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
31static nir_alu_instr *
32get_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_mov) ? alu : NULL;
39}
40
41static bool
42matching_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 always 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
66static bool
67remove_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 if (nir_src_is_undef(src->src) &&
102                    nir_block_dominates(def->parent_instr->block, src->pred)) {
103            /* Ignore this undef source. */
104         } else {
105            if (src->src.ssa != def && !matching_mov(mov, src->src.ssa)) {
106               srcs_same = false;
107               break;
108            }
109         }
110      }
111
112      if (!srcs_same)
113         continue;
114
115      if (!def) {
116         /* In this case, the phi had no sources. So turn it into an undef. */
117
118         b->cursor = nir_after_phis(block);
119         def = nir_ssa_undef(b, phi->dest.ssa.num_components,
120                             phi->dest.ssa.bit_size);
121      } else if (mov) {
122         /* If the sources were all movs from the same source with the same
123          * swizzle, then we can't just pick a random move because it may not
124          * dominate the phi node. Instead, we need to emit our own move after
125          * the phi which uses the shared source, and rewrite uses of the phi
126          * to use the move instead. This is ok, because while the movs may
127          * not all dominate the phi node, their shared source does.
128          */
129
130         b->cursor = nir_after_phis(block);
131         def = nir_mov_alu(b, mov->src[0], def->num_components);
132      }
133
134      assert(phi->dest.is_ssa);
135      nir_ssa_def_rewrite_uses(&phi->dest.ssa, def);
136      nir_instr_remove(instr);
137
138      progress = true;
139   }
140
141   return progress;
142}
143
144bool
145nir_opt_remove_phis_block(nir_block *block)
146{
147   nir_builder b;
148   nir_builder_init(&b, nir_cf_node_get_function(&block->cf_node));
149   return remove_phis_block(block, &b);
150}
151
152static bool
153nir_opt_remove_phis_impl(nir_function_impl *impl)
154{
155   bool progress = false;
156   nir_builder bld;
157   nir_builder_init(&bld, impl);
158
159   nir_metadata_require(impl, nir_metadata_dominance);
160
161   nir_foreach_block(block, impl) {
162      progress |= remove_phis_block(block, &bld);
163   }
164
165   if (progress) {
166      nir_metadata_preserve(impl, nir_metadata_block_index |
167                                  nir_metadata_dominance);
168   } else {
169      nir_metadata_preserve(impl, nir_metadata_all);
170   }
171
172   return progress;
173}
174
175bool
176nir_opt_remove_phis(nir_shader *shader)
177{
178   bool progress = false;
179
180   nir_foreach_function(function, shader)
181      if (function->impl)
182         progress = nir_opt_remove_phis_impl(function->impl) || progress;
183
184   return progress;
185}
186
187