Re: [stage1][PATCH] Lower VEC_COND_EXPR into internal functions.

[Richard Biener <richard.guenther@gmail.com>]

On June 3, 2020 8:23:14 PM GMT+02:00, Segher Boessenkool <segher@kernel.crashing.org> wrote:
>On Wed, Jun 03, 2020 at 07:23:47PM +0200, Richard Biener wrote:
>> >>   mask = vec_cmp of the comparison
>> >>   true_masked = true_op & mask;
>> >>   false_masked = false_op & ~mask;
>> >>   result = true_masked | false_masked;
>> >> 
>> >> but I believe this would be dead code never triggered.
>> >
>> >But that would be the generic code as well?  Is that not useful to
>have
>> >in any case?
>> 
>> Sure. If you remove the vcond patterns from your port the vectorizer
>will do this transparently for you. So if you do not actually have a
>more clever way of representing this in the ISA there's no point of the
>vcond patterns. (though I think the vec_cmp ones didn't originally
>exist) 
>
>So why can the expander not just do that whenever the patterns FAIL as
>well?

It could but all the vectorizer costing assumed it goes the 'cheaper' way. So this is kind of a sanity check. And what when vec_cmp expansion fails as well? Resort to scalar soft FP support as ultimate fallback? That sounds very wrong as a auto vectorization result... 

>> The point is the vectorizer relies on a optab query for querying
>backend support and power claims vcond support here. If you then FAIL
>you have lied. (not in your interpretation of the pattern docs but in
>the implementations since introduction of vcond named patterns) 
>
>Almost all RTL patterns are allowed to FAIL, and that is a very good
>thing.  If the vectoriser does not allow that, *it* is buggy.

Your opinion. Please suggest a better way to query target vector capabilities. 

>> So if you're happy I'll document explicitly that vector named
>patterns may not FAIL. 
>
>That will not work in general at all, no.  Please document it for only
>those RTL patterns you need it for (and it is documented per pattern
>currently, anyway).

Sure, will do. But as Richard said, the documented list is the other way around. 
At least that was my interpretation. 

All vectorizer queried optabs have this constraint BTW.

Richard. 

>
>Segher