1 /* Generic dominator tree walker
2 Copyright (C) 2003, 2004, 2005 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 2, 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 COPYING. If not, write to
19 the Free Software Foundation, 51 Franklin Street, Fifth Floor,
20 Boston, MA 02110-1301, USA. */
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 a 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
walk_dominator_tree(struct dom_walk_data * walk_data,basic_block bb)143 walk_dominator_tree (struct dom_walk_data *walk_data, basic_block bb)
144 {
145 void *bd = NULL;
146 basic_block dest;
147 block_stmt_iterator bsi;
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 || bb == ENTRY_BLOCK_PTR)
156 {
157 /* If block BB is not interesting to the caller, then none of the
158 callbacks that walk the statements in BB are going to be
159 executed. */
160 is_interesting = walk_data->interesting_blocks == NULL
161 || TEST_BIT (walk_data->interesting_blocks,
162 bb->index);
163
164 /* Callback to initialize the local data structure. */
165 if (walk_data->initialize_block_local_data)
166 {
167 bool recycled;
168
169 /* First get some local data, reusing any local data pointer we may
170 have saved. */
171 if (VEC_length (void_p, walk_data->free_block_data) > 0)
172 {
173 bd = VEC_pop (void_p, walk_data->free_block_data);
174 recycled = 1;
175 }
176 else
177 {
178 bd = xcalloc (1, walk_data->block_local_data_size);
179 recycled = 0;
180 }
181
182 /* Push the local data into the local data stack. */
183 VEC_safe_push (void_p, heap, walk_data->block_data_stack, bd);
184
185 /* Call the initializer. */
186 walk_data->initialize_block_local_data (walk_data, bb,
187 recycled);
188
189 }
190
191 /* Callback for operations to execute before we have walked the
192 dominator children, but before we walk statements. */
193 if (walk_data->before_dom_children_before_stmts)
194 (*walk_data->before_dom_children_before_stmts) (walk_data, bb);
195
196 /* Statement walk before walking dominator children. */
197 if (is_interesting && walk_data->before_dom_children_walk_stmts)
198 {
199 if (walk_data->walk_stmts_backward)
200 for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
201 (*walk_data->before_dom_children_walk_stmts) (walk_data, bb,
202 bsi);
203 else
204 for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
205 (*walk_data->before_dom_children_walk_stmts) (walk_data, bb,
206 bsi);
207 }
208
209 /* Callback for operations to execute before we have walked the
210 dominator children, and after we walk statements. */
211 if (walk_data->before_dom_children_after_stmts)
212 (*walk_data->before_dom_children_after_stmts) (walk_data, bb);
213
214 /* Mark the current BB to be popped out of the recursion stack
215 once childs are processed. */
216 worklist[sp++] = bb;
217 worklist[sp++] = NULL;
218
219 for (dest = first_dom_son (walk_data->dom_direction, bb);
220 dest; dest = next_dom_son (walk_data->dom_direction, dest))
221 worklist[sp++] = dest;
222 }
223 /* NULL is used to signalize pop operation in recursion stack. */
224 while (sp > 0 && !worklist[sp - 1])
225 {
226 --sp;
227 bb = worklist[--sp];
228 is_interesting = walk_data->interesting_blocks == NULL
229 || TEST_BIT (walk_data->interesting_blocks,
230 bb->index);
231 /* Callback for operations to execute after we have walked the
232 dominator children, but before we walk statements. */
233 if (walk_data->after_dom_children_before_stmts)
234 (*walk_data->after_dom_children_before_stmts) (walk_data, bb);
235
236 /* Statement walk after walking dominator children. */
237 if (is_interesting && walk_data->after_dom_children_walk_stmts)
238 {
239 if (walk_data->walk_stmts_backward)
240 for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
241 (*walk_data->after_dom_children_walk_stmts) (walk_data, bb,
242 bsi);
243 else
244 for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
245 (*walk_data->after_dom_children_walk_stmts) (walk_data, bb,
246 bsi);
247 }
248
249 /* Callback for operations to execute after we have walked the
250 dominator children and after we have walked statements. */
251 if (walk_data->after_dom_children_after_stmts)
252 (*walk_data->after_dom_children_after_stmts) (walk_data, bb);
253
254 if (walk_data->initialize_block_local_data)
255 {
256 /* And finally pop the record off the block local data stack. */
257 bd = VEC_pop (void_p, walk_data->block_data_stack);
258 /* And save the block data so that we can re-use it. */
259 VEC_safe_push (void_p, heap, walk_data->free_block_data, bd);
260 }
261 }
262 if (sp)
263 bb = worklist[--sp];
264 else
265 break;
266 }
267 free (worklist);
268 }
269
270 void
init_walk_dominator_tree(struct dom_walk_data * walk_data)271 init_walk_dominator_tree (struct dom_walk_data *walk_data)
272 {
273 walk_data->free_block_data = NULL;
274 walk_data->block_data_stack = NULL;
275 }
276
277 void
fini_walk_dominator_tree(struct dom_walk_data * walk_data)278 fini_walk_dominator_tree (struct dom_walk_data *walk_data)
279 {
280 if (walk_data->initialize_block_local_data)
281 {
282 while (VEC_length (void_p, walk_data->free_block_data) > 0)
283 free (VEC_pop (void_p, walk_data->free_block_data));
284 }
285
286 VEC_free (void_p, heap, walk_data->free_block_data);
287 VEC_free (void_p, heap, walk_data->block_data_stack);
288 }
289