mesa/src/compiler/nir/nir_lower_const_arrays_to_uniforms.c

/*
 * Copyright © 2021 Valve Corporation
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice (including the next
 * paragraph) shall be included in all copies or substantial portions of the
 * Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 */

/**
 * Lower constant arrays to uniform arrays.
 *
 * Some driver backends (such as i965 and nouveau) don't handle constant arrays
 * gracefully, instead treating them as ordinary writable temporary arrays.
 * Since arrays can be large, this often means spilling them to scratch memory,
 * which usually involves a large number of instructions.
 *
 * This must be called prior to gl_nir_set_uniform_initializers(); we need the
 * linker to process our new uniform's constant initializer.
 *
 * This should be called after optimizations, since those can result in
 * splitting and removing arrays that are indexed by constant expressions.
 */
#include "nir.h"
#include "nir_builder.h"
#include "nir_deref.h"

struct var_info {
   nir_variable *var;

   bool is_constant;
   bool found_read;

   /* Block that has all the variable stores.  All the blocks with reads
    * should be dominated by this block.
    */
   nir_block *block;
};

static void
set_const_initialiser(nir_deref_instr **p, nir_constant *top_level_init,
                      nir_src *const_src, unsigned writemask)
{
   assert(*p);

   nir_constant *ret = top_level_init;
   for ( ; *p; p++) {
      if ((*p)->deref_type == nir_deref_type_array) {
         assert(nir_src_is_const((*p)->arr.index));

         uint64_t idx = nir_src_as_uint((*p)->arr.index);

         /* Just return if this is an out of bounds write */
         if (idx >= ret->num_elements)
            return;

         ret = ret->elements[idx];
      } else if ((*p)->deref_type == nir_deref_type_struct) {
         ret = ret->elements[(*p)->strct.index];
      } else {
         unreachable("Unsupported deref type");
      }
   }

   /* Now that we have selected the corrent nir_constant we copy the constant
    * values to it.
    */
   assert(const_src->is_ssa);
   nir_instr *src_instr = const_src->ssa->parent_instr;
   assert(src_instr->type == nir_instr_type_load_const);
   nir_load_const_instr* load = nir_instr_as_load_const(src_instr);

   for (unsigned i = 0; i < load->def.num_components; i++) {
      if (!(writemask & (1 << i)))
         continue;

      memcpy(ret->values + i, load->value + i, sizeof(*load->value));
   }

   return;
}

static nir_constant *
rebuild_const_array_initialiser(const struct glsl_type *type, void *mem_ctx)
{
   nir_constant *ret = rzalloc(mem_ctx, nir_constant);

   if (glsl_type_is_matrix(type) && glsl_get_matrix_columns(type) > 1) {
      ret->num_elements = glsl_get_matrix_columns(type);
      ret->elements = ralloc_array(mem_ctx, nir_constant *, ret->num_elements);

      for (unsigned i = 0; i < ret->num_elements; i++) {
         ret->elements[i] = rzalloc(mem_ctx, nir_constant);
      }

      return ret;
   }

   if (glsl_type_is_array(type) || glsl_type_is_struct(type)) {
      ret->num_elements = glsl_get_length(type);
      ret->elements = ralloc_array(mem_ctx, nir_constant *, ret->num_elements);

      for (unsigned i = 0; i < ret->num_elements; i++) {
         if (glsl_type_is_array(type)) {
            ret->elements[i] =
               rebuild_const_array_initialiser(glsl_get_array_element(type), mem_ctx);
         } else {
            ret->elements[i] =
               rebuild_const_array_initialiser(glsl_get_struct_field(type, i), mem_ctx);
         }
      }
   }

   return ret;
}

static bool
lower_const_array_to_uniform(nir_shader *shader, struct var_info *info,
                             struct hash_table *const_array_vars,
                             unsigned *free_uni_components,
                             unsigned *const_count, bool *progress)
{
   nir_variable *var = info->var;

   if (!info->is_constant)
      return true;

   if (!glsl_type_is_array(var->type))
      return true;

   /* TODO: Add support for 8bit and 16bit types */
   if (!glsl_type_is_32bit(glsl_without_array(var->type)) &&
       !glsl_type_is_64bit(glsl_without_array(var->type)))
      return true;

