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6de9cd9a | 1 | /* Generic dominator tree walker |
a945c346 | 2 | Copyright (C) 2003-2024 Free Software Foundation, Inc. |
6de9cd9a DN |
3 | Contributed by Diego Novillo <dnovillo@redhat.com> |
4 | ||
5 | This file is part of GCC. | |
6 | ||
7 | GCC is free software; you can redistribute it and/or modify | |
8 | it under the terms of the GNU General Public License as published by | |
9dcd6f09 | 9 | the Free Software Foundation; either version 3, or (at your option) |
6de9cd9a DN |
10 | any later version. |
11 | ||
12 | GCC is distributed in the hope that it will be useful, | |
13 | but WITHOUT ANY WARRANTY; without even the implied warranty of | |
14 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
15 | GNU General Public License for more details. | |
16 | ||
17 | You should have received a copy of the GNU General Public License | |
9dcd6f09 NC |
18 | along with GCC; see the file COPYING3. If not see |
19 | <http://www.gnu.org/licenses/>. */ | |
6de9cd9a DN |
20 | |
21 | #include "config.h" | |
22 | #include "system.h" | |
23 | #include "coretypes.h" | |
c7131fb2 | 24 | #include "backend.h" |
60393bbc | 25 | #include "cfganal.h" |
6de9cd9a | 26 | #include "domwalk.h" |
3daacdcd | 27 | #include "dumpfile.h" |
6de9cd9a | 28 | |
b8698a0f | 29 | /* This file implements a generic walker for dominator trees. |
6de9cd9a DN |
30 | |
31 | To understand the dominator walker one must first have a grasp of dominators, | |
32 | immediate dominators and the dominator tree. | |
33 | ||
34 | Dominators | |
35 | A block B1 is said to dominate B2 if every path from the entry to B2 must | |
36 | pass through B1. Given the dominance relationship, we can proceed to | |
37 | compute immediate dominators. Note it is not important whether or not | |
38 | our definition allows a block to dominate itself. | |
39 | ||
40 | Immediate Dominators: | |
41 | Every block in the CFG has no more than one immediate dominator. The | |
42 | immediate dominator of block BB must dominate BB and must not dominate | |
43 | any other dominator of BB and must not be BB itself. | |
44 | ||
45 | Dominator tree: | |
46 | If we then construct a tree where each node is a basic block and there | |
47 | is an edge from each block's immediate dominator to the block itself, then | |
48 | we have a dominator tree. | |
49 | ||
50 | ||
51 | [ Note this walker can also walk the post-dominator tree, which is | |
454ff5cb | 52 | defined in a similar manner. i.e., block B1 is said to post-dominate |
6de9cd9a DN |
53 | block B2 if all paths from B2 to the exit block must pass through |
54 | B1. ] | |
55 | ||
56 | For example, given the CFG | |
57 | ||
58 | 1 | |
59 | | | |
60 | 2 | |
61 | / \ | |
62 | 3 4 | |
63 | / \ | |
64 | +---------->5 6 | |
65 | | / \ / | |
66 | | +--->8 7 | |
67 | | | / | | |
68 | | +--9 11 | |
69 | | / | | |
70 | +--- 10 ---> 12 | |
b8698a0f L |
71 | |
72 | ||
6de9cd9a DN |
73 | We have a dominator tree which looks like |
74 | ||
75 | 1 | |
76 | | | |
77 | 2 | |
78 | / \ | |
79 | / \ | |
80 | 3 4 | |
81 | / / \ \ | |
82 | | | | | | |
83 | 5 6 7 12 | |
84 | | | | |
85 | 8 11 | |
86 | | | |
87 | 9 | |
88 | | | |
89 | 10 | |
b8698a0f L |
90 | |
91 | ||
92 | ||
6de9cd9a DN |
93 | The dominator tree is the basis for a number of analysis, transformation |
94 | and optimization algorithms that operate on a semi-global basis. | |
b8698a0f | 95 | |
6de9cd9a DN |
96 | The dominator walker is a generic routine which visits blocks in the CFG |
97 | via a depth first search of the dominator tree. In the example above | |
