On Tue, Sep 22, 2015 at 10:01 PM, Jason Ekstrand <ja...@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 <matts...@gmail.com>
Thanks for digging into this more. It seems to always lead to
something like this. :)
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