[Mesa-dev] [PATCH 6/6] ir_to_mesa: Try to avoid emitting a MOV_SAT to saturate an expression tree.

[Ian Romanick]

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On 07/29/2011 12:48 PM, Eric Anholt wrote:
> Fixes a regression in codegen quality for ff_fragment_shader
> conversion to GLSL -- glean texCombine produces 7.5% fewer Mesa IR
> instructions.
> ---
>  src/mesa/program/ir_to_mesa.cpp |   28 ++++++++++++++++++++++++----
>  1 files changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/src/mesa/program/ir_to_mesa.cpp b/src/mesa/program/ir_to_mesa.cpp
> index 8b4a535..f82b048 100644
> --- a/src/mesa/program/ir_to_mesa.cpp
> +++ b/src/mesa/program/ir_to_mesa.cpp
> @@ -925,13 +925,33 @@ ir_to_mesa_visitor::try_emit_sat(ir_expression *ir)
>     if (!sat_src)
>        return false;
>  
> +   ir_to_mesa_instruction *new_inst;
> +

Why not move this declaration down to the one place new_inst is set?

>     sat_src->accept(this);
>     src_reg src = this->result;
>  
> -   this->result = get_temp(ir->type);
> -   ir_to_mesa_instruction *inst;
> -   inst = emit(ir, OPCODE_MOV, dst_reg(this->result), src);
> -   inst->saturate = true;
> +   /* If we generated an expression instruction into a temporary in
> +    * processing the saturate's operand, apply the saturate to that
> +    * instruction.  Otherwise, generate a MOV to do the saturate.
> +    *
> +    * Note that we have to be careful to only do this optimization if
> +    * the instruction in question was in generating src->result -- for
> +    * example, ir_dereference_array would generate a MUL to create the
> +    * reladdr and return us a src reg using that reladdr, and that MUL
> +    * not the thing we're trying to saturate.

The "and that MUL.." bit doesn't parse for me.

> +    */
> +   ir_expression *sat_src_expr = sat_src->as_expression();
> +   new_inst = (ir_to_mesa_instruction *)this->instructions.get_tail();
> +   if (sat_src_expr && (sat_src_expr->operation == ir_binop_mul ||
> +			sat_src_expr->operation == ir_binop_add ||
> +			sat_src_expr->operation == ir_binop_dot)) {
> +      new_inst->saturate = true;
> +   } else {
> +      this->result = get_temp(ir->type);
> +      ir_to_mesa_instruction *inst;
> +      inst = emit(ir, OPCODE_MOV, dst_reg(this->result), src);
> +      inst->saturate = true;
> +   }
>  
>     return true;
>  }
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