98 | the dominator walker might visit blocks in the following order | |
99 | 1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12. | |
b8698a0f | 100 | |
6de9cd9a DN |
101 | The dominator walker has a number of callbacks to perform actions |
102 | during the walk of the dominator tree. There are two callbacks | |
103 | which walk statements, one before visiting the dominator children, | |
b8698a0f | 104 | one after visiting the dominator children. There is a callback |
6de9cd9a DN |
105 | before and after each statement walk callback. In addition, the |
106 | dominator walker manages allocation/deallocation of data structures | |
107 | which are local to each block visited. | |
b8698a0f | 108 | |
6de9cd9a DN |
109 | The dominator walker is meant to provide a generic means to build a pass |
110 | which can analyze or transform/optimize a function based on walking | |
111 | the dominator tree. One simply fills in the dominator walker data | |
112 | structure with the appropriate callbacks and calls the walker. | |
b8698a0f | 113 | |
6de9cd9a DN |
114 | We currently use the dominator walker to prune the set of variables |
115 | which might need PHI nodes (which can greatly improve compile-time | |
116 | performance in some cases). | |
b8698a0f | 117 | |
6de9cd9a DN |
118 | We also use the dominator walker to rewrite the function into SSA form |
119 | which reduces code duplication since the rewriting phase is inherently | |
120 | a walk of the dominator tree. | |
121 | ||
110abdbc | 122 | And (of course), we use the dominator walker to drive our dominator |
6de9cd9a DN |
123 | optimizer, which is a semi-global optimizer. |
124 | ||
125 | TODO: | |
126 | ||
127 | Walking statements is based on the block statement iterator abstraction, | |
128 | which is currently an abstraction over walking tree statements. Thus | |
129 | the dominator walker is currently only useful for trees. */ | |
130 | ||
076b4605 | 131 | static int |
51007dc1 | 132 | cmp_bb_postorder (const void *a, const void *b, void *data) |
076b4605 | 133 | { |
e5df270e AM |
134 | basic_block bb1 = *(const basic_block *)(a); |
135 | basic_block bb2 = *(const basic_block *)(b); | |
51007dc1 | 136 | int *bb_postorder = (int *)data; |
076b4605 | 137 | /* Place higher completion number first (pop off lower number first). */ |
e5df270e AM |
138 | return bb_postorder[bb2->index] - bb_postorder[bb1->index]; |
139 | } | |
140 | ||
141 | /* Permute array BBS of N basic blocks in postorder, | |
142 | i.e. by descending number in BB_POSTORDER array. */ | |
143 | ||
144 | static void | |
51007dc1 | 145 | sort_bbs_postorder (basic_block *bbs, int n, int *bb_postorder) |
e5df270e | 146 | { |
22d9c880 | 147 | if (LIKELY (n == 2)) |
e5df270e AM |
148 | { |
149 | basic_block bb0 = bbs[0], bb1 = bbs[1]; | |
150 | if (bb_postorder[bb0->index] < bb_postorder[bb1->index]) | |
151 | bbs[0] = bb1, bbs[1] = bb0; | |
152 | } | |
22d9c880 | 153 | else if (LIKELY (n == 3)) |
e5df270e AM |
154 | { |
155 | basic_block bb0 = bbs[0], bb1 = bbs[1], bb2 = bbs[2]; | |
156 | if (bb_postorder[bb0->index] < bb_postorder[bb1->index]) | |
157 | std::swap (bb0, bb1); | |
158 | if (bb_postorder[bb1->index] < bb_postorder[bb2->index]) | |
159 | { | |
160 | std::swap (bb1, bb2); | |
161 | if (bb_postorder[bb0->index] < bb_postorder[bb1->index]) | |
162 | std::swap (bb0, bb1); | |
163 | } | |
164 | bbs[0] = bb0, bbs[1] = bb1, bbs[2] = bb2; | |
165 | } | |
166 | else | |
51007dc1 | 167 | gcc_sort_r (bbs, n, sizeof *bbs, cmp_bb_postorder, bb_postorder); |
076b4605 RB |
168 | } |
169 | ||
9972bbbc DM |
170 | /* Set EDGE_EXECUTABLE on every edge within FN's CFG. */ |
