1 /* Generic dominator tree walker
2 Copyright (C) 2003, 2004, 2005, 2007, 2008, 2010 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 "basic-block.h"
27 #include "domwalk.h"
28 #include "sbitmap.h"
29
30 /* This file implements a generic walker for dominator trees.
31
32 To understand the dominator walker one must first have a grasp of dominators,
33 immediate dominators and the dominator tree.
34
35 Dominators
36 A block B1 is said to dominate B2 if every path from the entry to B2 must
37 pass through B1. Given the dominance relationship, we can proceed to
38 compute immediate dominators. Note it is not important whether or not
39 our definition allows a block to dominate itself.
40
41 Immediate Dominators:
42 Every block in the CFG has no more than one immediate dominator. The
43 immediate dominator of block BB must dominate BB and must not dominate
44 any other dominator of BB and must not be BB itself.
45
46 Dominator tree:
47 If we then construct a tree where each node is a basic block and there
48 is an edge from each block's immediate dominator to the block itself, then
49 we have a dominator tree.
50
51
52 [ Note this walker can also walk the post-dominator tree, which is
53 defined in a similar manner. i.e., block B1 is said to post-dominate
54 block B2 if all paths from B2 to the exit block must pass through
55 B1. ]
56
57 For example, given the CFG
58
59 1
60 |
61 2
62 / \
63 3 4
64 / \
65 +---------->5 6
66 | / \ /
67 | +--->8 7
68 | | / |
69 | +--9 11
70 | / |
71 +--- 10 ---> 12
72
73
74 We have a dominator tree which looks like
75
76 1
77 |
78 2
79 / \
80 / \
81 3 4
82 / / \ \
83 | | | |
84 5 6 7 12
85 | |
86 8 11
87 |
88 9
89 |
90 10
91
92
93
94 The dominator tree is the basis for a number of analysis, transformation
95 and optimization algorithms that operate on a semi-global basis.
96
97 The dominator walker is a generic routine which visits blocks in the CFG
98 via a depth first search of the dominator tree. In the example above
99 the dominator walker might visit blocks in the following order
100 1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12.
101
102 The dominator walker has a number of callbacks to perform actions
103 during the walk of the dominator tree. There are two callbacks
104 which walk statements, one before visiting the dominator children,
105 one after visiting the dominator children. There is a callback
106 before and after each statement walk callback. In addition, the
107 dominator walker manages allocation/deallocation of data structures
108 which are local to each block visited.
109
110 The dominator walker is meant to provide a generic means to build a pass
111 which can analyze or transform/optimize a function based on walking
112 the dominator tree. One simply fills in the dominator walker data
113 structure with the appropriate callbacks and calls the walker.
114
115 We currently use the dominator walker to prune the set of variables
116 which might need PHI nodes (which can greatly improve compile-time
117 performance in some cases).
118
119 We also use the dominator walker to rewrite the function into SSA form
120 which reduces code duplication since the rewriting phase is inherently
121 a walk of the dominator tree.
122
123 And (of course), we use the dominator walker to drive our dominator
124 optimizer, which is a semi-global optimizer.
125
126 TODO:
127
128 Walking statements is based on the block statement iterator abstraction,
129 which is currently an abstraction over walking tree statements. Thus
130 the dominator walker is currently only useful for trees. */
131
132 /* Recursively walk the dominator tree.
133
134 WALK_DATA contains a set of callbacks to perform pass-specific
135 actions during the dominator walk as well as a stack of block local
136 data maintained during the dominator walk.
