[Mesa-dev] [PATCH 4/5] nir: Recognize mul(b2f(a), b2f(b)) as a logical AND.

[Jason Ekstrand]

On Wed, Mar 18, 2015 at 9:31 PM, Matt Turner <mattst88 at gmail.com> wrote:
> On Wed, Mar 18, 2015 at 9:27 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>> On Wed, Mar 18, 2015 at 8:59 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
>>> On Wed, Mar 18, 2015 at 11:35 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>>>>
>>>> On Mar 18, 2015 8:32 PM, "Matt Turner" <mattst88 at gmail.com> wrote:
>>>>>
>>>>> On Wed, Mar 18, 2015 at 7:39 PM, Jason Ekstrand <jason at jlekstrand.net>
>>>>> wrote:
>>>>> > On Wed, Mar 18, 2015 at 11:37 AM, Matt Turner <mattst88 at gmail.com>
>>>>> > wrote:
>>>>> >> Transform this into b2f(and(a, b)).
>>>>> >>
>>>>> >> total instructions in shared programs: 6205448 -> 6204391 (-0.02%)
>>>>> >> instructions in affected programs:     284030 -> 282973 (-0.37%)
>>>>> >> helped:                                903
>>>>> >> HURT:                                  6
>>>>> >> ---
>>>>> >>  src/glsl/nir/nir_opt_algebraic.py | 2 ++
>>>>> >>  1 file changed, 2 insertions(+)
>>>>> >>
>>>>> >> diff --git a/src/glsl/nir/nir_opt_algebraic.py
>>>>> >> b/src/glsl/nir/nir_opt_algebraic.py
>>>>> >> index ef855aa..f956edf 100644
>>>>> >> --- a/src/glsl/nir/nir_opt_algebraic.py
>>>>> >> +++ b/src/glsl/nir/nir_opt_algebraic.py
>>>>> >> @@ -95,6 +95,8 @@ optimizations = [
>>>>> >>     (('fsat', a), ('fmin', ('fmax', a, 0.0), 1.0),
>>>>> >> 'options->lower_fsat'),
>>>>> >>     (('fsat', ('fsat', a)), ('fsat', a)),
>>>>> >>     (('fmin', ('fmax', ('fmin', ('fmax', a, 0.0), 1.0), 0.0), 1.0),
>>>>> >> ('fmin', ('fmax', a, 0.0), 1.0)),
>>>>> >> +   # Emulating booleans
>>>>> >> +   (('fmul', ('b2f', a), ('b2f', b)), ('b2f', ('iand', a, b))),
>>>>> >
>>>>> > Those are only equivalent if the sources are known booleans.
>>>>> > Otherwise, no dice.
>>>>>
>>>>> Well... they're the source of a b2f. Are you saying that's not sufficient?
>>>>
>>>> No, that's not.  Fortunately, @bool should solve it for you in all of the
>>>> cases you care about.
>>>
>>> I think Matt has a point here. There's not much point in defining how
>>> b2f should work for things that aren't bools, and I'm fine with
>>> transforms that produce "bad"/undefined results when the input isn't 0
>>> or ~0. We should never get into that situation anyways. This is
>>> different from the issue that compare instructions always have to
>>> produce 0 or ~0, although both do stem from the fact that NIR doesn't
>>> have a bool type.
>>
>> Ok... In that case, we need to define some things better.  For
>> instance, bcsel is currently defined as "if nonzero do this else that"
>> which is very different from "if ~0 do this else if 0 do that else...
>
> The only place I see any comments about bcsel is in nir_opcodes.py, which says:
>
> # Conditional Select
> #
> # A vector conditional select instruction (like ?:, but operating per-
> # component on vectors). There are two versions, one for floating point
> # bools (0.0 vs 1.0) and one for integer bools (0 vs ~0).
>
> I'd be great if we documented the opcodes much in the same way TGSI's
> opcodes are documented. I get the sense that the documentation you're
> referring to is only in your brain. :)

Yes, official documentation would be good.  However, what I said above
is what constant folding does as well as the i965 backend (we do a
mov.nz to get the source).  Also, I thought there were other places
where we explicitly say 0 vs. non-zero.

In any case, we need to define these things better and I'm 100% ok
defining them as 0 vs ~0 and if you pass anything else, it's
undefined.  That would sure make optimizations easier.

>> who knows?"  I'm ok with either but we need to be clear.  If we stop
>> using the sloppy version that may mean that when translating from
>> other source languages that may not be strongly typed we'll have to do
>> a ine with zero to fix it up.
>> --Jason