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

[Patrick Baggett]

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

> +   # 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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