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 bool
move_source(nir_src * src,nir_block * block,nir_instr * before,nir_move_options options)55 move_source(nir_src *src, nir_block *block, nir_instr *before, nir_move_options options)
56 {
57 if (!src->is_ssa)
58 return false;
59
60 nir_instr *src_instr = src->ssa->parent_instr;
61
62 if (src_instr->block == block && nir_can_move_instr(src_instr, options)) {
63 exec_node_remove(&src_instr->node);
64
65 if (before)
66 exec_node_insert_node_before(&before->node, &src_instr->node);
67 else
68 exec_list_push_tail(&block->instr_list, &src_instr->node);
69
70 return true;
71 }
72 return false;
73 }
74
75 struct source_cb_data {
76 bool *progress;
77 nir_move_options options;
78 };
79
80 static bool
move_source_cb(nir_src * src,void * data_ptr)81 move_source_cb(nir_src *src, void *data_ptr)
82 {
83 struct source_cb_data data = *(struct source_cb_data*)data_ptr;
84
85 nir_instr *instr = src->parent_instr;
86 if (move_source(src, instr->block, instr, data.options))
87 *data.progress = true;
88
89 return true; /* nir_foreach_src should keep going */
90 }
91
92 static bool
move(nir_block * block,nir_move_options options)93 move(nir_block *block, nir_move_options options)
94 {
95 bool progress = false;
96
97 /* We use a simple approach: walk instructions backwards.
98 *
99 * If the instruction's source is a comparison from the same block,
100 * simply move it here. This may break SSA if it's used earlier in
101 * the block as well. However, as we walk backwards, we'll find the
102 * earlier use and move it again, further up. It eventually ends up
103 * dominating all uses again, restoring SSA form.
104 *
105 * Before walking instructions, we consider the if-condition at the
106 * end of the block, if one exists. It's effectively a use at the
107 * bottom of the block.
108 */
109 nir_if *iff = nir_block_get_following_if(block);
110 if (iff) {
111 progress |= move_source(&iff->condition, block, NULL, options);
112 }
113
114 nir_foreach_instr_reverse(instr, block) {
115 /* The sources of phi instructions happen after the predecessor block
116 * but before this block. (Yes, that's between blocks). This means
117 * that we don't need to move them in order for them to be correct.
118 * We could move them to encourage comparisons that are used in a phi to
119 * the end of the block, doing so correctly would make the pass
120 * substantially more complicated and wouldn't gain us anything since
121 * the phi can't use a flag value anyway.
122 */
123
124 if (instr->type == nir_instr_type_phi) {
125 /* We're going backwards so everything else is a phi too */
126 break;
127 } else if (instr->type == nir_instr_type_alu) {
128 /* Walk ALU instruction sources backwards so that bcsel's boolean
129 * condition is processed last for when comparisons are being moved.
130 */
131 nir_alu_instr *alu = nir_instr_as_alu(instr);
132 for (int i = nir_op_infos[alu->op].num_inputs - 1; i >= 0; i--) {
133 progress |= move_source(&alu->src[i].src, block, instr, options);
134 }
135 } else {
136 struct source_cb_data data;
137 data.progress = &progress;
138 data.options = options;
139 nir_foreach_src(instr, move_source_cb, &data);
140 }
141 }
142
143 return progress;
144 }
145
146 bool
nir_opt_move(nir_shader * shader,nir_move_options options)147 nir_opt_move(nir_shader *shader, nir_move_options options)
148 {
149 bool progress = false;
150
151 nir_foreach_function(func, shader) {
152 if (!func->impl)
153 continue;
154
155 bool impl_progress = false;
156 nir_foreach_block(block, func->impl) {
157 if (move(block, options))
158 impl_progress = true;
159 }
160
161 if (impl_progress) {
162 nir_metadata_preserve(func->impl, nir_metadata_block_index |
163 nir_metadata_dominance |
164 nir_metadata_live_ssa_defs);
165 progress = true;
166 } else {
167 nir_metadata_preserve(func->impl, nir_metadata_all);
168 }
169 }
170
171 return progress;
172 }
173