[Mesa-dev] [PATCH v2 7/7] i965: make use of nir linking

[Kenneth Graunke]

On Monday, September 25, 2017 6:23:13 PM PDT Timothy Arceri wrote:
> For now linking is just removing unused varyings between stages.
> 
> shader-db results BDW:
> 
> total instructions in shared programs: 13198288 -> 13191693 (-0.05%)
> instructions in affected programs: 48325 -> 41730 (-13.65%)
> helped: 473
> HURT: 0
> 
> total cycles in shared programs: 541184926 -> 541159260 (-0.00%)
> cycles in affected programs: 213238 -> 187572 (-12.04%)
> helped: 435
> HURT: 8
> 
> V2:
> - lower indirects on demoted inputs as well as outputs.
> ---
>  src/mesa/drivers/dri/i965/brw_link.cpp | 56 ++++++++++++++++++++++++++++++++++
>  1 file changed, 56 insertions(+)
> 
> diff --git a/src/mesa/drivers/dri/i965/brw_link.cpp b/src/mesa/drivers/dri/i965/brw_link.cpp
> index b7fab8d7a25..9ddf0230183 100644
> --- a/src/mesa/drivers/dri/i965/brw_link.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_link.cpp
> @@ -253,6 +253,62 @@ brw_link_shader(struct gl_context *ctx, struct gl_shader_program *shProg)
>                                   compiler->scalar_stage[stage]);
>     }
>  
> +   /* Determine first and last stage. */
> +   unsigned first = MESA_SHADER_STAGES;
> +   unsigned last = 0;
> +   for (unsigned i = 0; i < MESA_SHADER_STAGES; i++) {
> +      if (!shProg->_LinkedShaders[i])
> +         continue;
> +      if (first == MESA_SHADER_STAGES)
> +         first = i;
> +      last = i;
> +   }
> +
> +   /* Linking the stages in the opposite order (from fragment to vertex)
> +    * ensures that inter-shader outputs written to in an earlier stage
> +    * are eliminated if they are (transitively) not used in a later
> +    * stage.
> +    */
> +    if (first != last) {
> +       int next = last;
> +       for (int i = next - 1; i >= 0; i--) {
> +          if (shProg->_LinkedShaders[i] == NULL)
> +             continue;
> +
> +            nir_shader *producer = shProg->_LinkedShaders[i]->Program->nir;
> +            nir_shader *consumer = shProg->_LinkedShaders[next]->Program->nir;
> +
> +            nir_remove_dead_variables(producer, nir_var_shader_out);
> +            nir_remove_dead_variables(consumer, nir_var_shader_in);
> +
> +            if (nir_remove_unused_varyings(producer, consumer)) {
> +               nir_lower_global_vars_to_local(producer);
> +               nir_lower_global_vars_to_local(consumer);
> +
> +               nir_variable_mode indirect_mask = (nir_variable_mode) 0;
> +               if (compiler->glsl_compiler_options[i].EmitNoIndirectTemp)
> +                  indirect_mask = (nir_variable_mode) nir_var_local;
> +
> +               /* The backend might not be able to handle indirects on
> +                * temporaries so we need to lower indirects on any of the
> +                * varyings we have demoted here.
> +                */
> +               nir_lower_indirect_derefs(producer, indirect_mask);
> +               nir_lower_indirect_derefs(consumer, indirect_mask);
> +
> +               const bool p_is_scalar = compiler->scalar_stage[producer->stage];
> +               shProg->_LinkedShaders[i]->Program->nir =
> +                 brw_nir_optimize(producer, compiler, p_is_scalar);
> +
> +               const bool c_is_scalar = compiler->scalar_stage[producer->stage];
> +               shProg->_LinkedShaders[next]->Program->nir =
> +                 brw_nir_optimize(consumer, compiler, c_is_scalar);
> +            }
> +
> +            next = i;
> +       }
> +    }
> +
>     for (stage = 0; stage < ARRAY_SIZE(shProg->_LinkedShaders); stage++) {
>        struct gl_linked_shader *shader = shProg->_LinkedShaders[stage];
>        if (!shader)
> 

A couple more thoughts:

1. We're calling brw_nir_optimize even more now...it might make sense to
   drop it from brw_preprocess_nir, drop it here, and put it once in the
   final (third) loop, just before brw_shader_gather_info.  At least,
   it'd be interesting how that affects compile times / quality...

2. It would be great to use this in anv as well, especially given that
   it has no GLSL IR linking to do this sort of stuff.  Plus, the SPIR-V
   might be compiled independently...and when you build the pipeline,
   you have all the stages and can do cross-stage linking optimizations
   like this.

Series looks great though, and is:
Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>

Thanks Tim!
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