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    calculate_live_intervals();
  43
  44    int num_vars = live_intervals->num_vars;
  45    BITSET_WORD *live = rzalloc_array(NULL, BITSET_WORD, BITSET_WORDS(num_vars));
  46    BITSET_WORD *flag_live = rzalloc_array(NULL, BITSET_WORD, 1);
  47
  48    foreach_block_reverse_safe(block, cfg) {
  49       memcpy(live, live_intervals->block_data[block->num].liveout,
  50              sizeof(BITSET_WORD) * BITSET_WORDS(num_vars));
  51       memcpy(flag_live, live_intervals->block_data[block->num].flag_liveout,
  52              sizeof(BITSET_WORD));
  53
  54       foreach_inst_in_block_reverse_safe(fs_inst, inst, block) {
  55          if (inst->dst.file == VGRF && !inst->has_side_effects()) {
  56             const unsigned var = live_intervals->var_from_reg(inst->dst);
  57             bool result_live = false;
  58
  59             for (unsigned i = 0; i < regs_written(inst); i++)
  60                result_live |= BITSET_TEST(live, var + i);
  61
  62             if (!result_live) {
  63                progress = true;
  64
  65                if (inst->writes_accumulator || inst->flags_written()) {
  66                   inst->dst = fs_reg(retype(brw_null_reg(), inst->dst.type));
  67                } else {
  68                   inst->opcode = BRW_OPCODE_NOP;
  69                }
  70             }
  71          }
  72
  73          if (inst->dst.is_null() &&
  74              !inst->is_control_flow() &&
  75              !inst->has_side_effects() &&
  76              !(flag_live[0] & inst->flags_written()) &&
  77              !inst->writes_accumulator) {
  78             inst->opcode = BRW_OPCODE_NOP;
  79             progress = true;
  80          }
  81
  82          if (inst->dst.file == VGRF) {
  83             if (!inst->is_partial_write()) {
  84                int var = live_intervals->var_from_reg(inst->dst);
  85                for (unsigned i = 0; i < regs_written(inst); i++) {
  86                   BITSET_CLEAR(live, var + i);
  87                }
  88             }
  89          }
  90
  91          if (!inst->predicate && inst->exec_size >= 8)
  92             flag_live[0] &= ~inst->flags_written();
  93
  94          if (inst->opcode == BRW_OPCODE_NOP) {
  95             inst->remove(block);
  96             continue;
  97          }
  98
  99          for (int i = 0; i < inst->sources; i++) {
 100             if (inst->src[i].file == VGRF) {
 101                int var = live_intervals->var_from_reg(inst->src[i]);
 102
 103                for (unsigned j = 0; j < regs_read(inst, i); j++) {
 104                   BITSET_SET(live, var + j);
 105                }
 106             }
 107          }
 108
 109          flag_live[0] |= inst->flags_read(devinfo);
 110       }
 111    }
 112
 113    ralloc_free(live);
 114    ralloc_free(flag_live);
 115
 116    if (progress)
 117       invalidate_live_intervals();
 118
 119    return progress;
 120 }