1 /* Generic dominator tree walker
2 Copyright (C) 2003-2019 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_reachability (reachability),
194 m_user_bb_to_rpo (bb_index_to_rpo != NULL),
195 m_unreachable_dom (NULL),
196 m_bb_to_rpo (bb_index_to_rpo)
197 {
198 }
199
200 /* Destructor. */
201
~dom_walker()202 dom_walker::~dom_walker ()
203 {
204 if (! m_user_bb_to_rpo)
205 free (m_bb_to_rpo);
206 }
207
208 /* Return TRUE if BB is reachable, false otherwise. */
209
210 bool
bb_reachable(struct function * fun,basic_block bb)211 dom_walker::bb_reachable (struct function *fun, basic_block bb)
212 {
213 /* If we're not skipping unreachable blocks, then assume everything
214 is reachable. */
215 if (m_reachability == ALL_BLOCKS)
216 return true;
217
218 /* If any of the predecessor edges that do not come from blocks dominated
219 by us are still marked as possibly executable consider this block
220 reachable. */
221 bool reachable = false;
222 if (!m_unreachable_dom)
223 {
224 reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun);
225 edge_iterator ei;
226 edge e;
227 FOR_EACH_EDGE (e, ei, bb->preds)
228 if (!dominated_by_p (CDI_DOMINATORS, e->src, bb))
229 reachable |= (e->flags & EDGE_EXECUTABLE);
230 }
231
232 return reachable;
233 }
234
235 /* BB has been determined to be unreachable. Propagate that property
236 to incoming and outgoing edges of BB as appropriate. */
237
238 void
propagate_unreachable_to_edges(basic_block bb,FILE * dump_file,dump_flags_t dump_flags)239 dom_walker::propagate_unreachable_to_edges (basic_block bb,
240 FILE *dump_file,
241 dump_flags_t dump_flags)
242 {
243 if (dump_file && (dump_flags & TDF_DETAILS))
244 fprintf (dump_file, "Marking all outgoing edges of unreachable "
245 "BB %d as not executable\n", bb->index);
246
247 edge_iterator ei;
248 edge e;
249 FOR_EACH_EDGE (e, ei, bb->succs)
250 e->flags &= ~EDGE_EXECUTABLE;
251
252 FOR_EACH_EDGE (e, ei, bb->preds)
253 {
254 if (dominated_by_p (CDI_DOMINATORS, e->src, bb))
255 {
256 if (dump_file && (dump_flags & TDF_DETAILS))
257 fprintf (dump_file, "Marking backedge from BB %d into "
258 "unreachable BB %d as not executable\n",
259 e->src->index, bb->index);
260 e->flags &= ~EDGE_EXECUTABLE;
261 }
262 }
263
264 if (!m_unreachable_dom)
265 m_unreachable_dom = bb;
266 }
267
268 const edge dom_walker::STOP = (edge)-1;
269
270 /* Recursively walk the dominator tree.
271 BB is the basic block we are currently visiting. */
272
273 void
walk(basic_block bb)274 dom_walker::walk (basic_block bb)
275 {
276 /* Compute the basic-block index to RPO mapping lazily. */
277 if (!m_bb_to_rpo
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
293 basic_block dest;
294 basic_block *worklist = XNEWVEC (basic_block,
295 n_basic_blocks_for_fn (cfun) * 2);
296 int sp = 0;
297 bb_postorder = m_bb_to_rpo;
298
299 while (true)
300 {
301 /* Don't worry about unreachable blocks. */
302 if (EDGE_COUNT (bb->preds) > 0
303 || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
304 || bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
305 {
306 edge taken_edge = NULL;
307
308 /* Callback for subclasses to do custom things before we have walked
309 the dominator children, but before we walk statements. */
310 if (this->bb_reachable (cfun, bb))
311 {
312 taken_edge = before_dom_children (bb);
313 if (taken_edge && taken_edge != STOP)
314 {
315 edge_iterator ei;
316 edge e;
317 FOR_EACH_EDGE (e, ei, bb->succs)
318 if (e != taken_edge)
319 e->flags &= ~EDGE_EXECUTABLE;
320 }
321 }
322 else
323 propagate_unreachable_to_edges (bb, dump_file, dump_flags);
324
325 /* Mark the current BB to be popped out of the recursion stack
326 once children are processed. */
327 worklist[sp++] = bb;
328 worklist[sp++] = NULL;
329
330 /* If the callback returned NONE then we are supposed to
331 stop and not even propagate EDGE_EXECUTABLE further. */
332 if (taken_edge != STOP)
333 {
334 int saved_sp = sp;
335 for (dest = first_dom_son (m_dom_direction, bb);
336 dest; dest = next_dom_son (m_dom_direction, dest))
337 worklist[sp++] = dest;
338 /* Sort worklist after RPO order if requested. */
339 if (sp - saved_sp > 1
340 && m_dom_direction == CDI_DOMINATORS
341 && m_bb_to_rpo)
342 sort_bbs_postorder (&worklist[saved_sp], sp - saved_sp);
343 }
344 }
345 /* NULL is used to mark pop operations in the recursion stack. */
346 while (sp > 0 && !worklist[sp - 1])
347 {
348 --sp;
349 bb = worklist[--sp];
350
351 /* Callback allowing subclasses to do custom things after we have
352 walked dominator children, but before we walk statements. */
353 if (bb_reachable (cfun, bb))
354 after_dom_children (bb);
355 else if (m_unreachable_dom == bb)
356 m_unreachable_dom = NULL;
357 }
358 if (sp)
359 bb = worklist[--sp];
360 else
361 break;
362 }
363 bb_postorder = NULL;
364 free (worklist);
365 }
366