> operation on the active elements and then a merge of the inactive ones.
> I'd think the proposed vec_predicate would be a superset of vec_merge
> but I was hoping to introduce it without rewriting half of the x86 code ;)
>
> vec_select's more advanced generic part/element selection and permutes would
> be
> difficult to incorporate I think.
>
> > Why two predicates? Can you explain?
>
> Like an outer length predicate and a nested mask predicate each with specific
> policy, basically nesting the whole insn:
>
> (define_insn "*addv4si"
> [(set (match_operand:V4SI 0 "register_operand" ...)
> (if_then_else:V4BI
> (vec_predicate
> ("register_or_immediate" # length)
> (... # bias?)
> ( # "tail_policy"))
> (if_then_else:V4BI
> (vec_predicate
> ("register_or_immediate" # mask)
> ( # "mask_policy"))
> (plus:V4SI
> (match_operand 1)
> (match_operand 2)...
Ah, OK.
> sve and x86 would only have the inner mask-predicated insn. But I strongly
> prefer no nesting.
So it would be sth like
(vec_predicate:V4BI
(and:V4BI
(reg:V4BI ..)
(whilelt:V4BI (reg:SI ..)))
for a combined mask (reg:V4BI) and length (reg:SI)? Or are you
thinking of "unstructured" (aka, sort-of-UNSPEC)
(vec_predicate:V4BI
(reg:V4BI ...)
(reg:SI ...)
...
and on targets w/o length use, what, NULL_RTX?
Richard.
--
Richard Biener <[email protected]> SUSE Software Solutions Germany
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