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_fs.h"
25 #include "brw_cfg.h"
26
27 /** @file brw_fs_sel_peephole.cpp
28 *
29 * This file contains the opt_peephole_sel() optimization pass that replaces
30 * MOV instructions to the same destination in the "then" and "else" bodies of
31 * an if statement with SEL instructions.
32 */
33
34 /* Four MOVs seems to be pretty typical, so I picked the next power of two in
35 * the hopes that it would handle almost anything possible in a single
36 * pass.
37 */
38 #define MAX_MOVS 8 /**< The maximum number of MOVs to attempt to match. */
39
40 using namespace brw;
41
42 /**
43 * Scans forwards from an IF counting consecutive MOV instructions in the
44 * "then" and "else" blocks of the if statement.
45 *
46 * A pointer to the bblock_t following the IF is passed as the <then_block>
47 * argument. The function stores pointers to the MOV instructions in the
48 * <then_mov> and <else_mov> arrays.
49 *
50 * \return the minimum number of MOVs found in the two branches or zero if
51 * an error occurred.
52 *
53 * E.g.:
54 * IF ...
55 * then_mov[0] = MOV g4, ...
56 * then_mov[1] = MOV g5, ...
57 * then_mov[2] = MOV g6, ...
58 * ELSE ...
59 * else_mov[0] = MOV g4, ...
60 * else_mov[1] = MOV g5, ...
61 * else_mov[2] = MOV g7, ...
62 * ENDIF
63 * returns 3.
64 */
65 static int
count_movs_from_if(fs_inst * then_mov[MAX_MOVS],fs_inst * else_mov[MAX_MOVS],bblock_t * then_block,bblock_t * else_block)66 count_movs_from_if(fs_inst *then_mov[MAX_MOVS], fs_inst *else_mov[MAX_MOVS],
67 bblock_t *then_block, bblock_t *else_block)
68 {
69 int then_movs = 0;
70 foreach_inst_in_block(fs_inst, inst, then_block) {
71 if (then_movs == MAX_MOVS || inst->opcode != BRW_OPCODE_MOV ||
72 inst->flags_written())
73 break;
74
75 then_mov[then_movs] = inst;
76 then_movs++;
77 }
78
79 int else_movs = 0;
80 foreach_inst_in_block(fs_inst, inst, else_block) {
81 if (else_movs == MAX_MOVS || inst->opcode != BRW_OPCODE_MOV ||
82 inst->flags_written())
83 break;
84
85 else_mov[else_movs] = inst;
86 else_movs++;
87 }
88
89 return MIN2(then_movs, else_movs);
90 }
91
92 /**
93 * Try to replace IF/MOV+/ELSE/MOV+/ENDIF with SEL.
94 *
95 * Many GLSL shaders contain the following pattern:
96 *
97 * x = condition ? foo : bar
98 *
99 * or
100 *
101 * if (...) a.xyzw = foo.xyzw;
102 * else a.xyzw = bar.xyzw;
103 *
104 * The compiler emits an ir_if tree for this, since each subexpression might be
105 * a complex tree that could have side-effects or short-circuit logic.
106 *
107 * However, the common case is to simply select one of two constants or
108 * variable values---which is exactly what SEL is for. In this case, the
109 * assembly looks like:
110 *
111 * (+f0) IF
112 * MOV dst src0
113 * ...
114 * ELSE
115 * MOV dst src1
116 * ...
117 * ENDIF
118 *
119 * where each pair of MOVs to a common destination and can be easily translated
120 * into
121 *
122 * (+f0) SEL dst src0 src1
123 *
124 * If src0 is an immediate value, we promote it to a temporary GRF.
125 */
126 bool
opt_peephole_sel()127 fs_visitor::opt_peephole_sel()
128 {
129 bool progress = false;
130
131 foreach_block (block, cfg) {
132 /* IF instructions, by definition, can only be found at the ends of
133 * basic blocks.
134 */
135 fs_inst *if_inst = (fs_inst *)block->end();
136 if (if_inst->opcode != BRW_OPCODE_IF)
137 continue;
138
139 fs_inst *else_mov[MAX_MOVS] = { NULL };
140 fs_inst *then_mov[MAX_MOVS] = { NULL };
141
142 bblock_t *then_block = block->next();
143 bblock_t *else_block = NULL;
144 foreach_list_typed(bblock_link, child, link, &block->children) {
145 if (child->block != then_block) {
146 if (child->block->prev()->end()->opcode == BRW_OPCODE_ELSE) {
147 else_block = child->block;
148 }
149 break;
150 }
151 }
152 if (else_block == NULL)
153 continue;
154
155 int movs = count_movs_from_if(then_mov, else_mov, then_block, else_block);
156
157 if (movs == 0)
158 continue;
159
160 /* Generate SEL instructions for pairs of MOVs to a common destination. */
161 for (int i = 0; i < movs; i++) {
162 if (!then_mov[i] || !else_mov[i])
163 break;
164
165 /* Check that the MOVs are the right form. */
166 if (!then_mov[i]->dst.equals(else_mov[i]->dst) ||
167 then_mov[i]->exec_size != else_mov[i]->exec_size ||
168 then_mov[i]->group != else_mov[i]->group ||
169 then_mov[i]->force_writemask_all != else_mov[i]->force_writemask_all ||
170 then_mov[i]->is_partial_write() ||
171 else_mov[i]->is_partial_write() ||
172 then_mov[i]->conditional_mod != BRW_CONDITIONAL_NONE ||
173 else_mov[i]->conditional_mod != BRW_CONDITIONAL_NONE) {
174 movs = i;
175 break;
176 }
177
178 /* Check that source types for mov operations match. */
179 if (then_mov[i]->src[0].type != else_mov[i]->src[0].type) {
180 movs = i;
181 break;
182 }
183 }
184
185 if (movs == 0)
186 continue;
187
188 for (int i = 0; i < movs; i++) {
189 const fs_builder ibld = fs_builder(this, then_block, then_mov[i])
190 .at(block, if_inst);
191
192 if (then_mov[i]->src[0].equals(else_mov[i]->src[0])) {
193 ibld.MOV(then_mov[i]->dst, then_mov[i]->src[0]);
194 } else {
195 /* Only the last source register can be a constant, so if the MOV
196 * in the "then" clause uses a constant, we need to put it in a
197 * temporary.
198 */
199 fs_reg src0(then_mov[i]->src[0]);
200 if (src0.file == IMM) {
201 src0 = ibld.vgrf(then_mov[i]->src[0].type);
202 ibld.MOV(src0, then_mov[i]->src[0]);
203 }
204
205 /* 64-bit immediates can't be placed in src1. */
206 fs_reg src1(else_mov[i]->src[0]);
207 if (src1.file == IMM && type_sz(src1.type) == 8) {
208 src1 = ibld.vgrf(else_mov[i]->src[0].type);
209 ibld.MOV(src1, else_mov[i]->src[0]);
210 }
211
212 set_predicate_inv(if_inst->predicate, if_inst->predicate_inverse,
213 ibld.SEL(then_mov[i]->dst, src0, src1));
214 }
215
216 then_mov[i]->remove(then_block);
217 else_mov[i]->remove(else_block);
218 }
219
220 progress = true;
221 }
222
223 if (progress)
224 invalidate_analysis(DEPENDENCY_INSTRUCTIONS | DEPENDENCY_VARIABLES);
225
226 return progress;
227 }
228