[Mesa-dev] [PATCH 3/3] glsl: Optimize division by two with a right shift.

[Roland Scheidegger]

Am 25.02.2014 00:00, schrieb Matt Turner:
> ---
>  src/glsl/opt_algebraic.cpp | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp
> index 778638c..50342ac 100644
> --- a/src/glsl/opt_algebraic.cpp
> +++ b/src/glsl/opt_algebraic.cpp
> @@ -421,6 +421,9 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
>        }
>        if (is_vec_one(op_const[1]))
>  	 return ir->operands[0];
> +      if (is_vec_two(op_const[1]) && ir->type->is_integer()) {
> +         return rshift(ir->operands[0], new(mem_ctx) ir_constant(1));
> +      }
>        break;
>  
>     case ir_binop_dot:
> 

Does glsl require any particular rounding behavior for negative numbers
(like c99 does, which wouldn't work with shift emulation)? A quick look
didn't find anything but it might be implied somewhere...

Also, if you do this for two, wouldn't it make sense to extend that to
power-of-two numbers or is that too much cpu work?

Roland