1 /*
2 * Copyright © 2016 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 "nir.h"
25
26 /**
27 * \file nir_opt_move_comparisons.c
28 *
29 * This pass moves ALU comparison operations just before their first use.
30 *
31 * It only moves instructions within a single basic block; cross-block
32 * movement is left to global code motion.
33 *
34 * Many GPUs generate condition codes for comparisons, and use predication
35 * for conditional selects and control flow. In a sequence such as:
36 *
37 * vec1 32 ssa_1 = flt a b
38 * <some other operations>
39 * vec1 32 ssa_2 = bcsel ssa_1 c d
40 *
41 * the backend would likely do the comparison, producing condition codes,
42 * then save those to a boolean value. The intervening operations might
43 * trash the condition codes. Then, in order to do the bcsel, it would
44 * need to re-populate the condition code register based on the boolean.
45 *
46 * By moving the comparison just before the bcsel, the condition codes could
47 * be used directly. This eliminates the need to reload them from the boolean
48 * (generally eliminating an instruction). It may also eliminate the need to
49 * create a boolean value altogether (unless it's used elsewhere), which could
50 * lower register pressure.
51 */
52
53 static bool
is_comparison(nir_op op)54 is_comparison(nir_op op)
55 {
56 switch (op) {
57 case nir_op_flt:
58 case nir_op_fge:
59 case nir_op_feq:
60 case nir_op_fne:
61 case nir_op_ilt:
62 case nir_op_ult:
63 case nir_op_ige:
64 case nir_op_uge:
65 case nir_op_ieq:
66 case nir_op_ine:
67 case nir_op_i2b:
68 case nir_op_f2b:
69 case nir_op_inot:
70 case nir_op_fnot:
71 return true;
72 default:
73 return false;
74 }
75 }
76
77 static bool
move_comparison_source(nir_src * src,nir_block * block,nir_instr * before)78 move_comparison_source(nir_src *src, nir_block *block, nir_instr *before)
79 {
80 if (!src->is_ssa)
81 return false;
82
83 nir_instr *src_instr = src->ssa->parent_instr;
84
85 if (src_instr->block == block &&
86 src_instr->type == nir_instr_type_alu &&
87 is_comparison(nir_instr_as_alu(src_instr)->op)) {
88
89 exec_node_remove(&src_instr->node);
90
91 if (before)
92 exec_node_insert_node_before(&before->node, &src_instr->node);
93 else
94 exec_list_push_tail(&block->instr_list, &src_instr->node);
95
96 return true;
97 }
98
99 return false;
100 }
101
102 static bool
move_comparison_source_cb(nir_src * src,void * data)103 move_comparison_source_cb(nir_src *src, void *data)
104 {
105 bool *progress = data;
106
107 nir_instr *instr = src->parent_instr;
108 if (move_comparison_source(src, instr->block, instr))
109 *progress = true;
110
111 return true; /* nir_foreach_src should keep going */
112 }
113
114 static bool
move_comparisons(nir_block * block)115 move_comparisons(nir_block *block)
116 {
117 bool progress = false;
118
119 /* We use a simple approach: walk instructions backwards.
120 *
121 * If the instruction's source is a comparison from the same block,
122 * simply move it here. This may break SSA if it's used earlier in
123 * the block as well. However, as we walk backwards, we'll find the
124 * earlier use and move it again, further up. It eventually ends up
125 * dominating all uses again, restoring SSA form.
126 *
127 * Before walking instructions, we consider the if-condition at the
128 * end of the block, if one exists. It's effectively a use at the
129 * bottom of the block.
130 */
131 nir_if *iff = nir_block_get_following_if(block);
132 if (iff) {
133 progress |= move_comparison_source(&iff->condition, block, NULL);
134 }
135
136 nir_foreach_instr_reverse(instr, block) {
137 /* The sources of phi instructions happen after the predecessor block
138 * but before this block. (Yes, that's between blocks). This means
139 * that we don't need to move them in order for them to be correct.
140 * We could move them to encourage comparisons that are used in a phi to
141 * the end of the block, doing so correctly would make the pass
142 * substantially more complicated and wouldn't gain us anything since
143 * the phi can't use a flag value anyway.
144 */
145 if (instr->type == nir_instr_type_phi) {
146 /* We're going backwards so everything else is a phi too */
147 break;
148 } else if (instr->type == nir_instr_type_alu) {
149 /* Walk ALU instruction sources backwards so that bcsel's boolean
150 * condition is processed last.
151 */
152 nir_alu_instr *alu = nir_instr_as_alu(instr);
153 for (int i = nir_op_infos[alu->op].num_inputs - 1; i >= 0; i--) {
154 progress |= move_comparison_source(&alu->src[i].src,
155 block, instr);
156 }
157 } else {
158 nir_foreach_src(instr, move_comparison_source_cb, &progress);
159 }
160 }
161
162 return progress;
163 }
164
165 bool
nir_opt_move_comparisons(nir_shader * shader)166 nir_opt_move_comparisons(nir_shader *shader)
167 {
168 bool progress = false;
169
170 nir_foreach_function(func, shader) {
171 if (!func->impl)
172 continue;
173
174 nir_foreach_block(block, func->impl) {
175 if (move_comparisons(block)) {
176 nir_metadata_preserve(func->impl, nir_metadata_block_index |
177 nir_metadata_dominance |
178 nir_metadata_live_ssa_defs);
179 progress = true;
180 }
181 }
182 }
183
184 return progress;
185 }
186