[Mesa-dev] [PATCH] nir/algebraic: Replace a-fract(a) with floor(a)

[Timothy Arceri]

On 23/2/19 4:09 pm, Ian Romanick wrote:
> From: Ian Romanick <ian.d.romanick at intel.com>
> 
> I noticed this while looking at a shader that was affected by Tim's
> "more loop unrolling" series.
> 
> All Gen6+ platforms had similar results. (Skylake shown)
> total instructions in shared programs: 15437001 -> 15435259 (-0.01%)
> instructions in affected programs: 213651 -> 211909 (-0.82%)
> helped: 988
> HURT: 0
> helped stats (abs) min: 1 max: 27 x̄: 1.76 x̃: 1
> helped stats (rel) min: 0.15% max: 11.54% x̄: 1.14% x̃: 0.59%
> 95% mean confidence interval for instructions value: -1.89 -1.63
> 95% mean confidence interval for instructions %-change: -1.23% -1.05%
> Instructions are helped.
> 
> total cycles in shared programs: 383007378 -> 382997063 (<.01%)
> cycles in affected programs: 1650825 -> 1640510 (-0.62%)
> helped: 679
> HURT: 302

Why the hurt on Gen6+ is this something that should be in the late 
optimisations pass?


> helped stats (abs) min: 1 max: 348 x̄: 23.39 x̃: 14
> helped stats (rel) min: 0.04% max: 28.77% x̄: 1.61% x̃: 0.98%
> HURT stats (abs)   min: 1 max: 250 x̄: 18.43 x̃: 7
> HURT stats (rel)   min: 0.04% max: 25.86% x̄: 1.41% x̃: 0.53%
> 95% mean confidence interval for cycles value: -13.05 -7.98
> 95% mean confidence interval for cycles %-change: -0.86% -0.50%
> Cycles are helped.
> 
> Iron Lake and GM45 had similar results. (GM45 shown)
> total instructions in shared programs: 5043616 -> 5043010 (-0.01%)
> instructions in affected programs: 119691 -> 119085 (-0.51%)
> helped: 432
> HURT: 0
> helped stats (abs) min: 1 max: 27 x̄: 1.40 x̃: 1
> helped stats (rel) min: 0.10% max: 8.11% x̄: 0.66% x̃: 0.39%
> 95% mean confidence interval for instructions value: -1.58 -1.23
> 95% mean confidence interval for instructions %-change: -0.72% -0.59%
> Instructions are helped.
> 
> total cycles in shared programs: 128139812 -> 128135762 (<.01%)
> cycles in affected programs: 3829724 -> 3825674 (-0.11%)
> helped: 602
> HURT: 0
> helped stats (abs) min: 2 max: 486 x̄: 6.73 x̃: 6
> helped stats (rel) min: 0.02% max: 4.85% x̄: 0.19% x̃: 0.10%
> 95% mean confidence interval for cycles value: -8.40 -5.05
> 95% mean confidence interval for cycles %-change: -0.22% -0.16%
> Cycles are helped.
> ---
>   src/compiler/nir/nir_opt_algebraic.py | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
> index ba27d702b5d..c8fc938cc8f 100644
> --- a/src/compiler/nir/nir_opt_algebraic.py
> +++ b/src/compiler/nir/nir_opt_algebraic.py
> @@ -127,6 +127,7 @@ optimizations = [
>      (('flrp at 32', a, b, c), ('fadd', ('fmul', c, ('fsub', b, a)), a), 'options->lower_flrp32'),
>      (('flrp at 64', a, b, c), ('fadd', ('fmul', c, ('fsub', b, a)), a), 'options->lower_flrp64'),
>      (('ffloor', a), ('fsub', a, ('ffract', a)), 'options->lower_ffloor'),
> +   (('fadd', a, ('fneg', ('ffract', a))), ('ffloor', a), '!options->lower_ffloor'),
>      (('ffract', a), ('fsub', a, ('ffloor', a)), 'options->lower_ffract'),
>      (('fceil', a), ('fneg', ('ffloor', ('fneg', a))), 'options->lower_fceil'),
>      (('~fadd', ('fmul', a, ('fadd', 1.0, ('fneg', ('b2f', 'c at 1')))), ('fmul', b, ('b2f', c))), ('bcsel', c, b, a), 'options->lower_flrp32'),
>