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 v3d_opt_dead_code.c
26 *
27 * This is a simple dead code eliminator for SSA values in VIR.
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->VIR.
36 */
37
38 #include "v3d_compiler.h"
39
40 static bool debug;
41
42 static void
dce(struct v3d_compile * c,struct qinst * inst)43 dce(struct v3d_compile *c, struct qinst *inst)
44 {
45 if (debug) {
46 fprintf(stderr, "Removing: ");
47 vir_dump_inst(c, inst);
48 fprintf(stderr, "\n");
49 }
50 assert(!v3d_qpu_writes_flags(&inst->qpu));
51 vir_remove_instruction(c, inst);
52 }
53
54 static bool
has_nonremovable_reads(struct v3d_compile * c,struct qinst * inst)55 has_nonremovable_reads(struct v3d_compile *c, struct qinst *inst)
56 {
57 for (int i = 0; i < vir_get_nsrc(inst); i++) {
58 if (inst->src[i].file == QFILE_VPM)
59 return true;
60 }
61
62 return false;
63 }
64
65 static bool
can_write_to_null(struct v3d_compile * c,struct qinst * inst)66 can_write_to_null(struct v3d_compile *c, struct qinst *inst)
67 {
68 /* The SFU instructions must write to a physical register. */
69 if (c->devinfo->ver >= 41 && v3d_qpu_uses_sfu(&inst->qpu))
70 return false;
71
72 return true;
73 }
74
75 static void
vir_dce_flags(struct v3d_compile * c,struct qinst * inst)76 vir_dce_flags(struct v3d_compile *c, struct qinst *inst)
77 {
78 if (debug) {
79 fprintf(stderr,
80 "Removing flags write from: ");
81 vir_dump_inst(c, inst);
82 fprintf(stderr, "\n");
83 }
84
85 assert(inst->qpu.type == V3D_QPU_INSTR_TYPE_ALU);
86
87 inst->qpu.flags.apf = V3D_QPU_PF_NONE;
88 inst->qpu.flags.mpf = V3D_QPU_PF_NONE;
89 inst->qpu.flags.auf = V3D_QPU_UF_NONE;
90 inst->qpu.flags.muf = V3D_QPU_UF_NONE;
91 }
92
93 bool
vir_opt_dead_code(struct v3d_compile * c)94 vir_opt_dead_code(struct v3d_compile *c)
95 {
96 bool progress = false;
97 bool *used = calloc(c->num_temps, sizeof(bool));
98
99 /* Defuse the "are you removing the cursor?" assertion in the core.
100 * You'll need to set up a new cursor for any new instructions after
101 * doing DCE (which we would expect, anyway).
102 */
103 c->cursor.link = NULL;
104
105 vir_for_each_inst_inorder(inst, c) {
106 for (int i = 0; i < vir_get_nsrc(inst); i++) {
107 if (inst->src[i].file == QFILE_TEMP)
108 used[inst->src[i].index] = true;
109 }
110 }
111
112 vir_for_each_block(block, c) {
113 struct qinst *last_flags_write = NULL;
114
115 vir_for_each_inst_safe(inst, block) {
116 /* If this instruction reads the flags, we can't
117 * remove the flags generation for it.
118 */
119 if (v3d_qpu_reads_flags(&inst->qpu))
120 last_flags_write = NULL;
121
122 if (inst->dst.file != QFILE_NULL &&
123 !(inst->dst.file == QFILE_TEMP &&
124 !used[inst->dst.index])) {
125 continue;
126 }
127
128 if (vir_has_side_effects(c, inst))
129 continue;
130
131 if (v3d_qpu_writes_flags(&inst->qpu)) {
132 /* If we obscure a previous flags write,
133 * drop it.
134 */
135 if (last_flags_write &&
136 (inst->qpu.flags.apf != V3D_QPU_PF_NONE ||
137 inst->qpu.flags.mpf != V3D_QPU_PF_NONE)) {
138 vir_dce_flags(c, last_flags_write);
139 progress = true;
140 }
141
142 last_flags_write = inst;
143 }
144
145 if (v3d_qpu_writes_flags(&inst->qpu) ||
146 has_nonremovable_reads(c, inst)) {
147 /* If we can't remove the instruction, but we
148 * don't need its destination value, just
149 * remove the destination. The register
150 * allocator would trivially color it and it
151 * wouldn't cause any register pressure, but
152 * it's nicer to read the VIR code without
153 * unused destination regs.
154 */
155 if (inst->dst.file == QFILE_TEMP &&
156 can_write_to_null(c, inst)) {
157 if (debug) {
158 fprintf(stderr,
159 "Removing dst from: ");
160 vir_dump_inst(c, inst);
161 fprintf(stderr, "\n");
162 }
163 c->defs[inst->dst.index] = NULL;
164 inst->dst.file = QFILE_NULL;
165 progress = true;
166 }
167 continue;
168 }
169
170 assert(inst != last_flags_write);
171 dce(c, inst);
172 progress = true;
173 continue;
174 }
175 }
176
177 free(used);
178
179 return progress;
180 }
181