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

[Eric Anholt]

Connor Abbott <cwabbott0 at gmail.com> writes:

> On Wed, Aug 10, 2016 at 1:53 PM, Eric Anholt <eric at anholt.net> wrote:
>> Kenneth Graunke <kenneth at whitecape.org> writes:
>>
>>> 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)))),
>>
>> Isn't bcsel's first arg guaranteed to be 0 or ~0?  I thought that was
>> how nir's bcsel (and b2f) worked.  If not, it would be good to see this
>> documented.
>
> Yes, any ALU operation that takes a boolean input can expect it to be
> 0 or ~0. If that isn't true, then something else in the compiler has
> messed up. So I think we can replace 'a != 0' with 'a' and 'a == 0'
> with '!a'.

Yeah.  So, I like the contents of this series a lot, but it would be
nice to see some of the @bool annotations dropped.
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