[Mesa-dev] [PATCH] i965/cs: Don't use a thread channel ID for small local sizes

[Ian Romanick]

On 10/19/2016 12:45 PM, Jordan Justen wrote:
> When the local group size is 8 or less, we will execute the program at
> most 1 time. Therefore, the local channel ID will always be 0. By
> using a constant 0 in this case we can prevent using push constant
> data.
> 
> This is not expected to be common a occurance in real applications,
> but it has been seen in tests.
> 
> We could extend this optimization to 16 and 32 for SIMD16 and SIMD32,
> but it gets a bit more complicated, because this optimization is
> currently being done early on, before we have decided the SIMD size.
> 
> Signed-off-by: Jordan Justen <jordan.l.justen at intel.com>
> ---
>  src/mesa/drivers/dri/i965/brw_nir_intrinsics.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/brw_nir_intrinsics.c b/src/mesa/drivers/dri/i965/brw_nir_intrinsics.c
> index 9ae161f..60f302e 100644
> --- a/src/mesa/drivers/dri/i965/brw_nir_intrinsics.c
> +++ b/src/mesa/drivers/dri/i965/brw_nir_intrinsics.c
> @@ -39,12 +39,21 @@ struct lower_intrinsics_state {
>  static nir_ssa_def *
>  read_thread_local_id(struct lower_intrinsics_state *state)
>  {
> +   nir_builder *b = &state->builder;
> +   nir_shader *nir = state->nir;
> +   const unsigned *sizes = nir->info.cs.local_size;
> +   const group_size = sizes[0] * sizes[1] * sizes[2];
           ^
            Isn't this missing a type?  Did you test this? :)

> +
> +   /* Some programs have local_size dimensions so small that the thread local
> +    * ID will always be 0.
> +    */
> +   if (group_size <= 8)
> +      return nir_imm_int(b, 0);
> +
>     assert(state->cs_prog_data->thread_local_id_index >= 0);
>     state->cs_thread_id_used = true;
>     const int id_index = state->cs_prog_data->thread_local_id_index;
>  
> -   nir_builder *b = &state->builder;
> -   nir_shader *nir = state->nir;
>     nir_intrinsic_instr *load =
>        nir_intrinsic_instr_create(nir, nir_intrinsic_load_uniform);
>     load->num_components = 1;
>