1 /*
2 * Copyright © 2016 Intel Corporation
3 * Copyright © 2019 Valve Corporation
4 *
5 * Permission is hereby granted, free of charge, to any person obtaining a
6 * copy of this software and associated documentation files (the "Software"),
7 * to deal in the Software without restriction, including without limitation
8 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
9 * and/or sell copies of the Software, and to permit persons to whom the
10 * Software is furnished to do so, subject to the following conditions:
11 *
12 * The above copyright notice and this permission notice (including the next
13 * paragraph) shall be included in all copies or substantial portions of the
14 * Software.
15 *
16 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
17 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
18 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
19 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
20 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
21 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
22 * IN THE SOFTWARE.
23 */
24
25 #include "nir.h"
26
27 /**
28 * \file nir_opt_move.c
29 *
30 * This pass can move various operations just before their first use inside the
31 * same basic block. Usually this is to reduce register usage. It's probably
32 * not a good idea to use this in an optimization loop.
33 *
34 * Moving comparisons is useful because many GPUs generate condition codes
35 * for comparisons, and use predication for conditional selects and control
36 * flow. In a sequence such as:
37 *
38 * vec1 32 ssa_1 = flt a b
39 * <some other operations>
40 * vec1 32 ssa_2 = bcsel ssa_1 c d
41 *
42 * the backend would likely do the comparison, producing condition codes,
43 * then save those to a boolean value. The intervening operations might
44 * trash the condition codes. Then, in order to do the bcsel, it would
45 * need to re-populate the condition code register based on the boolean.
46 *
47 * By moving the comparison just before the bcsel, the condition codes could
48 * be used directly. This eliminates the need to reload them from the boolean
49 * (generally eliminating an instruction). It may also eliminate the need to
50 * create a boolean value altogether (unless it's used elsewhere), which could
51 * lower register pressure.
52 */
53
54 static ALWAYS_INLINE bool
src_is_ssa(nir_src * src,void * state)55 src_is_ssa(nir_src *src, void *state)
56 {
57 return src->is_ssa;
58 }
59
60 static ALWAYS_INLINE bool
instr_reads_register(nir_instr * instr)61 instr_reads_register(nir_instr *instr)
62 {
63 return !nir_foreach_src(instr, src_is_ssa, NULL);
64 }
65
66 static bool
nir_opt_move_block(nir_block * block,nir_move_options options)67 nir_opt_move_block(nir_block *block, nir_move_options options)
68 {
69 bool progress = false;
70 nir_instr *last_instr = nir_block_ends_in_jump(block) ?
71 nir_block_last_instr(block) : NULL;
72 const nir_if *iff = nir_block_get_following_if(block);
73 const nir_instr *if_cond_instr = iff ? iff->condition.parent_instr : NULL;
74
75 /* Walk the instructions backwards.
76 * The instructions get indexed while iterating.
77 * For each instruction which can be moved, find the earliest user
78 * and insert the instruction before it.
79 * If multiple instructions have the same user,
80 * the original order is kept.
81 */
82 unsigned index = 1;
83 unsigned last_reg_def_index = 0;
84 nir_foreach_instr_reverse_safe(instr, block) {
85 instr->index = index++;
86
87 /* Don't move register defs */
88 if (nir_instr_def_is_register(instr)) {
89 last_reg_def_index = instr->index;
90 continue;
91 }
92
93 /* Check if this instruction can be moved downwards */
94 if (!nir_can_move_instr(instr, options))
95 continue;
96
97 /* Check all users in this block which is the first */
98 const nir_ssa_def *def = nir_instr_ssa_def(instr);
99 nir_instr *first_user = instr == if_cond_instr ? NULL : last_instr;
100 nir_foreach_use(use, def) {
101 nir_instr *parent = use->parent_instr;
102 if (parent->type == nir_instr_type_phi || parent->block != block)
103 continue;
104 if (!first_user || parent->index > first_user->index)
105 first_user = parent;
106 }
107
108 if (first_user) {
109 /* Check predecessor instructions for the same index to keep the order */
110 while (nir_instr_prev(first_user)->index == first_user->index)
111 first_user = nir_instr_prev(first_user);
112
113 /* check if the user is already the immediate successor */
114 if (nir_instr_prev(first_user) == instr)
115 continue;
116
117 /* Don't move register reads past register defs */
118 if (first_user->index < last_reg_def_index &&
119 instr_reads_register(instr)) {
120 continue;
121 }
122
123 /* Insert the instruction before it's first user */
124 exec_node_remove(&instr->node);
125 instr->index = first_user->index;
126 exec_node_insert_node_before(&first_user->node, &instr->node);
127 progress = true;
128 continue;
129 }
130
131 /* No user was found in this block:
132 * This instruction will be moved to the end of the block.
133 */
134 assert(nir_block_last_instr(block)->type != nir_instr_type_jump);
135 if (instr == nir_block_last_instr(block))
136 continue;
137
138 exec_node_remove(&instr->node);
139 instr->index = 0;
140 exec_list_push_tail(&block->instr_list, &instr->node);
141
142 /* update last_instr */
143 last_instr = instr;
144
145 progress = true;
146 }
147
148 return progress;
149 }
150
151 bool
nir_opt_move(nir_shader * shader,nir_move_options options)152 nir_opt_move(nir_shader *shader, nir_move_options options)
153 {
154 bool progress = false;
155
156 nir_foreach_function(func, shader) {
157 if (!func->impl)
158 continue;
159
160 bool impl_progress = false;
161 nir_foreach_block(block, func->impl) {
162 if (nir_opt_move_block(block, options))
163 impl_progress = true;
164 }
165
166 if (impl_progress) {
167 nir_metadata_preserve(func->impl, nir_metadata_block_index |
168 nir_metadata_dominance |
169 nir_metadata_live_ssa_defs);
170 progress = true;
171 } else {
172 nir_metadata_preserve(func->impl, nir_metadata_all);
173 }
174 }
175
176 return progress;
177 }
178