[Mesa-dev] [PATCH 1/4] nir: Do basic constant reassociation.

[Jason Ekstrand]

On Fri, Apr 8, 2016 at 8:21 AM, Eduardo Lima Mitev <elima at igalia.com> wrote:

> On 04/08/2016 01:35 AM, Kenneth Graunke wrote:
>
>> Many shaders contain expression trees of the form:
>>
>>      const_1 * (value * const_2)
>>
>> Reorganizing these to
>>
>>      (const_1 * const_2) * value
>>
>> will allow constant folding to combine the constants.  Sometimes, these
>> constants are 2 and 0.5, so we can remove a multiply altogether.  Other
>> times, it can create more immediate constants, which can actually hurt.
>>
>
Further justification would be that the original was 4 NIR instructions: 2
load_const and 2 multiplies.  Once constant folding takes over, we are
again left with 4 NIR instructions: 2 load_const and 2 multiplies.  The
difference is that we now have (value * const_2) and (value * (const_1 *
const_2)).  So we have no more instructions in the end and, hopefully, the
(value * const_2) goes away.  Obviously, the emperical evidence says this
actually hurts sometimes, but that's not surprising.


> Finding a good balance here is tricky.  While much more could be done,
>> this simple patch seems to have a lot of positive benefit while having
>> a low downside.
>>
>>
> Makes sense.
>
> Reviewed-by: Eduardo Lima Mitev <elima at igalia.com>
>
> shader-db results on Broadwell:
>>
>> total instructions in shared programs: 8963768 -> 8961369 (-0.03%)
>> instructions in affected programs: 438318 -> 435919 (-0.55%)
>> helped: 1502
>> HURT: 245
>>
>> total cycles in shared programs: 71527354 -> 71421516 (-0.15%)
>> cycles in affected programs: 11541788 -> 11435950 (-0.92%)
>> helped: 3445
>> HURT: 1224
>>
>>
> FWIW, these are my results on HSW:
>
> total instructions in shared programs: 6223995 -> 6222470 (-0.02%)
> instructions in affected programs: 300945 -> 299420 (-0.51%)
> helped: 1231
> HURT: 240
>
> total cycles in shared programs: 56809432 -> 56615552 (-0.34%)
> cycles in affected programs: 7980160 -> 7786280 (-2.43%)
> helped: 2087
> HURT: 634
>
> Eduardo
>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
>> ---
>>   src/compiler/nir/nir_opt_algebraic.py | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/src/compiler/nir/nir_opt_algebraic.py
>> b/src/compiler/nir/nir_opt_algebraic.py
>> index e72b4a7..420d9d9 100644
>> --- a/src/compiler/nir/nir_opt_algebraic.py
>> +++ b/src/compiler/nir/nir_opt_algebraic.py
>> @@ -274,6 +274,14 @@ optimizations = [
>>      (('fmul', ('fneg', a), b), ('fneg', ('fmul', a, b))),
>>      (('imul', ('ineg', a), b), ('ineg', ('imul', a, b))),
>>
>> +   # Reassociate constants in add/mul chains so they can be folded
>> together.
>> +   # For now, we only handle cases where the constants are separated by
>> +   # a single non-constant.  We could do better eventually.
>> +   (('~fmul', '#a', ('fmul', b, '#c')), ('fmul', ('fmul', a, c), b)),
>> +   (('imul', '#a', ('imul', b, '#c')), ('imul', ('imul', a, c), b)),
>> +   (('~fadd', '#a', ('fadd', b, '#c')), ('fadd', ('fadd', a, c), b)),
>> +   (('iadd', '#a', ('iadd', b, '#c')), ('iadd', ('iadd', a, c), b)),
>> +
>>      # Misc. lowering
>>      (('fmod', a, b), ('fsub', a, ('fmul', b, ('ffloor', ('fdiv', a,
>> b)))), 'options->lower_fmod'),
>>      (('uadd_carry', a, b), ('b2i', ('ult', ('iadd', a, b), a)),
>> 'options->lower_uadd_carry'),
>>
>>
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