1 /* Dead store elimination
2 Copyright (C) 2004-2013 Free Software Foundation, Inc.
3
4 This file is part of GCC.
5
6 GCC is free software; you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 3, or (at your option)
9 any later version.
10
11 GCC is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
15
16 You should have received a copy of the GNU General Public License
17 along with GCC; see the file COPYING3. If not see
18 <http://www.gnu.org/licenses/>. */
19
20 #include "config.h"
21 #include "system.h"
22 #include "coretypes.h"
23 #include "tm.h"
24 #include "ggc.h"
25 #include "tree.h"
26 #include "tm_p.h"
27 #include "basic-block.h"
28 #include "gimple-pretty-print.h"
29 #include "tree-flow.h"
30 #include "tree-pass.h"
31 #include "domwalk.h"
32 #include "flags.h"
33 #include "langhooks.h"
34
35 /* This file implements dead store elimination.
36
37 A dead store is a store into a memory location which will later be
38 overwritten by another store without any intervening loads. In this
39 case the earlier store can be deleted.
40
41 In our SSA + virtual operand world we use immediate uses of virtual
42 operands to detect dead stores. If a store's virtual definition
43 is used precisely once by a later store to the same location which
44 post dominates the first store, then the first store is dead.
45
46 The single use of the store's virtual definition ensures that
47 there are no intervening aliased loads and the requirement that
48 the second load post dominate the first ensures that if the earlier
49 store executes, then the later stores will execute before the function
50 exits.
51
52 It may help to think of this as first moving the earlier store to
53 the point immediately before the later store. Again, the single
54 use of the virtual definition and the post-dominance relationship
55 ensure that such movement would be safe. Clearly if there are
56 back to back stores, then the second is redundant.
57
58 Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"
59 may also help in understanding this code since it discusses the
60 relationship between dead store and redundant load elimination. In
61 fact, they are the same transformation applied to different views of
62 the CFG. */
63
64
65 /* Bitmap of blocks that have had EH statements cleaned. We should
66 remove their dead edges eventually. */
67 static bitmap need_eh_cleanup;
68
69 static bool gate_dse (void);
70 static unsigned int tree_ssa_dse (void);
71 static void dse_enter_block (struct dom_walk_data *, basic_block);
72
73
74 /* A helper of dse_optimize_stmt.
75 Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that
76 may prove STMT to be dead.
77 Return TRUE if the above conditions are met, otherwise FALSE. */
78
79 static bool
dse_possible_dead_store_p(gimple stmt,gimple * use_stmt)80 dse_possible_dead_store_p (gimple stmt, gimple *use_stmt)
81 {
82 gimple temp;
83 unsigned cnt = 0;
84
85 *use_stmt = NULL;
86
87 /* Find the first dominated statement that clobbers (part of) the
88 memory stmt stores to with no intermediate statement that may use
89 part of the memory stmt stores. That is, find a store that may
90 prove stmt to be a dead store. */
91 temp = stmt;
92 do
93 {
94 gimple use_stmt, defvar_def;
95 imm_use_iterator ui;
96 bool fail = false;
97 tree defvar;
98
99 /* Limit stmt walking to be linear in the number of possibly
100 dead stores. */
101 if (++cnt > 256)
102 return false;
103
104 if (gimple_code (temp) == GIMPLE_PHI)
105 defvar = PHI_RESULT (temp);
106 else
107 defvar = gimple_vdef (temp);
108 defvar_def = temp;
109 temp = NULL;
110 FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
111 {
112 cnt++;
113
114 /* If we ever reach our DSE candidate stmt again fail. We
115 cannot handle dead stores in loops. */
116 if (use_stmt == stmt)
117 {
118 fail = true;
119 BREAK_FROM_IMM_USE_STMT (ui);
120 }
121 /* In simple cases we can look through PHI nodes, but we
122 have to be careful with loops and with memory references
123 containing operands that are also operands of PHI nodes.
