[Mesa-dev] [PATCH] i965/fs: Drop no-op shifts by 0.

[Erik Faye-Lund]

Why is this tagged as i965/fs, when everything seems to happen in the
glsl-optimizer?

On Thu, Oct 24, 2013 at 5:53 PM, Eric Anholt <eric at anholt.net> wrote:
> I noticed this in a shader in Unigine Heaven that was spilling.  While it
> doesn't really reduce register pressure, it shaves a few instructions
> anyway (7955 -> 7882).
> ---
>  src/glsl/opt_algebraic.cpp | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp
> index 37b2f02..ff06cfc 100644
> --- a/src/glsl/opt_algebraic.cpp
> +++ b/src/glsl/opt_algebraic.cpp
> @@ -387,6 +387,14 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
>        }
>        break;
>
> +   case ir_binop_rshift:
> +   case ir_binop_lshift:
> +      if (is_vec_zero(op_const[0]))
> +         return ir->operands[1];
> +      else if (is_vec_zero(op_const[1]))
> +         return ir->operands[0];
> +      break;
> +

Maybe update progress inside the conditionals also?

But wait a minute. x shifted by 0 is x, so the latter part looks
correct. But the first conditional seems to assume that 0 sifted by x
is x, but it's really 0, no? Shouldn't both cases return
ir->operands[0]? What am I missing?