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  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
270void
271init_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
277void
278fini_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