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

[Ian Romanick]

On 03/10/2016 01:24 PM, Patrick Baggett wrote:
> 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.

How about if I change it to

   # 0.0 != fabs(a)  Since fabs(a) >= 0, 0 <= fabs(a) must be true

I think it's trivial to see how to get from "0 < fabs(a)" to "0 !=
fabs(a)" based on that.

>>> +   # 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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