171 | ||
172 | void | |
173 | set_all_edges_as_executable (function *fn) | |
174 | { | |
175 | basic_block bb; | |
176 | FOR_ALL_BB_FN (bb, fn) | |
177 | { | |
178 | edge_iterator ei; | |
179 | edge e; | |
180 | FOR_EACH_EDGE (e, ei, bb->succs) | |
181 | e->flags |= EDGE_EXECUTABLE; | |
182 | } | |
183 | } | |
184 | ||
185 | /* Constructor for a dom walker. */ | |
3daacdcd | 186 | |
3daacdcd | 187 | dom_walker::dom_walker (cdi_direction direction, |
9972bbbc | 188 | enum reachability reachability, |
d2552094 | 189 | int *bb_index_to_rpo) |
3daacdcd | 190 | : m_dom_direction (direction), |
e37240b0 RB |
191 | m_reachability (reachability), |
192 | m_user_bb_to_rpo (bb_index_to_rpo != NULL), | |
d2552094 | 193 | m_unreachable_dom (NULL), |
aea32943 RB |
194 | m_bb_to_rpo (bb_index_to_rpo == (int *)(uintptr_t)-1 |
195 | ? NULL : bb_index_to_rpo) | |
3daacdcd | 196 | { |
3daacdcd JL |
197 | } |
198 | ||
d2552094 RB |
199 | /* Destructor. */ |
200 | ||
201 | dom_walker::~dom_walker () | |
202 | { | |
203 | if (! m_user_bb_to_rpo) | |
204 | free (m_bb_to_rpo); | |
205 | } | |
206 | ||
3daacdcd JL |
207 | /* Return TRUE if BB is reachable, false otherwise. */ |
208 | ||
209 | bool | |
210 | dom_walker::bb_reachable (struct function *fun, basic_block bb) | |
211 | { | |
212 | /* If we're not skipping unreachable blocks, then assume everything | |
213 | is reachable. */ | |
e37240b0 | 214 | if (m_reachability == ALL_BLOCKS) |
3daacdcd JL |
215 | return true; |
216 | ||
217 | /* If any of the predecessor edges that do not come from blocks dominated | |
218 | by us are still marked as possibly executable consider this block | |
219 | reachable. */ | |
220 | bool reachable = false; | |
221 | if (!m_unreachable_dom) | |
222 | { | |
223 | reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun); | |
224 | edge_iterator ei; | |
225 | edge e; | |
226 | FOR_EACH_EDGE (e, ei, bb->preds) | |
227 | if (!dominated_by_p (CDI_DOMINATORS, e->src, bb)) | |
228 | reachable |= (e->flags & EDGE_EXECUTABLE); | |
229 | } | |
230 | ||
231 | return reachable; | |
232 | } | |
233 | ||
234 | /* BB has been determined to be unreachable. Propagate that property | |
235 | to incoming and outgoing edges of BB as appropriate. */ | |
236 | ||
237 | void | |
238 | dom_walker::propagate_unreachable_to_edges (basic_block bb, | |
239 | FILE *dump_file, | |
1a817418 | 240 | dump_flags_t dump_flags) |
3daacdcd JL |
241 | { |
242 | if (dump_file && (dump_flags & TDF_DETAILS)) | |
243 | fprintf (dump_file, "Marking all outgoing edges of unreachable " | |
244 | "BB %d as not executable\n", bb->index); | |
245 | ||
246 | edge_iterator ei; | |
247 | edge e; | |
248 | FOR_EACH_EDGE (e, ei, bb->succs) | |
249 | e->flags &= ~EDGE_EXECUTABLE; | |
250 | ||
251 | FOR_EACH_EDGE (e, ei, bb->preds) | |
252 | { | |
253 | if (dominated_by_p (CDI_DOMINATORS, e->src, bb)) | |
254 | { | |
255 | if (dump_file && (dump_flags & TDF_DETAILS)) | |
256 | fprintf (dump_file, "Marking backedge from BB %d into " | |
257 | "unreachable BB %d as not executable\n", | |
258 | e->src->index, bb->index); | |
259 | e->flags &= ~EDGE_EXECUTABLE; | |
260 | } | |
261 | } | |
262 | ||
263 | if (!m_unreachable_dom) | |
264 | m_unreachable_dom = bb; | |
265 | } | |
266 | ||
d2552094 RB |
267 | const edge dom_walker::STOP = (edge)-1; |
268 | ||
6de9cd9a | 269 | /* Recursively walk the dominator tree. |
6de9cd9a DN |
270 | BB is the basic block we are currently visiting. */ |
271 | ||
272 | void | |
4d9192b5 | 273 | dom_walker::walk (basic_block bb) |
6de9cd9a | 274 | { |
e37240b0 | 275 | /* Compute the basic-block index to RPO mapping lazily. */ |
