src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp

   1 /*
   2  * Copyright © 2013 Intel Corporation
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining a
   5  * copy of this software and associated documentation files (the "Software"),
   6  * to deal in the Software without restriction, including without limitation
   7  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   8  * and/or sell copies of the Software, and to permit persons to whom the
   9  * Software is furnished to do so, subject to the following conditions:
  10  *
  11  * The above copyright notice and this permission notice (including the next
  12  * paragraph) shall be included in all copies or substantial portions of the
  13  * Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  16  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  18  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  19  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  20  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  21  * IN THE SOFTWARE.
  22  */
  23
  24 /** @file brw_dead_control_flow.cpp
  25  *
  26  * This file implements the dead control flow elimination optimization pass.
  27  */
  28
  29 #include "brw_shader.h"
  30 #include "brw_cfg.h"
  31
  32 /* Look for and eliminate dead control flow:
  33  *
  34  *   - if/endif
  35  *   - else in else/endif
  36  *   - then in if/else/endif
  37  */
  38 bool
  39 dead_control_flow_eliminate(backend_shader *s)
  40 {
  41    bool progress = false;
  42
  43    foreach_block_safe (block, s->cfg) {
  44       bblock_t *prev_block = block->prev();
  45       backend_instruction *const inst = block->start();
  46       backend_instruction *const prev_inst = prev_block->end();
  47
  48       /* ENDIF instructions, by definition, can only be found at the start of
  49        * basic blocks.
  50        */
  51       if (inst->opcode == BRW_OPCODE_ENDIF &&
  52           prev_inst->opcode == BRW_OPCODE_ELSE) {
  53          bblock_t *const else_block = prev_block;
  54          backend_instruction *const else_inst = prev_inst;
  55
  56          else_inst->remove(else_block);
  57          progress = true;
  58       } else if (inst->opcode == BRW_OPCODE_ENDIF &&
  59                  prev_inst->opcode == BRW_OPCODE_IF) {
  60          bblock_t *const endif_block = block;
  61          bblock_t *const if_block = prev_block;
  62          backend_instruction *const endif_inst = inst;
  63          backend_instruction *const if_inst = prev_inst;
  64
  65          bblock_t *earlier_block = NULL, *later_block = NULL;
  66
  67          if (if_block->start_ip == if_block->end_ip) {
  68             earlier_block = if_block->prev();
  69          } else {
  70             earlier_block = if_block;
  71          }
  72          if_inst->remove(if_block);
  73
  74          if (endif_block->start_ip == endif_block->end_ip) {
  75             later_block = endif_block->next();
  76          } else {
  77             later_block = endif_block;
  78          }
  79          endif_inst->remove(endif_block);
  80
  81          assert((earlier_block == NULL) == (later_block == NULL));
  82          if (earlier_block && earlier_block->can_combine_with(later_block)) {
  83             earlier_block->combine_with(later_block);
  84
  85             /* If ENDIF was in its own block, then we've now deleted it and
  86              * merged the two surrounding blocks, the latter of which the
  87              * __next block pointer was pointing to.
  88              */
  89             if (endif_block != later_block) {
  90                __next = earlier_block->next();
  91             }
  92          }
  93
  94          progress = true;
  95       } else if (inst->opcode == BRW_OPCODE_ELSE &&
  96                  prev_inst->opcode == BRW_OPCODE_IF) {
  97          bblock_t *const else_block = block;
  98          backend_instruction *const if_inst = prev_inst;
  99          backend_instruction *const else_inst = inst;
 100
 101          /* Since the else-branch is becoming the new then-branch, the
 102           * condition has to be inverted.
 103           */
 104          if_inst->predicate_inverse = !if_inst->predicate_inverse;
 105          else_inst->remove(else_block);
 106
 107          progress = true;
 108       }
 109    }
 110
 111    if (progress)
 112       s->invalidate_live_intervals();
 113
 114    return progress;
 115 }