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author kent <kent@cr.ie.u-ryukyu.ac.jp>
date Fri, 17 Jul 2009 14:47:48 +0900
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1 /* Generic dominator tree walker
2 Copyright (C) 2003, 2004, 2005, 2007, 2008 Free Software Foundation,
3 Inc.
4 Contributed by Diego Novillo <dnovillo@redhat.com>
5
6 This file is part of GCC.
7
8 GCC is free software; you can redistribute it and/or modify
9 it under the terms of the GNU General Public License as published by
10 the Free Software Foundation; either version 3, or (at your option)
11 any later version.
12
13 GCC is distributed in the hope that it will be useful,
14 but WITHOUT ANY WARRANTY; without even the implied warranty of
15 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
16 GNU General Public License for more details.
17
18 You should have received a copy of the GNU General Public License
19 along with GCC; see the file COPYING3. If not see
20 <http://www.gnu.org/licenses/>. */
21
22 #include "config.h"
23 #include "system.h"
24 #include "coretypes.h"
25 #include "tm.h"
26 #include "tree.h"
27 #include "basic-block.h"
28 #include "tree-flow.h"
29 #include "domwalk.h"
30 #include "ggc.h"
31
32 /* This file implements a generic walker for dominator trees.
33
34 To understand the dominator walker one must first have a grasp of dominators,
35 immediate dominators and the dominator tree.
36
37 Dominators
38 A block B1 is said to dominate B2 if every path from the entry to B2 must
39 pass through B1. Given the dominance relationship, we can proceed to
40 compute immediate dominators. Note it is not important whether or not
41 our definition allows a block to dominate itself.
42
43 Immediate Dominators:
44 Every block in the CFG has no more than one immediate dominator. The
45 immediate dominator of block BB must dominate BB and must not dominate
46 any other dominator of BB and must not be BB itself.
47
48 Dominator tree:
49 If we then construct a tree where each node is a basic block and there
50 is an edge from each block's immediate dominator to the block itself, then
51 we have a dominator tree.
52
53
54 [ Note this walker can also walk the post-dominator tree, which is
55 defined in a similar manner. i.e., block B1 is said to post-dominate
56 block B2 if all paths from B2 to the exit block must pass through
57 B1. ]
58
59 For example, given the CFG
60
61 1
62 |
63 2
64 / \
65 3 4
66 / \
67 +---------->5 6
68 | / \ /
69 | +--->8 7
70 | | / |
71 | +--9 11
72 | / |
73 +--- 10 ---> 12
74
75
76 We have a dominator tree which looks like
77
78 1
79 |
80 2
81 / \
82 / \
83 3 4
84 / / \ \
85 | | | |
86 5 6 7 12
87 | |
88 8 11
89 |
90 9
91 |
92 10
93
94
95
96 The dominator tree is the basis for a number of analysis, transformation
97 and optimization algorithms that operate on a semi-global basis.
98
99 The dominator walker is a generic routine which visits blocks in the CFG
100 via a depth first search of the dominator tree. In the example above
101 the dominator walker might visit blocks in the following order
102 1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12.
103
104 The dominator walker has a number of callbacks to perform actions
105 during the walk of the dominator tree. There are two callbacks
106 which walk statements, one before visiting the dominator children,
107 one after visiting the dominator children. There is a callback
108 before and after each statement walk callback. In addition, the
109 dominator walker manages allocation/deallocation of data structures
110 which are local to each block visited.
111
112 The dominator walker is meant to provide a generic means to build a pass
113 which can analyze or transform/optimize a function based on walking
114 the dominator tree. One simply fills in the dominator walker data
115 structure with the appropriate callbacks and calls the walker.
116
117 We currently use the dominator walker to prune the set of variables
118 which might need PHI nodes (which can greatly improve compile-time
119 performance in some cases).
120
121 We also use the dominator walker to rewrite the function into SSA form
122 which reduces code duplication since the rewriting phase is inherently
123 a walk of the dominator tree.
124
125 And (of course), we use the dominator walker to drive our dominator
126 optimizer, which is a semi-global optimizer.
127
128 TODO:
129
130 Walking statements is based on the block statement iterator abstraction,
131 which is currently an abstraction over walking tree statements. Thus
132 the dominator walker is currently only useful for trees. */
133
134 /* Recursively walk the dominator tree.
135
136 WALK_DATA contains a set of callbacks to perform pass-specific
137 actions during the dominator walk as well as a stack of block local
138 data maintained during the dominator walk.
