[Mesa-dev] [PATCH v2] nir: add new linking opt nir_move_out_const_to_consumer()

[Timothy Arceri]

On 9/11/18 6:53 am, Samuel Pitoiset wrote:
> On 11/7/18 9:36 AM, Timothy Arceri wrote:
>> This pass moves constant outputs to the consuming shader stage
>> where possible.
>>
>> V2: limit pass to scalars for now
>> ---
>>
>>   V2 doesn't change any shader-db/vkpipeline-db results as all 32bit
>>   varyings that we don't skip are already scalar. V2 just avoids a
>>   potential bug with doubles as we don't currently split those in
>>   nir_lower_io_to_scalar_early().
>>
>>   src/compiler/nir/nir.h                    |   2 +
>>   src/compiler/nir/nir_linking_helpers.c    | 110 ++++++++++++++++++++++
>>   src/mesa/state_tracker/st_glsl_to_nir.cpp |   2 +-
>>   3 files changed, 113 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
>> index a0ae9a4430e..0b8c8578953 100644
>> --- a/src/compiler/nir/nir.h
>> +++ b/src/compiler/nir/nir.h
>> @@ -2800,6 +2800,8 @@ bool nir_remove_unused_io_vars(nir_shader 
>> *shader, struct exec_list *var_list,
>>   void nir_compact_varyings(nir_shader *producer, nir_shader *consumer,
>>                             bool default_to_smooth_interp);
>>   void nir_link_xfb_varyings(nir_shader *producer, nir_shader *consumer);
>> +bool nir_move_out_const_to_consumer(nir_shader *producer,
>> +                                    nir_shader *consumer);
>>   typedef enum {
>>      /* If set, this forces all non-flat fragment shader inputs to be
>> diff --git a/src/compiler/nir/nir_linking_helpers.c 
>> b/src/compiler/nir/nir_linking_helpers.c
>> index de6f2481def..a03c327a12f 100644
>> --- a/src/compiler/nir/nir_linking_helpers.c
>> +++ b/src/compiler/nir/nir_linking_helpers.c
>> @@ -22,6 +22,7 @@
>>    */
>>   #include "nir.h"
>> +#include "nir_builder.h"
>>   #include "util/set.h"
>>   #include "util/hash_table.h"
>> @@ -556,3 +557,112 @@ nir_link_xfb_varyings(nir_shader *producer, 
>> nir_shader *consumer)
>>         }
>>      }
>>   }
>> +
>> +static bool
>> +try_replace_constant_input(nir_shader *shader,
>> +                           nir_intrinsic_instr *store_intr)
>> +{
>> +   nir_variable *out_var =
>> +      
>> nir_deref_instr_get_variable(nir_src_as_deref(store_intr->src[0]));
>> +
>> +   if (out_var->data.mode != nir_var_shader_out)
>> +      return false;
>> +
>> +   /* Skip types that require more complex handling.
>> +    * TODO: add support for these types.
>> +    */
>> +   if (glsl_type_is_array(out_var->type) ||
>> +       glsl_type_is_dual_slot(out_var->type) ||
>> +       glsl_type_is_matrix(out_var->type) ||
>> +       glsl_type_is_struct(out_var->type))
>> +      return false;
>> +
>> +   /* Limit this pass to scalars for now to keep things simple. Most 
>> varyings
>> +    * should have been lowered to scalars at this point anyway.
>> +    */
>> +   if (store_intr->num_components != 1)
>> +      return false;
>> +
>> +   if (out_var->data.location < VARYING_SLOT_VAR0 ||
>> +       out_var->data.location - VARYING_SLOT_VAR0 >= MAX_VARYING)
>> +      return false;
>> +
>> +   nir_function_impl *impl = nir_shader_get_entrypoint(shader);
>> +
>> +   nir_builder b;
>> +   nir_builder_init(&b, impl);
>> +
>> +   bool progress = false;
>> +   nir_foreach_block(block, impl) {
>> +      nir_foreach_instr(instr, block) {
>> +         if (instr->type != nir_instr_type_intrinsic)
>> +            continue;
>> +
>> +         nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
>> +         if (intr->intrinsic != nir_intrinsic_load_deref)
>> +            continue;
>> +
>> +         nir_variable *in_var =
>> +            
>> nir_deref_instr_get_variable(nir_src_as_deref(intr->src[0]));
>> +
>> +         if (in_var->data.mode != nir_var_shader_in)
>> +            continue;
>> +
>> +         if (in_var->data.location != out_var->data.location ||
>> +             in_var->data.location_frac != out_var->data.location_frac)
>> +            continue;
>> +
>> +         b.cursor = nir_before_instr(instr);
>> +
>> +         nir_load_const_instr *out_const =
>> +            
>> nir_instr_as_load_const(store_intr->src[1].ssa->parent_instr);
>> +
>> +         /* Add new const to replace the input */
>> +         nir_ssa_def *nconst = nir_build_imm(&b, 
>> store_intr->num_components,
>> +                                             intr->dest.ssa.bit_size,
>> +                                             out_const->value);
>> +
>> +         nir_ssa_def_rewrite_uses(&intr->dest.ssa, 
>> nir_src_for_ssa(nconst));
>> +
>> +         progress = true;
>> +      }
>> +   }
>> +
>> +   return progress;
>> +}
>> +
>> +bool
>> +nir_move_out_const_to_consumer(nir_shader *producer, nir_shader 
>> *consumer)
>> +{
>> +   /* TODO: Add support for more shader stage combinations */
>> +   if (consumer->info.stage != MESA_SHADER_FRAGMENT ||
>> +       (producer->info.stage != MESA_SHADER_VERTEX &&
>> +        producer->info.stage != MESA_SHADER_TESS_EVAL))
>> +      return false;
> 
> I would suggest to only enable it for VS->FS for now because I think 
> it's the most important point, also because it might be trickier for 
> other stages.

