nir_opt_dead_cf.c revision 8c4d9afb7e770f312079994994f42ddebf7e641c 
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 is when 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_list_extract(&list, cf_list);
132   nir_cf_reinsert(&list, nir_after_cf_node(&if_stmt->cf_node));
133   nir_cf_node_remove(&if_stmt->cf_node);
134}
135
136static bool
137cf_node_has_side_effects(nir_cf_node *node)
138{
139   nir_foreach_block_in_cf_node(block, node) {
140      nir_foreach_instr(instr, block) {
141         if (instr->type == nir_instr_type_call)
142            return true;
143
144         /* Return instructions can cause us to skip over other side-effecting
145          * instructions after the loop, so consider them to have side effects
146          * here.
147          */
148
149         if (instr->type == nir_instr_type_jump &&
150             nir_instr_as_jump(instr)->type == nir_jump_return)
151            return true;
152
153         if (instr->type != nir_instr_type_intrinsic)
154            continue;
155
156         nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
157         if (!(nir_intrinsic_infos[intrin->intrinsic].flags &
158             NIR_INTRINSIC_CAN_ELIMINATE))
159            return true;
160      }
161   }
162
163   return false;
164}
165
166static bool
167def_not_live_out(nir_ssa_def *def, void *state)
168{
169   nir_block *after = state;
170
171   return !BITSET_TEST(after->live_in, def->live_index);
172}
173
174/*
175 * Test if a loop is dead. A loop is dead if:
176 *
177 * 1) It has no side effects (i.e. intrinsics which could possibly affect the
178 * state of the program aside from producing an SSA value, indicated by a lack
179 * of NIR_INTRINSIC_CAN_ELIMINATE).
180 *
181 * 2) It has no phi nodes after it, since those indicate values inside the
182 * loop being used after the loop.
183 *
184 * 3) If there are no phi nodes after the loop, then the only way a value
185 * defined inside the loop can be used outside the loop is if its definition
186 * dominates the block after the loop. If none of the definitions that
187 * dominate the loop exit are used outside the loop, then the loop is dead
188 * and it can be deleted.
189 */
190
191static bool
192loop_is_dead(nir_loop *loop)
193{
194   nir_block *before = nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
195   nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&loop->cf_node));
196
197   if (!exec_list_is_empty(&after->instr_list) &&
198       nir_block_first_instr(after)->type == nir_instr_type_phi)
199      return false;
200
201   if (cf_node_has_side_effects(&loop->cf_node))
202      return false;
203
204   nir_function_impl *impl = nir_cf_node_get_function(&loop->cf_node);
205   nir_metadata_require(impl, nir_metadata_live_ssa_defs |
206                              nir_metadata_dominance);
207
208   for (nir_block *cur = after->imm_dom; cur != before; cur = cur->imm_dom) {
209      nir_foreach_instr(instr, cur) {
210         if (!nir_foreach_ssa_def(instr, def_not_live_out, after))
211            return false;
212      }
213   }
214
215   return true;
216}
217
218static bool
219dead_cf_block(nir_block *block)
220{
221   nir_if *following_if = nir_block_get_following_if(block);
222   if (following_if) {
223      nir_const_value *const_value =
224         nir_src_as_const_value(following_if->condition);
225
226      if (!const_value)
227         return false;
228
229      opt_constant_if(following_if, const_value->u32[0] != 0);
230      return true;
231   }
232
233   nir_loop *following_loop = nir_block_get_following_loop(block);
234   if (!following_loop)
235      return false;
236
237   if (!loop_is_dead(following_loop))
238      return false;
239
240   nir_cf_node_remove(&following_loop->cf_node);
241   return true;
242}
243
244static bool
245ends_in_jump(nir_block *block)
246{
247   if (exec_list_is_empty(&block->instr_list))
248      return false;
249
250   nir_instr *instr = nir_block_last_instr(block);
251   return instr->type == nir_instr_type_jump;
252}
253
254static bool
255dead_cf_list(struct exec_list *list, bool *list_ends_in_jump)
256{
257   bool progress = false;
258   *list_ends_in_jump = false;
259
260   nir_cf_node *prev = NULL;
261
262   foreach_list_typed(nir_cf_node, cur, node, list) {
263      switch (cur->type) {
264      case nir_cf_node_block: {
265         nir_block *block = nir_cf_node_as_block(cur);
266         if (dead_cf_block(block)) {
267            /* We just deleted the if or loop after this block, so we may have
268             * deleted the block before or after it -- which one is an
269             * implementation detail. Therefore, to recover the place we were
270             * at, we have to use the previous cf_node.
271             */
272
273            if (prev) {
274               cur = nir_cf_node_next(prev);
275            } else {
276               cur = exec_node_data(nir_cf_node, exec_list_get_head(list),
277                                    node);
278            }
279
280            block = nir_cf_node_as_block(cur);
281
282            progress = true;
283         }
284
285         if (ends_in_jump(block)) {
286            *list_ends_in_jump = true;
287
288            if (!exec_node_is_tail_sentinel(cur->node.next)) {
289               remove_after_cf_node(cur);
290               return true;
291            }
292         }
293
294         break;
295      }
296
297      case nir_cf_node_if: {
298         nir_if *if_stmt = nir_cf_node_as_if(cur);
299         bool then_ends_in_jump, else_ends_in_jump;
300         progress |= dead_cf_list(&if_stmt->then_list, &then_ends_in_jump);
301         progress |= dead_cf_list(&if_stmt->else_list, &else_ends_in_jump);
302
303         if (then_ends_in_jump && else_ends_in_jump) {
304            *list_ends_in_jump = true;
305            nir_block *next = nir_cf_node_as_block(nir_cf_node_next(cur));
306            if (!exec_list_is_empty(&next->instr_list) ||
307                !exec_node_is_tail_sentinel(next->cf_node.node.next)) {
308               remove_after_cf_node(cur);
309               return true;
310            }
311         }
312
313         break;
314      }
315
316      case nir_cf_node_loop: {
317         nir_loop *loop = nir_cf_node_as_loop(cur);
318         bool dummy;
319         progress |= dead_cf_list(&loop->body, &dummy);
320
321         break;
322      }
323
324      default:
325         unreachable("unknown cf node type");
326      }
327
328      prev = cur;
329   }
330
331   return progress;
332}
333
334static bool
335opt_dead_cf_impl(nir_function_impl *impl)
336{
337   bool dummy;
338   bool progress = dead_cf_list(&impl->body, &dummy);
339
340   if (progress)
341      nir_metadata_preserve(impl, nir_metadata_none);
342
343   return progress;
344}
345
346bool
347nir_opt_dead_cf(nir_shader *shader)
348{
349   bool progress = false;
350
351   nir_foreach_function(function, shader)
352      if (function->impl)
353         progress |= opt_dead_cf_impl(function->impl);
354
355   return progress;
356}
357