137
138 BB is the basic block we are currently visiting. */
139
140 void
walk_dominator_tree(struct dom_walk_data * walk_data,basic_block bb)141 walk_dominator_tree (struct dom_walk_data *walk_data, basic_block bb)
142 {
143 void *bd = NULL;
144 basic_block dest;
145 basic_block *worklist = XNEWVEC (basic_block, n_basic_blocks * 2);
146 int sp = 0;
147 sbitmap visited = sbitmap_alloc (last_basic_block + 1);
148 sbitmap_zero (visited);
149 SET_BIT (visited, ENTRY_BLOCK_PTR->index);
150
151 while (true)
152 {
153 /* Don't worry about unreachable blocks. */
154 if (EDGE_COUNT (bb->preds) > 0
155 || bb == ENTRY_BLOCK_PTR
156 || bb == EXIT_BLOCK_PTR)
157 {
158 /* Callback to initialize the local data structure. */
159 if (walk_data->initialize_block_local_data)
160 {
161 bool recycled;
162
163 /* First get some local data, reusing any local data
164 pointer we may have saved. */
165 if (VEC_length (void_p, walk_data->free_block_data) > 0)
166 {
167 bd = VEC_pop (void_p, walk_data->free_block_data);
168 recycled = 1;
169 }
170 else
171 {
172 bd = xcalloc (1, walk_data->block_local_data_size);
173 recycled = 0;
174 }
175
176 /* Push the local data into the local data stack. */
177 VEC_safe_push (void_p, heap, walk_data->block_data_stack, bd);
178
179 /* Call the initializer. */
180 walk_data->initialize_block_local_data (walk_data, bb,
181 recycled);
182
183 }
184
185 /* Callback for operations to execute before we have walked the
186 dominator children, but before we walk statements. */
187 if (walk_data->before_dom_children)
188 (*walk_data->before_dom_children) (walk_data, bb);
189
190 SET_BIT (visited, bb->index);
191
192 /* Mark the current BB to be popped out of the recursion stack
193 once children are processed. */
194 worklist[sp++] = bb;
195 worklist[sp++] = NULL;
196
197 for (dest = first_dom_son (walk_data->dom_direction, bb);
198 dest; dest = next_dom_son (walk_data->dom_direction, dest))
199 worklist[sp++] = dest;
200 }
201 /* NULL is used to mark pop operations in the recursion stack. */
202 while (sp > 0 && !worklist[sp - 1])
203 {
204 --sp;
205 bb = worklist[--sp];
206
207 /* Callback for operations to execute after we have walked the
208 dominator children, but before we walk statements. */
209 if (walk_data->after_dom_children)
210 (*walk_data->after_dom_children) (walk_data, bb);
211
212 if (walk_data->initialize_block_local_data)
213 {
214 /* And finally pop the record off the block local data stack. */
215 bd = VEC_pop (void_p, walk_data->block_data_stack);
216 /* And save the block data so that we can re-use it. */
217 VEC_safe_push (void_p, heap, walk_data->free_block_data, bd);
218 }
219 }
220 if (sp)
221 {
222 int spp;
223 spp = sp - 1;
224 if (walk_data->dom_direction == CDI_DOMINATORS)
225 /* Find the dominator son that has all its predecessors
226 visited and continue with that. */
227 while (1)
228 {
229 edge_iterator ei;
230 edge e;
231 bool found = true;
232 bb = worklist[spp];
233 FOR_EACH_EDGE (e, ei, bb->preds)
234 {
235 if (!dominated_by_p (CDI_DOMINATORS, e->src, e->dest)
236 && !TEST_BIT (visited, e->src->index))
237 {
238 found = false;
239 break;
240 }
241 }
242 if (found)
243 break;
244 /* If we didn't find a dom child with all visited
245 predecessors just use the candidate we were checking.
246 This happens for candidates in irreducible loops. */
247 if (!worklist[spp - 1])
248 break;
249 --spp;
250 }
251 bb = worklist[spp];
252 worklist[spp] = worklist[--sp];
253 }
254 else
255 break;
256 }
257 free (worklist);
258 sbitmap_free (visited);
259 }
260
261 void
init_walk_dominator_tree(struct dom_walk_data * walk_data)262 init_walk_dominator_tree (struct dom_walk_data *walk_data)
263 {
264 walk_data->free_block_data = NULL;
265 walk_data->block_data_stack = NULL;
266 }
267
268 void
fini_walk_dominator_tree(struct dom_walk_data * walk_data)269 fini_walk_dominator_tree (struct dom_walk_data *walk_data)
270 {
271 if (walk_data->initialize_block_local_data)
272 {
273 while (VEC_length (void_p, walk_data->free_block_data) > 0)
274 free (VEC_pop (void_p, walk_data->free_block_data));
275 }
276
277 VEC_free (void_p, heap, walk_data->free_block_data);
278 VEC_free (void_p, heap, walk_data->block_data_stack);
279 }
280