[Mesa-dev] [PATCH 2/6] nir: Turn bcsel of +/- 1.0 and 0.0 into b2f sequences.

[Erik Faye-Lund]

On Wed, Aug 10, 2016 at 5:48 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> On Aug 10, 2016 1:02 AM, "Erik Faye-Lund" <kusmabite at gmail.com> wrote:
>>
>> On Wed, Aug 10, 2016 at 4:30 AM, Kenneth Graunke <kenneth at whitecape.org>
>> wrote:
>> > On Haswell (GL 3.3):
>> >
>> > total instructions in shared programs: 6208759 -> 6203860 (-0.08%)
>> > instructions in affected programs: 856541 -> 851642 (-0.57%)
>> > helped: 3157
>> > HURT: 113
>> > LOST:   7
>> > GAINED: 15
>> >
>> > On Broadwell (GL 4.4):
>> >
>> > total instructions in shared programs: 11637854 -> 11632016 (-0.05%)
>> > instructions in affected programs: 1055693 -> 1049855 (-0.55%)
>> > helped: 3900
>> > HURT: 176
>> > LOST:   1
>> > GAINED: 18
>> >
>> > Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
>> > ---
>> >  src/compiler/nir/nir_opt_algebraic.py | 4 ++++
>> >  1 file changed, 4 insertions(+)
>> >
>> > diff --git a/src/compiler/nir/nir_opt_algebraic.py
>> > b/src/compiler/nir/nir_opt_algebraic.py
>> > index 1cf614c..4e9896f 100644
>> > --- a/src/compiler/nir/nir_opt_algebraic.py
>> > +++ b/src/compiler/nir/nir_opt_algebraic.py
>> > @@ -251,6 +251,10 @@ optimizations = [
>> >     (('ieq', 'a at bool', False), ('inot', 'a')),
>> >     (('bcsel', a, True, False), ('ine', a, 0)),
>> >     (('bcsel', a, False, True), ('ieq', a, 0)),
>> > +   (('bcsel at 32', a, 1.0, 0.0), ('b2f', ('ine', a, 0))),
>> > +   (('bcsel at 32', a, 0.0, 1.0), ('b2f', ('ieq', a, 0))),
>> > +   (('bcsel at 32', a, -1.0, -0.0), ('fneg', ('b2f', ('ine', a, 0)))),
>> > +   (('bcsel at 32', a, -0.0, -1.0), ('fneg', ('b2f', ('ieq', a, 0)))),
>> >     (('bcsel', True, b, c), b),
>> >     (('bcsel', False, b, c), c),
>> >     # The result of this should be hit by constant propagation and, in
>> > the
>>
>> Same as the previous patch, this smells like intel-isms. Hardware that
>> has native bcsel with support for two inline immediates will do better
>> without.
>
> Why? If you're back-end handles b2f *worse* then bcsel of two components,
> then it's broken.   Also, we have a lot of optimization in NIR to help cut
> through "fake booleans" where shaders use 0.0 and 1.0 and math operations
> instead of actual booleans.  Recognising hand-coded b2f suddenly enables
> more of these optimization paths to run on the shader.  That 0.57% isn't all
> just immediates being removed.

You're right. I don't know what I was thinking.