aea32943 RB |
276 | if (!m_user_bb_to_rpo |
277 | && !m_bb_to_rpo | |
e37240b0 RB |
278 | && m_dom_direction == CDI_DOMINATORS) |
279 | { | |
280 | int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun)); | |
281 | int postorder_num = pre_and_rev_post_order_compute (NULL, postorder, | |
282 | true); | |
283 | m_bb_to_rpo = XNEWVEC (int, last_basic_block_for_fn (cfun)); | |
284 | for (int i = 0; i < postorder_num; ++i) | |
285 | m_bb_to_rpo[postorder[i]] = i; | |
286 | free (postorder); | |
287 | } | |
288 | ||
289 | /* Set up edge flags if need be. */ | |
290 | if (m_reachability == REACHABLE_BLOCKS) | |
291 | set_all_edges_as_executable (cfun); | |
292 | ||
6de9cd9a | 293 | basic_block dest; |
0cae8d31 DM |
294 | basic_block *worklist = XNEWVEC (basic_block, |
295 | n_basic_blocks_for_fn (cfun) * 2); | |
df648b94 | 296 | int sp = 0; |
0bca51f0 | 297 | |
df648b94 | 298 | while (true) |
6de9cd9a | 299 | { |
df648b94 | 300 | /* Don't worry about unreachable blocks. */ |
515f36eb | 301 | if (EDGE_COUNT (bb->preds) > 0 |
fefa31b5 DM |
302 | || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun) |
303 | || bb == EXIT_BLOCK_PTR_FOR_FN (cfun)) | |
6de9cd9a | 304 | { |
d2552094 | 305 | edge taken_edge = NULL; |
3daacdcd | 306 | |
4d9192b5 TS |
307 | /* Callback for subclasses to do custom things before we have walked |
308 | the dominator children, but before we walk statements. */ | |
3daacdcd JL |
309 | if (this->bb_reachable (cfun, bb)) |
310 | { | |
d2552094 RB |
311 | taken_edge = before_dom_children (bb); |
312 | if (taken_edge && taken_edge != STOP) | |
3daacdcd JL |
313 | { |
314 | edge_iterator ei; | |
315 | edge e; | |
316 | FOR_EACH_EDGE (e, ei, bb->succs) | |
317 | if (e != taken_edge) | |
318 | e->flags &= ~EDGE_EXECUTABLE; | |
319 | } | |
320 | } | |
321 | else | |
322 | propagate_unreachable_to_edges (bb, dump_file, dump_flags); | |
df648b94 JH |
323 | |
324 | /* Mark the current BB to be popped out of the recursion stack | |
fa10beec | 325 | once children are processed. */ |
df648b94 JH |
326 | worklist[sp++] = bb; |
327 | worklist[sp++] = NULL; | |
328 | ||
d2552094 RB |
329 | /* If the callback returned NONE then we are supposed to |
330 | stop and not even propagate EDGE_EXECUTABLE further. */ | |
331 | if (taken_edge != STOP) | |
332 | { | |
333 | int saved_sp = sp; | |
334 | for (dest = first_dom_son (m_dom_direction, bb); | |
335 | dest; dest = next_dom_son (m_dom_direction, dest)) | |
336 | worklist[sp++] = dest; | |
dc3b4a20 RB |
337 | /* Sort worklist after RPO order if requested. */ |
338 | if (sp - saved_sp > 1 | |
339 | && m_dom_direction == CDI_DOMINATORS | |
340 | && m_bb_to_rpo) | |
51007dc1 RB |
341 | sort_bbs_postorder (&worklist[saved_sp], sp - saved_sp, |
342 | m_bb_to_rpo); | |
d2552094 | 343 | } |
6de9cd9a | 344 | } |
ccf5c864 | 345 | /* NULL is used to mark pop operations in the recursion stack. */ |
df648b94 | 346 | while (sp > 0 && !worklist[sp - 1]) |
6de9cd9a | 347 | { |
df648b94 JH |
348 | --sp; |
349 | bb = worklist[--sp]; | |
df648b94 | 350 | |
4d9192b5 TS |
351 | /* Callback allowing subclasses to do custom things after we have |
352 | walked dominator children, but before we walk statements. */ | |
3daacdcd JL |
353 | if (bb_reachable (cfun, bb)) |
354 | after_dom_children (bb); | |
355 | else if (m_unreachable_dom == bb) | |
356 | m_unreachable_dom = NULL; | |
6de9cd9a | 357 | } |
df648b94 | 358 | if (sp) |
076b4605 | 359 | bb = worklist[--sp]; |
6de9cd9a | 360 | else |
df648b94 | 361 | break; |
6de9cd9a | 362 | } |
df648b94 | 363 | free (worklist); |
6de9cd9a | 364 | } |