nir_opt_dead_cf.c revision 21424e019d29424e2c09cf8ac197d294cf2edb5f 
1/*
2 * Copyright © 2014 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_control_flow.h"
30
31/*
32 * This file implements an optimization that deletes statically
33 * unreachable/dead code. In NIR, one way this can happen if if an if
34 * statement has a constant condition:
35 *
36 * if (true) {
37 *    ...
38 * }
39 *
40 * We delete the if statement and paste the contents of the always-executed
41 * branch into the surrounding control flow, possibly removing more code if
42 * the branch had a jump at the end.
43 *
44 * Another way is that control flow can end in a jump so that code after it
45 * never gets executed. In particular, this can happen after optimizing
46 * something like:
47 *
48 * if (true) {
49 *    ...
50 *    break;
51 * }
52 * ...
53 *
54 * We also consider the case where both branches of an if end in a jump, e.g.:
55 *
56 * if (...) {
57 *    break;
58 * } else {
59 *    continue;
60 * }
61 * ...
62 *
63 * Finally, we also handle removing useless loops, i.e. loops with no side
64 * effects and without any definitions that are used elsewhere. This case is a
65 * little different from the first two in that the code is actually run (it
66 * just never does anything), but there are similar issues with needing to
67 * be careful with restarting after deleting the cf_node (see dead_cf_list())
68 * so this is a convenient place to remove them.
69 */
70
71static void
72remove_after_cf_node(nir_cf_node *node)
73{
74   nir_cf_node *end = node;
75   while (!nir_cf_node_is_last(end))
76      end = nir_cf_node_next(end);
77
78   nir_cf_list list;
79   nir_cf_extract(&list, nir_after_cf_node(node), nir_after_cf_node(end));
80   nir_cf_delete(&list);
81}
82
83static void
84opt_constant_if(nir_if *if_stmt, bool condition)
85{
86   /* First, we need to remove any phi nodes after the if by rewriting uses to
87    * point to the correct source.
88    */
89   nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&if_stmt->cf_node));
90   nir_block *last_block =
91      nir_cf_node_as_block(condition ? nir_if_last_then_node(if_stmt)
92                                     : nir_if_last_else_node(if_stmt));
93
94   nir_foreach_instr_safe(instr, after) {
95      if (instr->type != nir_instr_type_phi)
96         break;
97
98      nir_phi_instr *phi = nir_instr_as_phi(instr);
99      nir_ssa_def *def = NULL;
100      nir_foreach_phi_src(phi_src, phi) {
101         if (phi_src->pred != last_block)
102            continue;
103
104         assert(phi_src->src.is_ssa);
105         def = phi_src->src.ssa;
106      }
107
108      assert(def);
109      assert(phi->dest.is_ssa);
110      nir_ssa_def_rewrite_uses(&phi->dest.ssa, nir_src_for_ssa(def));
111      nir_instr_remove(instr);
112   }
113
114   /* The control flow list we're about to paste in may include a jump at the
115    * end, and in that case we have to delete the rest of the control flow
116    * list after the if since it's unreachable and the validator will balk if
117    * we don't.
118    */
119
120   if (!exec_list_is_empty(&last_block->instr_list)) {
121      nir_instr *last_instr = nir_block_last_instr(last_block);
122      if (last_instr->type == nir_instr_type_jump)
123         remove_after_cf_node(&if_stmt->cf_node);
124   }
125
126   /* Finally, actually paste in the then or else branch and delete the if. */
127   struct exec_list *cf_list = condition ? &if_stmt->then_list
128                                         : &if_stmt->else_list;
129
130   nir_cf_list list;
131   nir_cf_extract(&list, nir_before_cf_list(cf_list),
132                  nir_after_cf_list(cf_list));
133   nir_cf_reinsert(&list, nir_after_cf_node(&if_stmt->cf_node));
134   nir_cf_node_remove(&if_stmt->cf_node);
135}
136
137static bool
138cf_node_has_side_effects(nir_cf_node *node)
139{
140   nir_foreach_block_in_cf_node(block, node) {
141      nir_foreach_instr(instr, block) {
142         if (instr->type == nir_instr_type_call)
143            return true;
144
145         /* Return instructions can cause us to skip over other side-effecting
146          * instructions after the loop, so consider them to have side effects
147          * here.
148          */
149
150         if (instr->type == nir_instr_type_jump &&
151             nir_instr_as_jump(instr)->type == nir_jump_return)
152            return true;
153
154         if (instr->type != nir_instr_type_intrinsic)
155            continue;
156
157         nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
158         if (!(nir_intrinsic_infos[intrin->intrinsic].flags &
159             NIR_INTRINSIC_CAN_ELIMINATE))
160            return true;
161      }
162   }
163
164   return false;
165}
166
167static bool
168def_not_live_out(nir_ssa_def *def, void *state)
169{
170   nir_block *after = state;
171
172   return !BITSET_TEST(after->live_in, def->live_index);
173}
174
175/*
176 * Test if a loop is dead. A loop is dead if:
177 *
178 * 1) It has no side effects (i.e. intrinsics which could possibly affect the
179 * state of the program aside from producing an SSA value, indicated by a lack
180 * of NIR_INTRINSIC_CAN_ELIMINATE).
