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6de9cd9a | 1 | /* Generic dominator tree walker |
fa10beec RW |
2 | Copyright (C) 2003, 2004, 2005, 2007, 2008 Free Software Foundation, |
3 | Inc. | |
6de9cd9a DN |
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 | |
9dcd6f09 | 10 | the Free Software Foundation; either version 3, or (at your option) |
6de9cd9a DN |
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 | |
9dcd6f09 NC |
19 | along with GCC; see the file COPYING3. If not see |
20 | <http://www.gnu.org/licenses/>. */ | |
6de9cd9a DN |
21 | |
22 | #include "config.h" | |
23 | #include "system.h" | |
24 | #include "coretypes.h" | |
25 | #include "tm.h" | |
6de9cd9a | 26 | #include "basic-block.h" |
6de9cd9a DN |
27 | #include "domwalk.h" |
28 | #include "ggc.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 | |
454ff5cb | 53 | defined in a similar manner. i.e., block B1 is said to post-dominate |
6de9cd9a DN |
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 | ||
110abdbc | 123 | And (of course), we use the dominator walker to drive our dominator |
6de9cd9a DN |
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 | |
141 | walk_dominator_tree (struct dom_walk_data *walk_data, basic_block bb) | |
142 | { | |
143 | void *bd = NULL; | |
144 | basic_block dest; | |
df648b94 JH |
145 | basic_block *worklist = XNEWVEC (basic_block, n_basic_blocks * 2); |
146 | int sp = 0; | |
0bca51f0 | 147 | |
df648b94 | 148 | while (true) |
6de9cd9a | 149 | { |
df648b94 | 150 | /* Don't worry about unreachable blocks. */ |
515f36eb RG |
151 | if (EDGE_COUNT (bb->preds) > 0 |
152 | || bb == ENTRY_BLOCK_PTR | |
153 | || bb == EXIT_BLOCK_PTR) | |
6de9cd9a | 154 | { |
df648b94 JH |
155 | /* Callback to initialize the local data structure. */ |
156 | if (walk_data->initialize_block_local_data) | |
157 | { | |
158 | bool recycled; | |
159 | ||
726a989a RB |
160 | /* First get some local data, reusing any local data |
161 | pointer we may have saved. */ | |
df648b94 JH |
162 | if (VEC_length (void_p, walk_data->free_block_data) > 0) |
163 | { | |
164 | bd = VEC_pop (void_p, walk_data->free_block_data); | |
165 | recycled = 1; | |
166 | } | |
167 | else | |
168 | { | |
169 | bd = xcalloc (1, walk_data->block_local_data_size); | |
170 | recycled = 0; | |
171 | } | |
172 | ||
173 | /* Push the local data into the local data stack. */ | |
174 | VEC_safe_push (void_p, heap, walk_data->block_data_stack, bd); | |
175 | ||
176 | /* Call the initializer. */ | |
177 | walk_data->initialize_block_local_data (walk_data, bb, | |
178 | recycled); | |
179 | ||
180 | } | |
181 | ||
182 | /* Callback for operations to execute before we have walked the | |
183 | dominator children, but before we walk statements. */ | |
ccf5c864 PB |
184 | if (walk_data->before_dom_children) |
185 | (*walk_data->before_dom_children) (walk_data, bb); | |
df648b94 JH |
186 | |
187 | /* Mark the current BB to be popped out of the recursion stack | |
fa10beec | 188 | once children are processed. */ |
df648b94 JH |
189 | worklist[sp++] = bb; |
190 | worklist[sp++] = NULL; | |
191 | ||
192 | for (dest = first_dom_son (walk_data->dom_direction, bb); | |
193 | dest; dest = next_dom_son (walk_data->dom_direction, dest)) | |
194 | worklist[sp++] = dest; | |
6de9cd9a | 195 | } |
ccf5c864 | 196 | /* NULL is used to mark pop operations in the recursion stack. */ |
df648b94 | 197 | while (sp > 0 && !worklist[sp - 1]) |
6de9cd9a | 198 | { |
df648b94 JH |
199 | --sp; |
200 | bb = worklist[--sp]; | |
df648b94 JH |
201 | |
202 | /* Callback for operations to execute after we have walked the | |
ccf5c864 PB |
203 | dominator children, but before we walk statements. */ |
204 | if (walk_data->after_dom_children) | |
205 | (*walk_data->after_dom_children) (walk_data, bb); | |
df648b94 JH |
206 | |
207 | if (walk_data->initialize_block_local_data) | |
208 | { | |
209 | /* And finally pop the record off the block local data stack. */ | |
210 | bd = VEC_pop (void_p, walk_data->block_data_stack); | |
211 | /* And save the block data so that we can re-use it. */ | |
212 | VEC_safe_push (void_p, heap, walk_data->free_block_data, bd); | |
213 | } | |
6de9cd9a | 214 | } |
df648b94 JH |
215 | if (sp) |
216 | bb = worklist[--sp]; | |
6de9cd9a | 217 | else |
df648b94 | 218 | break; |
6de9cd9a | 219 | } |
df648b94 | 220 | free (worklist); |
6de9cd9a DN |
221 | } |
222 | ||
223 | void | |
224 | init_walk_dominator_tree (struct dom_walk_data *walk_data) | |
225 | { | |
ea497bb8 KH |
226 | walk_data->free_block_data = NULL; |
227 | walk_data->block_data_stack = NULL; | |
6de9cd9a DN |
228 | } |
229 | ||
230 | void | |
231 | fini_walk_dominator_tree (struct dom_walk_data *walk_data) | |
232 | { | |
233 | if (walk_data->initialize_block_local_data) | |
234 | { | |
ea497bb8 KH |
235 | while (VEC_length (void_p, walk_data->free_block_data) > 0) |
236 | free (VEC_pop (void_p, walk_data->free_block_data)); | |
6de9cd9a | 237 | } |
ea497bb8 KH |
238 | |
239 | VEC_free (void_p, heap, walk_data->free_block_data); | |
240 | VEC_free (void_p, heap, walk_data->block_data_stack); | |
6de9cd9a | 241 | } |