139
140 BB is the basic block we are currently visiting. */
141
142 void
143 walk_dominator_tree (struct dom_walk_data *walk_data, basic_block bb)
144 {
145 void *bd = NULL;
146 basic_block dest;
147 gimple_stmt_iterator gsi;
148 bool is_interesting;
149 basic_block *worklist = XNEWVEC (basic_block, n_basic_blocks * 2);
150 int sp = 0;
151
152 while (true)
153 {
154 /* Don't worry about unreachable blocks. */
155 if (EDGE_COUNT (bb->preds) > 0
156 || bb == ENTRY_BLOCK_PTR
157 || bb == EXIT_BLOCK_PTR)
158 {
159 /* If block BB is not interesting to the caller, then none of the
160 callbacks that walk the statements in BB are going to be
161 executed. */
162 is_interesting = walk_data->interesting_blocks == NULL
163 || TEST_BIT (walk_data->interesting_blocks,
164 bb->index);
165
166 /* Callback to initialize the local data structure. */
167 if (walk_data->initialize_block_local_data)
168 {
169 bool recycled;
170
171 /* First get some local data, reusing any local data
172 pointer we may have saved. */
173 if (VEC_length (void_p, walk_data->free_block_data) > 0)
174 {
175 bd = VEC_pop (void_p, walk_data->free_block_data);
176 recycled = 1;
177 }
178 else
179 {
180 bd = xcalloc (1, walk_data->block_local_data_size);
181 recycled = 0;
182 }
183
184 /* Push the local data into the local data stack. */
185 VEC_safe_push (void_p, heap, walk_data->block_data_stack, bd);
186
187 /* Call the initializer. */
188 walk_data->initialize_block_local_data (walk_data, bb,
189 recycled);
190
191 }
192
193 /* Callback for operations to execute before we have walked the
194 dominator children, but before we walk statements. */
195 if (walk_data->before_dom_children_before_stmts)
196 (*walk_data->before_dom_children_before_stmts) (walk_data, bb);
197
198 /* Statement walk before walking dominator children. */
199 if (is_interesting && walk_data->before_dom_children_walk_stmts)
200 {
201 if (walk_data->walk_stmts_backward)
202 for (gsi = gsi_last (bb_seq (bb)); !gsi_end_p (gsi);
203 gsi_prev (&gsi))
204 (*walk_data->before_dom_children_walk_stmts) (walk_data, bb,
205 gsi);
206 else
207 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
208 (*walk_data->before_dom_children_walk_stmts) (walk_data, bb,
209 gsi);
210 }
211
212 /* Callback for operations to execute before we have walked the
213 dominator children, and after we walk statements. */
214 if (walk_data->before_dom_children_after_stmts)
215 (*walk_data->before_dom_children_after_stmts) (walk_data, bb);
216
217 /* Mark the current BB to be popped out of the recursion stack
218 once children are processed. */
219 worklist[sp++] = bb;
220 worklist[sp++] = NULL;
221
222 for (dest = first_dom_son (walk_data->dom_direction, bb);
223 dest; dest = next_dom_son (walk_data->dom_direction, dest))
224 worklist[sp++] = dest;
225 }
226 /* NULL is used to signalize pop operation in recursion stack. */
227 while (sp > 0 && !worklist[sp - 1])
228 {
229 --sp;
230 bb = worklist[--sp];
231 is_interesting = walk_data->interesting_blocks == NULL
232 || TEST_BIT (walk_data->interesting_blocks,
233 bb->index);
234 /* Callback for operations to execute after we have walked the
235 dominator children, but before we walk statements. */
236 if (walk_data->after_dom_children_before_stmts)
237 (*walk_data->after_dom_children_before_stmts) (walk_data, bb);
238
239 /* Statement walk after walking dominator children. */
240 if (is_interesting && walk_data->after_dom_children_walk_stmts)
241 {
242 if (walk_data->walk_stmts_backward)
243 for (gsi = gsi_last (bb_seq (bb)); !gsi_end_p (gsi);
244 gsi_prev (&gsi))
245 (*walk_data->after_dom_children_walk_stmts) (walk_data, bb,
246 gsi);
247 else
248 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
249 (*walk_data->after_dom_children_walk_stmts) (walk_data, bb,
250 gsi);
251 }
252
253 /* Callback for operations to execute after we have walked the
254 dominator children and after we have walked statements. */
255 if (walk_data->after_dom_children_after_stmts)
256 (*walk_data->after_dom_children_after_stmts) (walk_data, bb);
257
258 if (walk_data->initialize_block_local_data)
259 {
260 /* And finally pop the record off the block local data stack. */
261 bd = VEC_pop (void_p, walk_data->block_data_stack);
262 /* And save the block data so that we can re-use it. */
263 VEC_safe_push (void_p, heap, walk_data->free_block_data, bd);
264 }
265 }
266 if (sp)
267 bb = worklist[--sp];
268 else
269 break;
270 }
271 free (worklist);
272 }
273
274 void
275 init_walk_dominator_tree (struct dom_walk_data *walk_data)
276 {
277 walk_data->free_block_data = NULL;
278 walk_data->block_data_stack = NULL;
279 }
280
281 void
282 fini_walk_dominator_tree (struct dom_walk_data *walk_data)
283 {
284 if (walk_data->initialize_block_local_data)
285 {
286 while (VEC_length (void_p, walk_data->free_block_data) > 0)
287 free (VEC_pop (void_p, walk_data->free_block_data));
288 }
289
290 VEC_free (void_p, heap, walk_data->free_block_data);
291 VEC_free (void_p, heap, walk_data->block_data_stack);
292 }