1 /* Generic dominator tree walker
2 Copyright (C) 2003-2018 Free Software Foundation, Inc.
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
9 the Free Software Foundation; either version 3, or (at your option)
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
18 along with GCC; see the file COPYING3. If not see
19 <http://www.gnu.org/licenses/>. */
20
21 #include "config.h"
22 #include "system.h"
23 #include "coretypes.h"
24 #include "backend.h"
25 #include "cfganal.h"
26 #include "domwalk.h"
27 #include "dumpfile.h"
28
29 /* This file implements a generic walker for dominator trees.
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
52 defined in a similar manner. i.e., block B1 is said to post-dominate
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
71
72
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
90
91
92
93 The dominator tree is the basis for a number of analysis, transformation
94 and optimization algorithms that operate on a semi-global basis.
95
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.
100
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,
104 one after visiting the dominator children. There is a callback
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.
108
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.
113
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).
117
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
122 And (of course), we use the dominator walker to drive our dominator
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
131 /* Reverse postorder index of each basic block. */
132 static int *bb_postorder;
133
134 static int
cmp_bb_postorder(const void * a,const void * b)135 cmp_bb_postorder (const void *a, const void *b)
136 {
137 basic_block bb1 = *(const basic_block *)(a);
138 basic_block bb2 = *(const basic_block *)(b);
139 /* Place higher completion number first (pop off lower number first). */
140 return bb_postorder[bb2->index] - bb_postorder[bb1->index];
141 }
142
143 /* Permute array BBS of N basic blocks in postorder,
144 i.e. by descending number in BB_POSTORDER array. */
145
146 static void
sort_bbs_postorder(basic_block * bbs,int n)147 sort_bbs_postorder (basic_block *bbs, int n)
148 {
149 if (__builtin_expect (n == 2, true))
150 {
151 basic_block bb0 = bbs[0], bb1 = bbs[1];
152 if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
153 bbs[0] = bb1, bbs[1] = bb0;
154 }
155 else if (__builtin_expect (n == 3, true))
156 {
157 basic_block bb0 = bbs[0], bb1 = bbs[1], bb2 = bbs[2];
158 if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
159 std::swap (bb0, bb1);
160 if (bb_postorder[bb1->index] < bb_postorder[bb2->index])
161 {
162 std::swap (bb1, bb2);
163 if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
164 std::swap (bb0, bb1);
165 }
166 bbs[0] = bb0, bbs[1] = bb1, bbs[2] = bb2;
167 }
168 else
169 qsort (bbs, n, sizeof *bbs, cmp_bb_postorder);
170 }
171
172 /* Set EDGE_EXECUTABLE on every edge within FN's CFG. */
173
174 void
set_all_edges_as_executable(function * fn)175 set_all_edges_as_executable (function *fn)
176 {
177 basic_block bb;
178 FOR_ALL_BB_FN (bb, fn)
179 {
180 edge_iterator ei;
181 edge e;
182 FOR_EACH_EDGE (e, ei, bb->succs)
183 e->flags |= EDGE_EXECUTABLE;
184 }
185 }
186
187 /* Constructor for a dom walker. */
188
dom_walker(cdi_direction direction,enum reachability reachability,int * bb_index_to_rpo)189 dom_walker::dom_walker (cdi_direction direction,
190 enum reachability reachability,
191 int *bb_index_to_rpo)
192 : m_dom_direction (direction),
193 m_skip_unreachable_blocks (reachability != ALL_BLOCKS),
194 m_user_bb_to_rpo (true),
195 m_unreachable_dom (NULL),
196 m_bb_to_rpo (bb_index_to_rpo)
197 {
198 /* Set up edge flags if need be. */
199 switch (reachability)
200 {
201 default:
202 gcc_unreachable ();
203 case ALL_BLOCKS:
204 /* No need to touch edge flags. */
205 break;
206
207 case REACHABLE_BLOCKS:
208 set_all_edges_as_executable (cfun);
209 break;
210
211 case REACHABLE_BLOCKS_PRESERVING_FLAGS:
212 /* Preserve the edge flags. */
213 break;
214 }
215 }
216
217 /* Constructor for a dom walker. */
218
dom_walker(cdi_direction direction,enum reachability reachability)219 dom_walker::dom_walker (cdi_direction direction,
220 enum reachability reachability)
221 : m_dom_direction (direction),
222 m_skip_unreachable_blocks (reachability != ALL_BLOCKS),
223 m_user_bb_to_rpo (false),
224 m_unreachable_dom (NULL),
225 m_bb_to_rpo (NULL)
226 {
227 /* Compute the basic-block index to RPO mapping. */
228 if (direction == CDI_DOMINATORS)
229 {
230 int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
231 int postorder_num = pre_and_rev_post_order_compute (NULL, postorder,
232 true);
233 m_bb_to_rpo = XNEWVEC (int, last_basic_block_for_fn (cfun));
234 for (int i = 0; i < postorder_num; ++i)
