src/intel/compiler/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
  46       if (!prev_block)
  47          continue;
  48
  49       backend_instruction *const inst = block->start();
  50       backend_instruction *const prev_inst = prev_block->end();
  51
  52       /* ENDIF instructions, by definition, can only be found at the start of
  53        * basic blocks.
  54        */
  55       if (inst->opcode == BRW_OPCODE_ENDIF &&
  56           prev_inst->opcode == BRW_OPCODE_ELSE) {
  57          bblock_t *const else_block = prev_block;
  58          backend_instruction *const else_inst = prev_inst;
  59
  60          else_inst->remove(else_block);
  61          progress = true;
  62       } else if (inst->opcode == BRW_OPCODE_ENDIF &&
  63                  prev_inst->opcode == BRW_OPCODE_IF) {
  64          bblock_t *const endif_block = block;
  65          bblock_t *const if_block = prev_block;
  66          backend_instruction *const endif_inst = inst;
  67          backend_instruction *const if_inst = prev_inst;
  68
  69          bblock_t *earlier_block = NULL, *later_block = NULL;
  70
  71          if (if_block->start_ip == if_block->end_ip) {
  72             earlier_block = if_block->prev();
  73          } else {
  74             earlier_block = if_block;
  75          }
  76          if_inst->remove(if_block);
  77
  78          if (endif_block->start_ip == endif_block->end_ip) {
  79             later_block = endif_block->next();
  80          } else {
  81             later_block = endif_block;
  82          }
  83          endif_inst->remove(endif_block);
  84
  85          assert((earlier_block == NULL) == (later_block == NULL));
  86          if (earlier_block && earlier_block->can_combine_with(later_block)) {
  87             earlier_block->combine_with(later_block);
  88
  89             /* If ENDIF was in its own block, then we've now deleted it and
  90              * merged the two surrounding blocks, the latter of which the
  91              * __next block pointer was pointing to.
  92              */
  93             if (endif_block != later_block) {
  94                __next = earlier_block->next();
  95             }
  96          }
  97
  98          progress = true;
  99       } else if (inst->opcode == BRW_OPCODE_ELSE &&
 100                  prev_inst->opcode == BRW_OPCODE_IF) {
 101          bblock_t *const else_block = block;
 102          backend_instruction *const if_inst = prev_inst;
 103          backend_instruction *const else_inst = inst;
 104
 105          /* Since the else-branch is becoming the new then-branch, the
 106           * condition has to be inverted.
 107           */
 108          if_inst->predicate_inverse = !if_inst->predicate_inverse;
 109          else_inst->remove(else_block);
 110
 111          progress = true;
 112       }
 113    }
 114
 115    if (progress)
 116       s->invalidate_live_intervals();
 117
 118    return progress;
 119 }