[Mesa-dev] [PATCH v2] nir: Handle variables dependent on the local work group size.

[Jason Ekstrand]

On Sun, Nov 11, 2018 at 3:35 PM Plamena Manolova <
plamena.n.manolova at gmail.com> wrote:

> Lowering shader variables which depend on the local work group
> size being available in nir_lower_system_values is only possible
> if the local work group size isn't variable, otherwise this should
> be handled by the native driver (if it supports
> ARB_compute_variable_group_size).
>
> Signed-off-by: Plamena Manolova <plamena.n.manolova at gmail.com>
> ---
>  src/compiler/nir/nir_lower_system_values.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/src/compiler/nir/nir_lower_system_values.c
> b/src/compiler/nir/nir_lower_system_values.c
> index bde7eb1180..6fdf9f9c51 100644
> --- a/src/compiler/nir/nir_lower_system_values.c
> +++ b/src/compiler/nir/nir_lower_system_values.c
> @@ -77,6 +77,15 @@ convert_block(nir_block *block, nir_builder *b)
>            *    "The value of gl_GlobalInvocationID is equal to
>            *    gl_WorkGroupID * gl_WorkGroupSize + gl_LocalInvocationID"
>            */
> +
> +         /*
> +          * If the local work group size is variable we can't lower the
> global
> +          * invocation id here.
>

Why not?


> +          */
> +         if (b->shader->info.cs.local_size_variable) {
>

I didn't realize we'd added a bit for this.  At one point in time, Karol
had patches which had it keyed off of the size being zero.  Having a
separate bit is probably fine, it just surpised me.


> +            break;
> +         }
> +
>           nir_ssa_def *group_size = build_local_group_size(b);
>           nir_ssa_def *group_id = nir_load_work_group_id(b);
>           nir_ssa_def *local_id = nir_load_local_invocation_id(b);
> @@ -115,6 +124,11 @@ convert_block(nir_block *block, nir_builder *b)
>        }
>
>        case SYSTEM_VALUE_LOCAL_GROUP_SIZE: {
> +         /* If the local work group size is variable we can't lower it
> here */
> +         if (b->shader->info.cs.local_size_variable) {
> +            break;
> +         }
> +
>           sysval = build_local_group_size(b);
>           break;
>        }
> @@ -189,6 +203,14 @@ convert_block(nir_block *block, nir_builder *b)
>           break;
>
>        case SYSTEM_VALUE_GLOBAL_GROUP_SIZE: {
> +         /*
> +          * If the local work group size is variable we can't lower the
> global
> +          * group size here.
> +          */
> +         if (b->shader->info.cs.local_size_variable) {
> +            break;
> +         }
>

Why can't we lower the global size?  It seems like we would want the below
multiplication regardless of whether the local size is constant.


> +
>           nir_ssa_def *group_size = build_local_group_size(b);
>           nir_ssa_def *num_work_groups = nir_load_num_work_groups(b);
>           sysval = nir_imul(b, group_size, num_work_groups);
> --
> 2.11.0
>
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