[Mesa-dev] [PATCH 4/4] radv/gfx10: do not always execute a barrier before the second shader

[Bas Nieuwenhuizen]

On Wed, Jul 17, 2019 at 3:44 PM Samuel Pitoiset
<samuel.pitoiset at gmail.com> wrote:
>
> With NGG, empty waves may still be required to export data.
>
> This fixes dEQP-VK.ycbcr.format.*_unorm.geometry_*.
>
> Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
> ---
>  src/amd/vulkan/radv_nir_to_llvm.c | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/src/amd/vulkan/radv_nir_to_llvm.c b/src/amd/vulkan/radv_nir_to_llvm.c
> index 3e18303879e..7e623414adc 100644
> --- a/src/amd/vulkan/radv_nir_to_llvm.c
> +++ b/src/amd/vulkan/radv_nir_to_llvm.c
> @@ -4448,8 +4448,37 @@ LLVMModuleRef ac_translate_nir_to_llvm(struct ac_llvm_compiler *ac_llvm,
>                         declare_esgs_ring(&ctx);
>                 }
>
> -               if (i)
> +               bool nested_barrier = false;
> +
> +               if (i) {
> +                       if (shaders[i]->info.stage == MESA_SHADER_GEOMETRY &&
> +                           ctx.options->key.vs_common_out.as_ngg) {
> +                               nested_barrier = false;
> +                       } else {
> +                               nested_barrier = true;
> +                       }
> +               }

We can simplify this to

nested_barrier = i && (shaders[i]->info.stage != MESA_SHADER_GEOMETRY
|| !ctx.options->key.vs_common_out.as_ngg);

Otherwise r-b, I'm just surprised an s_barrier is okay.
> +
> +               if (nested_barrier) {
> +                       /* Execute a barrier before the second shader in
> +                        * a merged shader.
> +                        *
> +                        * Execute the barrier inside the conditional block,
> +                        * so that empty waves can jump directly to s_endpgm,
> +                        * which will also signal the barrier.
> +                        *
> +                        * This is possible in gfx9, because an empty wave
> +                        * for the second shader does not participate in
> +                        * the epilogue. With NGG, empty waves may still
> +                        * be required to export data (e.g. GS output vertices),
> +                        * so we cannot let them exit early.
> +                        *
> +                        * If the shader is TCS and the TCS epilog is present
> +                        * and contains a barrier, it will wait there and then
> +                        * reach s_endpgm.
> +                       */
>                         ac_emit_barrier(&ctx.ac, ctx.stage);
> +               }
>
>                 nir_foreach_variable(variable, &shaders[i]->outputs)
>                         scan_shader_output_decl(&ctx, variable, shaders[i], shaders[i]->info.stage);
> --
> 2.22.0
>
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