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
6This file is part of GCC.
7
8GCC is free software; you can redistribute it and/or modify
9it under the terms of the GNU General Public License as published by
10the Free Software Foundation; either version 3, or (at your option)
11any later version.
12
13GCC is distributed in the hope that it will be useful,
14but WITHOUT ANY WARRANTY; without even the implied warranty of
15MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
16GNU General Public License for more details.
17
18You should have received a copy of the GNU General Public License
19along 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 "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
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
140void
141walk_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
148  while (true)
149    {
150      /* Don't worry about unreachable blocks.  */
151      if (EDGE_COUNT (bb->preds) > 0
152	  || bb == ENTRY_BLOCK_PTR
153	  || bb == EXIT_BLOCK_PTR)
154	{
155	  /* Callback to initialize the local data structure.  */
156	  if (walk_data->initialize_block_local_data)
157	    {
158	      bool recycled;
159
160	      /* First get some local data, reusing any local data
161		 pointer we may have saved.  */
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.  */
184	  if (walk_data->before_dom_children)
185	    (*walk_data->before_dom_children) (walk_data, bb);
186
187	  /* Mark the current BB to be popped out of the recursion stack
188	     once children are processed.  */
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;
195	}
196      /* NULL is used to mark pop operations in the recursion stack.  */
197      while (sp > 0 && !worklist[sp - 1])
198	{
199	  --sp;
200	  bb = worklist[--sp];
201
202	  /* Callback for operations to execute after we have walked the
203	     dominator children, but before we walk statements.  */
204	  if (walk_data->after_dom_children)
205	    (*walk_data->after_dom_children) (walk_data, bb);
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	    }
214	}
215      if (sp)
216	bb = worklist[--sp];
217      else
218	break;
219    }
220  free (worklist);
221}
222
223void
224init_walk_dominator_tree (struct dom_walk_data *walk_data)
225{
226  walk_data->free_block_data = NULL;
227  walk_data->block_data_stack = NULL;
228}
229
230void
231fini_walk_dominator_tree (struct dom_walk_data *walk_data)
232{
233  if (walk_data->initialize_block_local_data)
234    {
235      while (VEC_length (void_p, walk_data->free_block_data) > 0)
236	free (VEC_pop (void_p, walk_data->free_block_data));
237    }
238
239  VEC_free (void_p, heap, walk_data->free_block_data);
240  VEC_free (void_p, heap, walk_data->block_data_stack);
241}
242