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  *   - if/else/endif
  37  *   - else in if/else
  38  */
  39 bool
  40 dead_control_flow_eliminate(backend_shader *s)
  41 {
  42    bool progress = false;
  43
  44    foreach_block_safe (block, s->cfg) {
  45       bblock_t *prev_block = block->prev();
  46       backend_instruction *const inst = block->start();
  47       backend_instruction *prev_inst = prev_block->end();
  48
  49       /* ENDIF instructions, by definition, can only be found at the start of
  50        * basic blocks.
  51        */
  52       if (inst->opcode == BRW_OPCODE_ENDIF) {
  53          bool found = false;
  54          bblock_t *if_block = NULL, *else_block = NULL, *endif_block = block;
  55          backend_instruction *endif_inst = inst;
  56
  57          backend_instruction *if_inst = NULL, *else_inst = NULL;
  58          if (prev_inst->opcode == BRW_OPCODE_ELSE) {
  59             else_inst = prev_inst;
  60             else_block = endif_block->prev();
  61             found = true;
  62
  63             if (else_block->start_ip == else_block->end_ip) {
  64                prev_block = prev_block->prev();
  65                prev_inst = prev_block->end();
  66             }
  67          }
  68
  69          if (prev_inst->opcode == BRW_OPCODE_IF) {
  70             if_inst = prev_inst;
  71             if_block = prev_block;
  72             found = true;
  73          } else {
  74             /* Don't remove the ENDIF if we didn't find a dead IF. */
  75             endif_inst = NULL;
  76          }
  77
  78          if (found) {
  79             bblock_t *earlier_block = NULL, *later_block = NULL;
  80
  81             if (if_inst) {
  82                if (if_block->start_ip == if_block->end_ip) {
  83                   earlier_block = if_block->prev();
  84                } else {
  85                   earlier_block = if_block;
  86                }
  87                if_inst->remove(if_block);
  88             }
  89
  90             if (else_inst) {
  91                else_inst->remove(else_block);
  92             }
  93
  94             if (endif_inst) {
  95                if (endif_block->start_ip == endif_block->end_ip) {
  96                   later_block = endif_block->next();
  97                } else {
  98                   later_block = endif_block;
  99                }
 100                endif_inst->remove(endif_block);
 101             }
 102
 103             assert((earlier_block == NULL) == (later_block == NULL));
 104             if (earlier_block && earlier_block->can_combine_with(later_block)) {
 105                earlier_block->combine_with(later_block);
 106
 107                /* If ENDIF was in its own block, then we've now deleted it and
 108                 * merged the two surrounding blocks, the latter of which the
 109                 * __next block pointer was pointing to.
 110                 */
 111                if (endif_block != later_block) {
 112                   __next = earlier_block->next();
 113                }
 114             }
 115
 116             progress = true;
 117          }
 118       } else if (inst->opcode == BRW_OPCODE_ELSE &&
 119                  prev_inst->opcode == BRW_OPCODE_IF) {
 120          bblock_t *const else_block = block;
 121          backend_instruction *const if_inst = prev_inst;
 122          backend_instruction *const else_inst = inst;
 123
 124          /* Since the else-branch is becoming the new then-branch, the
 125           * condition has to be inverted.
 126           */
 127          if_inst->predicate_inverse = !if_inst->predicate_inverse;
 128          else_inst->remove(else_block);
 129
 130          progress = true;
 131       }
 132    }
 133
 134    if (progress)
 135       s->invalidate_live_intervals();
 136
 137    return progress;
 138 }