brw_dead_control_flow.cpp

/*
 * Copyright © 2013 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.
 */

/** @file brw_dead_control_flow.cpp
 *
 * This file implements the dead control flow elimination optimization pass.
 */

#include "brw_shader.h"
#include "brw_cfg.h"

using namespace brw;

/* Look for and eliminate dead control flow:
 *
 *   - if/endif
 *   - else in else/endif
 *   - then in if/else/endif
 */
bool
dead_control_flow_eliminate(backend_shader *s)
{
   bool progress = false;

   foreach_block_safe (block, s->cfg) {
      bblock_t *prev_block = block->prev();

      if (!prev_block)
         continue;

      backend_instruction *const inst = block->start();
      backend_instruction *const prev_inst = prev_block->end();

      /* ENDIF instructions, by definition, can only be found at the start of
       * basic blocks.
       */
      if (inst->opcode == BRW_OPCODE_ENDIF &&
          prev_inst->opcode == BRW_OPCODE_ELSE) {
         bblock_t *const else_block = prev_block;
         backend_instruction *const else_inst = prev_inst;

         else_inst->remove(else_block);
         progress = true;
      } else if (inst->opcode == BRW_OPCODE_ENDIF &&
                 prev_inst->opcode == BRW_OPCODE_IF) {
         bblock_t *const endif_block = block;
         bblock_t *const if_block = prev_block;
         backend_instruction *const endif_inst = inst;
         backend_instruction *const if_inst = prev_inst;

         bblock_t *earlier_block = NULL, *later_block = NULL;

         if (if_block->start_ip == if_block->end_ip) {
            earlier_block = if_block->prev();
         } else {
            earlier_block = if_block;
         }
         if_inst->remove(if_block);

         if (endif_block->start_ip == endif_block->end_ip) {
            later_block = endif_block->next();
         } else {
            later_block = endif_block;
         }
         endif_inst->remove(endif_block);

         assert((earlier_block == NULL) == (later_block == NULL));
         if (earlier_block && earlier_block->can_combine_with(later_block)) {
            earlier_block->combine_with(later_block);

            /* If ENDIF was in its own block, then we've now deleted it and
             * merged the two surrounding blocks, the latter of which the
             * __next block pointer was pointing to.
             */
            if (endif_block != later_block) {
               __next = earlier_block->next();
            }
         }

         progress = true;
      } else if (inst->opcode == BRW_OPCODE_ELSE &&
                 prev_inst->opcode == BRW_OPCODE_IF) {
         bblock_t *const else_block = block;
         backend_instruction *const if_inst = prev_inst;
         backend_instruction *const else_inst = inst;

         /* Since the else-branch is becoming the new then-branch, the
          * condition has to be inverted.
          */
         if_inst->predicate_inverse = !if_inst->predicate_inverse;
         else_inst->remove(else_block);

         progress = true;
      }
   }

   if (progress)
      s->invalidate_analysis(DEPENDENCY_BLOCKS | DEPENDENCY_INSTRUCTIONS);

   return progress;
}