The only other thing this supports is TES->FS and there should be no 
problem handling this. I've been thinking about TCS->TES some more also 
and I'm more confident that this will be fine too, however I'll send 
this change as a follow up patch.

> 
>> +
>> +   bool progress = false;
>> +
>> +   nir_function_impl *impl = nir_shader_get_entrypoint(producer);
>> +
>> +   /* If we find a store in the last block of the producer we can be 
>> sure this
>> +    * is the only possible value for this output.
>> +    */
>> +   nir_block *last_block = nir_impl_last_block(impl);
>> +   nir_foreach_instr_reverse(instr, last_block) {
>> +      if (instr->type != nir_instr_type_intrinsic)
>> +         continue;
>> +
>> +      nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
>> +
>> +      if (intr->intrinsic != nir_intrinsic_store_deref)
>> +         continue;
>> +
>> +      if (intr->src[1].ssa->parent_instr->type != 
>> nir_instr_type_load_const) {
>> +         continue;
>> +      }
> 
> useless braces.
> 
>> +
>> +      progress |= try_replace_constant_input(consumer, intr);
>> +   }
>> +
>> +   return progress;
>> +}
>> diff --git a/src/mesa/state_tracker/st_glsl_to_nir.cpp 
>> b/src/mesa/state_tracker/st_glsl_to_nir.cpp
>> index dcf8c2b638e..9141acbc4df 100644
>> --- a/src/mesa/state_tracker/st_glsl_to_nir.cpp
>> +++ b/src/mesa/state_tracker/st_glsl_to_nir.cpp
>> @@ -757,7 +757,7 @@ st_link_nir(struct gl_context *ctx,
>>             */
>>            if (!prev_shader->sh.LinkedTransformFeedback)
>>               
>> nir_compact_varyings(shader_program->_LinkedShaders[prev]->Program->nir,
>> -                              nir, ctx->API != API_OPENGL_COMPAT);
>> +                                 nir, ctx->API != API_OPENGL_COMPAT);
>>         }
>>         prev = i;
>>      }
>>