]>
Commit | Line | Data |
---|---|---|
6de9cd9a | 1 | /* Generic dominator tree walker |
8d9254fc | 2 | Copyright (C) 2003-2020 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 AM |
146 | { |
147 | if (__builtin_expect (n == 2, true)) | |
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 | } | |
153 | else if (__builtin_expect (n == 3, true)) | |
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 RB |
193 | m_unreachable_dom (NULL), |
194 | m_bb_to_rpo (bb_index_to_rpo) | |
3daacdcd | 195 | { |
3daacdcd JL |
196 | } |
197 | ||
d2552094 RB |
198 | /* Destructor. */ |
199 | ||
200 | dom_walker::~dom_walker () | |
201 | { | |
202 | if (! m_user_bb_to_rpo) | |
203 | free (m_bb_to_rpo); | |
204 | } | |
205 | ||
3daacdcd JL |
206 | /* Return TRUE if BB is reachable, false otherwise. */ |
207 | ||
208 | bool | |
209 | dom_walker::bb_reachable (struct function *fun, basic_block bb) | |
210 | { | |
211 | /* If we're not skipping unreachable blocks, then assume everything | |
212 | is reachable. */ | |
e37240b0 | 213 | if (m_reachability == ALL_BLOCKS) |
3daacdcd JL |
214 | return true; |
215 | ||
216 | /* If any of the predecessor edges that do not come from blocks dominated | |
217 | by us are still marked as possibly executable consider this block | |
218 | reachable. */ | |
219 | bool reachable = false; | |
220 | if (!m_unreachable_dom) | |
221 | { | |
222 | reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun); | |
223 | edge_iterator ei; | |
224 | edge e; | |
225 | FOR_EACH_EDGE (e, ei, bb->preds) | |
226 | if (!dominated_by_p (CDI_DOMINATORS, e->src, bb)) | |
227 | reachable |= (e->flags & EDGE_EXECUTABLE); | |
228 | } | |
229 | ||
230 | return reachable; | |
231 | } | |
232 | ||
233 | /* BB has been determined to be unreachable. Propagate that property | |
234 | to incoming and outgoing edges of BB as appropriate. */ | |
235 | ||
236 | void | |
237 | dom_walker::propagate_unreachable_to_edges (basic_block bb, | |
238 | FILE *dump_file, | |
1a817418 | 239 | dump_flags_t dump_flags) |
3daacdcd JL |
240 | { |
241 | if (dump_file && (dump_flags & TDF_DETAILS)) | |
242 | fprintf (dump_file, "Marking all outgoing edges of unreachable " | |
243 | "BB %d as not executable\n", bb->index); | |
244 | ||
245 | edge_iterator ei; | |
246 | edge e; | |
247 | FOR_EACH_EDGE (e, ei, bb->succs) | |
248 | e->flags &= ~EDGE_EXECUTABLE; | |
249 | ||
250 | FOR_EACH_EDGE (e, ei, bb->preds) | |
251 | { | |
252 | if (dominated_by_p (CDI_DOMINATORS, e->src, bb)) | |
253 | { | |
254 | if (dump_file && (dump_flags & TDF_DETAILS)) | |
255 | fprintf (dump_file, "Marking backedge from BB %d into " | |
256 | "unreachable BB %d as not executable\n", | |
257 | e->src->index, bb->index); | |
258 | e->flags &= ~EDGE_EXECUTABLE; | |
259 | } | |
260 | } | |
261 | ||
262 | if (!m_unreachable_dom) | |
263 | m_unreachable_dom = bb; | |
264 | } | |
265 | ||
d2552094 RB |
266 | const edge dom_walker::STOP = (edge)-1; |
267 | ||
6de9cd9a | 268 | /* Recursively walk the dominator tree. |
6de9cd9a DN |
269 | BB is the basic block we are currently visiting. */ |
270 | ||
271 | void | |
4d9192b5 | 272 | dom_walker::walk (basic_block bb) |
6de9cd9a | 273 | { |
e37240b0 RB |
274 | /* Compute the basic-block index to RPO mapping lazily. */ |
275 | if (!m_bb_to_rpo | |
276 | && m_dom_direction == CDI_DOMINATORS) | |
277 | { | |
278 | int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun)); | |