   /* How many uniform component slots are required? */
   unsigned component_slots = glsl_get_component_slots(var->type);

   /* We would utilize more than is available, bail out. */
   if (component_slots > *free_uni_components)
      return false;

   *free_uni_components -= component_slots;

   /* In the very unlikely event of 4294967295 constant arrays in a single
    * shader, don't promote this to a uniform.
    */
   unsigned limit = ~0;
   if (*const_count == limit)
      return false;

   nir_variable *uni = rzalloc(shader, nir_variable);

   /* Rebuild constant initialiser */
   nir_constant *const_init = rebuild_const_array_initialiser(var->type, uni);

   /* Set constant initialiser */
   nir_function_impl *impl = nir_shader_get_entrypoint(shader);
   nir_foreach_block(block, impl) {
      nir_foreach_instr(instr, block) {
         if (instr->type != nir_instr_type_intrinsic)
            continue;

         nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
         assert(intrin->intrinsic != nir_intrinsic_copy_deref);
         if (intrin->intrinsic != nir_intrinsic_store_deref)
            continue;

         nir_deref_instr *deref = nir_src_as_deref(intrin->src[0]);
         nir_variable *deref_var = nir_deref_instr_get_variable(deref);
         if (var != deref_var)
            continue;

         nir_deref_path path;
         nir_deref_path_init(&path, deref, NULL);
         assert(path.path[0]->deref_type == nir_deref_type_var);

         nir_deref_instr **p = &path.path[1];
         set_const_initialiser(p, const_init, &intrin->src[1],
                               nir_intrinsic_write_mask(intrin));

         nir_deref_path_finish(&path);
      }
   }

   uni->constant_initializer = const_init;
   uni->data.how_declared = nir_var_hidden;
   uni->data.read_only = true;
   uni->data.mode = nir_var_uniform;
   uni->type = info->var->type;
   uni->name = ralloc_asprintf(uni, "constarray_%x_%u",
                               *const_count, shader->info.stage);

   nir_shader_add_variable(shader, uni);

   *const_count = *const_count + 1;

   _mesa_hash_table_insert(const_array_vars, info->var, uni);

   *progress = true;

   return true;
}

static unsigned
count_uniforms(nir_shader *shader)
{
   unsigned total = 0;

   nir_foreach_variable_with_modes(var, shader, nir_var_uniform) {
      total += glsl_get_component_slots(var->type);
   }

   return total;
}

bool
nir_lower_const_arrays_to_uniforms(nir_shader *shader,
                                   unsigned max_uniform_components)
{
   /* This only works with a single entrypoint */
   nir_function_impl *impl = nir_shader_get_entrypoint(shader);

   unsigned num_locals = nir_function_impl_index_vars(impl);
   if (num_locals == 0) {
      nir_shader_preserve_all_metadata(shader);
      return false;
   }

   bool progress = false;
   unsigned uniform_components = count_uniforms(shader);
   unsigned free_uni_components = max_uniform_components - uniform_components;
   unsigned const_count = 0;

   struct var_info *var_infos = ralloc_array(NULL, struct var_info, num_locals);
   nir_foreach_function_temp_variable(var, impl) {
      var_infos[var->index] = (struct var_info) {
         .var = var,
         .is_constant = true,
         .found_read = false,
      };
   }

   nir_metadata_require(impl, nir_metadata_dominance);

   struct hash_table *const_array_vars =
      _mesa_hash_table_create(NULL, _mesa_hash_pointer, _mesa_key_pointer_equal);

   /* First, walk through the shader and figure out what variables we can
    * lower to a uniform.
    */
   nir_foreach_block(block, impl) {
      nir_foreach_instr(instr, block) {
         if (instr->type == nir_instr_type_deref) {
            /* If we ever see a complex use of a deref_var, we have to assume
             * that variable is non-constant because we can't guarantee we
             * will find all of the writers of that variable.
             */
            nir_deref_instr *deref = nir_instr_as_deref(instr);
            if (deref->deref_type == nir_deref_type_var &&
                deref->var->data.mode == nir_var_function_temp &&
                nir_deref_instr_has_complex_use(deref, 0))
               var_infos[deref->var->index].is_constant = false;
            continue;
         }

         if (instr->type != nir_instr_type_intrinsic)
            continue;

         nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);

         bool src_is_const = false;
         nir_deref_instr *src_deref = NULL, *dst_deref = NULL;
         switch (intrin->intrinsic) {
         case nir_intrinsic_store_deref:
            dst_deref = nir_src_as_deref(intrin->src[0]);
            src_is_const = nir_src_is_const(intrin->src[1]);
            break;

         case nir_intrinsic_load_deref:
            src_deref = nir_src_as_deref(intrin->src[0]);
            break;

         case nir_intrinsic_copy_deref:
            assert(!"Lowering of copy_deref with const arrays to uniform is prohibited");
            break;

         default:
            continue;
         }

         if (dst_deref && nir_deref_mode_must_be(dst_deref, nir_var_function_temp)) {
            nir_variable *var = nir_deref_instr_get_variable(dst_deref);
            if (var == NULL)
               continue;

            assert(var->data.mode == nir_var_function_temp);

            struct var_info *info = &var_infos[var->index];
            if (!info->is_constant)
               continue;

            if (!info->block)
               info->block = block;

            /* We only consider variables constant if they only have constant
             * stores, all the stores come before any reads, and all stores
             * come from the same block.  We also can't handle indirect stores.
             */
            if (!src_is_const || info->found_read || block != info->block ||
                nir_deref_instr_has_indirect(dst_deref)) {
               info->is_constant = false;
            }
         }

         if (src_deref && nir_deref_mode_must_be(src_deref, nir_var_function_temp)) {
            nir_variable *var = nir_deref_instr_get_variable(src_deref);
            if (var == NULL)
               continue;

            assert(var->data.mode == nir_var_function_temp);

            /* We only consider variables constant if all the reads are
             * dominated by the block that writes to it.
             */
            struct var_info *info = &var_infos[var->index];
            if (!info->is_constant)
               continue;

            if (!info->block || !nir_block_dominates(info->block, block))
               info->is_constant = false;

            info->found_read = true;
         }
      }
   }

   /* Now lower the constants to uniforms */
   for (int i = 0; i < num_locals; i++) {
      struct var_info *info = &var_infos[i];
      if (!lower_const_array_to_uniform(shader, info, const_array_vars,
                                        &free_uni_components, &const_count,
                                        &progress))
         break;
   }

   /* Finally rewrite its uses */
   nir_builder b;
   nir_builder_init(&b, impl);
   nir_foreach_block(block, impl) {
      nir_foreach_instr_safe(instr, block) {

         if (instr->type != nir_instr_type_intrinsic)
            continue;

         nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
         if (intrin->intrinsic != nir_intrinsic_load_deref)
            continue;

         nir_deref_instr *deref = nir_src_as_deref(intrin->src[0]);
         nir_variable *var = nir_deref_instr_get_variable(deref);

         struct hash_entry *entry =
            _mesa_hash_table_search(const_array_vars, var);
         if (!entry)
            continue;

         b.cursor = nir_before_instr(instr);

         nir_variable *uni = (nir_variable *) entry->data;
         nir_deref_instr *new_deref_instr = nir_build_deref_var(&b, uni);

         nir_deref_path path;
         nir_deref_path_init(&path, deref, NULL);
         assert(path.path[0]->deref_type == nir_deref_type_var);

         nir_deref_instr **p = &path.path[1];
         for (; *p; p++) {
            if ((*p)->deref_type == nir_deref_type_array) {
               new_deref_instr = nir_build_deref_array(&b, new_deref_instr,
                                                       (*p)->arr.index.ssa);
            } else if ((*p)->deref_type == nir_deref_type_struct) {
               new_deref_instr = nir_build_deref_struct(&b, new_deref_instr,
                                                        (*p)->strct.index);
            } else {
               unreachable("Unsupported deref type");
            }
         }
         nir_deref_path_finish(&path);

         nir_ssa_def *new_def = nir_load_deref(&b, new_deref_instr);

         nir_ssa_def_rewrite_uses(&intrin->dest.ssa, new_def);
         nir_instr_remove(&intrin->instr);
      }
   }

   nir_metadata_preserve(impl, nir_metadata_block_index |
                               nir_metadata_dominance);

   ralloc_free(var_infos);
   _mesa_hash_table_destroy(const_array_vars, NULL);

   return progress;
}