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 using namespace brw;
  33
  34 /* Look for and eliminate dead control flow:
  35  *
  36  *   - if/endif
  37  *   - else in else/endif
  38  *   - then in if/else/endif
  39  */
  40 bool
  41 dead_control_flow_eliminate(backend_shader *s)
  42 {
  43    bool progress = false;
  44
  45    foreach_block_safe (block, s->cfg) {
  46       bblock_t *prev_block = block->prev();
  47
  48       if (!prev_block)
  49          continue;
  50
  51       backend_instruction *const inst = block->start();
  52       backend_instruction *const prev_inst = prev_block->end();
  53
  54       /* ENDIF instructions, by definition, can only be found at the start of
  55        * basic blocks.
  56        */
  57       if (inst->opcode == BRW_OPCODE_ENDIF &&
  58           prev_inst->opcode == BRW_OPCODE_ELSE) {
  59          bblock_t *const else_block = prev_block;
  60          backend_instruction *const else_inst = prev_inst;
  61
  62          else_inst->remove(else_block);
  63          progress = true;
  64       } else if (inst->opcode == BRW_OPCODE_ENDIF &&
  65                  prev_inst->opcode == BRW_OPCODE_IF) {
  66          bblock_t *const endif_block = block;
  67          bblock_t *const if_block = prev_block;
  68          backend_instruction *const endif_inst = inst;
  69          backend_instruction *const if_inst = prev_inst;
  70
  71          bblock_t *earlier_block = NULL, *later_block = NULL;
  72
  73          if (if_block->start_ip == if_block->end_ip) {
  74             earlier_block = if_block->prev();
  75          } else {
  76             earlier_block = if_block;
  77          }
  78          if_inst->remove(if_block);
  79
  80          if (endif_block->start_ip == endif_block->end_ip) {
  81             later_block = endif_block->next();
  82          } else {
  83             later_block = endif_block;
  84          }
  85          endif_inst->remove(endif_block);
  86
  87          assert((earlier_block == NULL) == (later_block == NULL));
  88          if (earlier_block && earlier_block->can_combine_with(later_block)) {
  89             earlier_block->combine_with(later_block);
  90
  91             /* If ENDIF was in its own block, then we've now deleted it and
  92              * merged the two surrounding blocks, the latter of which the
  93              * __next block pointer was pointing to.
  94              */
  95             if (endif_block != later_block) {
  96                __next = earlier_block->next();
  97             }
  98          }
  99
 100          progress = true;
 101       } else if (inst->opcode == BRW_OPCODE_ELSE &&
 102                  prev_inst->opcode == BRW_OPCODE_IF) {
 103          bblock_t *const else_block = block;
 104          backend_instruction *const if_inst = prev_inst;
 105          backend_instruction *const else_inst = inst;
 106
 107          /* Since the else-branch is becoming the new then-branch, the
 108           * condition has to be inverted.
 109           */
 110          if_inst->predicate_inverse = !if_inst->predicate_inverse;
 111          else_inst->remove(else_block);
 112
 113          progress = true;
 114       }
 115    }
 116
 117    if (progress)
 118       s->invalidate_analysis(DEPENDENCY_BLOCKS | DEPENDENCY_INSTRUCTIONS);
 119
 120    return progress;
 121 }