1/* Generic dominator tree walker
2 Copyright (C) 2003-2024 Free Software Foundation, Inc.
3 Contributed by Diego Novillo <dnovillo@redhat.com>
4
5This file is part of GCC.
6
7GCC is free software; you can redistribute it and/or modify
8it under the terms of the GNU General Public License as published by
9the Free Software Foundation; either version 3, or (at your option)
10any later version.
11
12GCC is distributed in the hope that it will be useful,
13but WITHOUT ANY WARRANTY; without even the implied warranty of
14MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15GNU General Public License for more details.
16
17You should have received a copy of the GNU General Public License
18along 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
131static int
132cmp_bb_postorder (const void *a, const void *b, void *data)
133{
134 basic_block bb1 = *(const basic_block *)(a);
135 basic_block bb2 = *(const basic_block *)(b);
136 int *bb_postorder = (int *)data;
137 /* Place higher completion number first (pop off lower number first). */
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
144static void
145sort_bbs_postorder (basic_block *bbs, int n, int *bb_postorder)
146{
147 if (LIKELY (n == 2))
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 (LIKELY (n == 3))
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 (a&: bb0, b&: bb1);
158 if (bb_postorder[bb1->index] < bb_postorder[bb2->index])
159 {
160 std::swap (a&: bb1, b&: bb2);
161 if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
162 std::swap (a&: bb0, b&: bb1);
163 }
164 bbs[0] = bb0, bbs[1] = bb1, bbs[2] = bb2;
165 }
166 else
167 gcc_sort_r (bbs, n, sizeof *bbs, cmp_bb_postorder, bb_postorder);
168}
169
170/* Set EDGE_EXECUTABLE on every edge within FN's CFG. */
171
172void
173set_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. */
186
187dom_walker::dom_walker (cdi_direction direction,
188 enum reachability reachability,
189 int *bb_index_to_rpo)
190 : m_dom_direction (direction),
191 m_reachability (reachability),
192 m_user_bb_to_rpo (bb_index_to_rpo != NULL),
193 m_unreachable_dom (NULL),
194 m_bb_to_rpo (bb_index_to_rpo == (int *)(uintptr_t)-1
195 ? NULL : bb_index_to_rpo)
196{
197}
198
199/* Destructor. */
200
201dom_walker::~dom_walker ()
202{
203 if (! m_user_bb_to_rpo)
204 free (ptr: m_bb_to_rpo);
205}
206
207/* Return TRUE if BB is reachable, false otherwise. */
208
209bool
210dom_walker::bb_reachable (struct function *fun, basic_block bb)
211{
212 /* If we're not skipping unreachable blocks, then assume everything
213 is reachable. */
214 if (m_reachability == ALL_BLOCKS)
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
237void
238dom_walker::propagate_unreachable_to_edges (basic_block bb,
239 FILE *dump_file,
240 dump_flags_t dump_flags)
241{
242 if (dump_file && (dump_flags & TDF_DETAILS))
243 fprintf (stream: dump_file, format: "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 (stream: dump_file, format: "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
267const edge dom_walker::STOP = (edge)-1;
268
269/* Recursively walk the dominator tree.
270 BB is the basic block we are currently visiting. */
271
272void
273dom_walker::walk (basic_block bb)
274{
275 /* Compute the basic-block index to RPO mapping lazily. */
276 if (!m_user_bb_to_rpo
277 && !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 (ptr: 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
298 while (true)
299 {
300 /* Don't worry about unreachable blocks. */
301 if (EDGE_COUNT (bb->preds) > 0
302 || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
303 || bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
304 {
305 edge taken_edge = NULL;
306
307 /* Callback for subclasses to do custom things before we have walked
308 the dominator children, but before we walk statements. */
309 if (this->bb_reachable (cfun, bb))
310 {
311 taken_edge = before_dom_children (bb);
312 if (taken_edge && taken_edge != STOP)
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);
323
324 /* Mark the current BB to be popped out of the recursion stack
325 once children are processed. */
326 worklist[sp++] = bb;
327 worklist[sp++] = NULL;
328
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;
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)
341 sort_bbs_postorder (bbs: &worklist[saved_sp], n: sp - saved_sp,
342 bb_postorder: m_bb_to_rpo);
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 free (ptr: worklist);
364}
365

source code of gcc/domwalk.cc