[Mesa-dev] [PATCH 08/10] nir: Simplify 0 < fabs(a)

[Patrick Baggett]

On Thu, Mar 10, 2016 at 3:08 PM, Patrick Baggett
<baggett.patrick at gmail.com> wrote:
> On Thu, Mar 10, 2016 at 12:25 PM, Ian Romanick <idr at freedesktop.org> wrote:
>> From: Ian Romanick <ian.d.romanick at intel.com>
>>
>> Sandy Bridge / Ivy Bridge / Haswell
>> total instructions in shared programs: 8462180 -> 8462174 (-0.00%)
>> instructions in affected programs: 564 -> 558 (-1.06%)
>> helped: 6
>> HURT: 0
>>
>> total cycles in shared programs: 117542462 -> 117542276 (-0.00%)
>> cycles in affected programs: 9768 -> 9582 (-1.90%)
>> helped: 12
>> HURT: 0
>>
>> Broadwell / Skylake
>> total instructions in shared programs: 8980833 -> 8980826 (-0.00%)
>> instructions in affected programs: 626 -> 619 (-1.12%)
>> helped: 7
>> HURT: 0
>>
>> total cycles in shared programs: 70077900 -> 70077714 (-0.00%)
>> cycles in affected programs: 9378 -> 9192 (-1.98%)
>> helped: 12
>> HURT: 0
>>
>> G45 and Ironlake showed no change.
>>
>> Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
>> ---
>>  src/compiler/nir/nir_opt_algebraic.py | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
>> index 4db8f84..1442ce8 100644
>> --- a/src/compiler/nir/nir_opt_algebraic.py
>> +++ b/src/compiler/nir/nir_opt_algebraic.py
>> @@ -108,6 +108,11 @@ optimizations = [
>>     # inot(a)
>>     (('fge', 0.0, ('b2f', a)), ('inot', a)),
>>
>> +   # 0.0 < fabs(a)
>> +   # 0.0 != fabs(a)  because fabs(a) must be >= 0
> I think this is wrong. Because >= 0.0 can mean that fabs(a) == 0.0 for
> some a, you can't say then fabs(a) != 0.0.
>
> Then, the counter-example is when a = 0.0
>
> 1) 0.0 != fabs(0.0)
> 2) 0.0 != 0.0
>
Rather, I mean the comment is wrong, but the conclusion that:
0 < fabs(a) <-> a != 0.0
is correct. You can just build a truth table or just observe that when
a == 0, 0 < 0 is false, and
when a != 0.0, fabs(a) will be > 0, so 0 < fabs(a) will be always true.



>> +   # 0.0 != a
>
>
>
>
>> +   (('flt', 0.0, ('fabs', a)), ('fne', a, 0.0)),
>> +
>>     (('fge', ('fneg', ('fabs', a)), 0.0), ('feq', a, 0.0)),
>>     (('bcsel', ('flt', a, b), a, b), ('fmin', a, b)),
>>     (('bcsel', ('flt', a, b), b, a), ('fmax', a, b)),
>> --
>> 2.5.0
>>
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