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