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

[Connor Abbott]

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'.

>
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