1/*
2 * Copyright © 2016 Intel Corporation
3 * Copyright © 2019 Valve Corporation
4 *
5 * Permission is hereby granted, free of charge, to any person obtaining a
6 * copy of this software and associated documentation files (the "Software"),
7 * to deal in the Software without restriction, including without limitation
8 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
9 * and/or sell copies of the Software, and to permit persons to whom the
10 * Software is furnished to do so, subject to the following conditions:
11 *
12 * The above copyright notice and this permission notice (including the next
13 * paragraph) shall be included in all copies or substantial portions of the
14 * Software.
15 *
16 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
17 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
18 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
19 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
20 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
21 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
22 * IN THE SOFTWARE.
23 */
24
25#include "nir.h"
26
27/**
28 * \file nir_opt_move.c
29 *
30 * This pass can move various operations just before their first use inside the
31 * same basic block. Usually this is to reduce register usage. It's probably
32 * not a good idea to use this in an optimization loop.
33 *
34 * Moving comparisons is useful because many GPUs generate condition codes
35 * for comparisons, and use predication for conditional selects and control
36 * flow.  In a sequence such as:
37 *
38 *     vec1 32 ssa_1 = flt a b
39 *     <some other operations>
40 *     vec1 32 ssa_2 = bcsel ssa_1 c d
41 *
42 * the backend would likely do the comparison, producing condition codes,
43 * then save those to a boolean value.  The intervening operations might
44 * trash the condition codes.  Then, in order to do the bcsel, it would
45 * need to re-populate the condition code register based on the boolean.
46 *
47 * By moving the comparison just before the bcsel, the condition codes could
48 * be used directly.  This eliminates the need to reload them from the boolean
49 * (generally eliminating an instruction).  It may also eliminate the need to
50 * create a boolean value altogether (unless it's used elsewhere), which could
51 * lower register pressure.
52 */
53
54static bool
55move_source(nir_src *src, nir_block *block, nir_instr *before, nir_move_options options)
56{
57   if (!src->is_ssa)
58      return false;
59
60   nir_instr *src_instr = src->ssa->parent_instr;
61
62   if (src_instr->block == block && nir_can_move_instr(src_instr, options)) {
63      exec_node_remove(&src_instr->node);
64
65      if (before)
66         exec_node_insert_node_before(&before->node, &src_instr->node);
67      else
68         exec_list_push_tail(&block->instr_list, &src_instr->node);
69
70      return true;
71   }
72   return false;
73}
74
75struct source_cb_data {
76   bool *progress;
77   nir_move_options options;
78};
79
80static bool
81move_source_cb(nir_src *src, void *data_ptr)
82{
83   struct source_cb_data data = *(struct source_cb_data*)data_ptr;
84
85   nir_instr *instr = src->parent_instr;
86   if (move_source(src, instr->block, instr, data.options))
87      *data.progress = true;
88
89   return true; /* nir_foreach_src should keep going */
90}
91
92static bool
93move(nir_block *block, nir_move_options options)
94{
95   bool progress = false;
96
97   /* We use a simple approach: walk instructions backwards.
98    *
99    * If the instruction's source is a comparison from the same block,
100    * simply move it here.  This may break SSA if it's used earlier in
101    * the block as well.  However, as we walk backwards, we'll find the
102    * earlier use and move it again, further up.  It eventually ends up
103    * dominating all uses again, restoring SSA form.
104    *
105    * Before walking instructions, we consider the if-condition at the
106    * end of the block, if one exists.  It's effectively a use at the
107    * bottom of the block.
108    */
109   nir_if *iff = nir_block_get_following_if(block);
110   if (iff) {
111      progress |= move_source(&iff->condition, block, NULL, options);
112   }
113
114   nir_foreach_instr_reverse(instr, block) {
115      /* The sources of phi instructions happen after the predecessor block
116       * but before this block.  (Yes, that's between blocks).  This means
117       * that we don't need to move them in order for them to be correct.
118       * We could move them to encourage comparisons that are used in a phi to
119       * the end of the block, doing so correctly would make the pass
120       * substantially more complicated and wouldn't gain us anything since
121       * the phi can't use a flag value anyway.
122       */
123
124      if (instr->type == nir_instr_type_phi) {
125         /* We're going backwards so everything else is a phi too */
126         break;
127      } else if (instr->type == nir_instr_type_alu) {
128         /* Walk ALU instruction sources backwards so that bcsel's boolean
129          * condition is processed last for when comparisons are being moved.
130          */
131         nir_alu_instr *alu = nir_instr_as_alu(instr);
132         for (int i = nir_op_infos[alu->op].num_inputs - 1; i >= 0; i--) {
133            progress |= move_source(&alu->src[i].src, block, instr, options);
134         }
135      } else {
136         struct source_cb_data data;
137         data.progress = &progress;
138         data.options = options;
139         nir_foreach_src(instr, move_source_cb, &data);
140      }
141   }
142
143   return progress;
144}
145
146bool
147nir_opt_move(nir_shader *shader, nir_move_options options)
148{
149   bool progress = false;
150
151   nir_foreach_function(func, shader) {
152      if (!func->impl)
153         continue;
154
155      bool impl_progress = false;
156      nir_foreach_block(block, func->impl) {
157         if (move(block, options))
158            impl_progress = true;
159      }
160
161      if (impl_progress) {
162         nir_metadata_preserve(func->impl, nir_metadata_block_index |
163                                           nir_metadata_dominance |
164                                           nir_metadata_live_ssa_defs);
165         progress = true;
166      } else {
167         nir_metadata_preserve(func->impl, nir_metadata_all);
168      }
169   }
170
171   return progress;
172}
173