1 /*
2 * Copyright © 2014 Broadcom
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 /**
25 * @file vc4_opt_dead_code.c
26 *
27 * This is a simple dead code eliminator for SSA values in QIR.
28 *
29 * It walks all the instructions finding what temps are used, then walks again
30 * to remove instructions writing unused temps.
31 *
32 * This is an inefficient implementation if you have long chains of
33 * instructions where the entire chain is dead, but we expect those to have
34 * been eliminated at the NIR level, and here we're just cleaning up small
35 * problems produced by NIR->QIR.
36 */
37
38 #include "vc4_qir.h"
39
40 static bool debug;
41
42 static void
dce(struct vc4_compile * c,struct qinst * inst)43 dce(struct vc4_compile *c, struct qinst *inst)
44 {
45 if (debug) {
46 fprintf(stderr, "Removing: ");
47 qir_dump_inst(c, inst);
48 fprintf(stderr, "\n");
49 }
50 assert(!inst->sf);
51 qir_remove_instruction(c, inst);
52 }
53
54 static bool
has_nonremovable_reads(struct vc4_compile * c,struct qinst * inst)55 has_nonremovable_reads(struct vc4_compile *c, struct qinst *inst)
56 {
57 for (int i = 0; i < qir_get_nsrc(inst); i++) {
58 if (inst->src[i].file == QFILE_VPM) {
59 uint32_t attr = inst->src[i].index / 4;
60 uint32_t offset = (inst->src[i].index % 4) * 4;
61
62 if (c->vattr_sizes[attr] != offset + 4)
63 return true;
64
65 /* Can't get rid of the last VPM read, or the
66 * simulator (at least) throws an error.
67 */
68 uint32_t total_size = 0;
69 for (uint32_t i = 0; i < ARRAY_SIZE(c->vattr_sizes); i++)
70 total_size += c->vattr_sizes[i];
71 if (total_size == 4)
72 return true;
73 }
74
75 if (inst->src[i].file == QFILE_VARY &&
76 c->input_slots[inst->src[i].index].slot == 0xff) {
77 return true;
78 }
79 }
80
81 return false;
82 }
83
84 bool
qir_opt_dead_code(struct vc4_compile * c)85 qir_opt_dead_code(struct vc4_compile *c)
86 {
87 bool progress = false;
88 bool *used = calloc(c->num_temps, sizeof(bool));
89
90 qir_for_each_inst_inorder(inst, c) {
91 for (int i = 0; i < qir_get_nsrc(inst); i++) {
92 if (inst->src[i].file == QFILE_TEMP)
93 used[inst->src[i].index] = true;
94 }
95 }
96
97 qir_for_each_block(block, c) {
98 qir_for_each_inst_safe(inst, block) {
99 if (inst->dst.file != QFILE_NULL &&
100 !(inst->dst.file == QFILE_TEMP &&
101 !used[inst->dst.index])) {
102 continue;
103 }
104
105 if (qir_has_side_effects(c, inst))
106 continue;
107
108 if (inst->sf ||
109 has_nonremovable_reads(c, inst)) {
110 /* If we can't remove the instruction, but we
111 * don't need its destination value, just
112 * remove the destination. The register
113 * allocator would trivially color it and it
114 * wouldn't cause any register pressure, but
115 * it's nicer to read the QIR code without
116 * unused destination regs.
117 */
118 if (inst->dst.file == QFILE_TEMP) {
119 if (debug) {
120 fprintf(stderr,
121 "Removing dst from: ");
122 qir_dump_inst(c, inst);
123 fprintf(stderr, "\n");
124 }
125 c->defs[inst->dst.index] = NULL;
126 inst->dst.file = QFILE_NULL;
127 progress = true;
128 }
129 continue;
130 }
131
132 for (int i = 0; i < qir_get_nsrc(inst); i++) {
133 if (inst->src[i].file != QFILE_VPM)
134 continue;
135 uint32_t attr = inst->src[i].index / 4;
136 uint32_t offset = (inst->src[i].index % 4) * 4;
137
138 if (c->vattr_sizes[attr] == offset + 4) {
139 c->num_inputs--;
140 c->vattr_sizes[attr] -= 4;
141 }
142 }
143
144 dce(c, inst);
145 progress = true;
146 continue;
147 }
148 }
149
150 free(used);
151
152 return progress;
153 }
154