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  */
  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 *if_block = NULL, *else_block = NULL, *endif_block = block;
  45       bool found = false;
  46
  47       /* ENDIF instructions, by definition, can only be found at the start of
  48        * basic blocks.
  49        */
  50       backend_instruction *endif_inst = endif_block->start();
  51       if (endif_inst->opcode != BRW_OPCODE_ENDIF)
  52          continue;
  53
  54       backend_instruction *if_inst = NULL, *else_inst = NULL;
  55       backend_instruction *prev_inst = endif_block->prev()->end();
  56       if (prev_inst->opcode == BRW_OPCODE_ELSE) {
  57          else_inst = prev_inst;
  58          else_block = endif_block->prev();
  59          found = true;
  60
  61          if (else_block->start_ip == else_block->end_ip)
  62             prev_inst = else_block->prev()->end();
  63       }
  64
  65       if (prev_inst->opcode == BRW_OPCODE_IF) {
  66          if_inst = prev_inst;
  67          if_block = else_block != NULL ? else_block->prev()
  68                                        : endif_block->prev();
  69          found = true;
  70       } else {
  71          /* Don't remove the ENDIF if we didn't find a dead IF. */
  72          endif_inst = NULL;
  73       }
  74
  75       if (found) {
  76          bblock_t *earlier_block = NULL, *later_block = NULL;
  77
  78          if (if_inst) {
  79             if (if_block->start_ip == if_block->end_ip) {
  80                earlier_block = if_block->prev();
  81             } else {
  82                earlier_block = if_block;
  83             }
  84             if_inst->remove(if_block);
  85          }
  86
  87          if (else_inst) {
  88             else_inst->remove(else_block);
  89          }
  90
  91          if (endif_inst) {
  92             if (endif_block->start_ip == endif_block->end_ip) {
  93                later_block = endif_block->next();
  94             } else {
  95                later_block = endif_block;
  96             }
  97             endif_inst->remove(endif_block);
  98          }
  99
 100          assert((earlier_block == NULL) == (later_block == NULL));
 101          if (earlier_block && earlier_block->can_combine_with(later_block)) {
 102             earlier_block->combine_with(later_block);
 103
 104             /* If ENDIF was in its own block, then we've now deleted it and
 105              * merged the two surrounding blocks, the latter of which the
 106              * __next block pointer was pointing to.
 107              */
 108             if (endif_block != later_block) {
 109                __next = earlier_block->next();
 110             }
 111          }
 112
 113          progress = true;
 114       }
 115    }
 116
 117    if (progress)
 118       s->invalidate_live_intervals();
 119
 120    return progress;
 121 }