[Mesa-dev] [PATCH 2/3] glsl: Optimize clamp(x, 0.0, b), where b < 1.0 as saturate(min(x, b))

[Ilia Mirkin]

On Tue, Jun 24, 2014 at 1:32 AM, Abdiel Janulgue
<abdiel.janulgue at linux.intel.com> wrote:
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue at linux.intel.com>
> ---
>  src/glsl/opt_algebraic.cpp | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp
> index 8d7609d..0d694b6 100644
> --- a/src/glsl/opt_algebraic.cpp
> +++ b/src/glsl/opt_algebraic.cpp
> @@ -589,6 +589,10 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
>              /* Found a min (max(x, 0), 1.0) */
>              if (outer_const->is_one() && inner_val_a->is_zero())
>                 return saturate(inner_val_b);
> +
> +            /* Found a min (max(x, 0.0) b), where b < 1.0 */
> +            if ((outer_const->get_float_component(0) < 1.0f) && inner_val_b->is_zero())
> +               return saturate(expr(ir_binop_min, inner_val_a, outer_const));

Not sure about other GPU's, but on reasonably modern nvidia gpu's, in
addition to being able to just saturate a single var, some other
instructions take a "saturate" modifier (like fp add/multiply). But
not min/max. If you were to write this as

min(saturate(a), b)

then it would be easier to propagate the saturate as a modifier to
whatever expression produces 'a' without having a clever optimization
that moves it through min/max.

Cheers,

  -ilia

>           }
>        }
>
> --
> 1.9.1
>
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