235 m_bb_to_rpo[postorder[i]] = i;
236 free (postorder);
237 }
238
239 /* Set up edge flags if need be. */
240 switch (reachability)
241 {
242 default:
243 gcc_unreachable ();
244 case ALL_BLOCKS:
245 /* No need to touch edge flags. */
246 break;
247
248 case REACHABLE_BLOCKS:
249 set_all_edges_as_executable (cfun);
250 break;
251
252 case REACHABLE_BLOCKS_PRESERVING_FLAGS:
253 /* Preserve the edge flags. */
254 break;
255 }
256 }
257
258 /* Destructor. */
259
~dom_walker()260 dom_walker::~dom_walker ()
261 {
262 if (! m_user_bb_to_rpo)
263 free (m_bb_to_rpo);
264 }
265
266 /* Return TRUE if BB is reachable, false otherwise. */
267
268 bool
bb_reachable(struct function * fun,basic_block bb)269 dom_walker::bb_reachable (struct function *fun, basic_block bb)
270 {
271 /* If we're not skipping unreachable blocks, then assume everything
272 is reachable. */
273 if (!m_skip_unreachable_blocks)
274 return true;
275
276 /* If any of the predecessor edges that do not come from blocks dominated
277 by us are still marked as possibly executable consider this block
278 reachable. */
279 bool reachable = false;
280 if (!m_unreachable_dom)
281 {
282 reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun);
283 edge_iterator ei;
284 edge e;
285 FOR_EACH_EDGE (e, ei, bb->preds)
286 if (!dominated_by_p (CDI_DOMINATORS, e->src, bb))
287 reachable |= (e->flags & EDGE_EXECUTABLE);
288 }
289
290 return reachable;
291 }
292
293 /* BB has been determined to be unreachable. Propagate that property
294 to incoming and outgoing edges of BB as appropriate. */
295
296 void
propagate_unreachable_to_edges(basic_block bb,FILE * dump_file,dump_flags_t dump_flags)297 dom_walker::propagate_unreachable_to_edges (basic_block bb,
298 FILE *dump_file,
299 dump_flags_t dump_flags)
300 {
301 if (dump_file && (dump_flags & TDF_DETAILS))
302 fprintf (dump_file, "Marking all outgoing edges of unreachable "
303 "BB %d as not executable\n", bb->index);
304
305 edge_iterator ei;
306 edge e;
307 FOR_EACH_EDGE (e, ei, bb->succs)
308 e->flags &= ~EDGE_EXECUTABLE;
309
310 FOR_EACH_EDGE (e, ei, bb->preds)
311 {
312 if (dominated_by_p (CDI_DOMINATORS, e->src, bb))
313 {
314 if (dump_file && (dump_flags & TDF_DETAILS))
315 fprintf (dump_file, "Marking backedge from BB %d into "
316 "unreachable BB %d as not executable\n",
317 e->src->index, bb->index);
318 e->flags &= ~EDGE_EXECUTABLE;
319 }
320 }
321
322 if (!m_unreachable_dom)
323 m_unreachable_dom = bb;
324 }
325
326 const edge dom_walker::STOP = (edge)-1;
327
328 /* Recursively walk the dominator tree.
329 BB is the basic block we are currently visiting. */
330
331 void
walk(basic_block bb)332 dom_walker::walk (basic_block bb)
333 {
334 basic_block dest;
335 basic_block *worklist = XNEWVEC (basic_block,
336 n_basic_blocks_for_fn (cfun) * 2);
337 int sp = 0;
338 bb_postorder = m_bb_to_rpo;
339
340 while (true)
341 {
342 /* Don't worry about unreachable blocks. */
343 if (EDGE_COUNT (bb->preds) > 0
344 || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
345 || bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
346 {
347 edge taken_edge = NULL;
348
349 /* Callback for subclasses to do custom things before we have walked
350 the dominator children, but before we walk statements. */
351 if (this->bb_reachable (cfun, bb))
352 {
353 taken_edge = before_dom_children (bb);
354 if (taken_edge && taken_edge != STOP)
355 {
356 edge_iterator ei;
357 edge e;
358 FOR_EACH_EDGE (e, ei, bb->succs)
359 if (e != taken_edge)
360 e->flags &= ~EDGE_EXECUTABLE;
361 }
362 }
363 else
364 propagate_unreachable_to_edges (bb, dump_file, dump_flags);
365
366 /* Mark the current BB to be popped out of the recursion stack
367 once children are processed. */
368 worklist[sp++] = bb;
369 worklist[sp++] = NULL;
370
371 /* If the callback returned NONE then we are supposed to
372 stop and not even propagate EDGE_EXECUTABLE further. */
373 if (taken_edge != STOP)
374 {
375 int saved_sp = sp;
376 for (dest = first_dom_son (m_dom_direction, bb);
377 dest; dest = next_dom_son (m_dom_direction, dest))
378 worklist[sp++] = dest;
379 /* Sort worklist after RPO order if requested. */
380 if (sp - saved_sp > 1
381 && m_dom_direction == CDI_DOMINATORS
382 && m_bb_to_rpo)
383 sort_bbs_postorder (&worklist[saved_sp], sp - saved_sp);
384 }
385 }
386 /* NULL is used to mark pop operations in the recursion stack. */
387 while (sp > 0 && !worklist[sp - 1])
388 {
389 --sp;
390 bb = worklist[--sp];
391
392 /* Callback allowing subclasses to do custom things after we have
393 walked dominator children, but before we walk statements. */
394 if (bb_reachable (cfun, bb))
395 after_dom_children (bb);
396 else if (m_unreachable_dom == bb)
397 m_unreachable_dom = NULL;
398 }
399 if (sp)
400 bb = worklist[--sp];
401 else
402 break;
403 }
404 bb_postorder = NULL;
405 free (worklist);
406 }
407