[Mesa-dev] [PATCH 1/5] i965/vec4: nir_emit_if doesn't need to predicate based on all the channels

[Jason Ekstrand]

Looking at the docs a bit, it looks like we should never have been
using predicate_normal for if's in the first place

Reviewed-by: Jason Ekstrand <jason.ekstrand at intel.com>

On Sat, Oct 10, 2015 at 4:24 AM, Alejandro Piñeiro <apinheiro at igalia.com> wrote:
> ---
>
> I already talked about this with Jason Ekstrand and Matt Turner
> privately, but just in case somebody else jump to the review:
>
> When using BRW_PREDICATE_NORMAL, the if will use all the channels of
> the register flag.  But nir_if only reads from one channel, so that
> is not needed. Another hint showing that this is safe: the MOV that
> put the condition on f0 is calling get_nir_src with just one
> component. That will return always a source with swizzle
> BRW_SWIZZLE_XXXX, so that component is the only to be used.
>
> This commit is not needed/solving anything per-se, but it is needed in
> order to be able to implement vec4_cmod_propagation with a good
> overall outcome.
>
>  src/mesa/drivers/dri/i965/brw_vec4_nir.cpp | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> index 41bd80d..e05745f 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> @@ -193,7 +193,9 @@ vec4_visitor::nir_emit_if(nir_if *if_stmt)
>     vec4_instruction *inst = emit(MOV(dst_null_d(), condition));
>     inst->conditional_mod = BRW_CONDITIONAL_NZ;
>
> -   emit(IF(BRW_PREDICATE_NORMAL));
> +   /* We can just predicate based on the X channel, as the condition only
> +    * reads from one channel */
> +   emit(IF(BRW_PREDICATE_ALIGN16_REPLICATE_X));
>
>     nir_emit_cf_list(&if_stmt->then_list);
>
> --
> 2.1.4
>
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