1 /*
2 * Copyright © 2015 Connor Abbott
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 * Authors:
24 * Connor Abbott (cwabbott0@gmail.com)
25 *
26 */
27
28 #include "nir.h"
29 #include "nir_builder.h"
30
31 static nir_alu_instr *
get_parent_mov(nir_ssa_def * ssa)32 get_parent_mov(nir_ssa_def *ssa)
33 {
34 if (ssa->parent_instr->type != nir_instr_type_alu)
35 return NULL;
36
37 nir_alu_instr *alu = nir_instr_as_alu(ssa->parent_instr);
38 return (alu->op == nir_op_imov || alu->op == nir_op_fmov) ? alu : NULL;
39 }
40
41 static bool
matching_mov(nir_alu_instr * mov1,nir_ssa_def * ssa)42 matching_mov(nir_alu_instr *mov1, nir_ssa_def *ssa)
43 {
44 if (!mov1)
45 return false;
46
47 nir_alu_instr *mov2 = get_parent_mov(ssa);
48 return mov2 && nir_alu_srcs_equal(mov1, mov2, 0, 0);
49 }
50
51 /*
52 * This is a pass for removing phi nodes that look like:
53 * a = phi(b, b, b, ...)
54 *
55 * Note that we can't ignore undef sources here, or else we may create a
56 * situation where the definition of b isn't dominated by its uses. We're
57 * allowed to do this since the definition of b must dominate all of the
58 * phi node's predecessors, which means it must dominate the phi node as well
59 * as all of the phi node's uses. In essence, the phi node acts as a copy
60 * instruction. b can't be another phi node in the same block, since the only
61 * time when phi nodes can source other phi nodes defined in the same block is
62 * at the loop header, and in that case one of the sources of the phi has to
63 * be from before the loop and that source can't be b.
64 */
65
66 static bool
remove_phis_block(nir_block * block,nir_builder * b)67 remove_phis_block(nir_block *block, nir_builder *b)
68 {
69 bool progress = false;
70
71 nir_foreach_instr_safe(instr, block) {
72 if (instr->type != nir_instr_type_phi)
73 break;
74
75 nir_phi_instr *phi = nir_instr_as_phi(instr);
76
77 nir_ssa_def *def = NULL;
78 nir_alu_instr *mov = NULL;
79 bool srcs_same = true;
80
81 nir_foreach_phi_src(src, phi) {
82 assert(src->src.is_ssa);
83
84 /* For phi nodes at the beginning of loops, we may encounter some
85 * sources from backedges that point back to the destination of the
86 * same phi, i.e. something like:
87 *
88 * a = phi(a, b, ...)
89 *
90 * We can safely ignore these sources, since if all of the normal
91 * sources point to the same definition, then that definition must
92 * still dominate the phi node, and the phi will still always take
93 * the value of that definition.
94 */
95 if (src->src.ssa == &phi->dest.ssa)
96 continue;
97
98 if (def == NULL) {
99 def = src->src.ssa;
100 mov = get_parent_mov(def);
101 } else {
102 if (src->src.ssa != def && !matching_mov(mov, src->src.ssa)) {
103 srcs_same = false;
104 break;
105 }
106 }
107 }
108
109 if (!srcs_same)
110 continue;
111
112 /* We must have found at least one definition, since there must be at
113 * least one forward edge.
114 */
115 assert(def != NULL);
116
117 if (mov) {
118 /* If the sources were all mov's from the same source with the same
119 * swizzle, then we can't just pick a random move because it may not
120 * dominate the phi node. Instead, we need to emit our own move after
121 * the phi which uses the shared source, and rewrite uses of the phi
122 * to use the move instead. This is ok, because while the mov's may
123 * not all dominate the phi node, their shared source does.
124 */
125
126 b->cursor = nir_after_phis(block);
127 def = mov->op == nir_op_imov ?
128 nir_imov_alu(b, mov->src[0], def->num_components) :
129 nir_fmov_alu(b, mov->src[0], def->num_components);
130 }
131
132 assert(phi->dest.is_ssa);
133 nir_ssa_def_rewrite_uses(&phi->dest.ssa, nir_src_for_ssa(def));
134 nir_instr_remove(instr);
135
136 progress = true;
137 }
138
139 return progress;
140 }
141
142 static bool
remove_phis_impl(nir_function_impl * impl)143 remove_phis_impl(nir_function_impl *impl)
144 {
145 bool progress = false;
146 nir_builder bld;
147 nir_builder_init(&bld, impl);
148
149 nir_foreach_block(block, impl) {
150 progress |= remove_phis_block(block, &bld);
151 }
152
153 if (progress) {
154 nir_metadata_preserve(impl, nir_metadata_block_index |
155 nir_metadata_dominance);
156 }
157
158 return progress;
159 }
160
161 bool
nir_opt_remove_phis(nir_shader * shader)162 nir_opt_remove_phis(nir_shader *shader)
163 {
164 bool progress = false;
165
166 nir_foreach_function(function, shader)
167 if (function->impl)
168 progress = remove_phis_impl(function->impl) || progress;
169
170 return progress;
171 }
172
173