181 *
182 * 2) It has no phi nodes after it, since those indicate values inside the
183 * loop being used after the loop.
184 *
185 * 3) If there are no phi nodes after the loop, then the only way a value
186 * defined inside the loop can be used outside the loop is if its definition
187 * dominates the block after the loop. If none of the definitions that
188 * dominate the loop exit are used outside the loop, then the loop is dead
189 * and it can be deleted.
190 */
191
192static bool
193loop_is_dead(nir_loop *loop)
194{
195   nir_block *before = nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
196   nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&loop->cf_node));
197
198   if (!exec_list_is_empty(&after->instr_list) &&
199       nir_block_first_instr(after)->type == nir_instr_type_phi)
200      return false;
201
202   if (cf_node_has_side_effects(&loop->cf_node))
203      return false;
204
205   nir_function_impl *impl = nir_cf_node_get_function(&loop->cf_node);
206   nir_metadata_require(impl, nir_metadata_live_ssa_defs |
207                              nir_metadata_dominance);
208
209   for (nir_block *cur = after->imm_dom; cur != before; cur = cur->imm_dom) {
210      nir_foreach_instr(instr, cur) {
211         if (!nir_foreach_ssa_def(instr, def_not_live_out, after))
212            return false;
213      }
214   }
215
216   return true;
217}
218
219static bool
220dead_cf_block(nir_block *block)
221{
222   nir_if *following_if = nir_block_get_following_if(block);
223   if (following_if) {
224      nir_const_value *const_value =
225         nir_src_as_const_value(following_if->condition);
226
227      if (!const_value)
228         return false;
229
230      opt_constant_if(following_if, const_value->u32[0] != 0);
231      return true;
232   }
233
234   nir_loop *following_loop = nir_block_get_following_loop(block);
235   if (!following_loop)
236      return false;
237
238   if (!loop_is_dead(following_loop))
239      return false;
240
241   nir_cf_node_remove(&following_loop->cf_node);
242   return true;
243}
244
245static bool
246ends_in_jump(nir_block *block)
247{
248   if (exec_list_is_empty(&block->instr_list))
249      return false;
250
251   nir_instr *instr = nir_block_last_instr(block);
252   return instr->type == nir_instr_type_jump;
253}
254
255static bool
256dead_cf_list(struct exec_list *list, bool *list_ends_in_jump)
257{
258   bool progress = false;
259   *list_ends_in_jump = false;
260
261   nir_cf_node *prev = NULL;
262
263   foreach_list_typed(nir_cf_node, cur, node, list) {
264      switch (cur->type) {
265      case nir_cf_node_block: {
266         nir_block *block = nir_cf_node_as_block(cur);
267         if (dead_cf_block(block)) {
268            /* We just deleted the if or loop after this block, so we may have
269             * deleted the block before or after it -- which one is an
270             * implementation detail. Therefore, to recover the place we were
271             * at, we have to use the previous cf_node.
272             */
273
274            if (prev) {
275               cur = nir_cf_node_next(prev);
276            } else {
277               cur = exec_node_data(nir_cf_node, exec_list_get_head(list),
278                                    node);
279            }
280
281            block = nir_cf_node_as_block(cur);
282
283            progress = true;
284         }
285
286         if (ends_in_jump(block)) {
287            *list_ends_in_jump = true;
288
289            if (!exec_node_is_tail_sentinel(cur->node.next)) {
290               remove_after_cf_node(cur);
291               return true;
292            }
293         }
294
295         break;
296      }
297
298      case nir_cf_node_if: {
299         nir_if *if_stmt = nir_cf_node_as_if(cur);
300         bool then_ends_in_jump, else_ends_in_jump;
301         progress |= dead_cf_list(&if_stmt->then_list, &then_ends_in_jump);
302         progress |= dead_cf_list(&if_stmt->else_list, &else_ends_in_jump);
303
304         if (then_ends_in_jump && else_ends_in_jump) {
305            *list_ends_in_jump = true;
306            nir_block *next = nir_cf_node_as_block(nir_cf_node_next(cur));
307            if (!exec_list_is_empty(&next->instr_list) ||
308                !exec_node_is_tail_sentinel(next->cf_node.node.next)) {
309               remove_after_cf_node(cur);
310               return true;
311            }
312         }
313
314         break;
315      }
316
317      case nir_cf_node_loop: {
318         nir_loop *loop = nir_cf_node_as_loop(cur);
319         bool dummy;
320         progress |= dead_cf_list(&loop->body, &dummy);
321
322         break;
323      }
324
325      default:
326         unreachable("unknown cf node type");
327      }
328
329      prev = cur;
330   }
331
332   return progress;
333}
334
335static bool
336opt_dead_cf_impl(nir_function_impl *impl)
337{
338   bool dummy;
339   bool progress = dead_cf_list(&impl->body, &dummy);
340
341   if (progress)
342      nir_metadata_preserve(impl, nir_metadata_none);
343
344   return progress;
345}
346
347bool
348nir_opt_dead_cf(nir_shader *shader)
349{
350   bool progress = false;
351
352   nir_foreach_function(function, shader)
353      if (function->impl)
354         progress |= opt_dead_cf_impl(function->impl);
355
356   return progress;
357}
358