[Mesa-dev] [PATCH 2/2] nir/lower_vec_to_movs: Don't emit unneeded movs

[Matt Turner]

On Tue, Sep 22, 2015 at 10:01 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> It's possible that, if a vecN operation is involved in a phi node, that we
> could end up moving from a register to itself.  If swizzling is involved,
> we need to emit the move but.  However, if there is no swizzling, then the
> mov is a no-op and we might as well not bother emitting it.
>
> Shader-db results on Haswell:
>
>    total instructions in shared programs: 6262536 -> 6259558 (-0.05%)
>    instructions in affected programs:     184780 -> 181802 (-1.61%)
>    helped:                                838
>    HURT:                                  0
> ---
>  src/glsl/nir/nir_lower_vec_to_movs.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/src/glsl/nir/nir_lower_vec_to_movs.c b/src/glsl/nir/nir_lower_vec_to_movs.c
> index 287f2bf..2039891 100644
> --- a/src/glsl/nir/nir_lower_vec_to_movs.c
> +++ b/src/glsl/nir/nir_lower_vec_to_movs.c
> @@ -83,7 +83,24 @@ insert_mov(nir_alu_instr *vec, unsigned start_idx, nir_shader *shader)
>        }
>     }
>
> -   nir_instr_insert_before(&vec->instr, &mov->instr);
> +   /* In some situations (if the vecN is involved in a phi-web), we can end
> +    * up with a mov from a register to itself.  Some of those channels may end
> +    * up doing nothing and there's no reason to have them as part of the mov.
> +    */
> +   if (src_matches_dest_reg(&mov->dest.dest, &mov->src[0].src) &&
> +       !mov->src[0].abs && !mov->src[0].negate) {
> +      for (unsigned i = 0; i < 4; i++) {
> +         if (mov->src[0].swizzle[i] == i)
> +            mov->dest.write_mask &= ~(1 << i);

I'd advise using braces around this statement.

Reviewed-by: Matt Turner <mattst88 at gmail.com>

Thanks for digging into this more. It seems to always lead to
something like this. :)