compiler/nir/nir_remove_dead_variables.c

/*
 * Copyright © 2014 Intel 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
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 *
 * Authors:
 *    Connor Abbott (cwabbott0@gmail.com)
 *
 */

#include "nir.h"
#include "nir_builder.h"

static bool
deref_used_for_not_store(nir_deref_instr *deref)
{
   nir_foreach_use(src, &deref->def) {
      switch (nir_src_parent_instr(src)->type) {
      case nir_instr_type_deref:
         if (deref_used_for_not_store(nir_instr_as_deref(nir_src_parent_instr(src))))
            return true;
         break;

      case nir_instr_type_intrinsic: {
         nir_intrinsic_instr *intrin =
            nir_instr_as_intrinsic(nir_src_parent_instr(src));
         /* The first source of copy and store intrinsics is the deref to
          * write.  Don't record those.
          */
         if ((intrin->intrinsic != nir_intrinsic_store_deref &&
              intrin->intrinsic != nir_intrinsic_copy_deref) ||
             src != &intrin->src[0])
            return true;
         break;
      }

      default:
         /* If it's used by any other instruction type (most likely a texture
          * or call instruction), consider it used.
          */
         return true;
      }
   }

   return false;
}

static void
add_var_use_deref(nir_deref_instr *deref, struct set *live)
{
   if (deref->deref_type != nir_deref_type_var)
      return;

   /* Since these local variables don't escape the shader, writing doesn't
    * make them live.  Only keep them if they are used by some intrinsic.
    */
   if ((deref->var->data.mode & (nir_var_function_temp |
                                 nir_var_shader_temp)) &&
       !deref_used_for_not_store(deref))
      return;

   /*
    * Shared memory blocks (interface type) alias each other, so be
    * conservative in that case.
    */
   if ((deref->var->data.mode & nir_var_mem_shared) &&
       !glsl_type_is_interface(deref->var->type) &&
       !deref_used_for_not_store(deref))
      return;

   nir_variable *var = deref->var;
   do {
      _mesa_set_add(live, var);
      /* Also mark the chain of variables used to initialize it. */
      var = var->pointer_initializer;
   } while (var);
}

static void
add_var_use_shader(nir_shader *shader, struct set *live, nir_variable_mode modes)
{
   nir_foreach_function_impl(impl, shader) {
      nir_foreach_block(block, impl) {
         nir_foreach_instr(instr, block) {
            if (instr->type == nir_instr_type_deref)
               add_var_use_deref(nir_instr_as_deref(instr), live);
         }
      }
   }
}

static bool
remove_dead_var_writes(nir_builder *b, nir_instr *instr, void *_)
{
   if (instr->type == nir_instr_type_deref) {
      nir_deref_instr *deref = nir_instr_as_deref(instr);
      if (deref->deref_type == nir_deref_type_cast &&
          !nir_deref_instr_parent(deref))
         return false;

      nir_variable_mode parent_modes;
      if (deref->deref_type == nir_deref_type_var)
         parent_modes = deref->var->data.mode;
      else
         parent_modes = nir_deref_instr_parent(deref)->modes;

      /* If the parent mode is 0, then it references a dead variable.
       * Flag this deref as dead and remove it.
       */
      if (parent_modes != 0)
         return false;

      deref->modes = 0;
   } else if (instr->type == nir_instr_type_intrinsic) {
      nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
      if (!(intrin->intrinsic == nir_intrinsic_copy_deref ||
            intrin->intrinsic == nir_intrinsic_store_deref) ||
          nir_src_as_deref(intrin->src[0])->modes != 0)
         return false;
   } else {
      return false;
   }

   nir_instr_remove(instr);
   return true;
}

static bool
remove_dead_vars(struct exec_list *var_list, nir_variable_mode modes,
                 struct set *live, const nir_remove_dead_variables_options *opts)
{
   bool progress = false;

   nir_foreach_variable_in_list_safe(var, var_list) {
      if (!(var->data.mode & modes))
         continue;

      if (opts && opts->can_remove_var &&
          !opts->can_remove_var(var, opts->can_remove_var_data))
         continue;

      struct set_entry *entry = _mesa_set_search(live, var);
      if (entry == NULL) {
         /* Mark this variable as dead by setting the mode to 0 */
         var->data.mode = 0;
         exec_node_remove(&var->node);
         progress = true;
      }
   }

   return progress;
}

bool
nir_remove_dead_variables(nir_shader *shader, nir_variable_mode modes,
                          const nir_remove_dead_variables_options *opts)
{
   bool progress = false;
   struct set *live = _mesa_pointer_set_create(NULL);

   add_var_use_shader(shader, live, modes);

   if (modes & ~nir_var_function_temp) {
      progress = remove_dead_vars(&shader->variables, modes,
                                  live, opts) ||
                 progress;
   }

   if (modes & nir_var_function_temp) {
      nir_foreach_function_impl(impl, shader) {
         if (remove_dead_vars(&impl->locals,
                              nir_var_function_temp,
                              live, opts))
            progress = true;
      }
   }

   _mesa_set_destroy(live, NULL);

   if (progress) {
      nir_shader_instructions_pass(shader, remove_dead_var_writes,
                                   nir_metadata_control_flow, NULL);
   } else {
      nir_shader_preserve_all_metadata(shader);
   }

   return progress;
}