1/* Generic dominator tree walker
2   Copyright (C) 2003, 2004, 2005 Free Software Foundation, Inc.
3   Contributed by Diego Novillo <dnovillo@redhat.com>
4
5This file is part of GCC.
6
7GCC is free software; you can redistribute it and/or modify
8it under the terms of the GNU General Public License as published by
9the Free Software Foundation; either version 2, or (at your option)
10any later version.
11
12GCC is distributed in the hope that it will be useful,
13but WITHOUT ANY WARRANTY; without even the implied warranty of
14MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
15GNU General Public License for more details.
16
17You should have received a copy of the GNU General Public License
18along with GCC; see the file COPYING.  If not, write to
19the Free Software Foundation, 51 Franklin Street, Fifth Floor,
20Boston, 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
142void
143walk_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
150  /* If block BB is not interesting to the caller, then none of the
151     callbacks that walk the statements in BB are going to be
152     executed.  */
153  is_interesting = bb->index < 0
154		   || walk_data->interesting_blocks == NULL
155		   || TEST_BIT (walk_data->interesting_blocks, bb->index);
156
157  /* Callback to initialize the local data structure.  */
158  if (walk_data->initialize_block_local_data)
159    {
160      bool recycled;
161
162      /* First get some local data, reusing any local data pointer we may
163	 have saved.  */
164      if (VEC_length (void_p, walk_data->free_block_data) > 0)
165	{
166	  bd = VEC_pop (void_p, walk_data->free_block_data);
167	  recycled = 1;
168	}
169      else
170	{
171	  bd = xcalloc (1, walk_data->block_local_data_size);
172	  recycled = 0;
173	}
174
175      /* Push the local data into the local data stack.  */
176      VEC_safe_push (void_p, heap, walk_data->block_data_stack, bd);
177
178      /* Call the initializer.  */
179      walk_data->initialize_block_local_data (walk_data, bb, recycled);
180
181    }
182
183  /* Callback for operations to execute before we have walked the
184     dominator children, but before we walk statements.  */
185  if (walk_data->before_dom_children_before_stmts)
186    (*walk_data->before_dom_children_before_stmts) (walk_data, bb);
187
188  /* Statement walk before walking dominator children.  */
189  if (is_interesting && walk_data->before_dom_children_walk_stmts)
190    {
191      if (walk_data->walk_stmts_backward)
192	for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
193	  (*walk_data->before_dom_children_walk_stmts) (walk_data, bb, bsi);
194      else
195	for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
196	  (*walk_data->before_dom_children_walk_stmts) (walk_data, bb, bsi);
197    }
198
199  /* Callback for operations to execute before we have walked the
200     dominator children, and after we walk statements.  */
201  if (walk_data->before_dom_children_after_stmts)
202    (*walk_data->before_dom_children_after_stmts) (walk_data, bb);
203
204  /* Recursively call ourselves on the dominator children of BB.  */
205  for (dest = first_dom_son (walk_data->dom_direction, bb);
206       dest;
207       dest = next_dom_son (walk_data->dom_direction, dest))
208    {
209      /* The destination block may have become unreachable, in
210	 which case there's no point in optimizing it.  */
211      if (EDGE_COUNT (dest->preds) > 0)
212	walk_dominator_tree (walk_data, dest);
213    }
214
215  /* Callback for operations to execute after we have walked the
216     dominator children, but before we walk statements.  */
217  if (walk_data->after_dom_children_before_stmts)
218    (*walk_data->after_dom_children_before_stmts) (walk_data, bb);
219
220  /* Statement walk after walking dominator children.  */
221  if (is_interesting && walk_data->after_dom_children_walk_stmts)
222    {
223      if (walk_data->walk_stmts_backward)
224	for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
225	  (*walk_data->after_dom_children_walk_stmts) (walk_data, bb, bsi);
226      else
227	for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
228	  (*walk_data->after_dom_children_walk_stmts) (walk_data, bb, bsi);
229    }
230
231  /* Callback for operations to execute after we have walked the
232     dominator children and after we have walked statements.  */
233  if (walk_data->after_dom_children_after_stmts)
234    (*walk_data->after_dom_children_after_stmts) (walk_data, bb);
235
236  if (walk_data->initialize_block_local_data)
237    {
238      /* And save the block data so that we can re-use it.  */
239      VEC_safe_push (void_p, heap, walk_data->free_block_data, bd);
240
241      /* And finally pop the record off the block local data stack.  */
242      VEC_pop (void_p, walk_data->block_data_stack);
243    }
244}
245
246void
247init_walk_dominator_tree (struct dom_walk_data *walk_data)
248{
249  walk_data->free_block_data = NULL;
250  walk_data->block_data_stack = NULL;
251}
252
253void
254fini_walk_dominator_tree (struct dom_walk_data *walk_data)
255{
256  if (walk_data->initialize_block_local_data)
257    {
258      while (VEC_length (void_p, walk_data->free_block_data) > 0)
259	free (VEC_pop (void_p, walk_data->free_block_data));
260    }
261
262  VEC_free (void_p, heap, walk_data->free_block_data);
263  VEC_free (void_p, heap, walk_data->block_data_stack);
264}
265