279 | int postorder_num = pre_and_rev_post_order_compute (NULL, postorder, | |
280 | true); | |
281 | m_bb_to_rpo = XNEWVEC (int, last_basic_block_for_fn (cfun)); | |
282 | for (int i = 0; i < postorder_num; ++i) | |
283 | m_bb_to_rpo[postorder[i]] = i; | |
284 | free (postorder); | |
285 | } | |
286 | ||
287 | /* Set up edge flags if need be. */ | |
288 | if (m_reachability == REACHABLE_BLOCKS) | |
289 | set_all_edges_as_executable (cfun); | |
290 | ||
6de9cd9a | 291 | basic_block dest; |
0cae8d31 DM |
292 | basic_block *worklist = XNEWVEC (basic_block, |
293 | n_basic_blocks_for_fn (cfun) * 2); | |
df648b94 | 294 | int sp = 0; |
0bca51f0 | 295 | |
df648b94 | 296 | while (true) |
6de9cd9a | 297 | { |
df648b94 | 298 | /* Don't worry about unreachable blocks. */ |
515f36eb | 299 | if (EDGE_COUNT (bb->preds) > 0 |
fefa31b5 DM |
300 | || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun) |
301 | || bb == EXIT_BLOCK_PTR_FOR_FN (cfun)) | |
6de9cd9a | 302 | { |
d2552094 | 303 | edge taken_edge = NULL; |
3daacdcd | 304 | |
4d9192b5 TS |
305 | /* Callback for subclasses to do custom things before we have walked |
306 | the dominator children, but before we walk statements. */ | |
3daacdcd JL |
307 | if (this->bb_reachable (cfun, bb)) |
308 | { | |
d2552094 RB |
309 | taken_edge = before_dom_children (bb); |
310 | if (taken_edge && taken_edge != STOP) | |
3daacdcd JL |
311 | { |
312 | edge_iterator ei; | |
313 | edge e; | |
314 | FOR_EACH_EDGE (e, ei, bb->succs) | |
315 | if (e != taken_edge) | |
316 | e->flags &= ~EDGE_EXECUTABLE; | |
317 | } | |
318 | } | |
319 | else | |
320 | propagate_unreachable_to_edges (bb, dump_file, dump_flags); | |
df648b94 JH |
321 | |
322 | /* Mark the current BB to be popped out of the recursion stack | |
fa10beec | 323 | once children are processed. */ |
df648b94 JH |
324 | worklist[sp++] = bb; |
325 | worklist[sp++] = NULL; | |
326 | ||
d2552094 RB |
327 | /* If the callback returned NONE then we are supposed to |
328 | stop and not even propagate EDGE_EXECUTABLE further. */ | |
329 | if (taken_edge != STOP) | |
330 | { | |
331 | int saved_sp = sp; | |
332 | for (dest = first_dom_son (m_dom_direction, bb); | |
333 | dest; dest = next_dom_son (m_dom_direction, dest)) | |
334 | worklist[sp++] = dest; | |
dc3b4a20 RB |
335 | /* Sort worklist after RPO order if requested. */ |
336 | if (sp - saved_sp > 1 | |
337 | && m_dom_direction == CDI_DOMINATORS | |
338 | && m_bb_to_rpo) | |
51007dc1 RB |
339 | sort_bbs_postorder (&worklist[saved_sp], sp - saved_sp, |
340 | m_bb_to_rpo); | |
d2552094 | 341 | } |
6de9cd9a | 342 | } |
ccf5c864 | 343 | /* NULL is used to mark pop operations in the recursion stack. */ |
df648b94 | 344 | while (sp > 0 && !worklist[sp - 1]) |
6de9cd9a | 345 | { |
df648b94 JH |
346 | --sp; |
347 | bb = worklist[--sp]; | |
df648b94 | 348 | |
4d9192b5 TS |
349 | /* Callback allowing subclasses to do custom things after we have |
350 | walked dominator children, but before we walk statements. */ | |
3daacdcd JL |
351 | if (bb_reachable (cfun, bb)) |
352 | after_dom_children (bb); | |
353 | else if (m_unreachable_dom == bb) | |
354 | m_unreachable_dom = NULL; | |
6de9cd9a | 355 | } |
df648b94 | 356 | if (sp) |
076b4605 | 357 | bb = worklist[--sp]; |
6de9cd9a | 358 | else |
df648b94 | 359 | break; |
6de9cd9a | 360 | } |
df648b94 | 361 | free (worklist); |
6de9cd9a | 362 | } |