src/intel/compiler/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 /**
  38  * Is it safe to eliminate the instruction?
  39  */
  40 static bool
  41 can_eliminate(const fs_inst *inst, BITSET_WORD *flag_live)
  42 {
  43     return !inst->is_control_flow() &&
  44            !inst->has_side_effects() &&
  45            !(flag_live[0] & inst->flags_written()) &&
  46            !inst->writes_accumulator;
  47 }
  48
  49 /**
  50  * Is it safe to omit the write, making the destination ARF null?
  51  */
  52 static bool
  53 can_omit_write(const fs_inst *inst)
  54 {
  55    switch (inst->opcode) {
  56    case SHADER_OPCODE_UNTYPED_ATOMIC:
  57    case SHADER_OPCODE_UNTYPED_ATOMIC_LOGICAL:
  58    case SHADER_OPCODE_UNTYPED_ATOMIC_FLOAT:
  59    case SHADER_OPCODE_UNTYPED_ATOMIC_FLOAT_LOGICAL:
  60    case SHADER_OPCODE_TYPED_ATOMIC:
  61    case SHADER_OPCODE_TYPED_ATOMIC_LOGICAL:
  62       return true;
  63    default:
  64       /* We can eliminate the destination write for ordinary instructions,
  65        * but not most SENDs.
  66        */
  67       if (inst->opcode < 128 && inst->mlen == 0)
  68          return true;
  69
  70       /* It might not be safe for other virtual opcodes. */
  71       return false;
  72    }
  73 }
  74
  75 bool
  76 fs_visitor::dead_code_eliminate()
  77 {
  78    bool progress = false;
  79
  80    calculate_live_intervals();
  81
  82    int num_vars = live_intervals->num_vars;
  83    BITSET_WORD *live = rzalloc_array(NULL, BITSET_WORD, BITSET_WORDS(num_vars));
  84    BITSET_WORD *flag_live = rzalloc_array(NULL, BITSET_WORD, 1);
  85
  86    foreach_block_reverse_safe(block, cfg) {
  87       memcpy(live, live_intervals->block_data[block->num].liveout,
  88              sizeof(BITSET_WORD) * BITSET_WORDS(num_vars));
  89       memcpy(flag_live, live_intervals->block_data[block->num].flag_liveout,
  90              sizeof(BITSET_WORD));
  91
  92       foreach_inst_in_block_reverse_safe(fs_inst, inst, block) {
  93          if (inst->dst.file == VGRF) {
  94             const unsigned var = live_intervals->var_from_reg(inst->dst);
  95             bool result_live = false;
  96
  97             for (unsigned i = 0; i < regs_written(inst); i++)
  98                result_live |= BITSET_TEST(live, var + i);
  99
 100             if (!result_live &&
 101                 (can_omit_write(inst) || can_eliminate(inst, flag_live))) {
 102                inst->dst = fs_reg(retype(brw_null_reg(), inst->dst.type));
 103                progress = true;
 104             }
 105          }
 106
 107          if (inst->dst.is_null() && can_eliminate(inst, flag_live)) {
 108             inst->opcode = BRW_OPCODE_NOP;
 109             progress = true;
 110          }
 111
 112          if (inst->dst.file == VGRF) {
 113             if (!inst->is_partial_write()) {
 114                int var = live_intervals->var_from_reg(inst->dst);
 115                for (unsigned i = 0; i < regs_written(inst); i++) {
 116                   BITSET_CLEAR(live, var + i);
 117                }
 118             }
 119          }
 120
 121          if (!inst->predicate && inst->exec_size >= 8)
 122             flag_live[0] &= ~inst->flags_written();
 123
 124          if (inst->opcode == BRW_OPCODE_NOP) {
 125             inst->remove(block);
 126             continue;
 127          }
 128
 129          for (int i = 0; i < inst->sources; i++) {
 130             if (inst->src[i].file == VGRF) {
 131                int var = live_intervals->var_from_reg(inst->src[i]);
 132
 133                for (unsigned j = 0; j < regs_read(inst, i); j++) {
 134                   BITSET_SET(live, var + j);
 135                }
 136             }
 137          }
 138
 139          flag_live[0] |= inst->flags_read(devinfo);
 140       }
 141    }
 142
 143    ralloc_free(live);
 144    ralloc_free(flag_live);
 145
 146    if (progress)
 147       invalidate_live_intervals();
 148
 149    return progress;
 150 }