* config/i386/i386.c (ix86_adjust_stack_and_probe_stack_clash): New.
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1 /* Generic dominator tree walker
2 Copyright (C) 2003-2017 Free Software Foundation, Inc.
3 Contributed by Diego Novillo <dnovillo@redhat.com>
5 This file is part of GCC.
7 GCC is free software; you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation; either version 3, or (at your option)
10 any later version.
12 GCC is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with GCC; see the file COPYING3. If not see
19 <http://www.gnu.org/licenses/>. */
21 #include "config.h"
22 #include "system.h"
23 #include "coretypes.h"
24 #include "backend.h"
25 #include "cfganal.h"
26 #include "domwalk.h"
27 #include "dumpfile.h"
29 /* This file implements a generic walker for dominator trees.
31 To understand the dominator walker one must first have a grasp of dominators,
32 immediate dominators and the dominator tree.
34 Dominators
35 A block B1 is said to dominate B2 if every path from the entry to B2 must
36 pass through B1. Given the dominance relationship, we can proceed to
37 compute immediate dominators. Note it is not important whether or not
38 our definition allows a block to dominate itself.
40 Immediate Dominators:
41 Every block in the CFG has no more than one immediate dominator. The
42 immediate dominator of block BB must dominate BB and must not dominate
43 any other dominator of BB and must not be BB itself.
45 Dominator tree:
46 If we then construct a tree where each node is a basic block and there
47 is an edge from each block's immediate dominator to the block itself, then
48 we have a dominator tree.
51 [ Note this walker can also walk the post-dominator tree, which is
52 defined in a similar manner. i.e., block B1 is said to post-dominate
53 block B2 if all paths from B2 to the exit block must pass through
54 B1. ]
56 For example, given the CFG
61 / \
62 3 4
63 / \
64 +---------->5 6
65 | / \ /
66 | +--->8 7
67 | | / |
68 | +--9 11
69 | / |
70 +--- 10 ---> 12
73 We have a dominator tree which looks like
78 / \
79 / \
80 3 4
81 / / \ \
82 | | | |
83 5 6 7 12
84 | |
85 8 11
93 The dominator tree is the basis for a number of analysis, transformation
94 and optimization algorithms that operate on a semi-global basis.
96 The dominator walker is a generic routine which visits blocks in the CFG
97 via a depth first search of the dominator tree. In the example above
98 the dominator walker might visit blocks in the following order
99 1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12.
101 The dominator walker has a number of callbacks to perform actions
102 during the walk of the dominator tree. There are two callbacks
103 which walk statements, one before visiting the dominator children,
104 one after visiting the dominator children. There is a callback
105 before and after each statement walk callback. In addition, the
106 dominator walker manages allocation/deallocation of data structures
107 which are local to each block visited.
109 The dominator walker is meant to provide a generic means to build a pass
110 which can analyze or transform/optimize a function based on walking
111 the dominator tree. One simply fills in the dominator walker data
112 structure with the appropriate callbacks and calls the walker.
114 We currently use the dominator walker to prune the set of variables
115 which might need PHI nodes (which can greatly improve compile-time
116 performance in some cases).
118 We also use the dominator walker to rewrite the function into SSA form
119 which reduces code duplication since the rewriting phase is inherently
120 a walk of the dominator tree.
122 And (of course), we use the dominator walker to drive our dominator
123 optimizer, which is a semi-global optimizer.
125 TODO:
127 Walking statements is based on the block statement iterator abstraction,
128 which is currently an abstraction over walking tree statements. Thus
129 the dominator walker is currently only useful for trees. */
131 /* Reverse postorder index of each basic block. */
132 static int *bb_postorder;
134 static int
135 cmp_bb_postorder (const void *a, const void *b)
137 basic_block bb1 = *(const basic_block *)(a);
138 basic_block bb2 = *(const basic_block *)(b);
139 /* Place higher completion number first (pop off lower number first). */
140 return bb_postorder[bb2->index] - bb_postorder[bb1->index];
143 /* Permute array BBS of N basic blocks in postorder,
144 i.e. by descending number in BB_POSTORDER array. */
146 static void
147 sort_bbs_postorder (basic_block *bbs, int n)
149 if (__builtin_expect (n == 2, true))
151 basic_block bb0 = bbs[0], bb1 = bbs[1];
152 if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
153 bbs[0] = bb1, bbs[1] = bb0;
155 else if (__builtin_expect (n == 3, true))
157 basic_block bb0 = bbs[0], bb1 = bbs[1], bb2 = bbs[2];
158 if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
159 std::swap (bb0, bb1);
160 if (bb_postorder[bb1->index] < bb_postorder[bb2->index])
162 std::swap (bb1, bb2);
163 if (bb_postorder[bb0->index] < bb_postorder[bb1->index])
164 std::swap (bb0, bb1);
166 bbs[0] = bb0, bbs[1] = bb1, bbs[2] = bb2;
168 else
169 qsort (bbs, n, sizeof *bbs, cmp_bb_postorder);
172 /* Constructor for a dom walker.
