[Mesa-dev] [PATCH] glsl: If an "if" has no "then" or "else" code left, remove it.

[Kenneth Graunke]

On 04/11/2012 06:33 PM, Eric Anholt wrote:
> Cuts 8/1068 instructions from glyphy's fragment shaders on i965.
> ---
>   src/glsl/opt_if_simplification.cpp |    8 ++++++++
>   1 file changed, 8 insertions(+)
>
> diff --git a/src/glsl/opt_if_simplification.cpp b/src/glsl/opt_if_simplification.cpp
> index 940dd08..7e88208 100644
> --- a/src/glsl/opt_if_simplification.cpp
> +++ b/src/glsl/opt_if_simplification.cpp
> @@ -66,6 +66,14 @@ do_if_simplification(exec_list *instructions)
>   ir_visitor_status
>   ir_if_simplification_visitor::visit_leave(ir_if *ir)
>   {
> +   /* If the if statement has nothing on either side, remove it. */
> +   if (ir->then_instructions.is_empty()&&
> +       ir->else_instructions.is_empty()) {
> +      ir->remove();
> +      this->made_progress = true;
> +      return visit_continue;
> +   }
> +
>      /* FINISHME: Ideally there would be a way to note that the condition results
>       * FINISHME: in a constant before processing both of the other subtrees.
>       * FINISHME: This can probably be done with some flags, but it would take

Hah :) Such an obviously good idea.

Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>

I was about to say "but wait, side effects in the condition!" but 
quickly remembered: we can't represent those anymore.  It's nice not to 
have to worry about that. :)