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

   1 /*
   2  * Copyright © 2014 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 #include "brw_fs.h"
  25 #include "brw_fs_live_variables.h"
  26 #include "brw_cfg.h"
  27
  28 /** @file brw_fs_dead_code_eliminate.cpp
  29  *
  30  * Dataflow-aware dead code elimination.
  31  *
  32  * Walks the instruction list from the bottom, removing instructions that
  33  * have results that both aren't used in later blocks and haven't been read
  34  * yet in the tail end of this block.
  35  */
  36
  37 bool
  38 fs_visitor::dead_code_eliminate()
  39 {
  40    bool progress = false;
  41
  42    cfg_t cfg(&instructions);
  43
  44    calculate_live_intervals();
  45
  46    int num_vars = live_intervals->num_vars;
  47    BITSET_WORD *live = ralloc_array(NULL, BITSET_WORD, BITSET_WORDS(num_vars));
  48
  49    for (int b = 0; b < cfg.num_blocks; b++) {
  50       bblock_t *block = cfg.blocks[b];
  51       memcpy(live, live_intervals->bd[b].liveout,
  52              sizeof(BITSET_WORD) * BITSET_WORDS(num_vars));
  53
  54       for (fs_inst *inst = (fs_inst *)block->end;
  55            inst != block->start->prev;
  56            inst = (fs_inst *)inst->prev) {
  57          if (inst->dst.file == GRF &&
  58              !inst->has_side_effects() &&
  59              !inst->writes_flag()) {
  60             bool result_live = false;
  61
  62             if (inst->regs_written == 1) {
  63                int var = live_intervals->var_from_reg(&inst->dst);
  64                result_live = BITSET_TEST(live, var);
  65             } else {
  66                int var = live_intervals->var_from_vgrf[inst->dst.reg];
  67                for (int i = 0; i < inst->regs_written; i++) {
  68                   result_live = result_live || BITSET_TEST(live, var + i);
  69                }
  70             }
  71
  72             if (!result_live) {
  73                progress = true;
  74
  75                if (inst->writes_accumulator) {
  76                   inst->dst = fs_reg(retype(brw_null_reg(), inst->dst.type));
  77                } else {
  78                   inst->opcode = BRW_OPCODE_NOP;
  79                   continue;
  80                }
  81             }
  82          }
  83
  84          if (inst->dst.file == GRF) {
  85             if (!inst->is_partial_write()) {
  86                int var = live_intervals->var_from_vgrf[inst->dst.reg];
  87                for (int i = 0; i < inst->regs_written; i++) {
  88                   BITSET_CLEAR(live, var + inst->dst.reg_offset + i);
  89                }
  90             }
  91          }
  92
  93          for (int i = 0; i < 3; i++) {
  94             if (inst->src[i].file == GRF) {
  95                int var = live_intervals->var_from_vgrf[inst->src[i].reg];
  96
  97                for (int j = 0; j < inst->regs_read(this, i); j++) {
  98                   BITSET_SET(live, var + inst->src[i].reg_offset + j);
  99                }
 100             }
 101          }
 102       }
 103    }
 104
 105    ralloc_free(live);
 106
 107    if (progress) {
 108       foreach_list_safe(node, &this->instructions) {
 109          fs_inst *inst = (fs_inst *)node;
 110
 111          if (inst->opcode == BRW_OPCODE_NOP) {
 112             inst->remove();
 113          }
 114       }
 115
 116       invalidate_live_intervals();
 117    }
 118
 119    return progress;
 120 }