[Mesa-dev] [PATCH] glsl: Optimize 1/exp(x) into exp(-x).

[Jason Ekstrand]

Looks fine to me but why are we still adding algebraic optimizations to
GLSL-IR?

Reviewed-by: Jason Ekstrand <jason.ekstrand at intel.com>
On Feb 10, 2015 12:36 PM, "Matt Turner" <mattst88 at gmail.com> wrote:

> Lots of shaders divide by exp2(...) which we turn into a multiplication
> by the reciprocal. We can avoid the reciprocal by simply negating exp2's
> argument.
>
> total instructions in shared programs: 5947154 -> 5946695 (-0.01%)
> instructions in affected programs:     118661 -> 118202 (-0.39%)
> helped:                                380
> ---
>  src/glsl/opt_algebraic.cpp | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp
> index c6f4a9c..616ed37 100644
> --- a/src/glsl/opt_algebraic.cpp
> +++ b/src/glsl/opt_algebraic.cpp
> @@ -747,6 +747,12 @@ ir_algebraic_visitor::handle_expression(ir_expression
> *ir)
>        if (op_expr[0] && op_expr[0]->operation == ir_unop_rcp)
>          return op_expr[0]->operands[0];
>
> +      if (op_expr[0] && (op_expr[0]->operation == ir_unop_exp2 ||
> +                         op_expr[0]->operation == ir_unop_exp)) {
> +         return new(mem_ctx) ir_expression(op_expr[0]->operation,
> ir->type,
> +                                           neg(op_expr[0]->operands[0]));
> +      }
> +
>        /* While ir_to_mesa.cpp will lower sqrt(x) to rcp(rsq(x)), it does
> so at
>         * its IR level, so we can always apply this transformation.
>         */
> --
> 2.0.5
>
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