174 If SKIP_UNREACHBLE_BLOCKS is true, then we need to set
175 EDGE_EXECUTABLE on every edge in the CFG. */
176 dom_walker::dom_walker (cdi_direction direction,
177 bool skip_unreachable_blocks)
178 : m_dom_direction (direction),
179 m_skip_unreachable_blocks (skip_unreachable_blocks),
180 m_unreachable_dom (NULL)
182 /* If we are not skipping unreachable blocks, then there is nothing
183 to do. */
184 if (!m_skip_unreachable_blocks)
185 return;
187 basic_block bb;
188 FOR_ALL_BB_FN (bb, cfun)
190 edge_iterator ei;
191 edge e;
192 FOR_EACH_EDGE (e, ei, bb->succs)
193 e->flags |= EDGE_EXECUTABLE;
197 /* Return TRUE if BB is reachable, false otherwise. */
199 bool
200 dom_walker::bb_reachable (struct function *fun, basic_block bb)
202 /* If we're not skipping unreachable blocks, then assume everything
203 is reachable. */
204 if (!m_skip_unreachable_blocks)
205 return true;
207 /* If any of the predecessor edges that do not come from blocks dominated
208 by us are still marked as possibly executable consider this block
209 reachable. */
210 bool reachable = false;
211 if (!m_unreachable_dom)
213 reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun);
214 edge_iterator ei;
215 edge e;
216 FOR_EACH_EDGE (e, ei, bb->preds)
217 if (!dominated_by_p (CDI_DOMINATORS, e->src, bb))
218 reachable |= (e->flags & EDGE_EXECUTABLE);
221 return reachable;
224 /* BB has been determined to be unreachable. Propagate that property
225 to incoming and outgoing edges of BB as appropriate. */
227 void
228 dom_walker::propagate_unreachable_to_edges (basic_block bb,
229 FILE *dump_file,
230 dump_flags_t dump_flags)
232 if (dump_file && (dump_flags & TDF_DETAILS))
233 fprintf (dump_file, "Marking all outgoing edges of unreachable "
234 "BB %d as not executable\n", bb->index);
236 edge_iterator ei;
237 edge e;
238 FOR_EACH_EDGE (e, ei, bb->succs)
239 e->flags &= ~EDGE_EXECUTABLE;
241 FOR_EACH_EDGE (e, ei, bb->preds)
243 if (dominated_by_p (CDI_DOMINATORS, e->src, bb))
245 if (dump_file && (dump_flags & TDF_DETAILS))
246 fprintf (dump_file, "Marking backedge from BB %d into "
247 "unreachable BB %d as not executable\n",
248 e->src->index, bb->index);
249 e->flags &= ~EDGE_EXECUTABLE;
253 if (!m_unreachable_dom)
254 m_unreachable_dom = bb;
257 /* Recursively walk the dominator tree.
258 BB is the basic block we are currently visiting. */
260 void
261 dom_walker::walk (basic_block bb)
263 basic_block dest;
264 basic_block *worklist = XNEWVEC (basic_block,
265 n_basic_blocks_for_fn (cfun) * 2);
266 int sp = 0;
267 int *postorder, postorder_num;
269 if (m_dom_direction == CDI_DOMINATORS)
271 postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
272 postorder_num = pre_and_rev_post_order_compute (NULL, postorder, true);
273 bb_postorder = XNEWVEC (int, last_basic_block_for_fn (cfun));
274 for (int i = 0; i < postorder_num; ++i)
275 bb_postorder[postorder[i]] = i;
276 free (postorder);
279 while (true)
281 /* Don't worry about unreachable blocks. */
282 if (EDGE_COUNT (bb->preds) > 0
283 || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun)
284 || bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
287 /* Callback for subclasses to do custom things before we have walked
288 the dominator children, but before we walk statements. */
289 if (this->bb_reachable (cfun, bb))
291 edge taken_edge = before_dom_children (bb);
292 if (taken_edge)
294 edge_iterator ei;
295 edge e;
296 FOR_EACH_EDGE (e, ei, bb->succs)
297 if (e != taken_edge)
298 e->flags &= ~EDGE_EXECUTABLE;
301 else
302 propagate_unreachable_to_edges (bb, dump_file, dump_flags);
304 /* Mark the current BB to be popped out of the recursion stack
305 once children are processed. */
306 worklist[sp++] = bb;
307 worklist[sp++] = NULL;
309 int saved_sp = sp;
310 for (dest = first_dom_son (m_dom_direction, bb);
311 dest; dest = next_dom_son (m_dom_direction, dest))
312 worklist[sp++] = dest;
313 if (sp - saved_sp > 1 && m_dom_direction == CDI_DOMINATORS)
314 sort_bbs_postorder (&worklist[saved_sp], sp - saved_sp);
316 /* NULL is used to mark pop operations in the recursion stack. */
317 while (sp > 0 && !worklist[sp - 1])
319 --sp;
320 bb = worklist[--sp];
322 /* Callback allowing subclasses to do custom things after we have
323 walked dominator children, but before we walk statements. */
324 if (bb_reachable (cfun, bb))
325 after_dom_children (bb);
326 else if (m_unreachable_dom == bb)
327 m_unreachable_dom = NULL;
329 if (sp)
330 bb = worklist[--sp];
331 else
332 break;
334 if (m_dom_direction == CDI_DOMINATORS)
336 free (bb_postorder);
337 bb_postorder = NULL;
339 free (worklist);