1 /*
2 * Copyright © 2013 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 "brw_shader.h"
25
26 using namespace brw;
27
28 /** @file brw_predicated_break.cpp
29 *
30 * Loops are often structured as
31 *
32 * loop:
33 * CMP.f0
34 * (+f0) IF
35 * BREAK
36 * ENDIF
37 * ...
38 * WHILE loop
39 *
40 * This peephole pass removes the IF and ENDIF instructions and predicates the
41 * BREAK, dropping two instructions from the loop body.
42 *
43 * If the loop was a DO { ... } WHILE loop, it looks like
44 *
45 * loop:
46 * ...
47 * CMP.f0
48 * (+f0) IF
49 * BREAK
50 * ENDIF
51 * WHILE loop
52 *
53 * and we can remove the BREAK instruction and predicate the WHILE.
54 */
55
56 #define MAX_NESTING 128
57
58 struct loop_continue_tracking {
59 BITSET_WORD has_continue[BITSET_WORDS(MAX_NESTING)];
60 unsigned depth;
61 };
62
63 static void
enter_loop(struct loop_continue_tracking * s)64 enter_loop(struct loop_continue_tracking *s)
65 {
66 s->depth++;
67
68 /* Any loops deeper than that maximum nesting will just re-use the last
69 * flag. This simplifies most of the code. MAX_NESTING is chosen to be
70 * large enough that it is unlikely to occur. Even if it does, the
71 * optimization that uses this tracking is unlikely to make much
72 * difference.
73 */
74 if (s->depth < MAX_NESTING)
75 BITSET_CLEAR(s->has_continue, s->depth);
76 }
77
78 static void
exit_loop(struct loop_continue_tracking * s)79 exit_loop(struct loop_continue_tracking *s)
80 {
81 assert(s->depth > 0);
82 s->depth--;
83 }
84
85 static void
set_continue(struct loop_continue_tracking * s)86 set_continue(struct loop_continue_tracking *s)
87 {
88 const unsigned i = MIN2(s->depth, MAX_NESTING - 1);
89
90 BITSET_SET(s->has_continue, i);
91 }
92
93 static bool
has_continue(const struct loop_continue_tracking * s)94 has_continue(const struct loop_continue_tracking *s)
95 {
96 const unsigned i = MIN2(s->depth, MAX_NESTING - 1);
97
98 return BITSET_TEST(s->has_continue, i);
99 }
100
101 bool
opt_predicated_break(backend_shader * s)102 opt_predicated_break(backend_shader *s)
103 {
104 bool progress = false;
105 struct loop_continue_tracking state = { {0, }, 0 };
106
107 foreach_block (block, s->cfg) {
108 /* DO instructions, by definition, can only be found at the beginning of
109 * basic blocks.
110 */
111 backend_instruction *const do_inst = block->start();
112
113 /* BREAK, CONTINUE, and WHILE instructions, by definition, can only be
114 * found at the ends of basic blocks.
115 */
116 backend_instruction *jump_inst = block->end();
117
118 if (do_inst->opcode == BRW_OPCODE_DO)
119 enter_loop(&state);
120
121 if (jump_inst->opcode == BRW_OPCODE_CONTINUE)
122 set_continue(&state);
123 else if (jump_inst->opcode == BRW_OPCODE_WHILE)
124 exit_loop(&state);
125
126 if (block->start_ip != block->end_ip)
127 continue;
128
129 if (jump_inst->opcode != BRW_OPCODE_BREAK &&
130 jump_inst->opcode != BRW_OPCODE_CONTINUE)
131 continue;
132
133 backend_instruction *if_inst = block->prev()->end();
134 if (if_inst->opcode != BRW_OPCODE_IF)
135 continue;
136
137 backend_instruction *endif_inst = block->next()->start();
138 if (endif_inst->opcode != BRW_OPCODE_ENDIF)
139 continue;
140
141 bblock_t *jump_block = block;
142 bblock_t *if_block = jump_block->prev();
143 bblock_t *endif_block = jump_block->next();
144
145 jump_inst->predicate = if_inst->predicate;
146 jump_inst->predicate_inverse = if_inst->predicate_inverse;
147
148 bblock_t *earlier_block = if_block;
149 if (if_block->start_ip == if_block->end_ip) {
150 earlier_block = if_block->prev();
151 }
152
153 if_inst->remove(if_block);
154
155 bblock_t *later_block = endif_block;
156 if (endif_block->start_ip == endif_block->end_ip) {
157 later_block = endif_block->next();
158 }
159 endif_inst->remove(endif_block);
160
161 if (!earlier_block->ends_with_control_flow()) {
162 earlier_block->children.make_empty();
163 earlier_block->add_successor(s->cfg->mem_ctx, jump_block,
164 bblock_link_logical);
165 }
166
167 if (!later_block->starts_with_control_flow()) {
168 later_block->parents.make_empty();
169 }
170 jump_block->add_successor(s->cfg->mem_ctx, later_block,
171 bblock_link_logical);
172
173 if (earlier_block->can_combine_with(jump_block)) {
174 earlier_block->combine_with(jump_block);
175
176 block = earlier_block;
177 }
178
179 /* Now look at the first instruction of the block following the BREAK. If
180 * it's a WHILE, we can delete the break, predicate the WHILE, and join
181 * the two basic blocks.
182 *
183 * This optimization can only be applied if the only instruction that
184 * can transfer control to the WHILE is the BREAK. If other paths can
185 * lead to the while, the flags may be in an unknown state, and the loop
186 * could terminate prematurely. This can occur if the loop contains a
187 * CONT instruction.
188 */
189 bblock_t *while_block = earlier_block->next();
190 backend_instruction *while_inst = while_block->start();
191
192 if (jump_inst->opcode == BRW_OPCODE_BREAK &&
193 while_inst->opcode == BRW_OPCODE_WHILE &&
194 while_inst->predicate == BRW_PREDICATE_NONE &&
195 !has_continue(&state)) {
196 jump_inst->remove(earlier_block);
197 while_inst->predicate = jump_inst->predicate;
198 while_inst->predicate_inverse = !jump_inst->predicate_inverse;
199
200 assert(earlier_block->can_combine_with(while_block));
201 earlier_block->combine_with(while_block);
202 }
203
204 progress = true;
205 }
206
207 if (progress)
208 s->invalidate_analysis(DEPENDENCY_BLOCKS | DEPENDENCY_INSTRUCTIONS);
209
210 return progress;
211 }
212