124 See gcc.c-torture/execute/20051110-*.c. */
125 else if (gimple_code (use_stmt) == GIMPLE_PHI)
126 {
127 if (temp
128 /* Make sure we are not in a loop latch block. */
129 || gimple_bb (stmt) == gimple_bb (use_stmt)
130 || dominated_by_p (CDI_DOMINATORS,
131 gimple_bb (stmt), gimple_bb (use_stmt))
132 /* We can look through PHIs to regions post-dominating
133 the DSE candidate stmt. */
134 || !dominated_by_p (CDI_POST_DOMINATORS,
135 gimple_bb (stmt), gimple_bb (use_stmt)))
136 {
137 fail = true;
138 BREAK_FROM_IMM_USE_STMT (ui);
139 }
140 /* Do not consider the PHI as use if it dominates the
141 stmt defining the virtual operand we are processing,
142 we have processed it already in this case. */
143 if (gimple_bb (defvar_def) != gimple_bb (use_stmt)
144 && !dominated_by_p (CDI_DOMINATORS,
145 gimple_bb (defvar_def),
146 gimple_bb (use_stmt)))
147 temp = use_stmt;
148 }
149 /* If the statement is a use the store is not dead. */
150 else if (ref_maybe_used_by_stmt_p (use_stmt,
151 gimple_assign_lhs (stmt)))
152 {
153 fail = true;
154 BREAK_FROM_IMM_USE_STMT (ui);
155 }
156 /* If this is a store, remember it or bail out if we have
157 multiple ones (the will be in different CFG parts then). */
158 else if (gimple_vdef (use_stmt))
159 {
160 if (temp)
161 {
162 fail = true;
163 BREAK_FROM_IMM_USE_STMT (ui);
164 }
165 temp = use_stmt;
166 }
167 }
168
169 if (fail)
170 return false;
171
172 /* If we didn't find any definition this means the store is dead
173 if it isn't a store to global reachable memory. In this case
174 just pretend the stmt makes itself dead. Otherwise fail. */
175 if (!temp)
176 {
177 if (stmt_may_clobber_global_p (stmt))
178 return false;
179
180 temp = stmt;
181 break;
182 }
183 }
184 /* We deliberately stop on clobbering statements and not only on
185 killing ones to make walking cheaper. Otherwise we can just
186 continue walking until both stores have equal reference trees. */
187 while (!stmt_may_clobber_ref_p (temp, gimple_assign_lhs (stmt)));
188
189 *use_stmt = temp;
190
191 return true;
192 }
193
194
195 /* Attempt to eliminate dead stores in the statement referenced by BSI.
196
197 A dead store is a store into a memory location which will later be
198 overwritten by another store without any intervening loads. In this
199 case the earlier store can be deleted.
200
201 In our SSA + virtual operand world we use immediate uses of virtual
202 operands to detect dead stores. If a store's virtual definition
203 is used precisely once by a later store to the same location which
204 post dominates the first store, then the first store is dead. */
205
206 static void
dse_optimize_stmt(gimple_stmt_iterator * gsi)207 dse_optimize_stmt (gimple_stmt_iterator *gsi)
208 {
209 gimple stmt = gsi_stmt (*gsi);
210
211 /* If this statement has no virtual defs, then there is nothing
212 to do. */
213 if (!gimple_vdef (stmt))
214 return;
215
216 /* We know we have virtual definitions. If this is a GIMPLE_ASSIGN
217 that's not also a function call, then record it into our table. */
218 if (is_gimple_call (stmt) && gimple_call_fndecl (stmt))
219 return;
220
221 if (gimple_has_volatile_ops (stmt))
222 return;
223
224 if (is_gimple_assign (stmt))
225 {
226 gimple use_stmt;
227
228 if (!dse_possible_dead_store_p (stmt, &use_stmt))
229 return;
230
231 /* If we have precisely one immediate use at this point and the
232 stores are to the same memory location or there is a chain of
233 virtual uses from stmt and the stmt which stores to that same
234 memory location, then we may have found redundant store. */
235 if ((gimple_has_lhs (use_stmt)
236 && (operand_equal_p (gimple_assign_lhs (stmt),
237 gimple_get_lhs (use_stmt), 0)))
238 || stmt_kills_ref_p (use_stmt, gimple_assign_lhs (stmt)))
239 {
240 basic_block bb;
241
242 /* If use_stmt is or might be a nop assignment, e.g. for
243 struct { ... } S a, b, *p; ...
244 b = a; b = b;
245 or
246 b = a; b = *p; where p might be &b,
247 or
248 *p = a; *p = b; where p might be &b,
249 or
250 *p = *u; *p = *v; where p might be v, then USE_STMT
251 acts as a use as well as definition, so store in STMT
252 is not dead. */
253 if (stmt != use_stmt
254 && ref_maybe_used_by_stmt_p (use_stmt, gimple_assign_lhs (stmt)))
255 return;
256
257 if (dump_file && (dump_flags & TDF_DETAILS))
258 {
259 fprintf (dump_file, " Deleted dead store '");
260 print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags, 0);
261 fprintf (dump_file, "'\n");
262 }
263
264 /* Then we need to fix the operand of the consuming stmt. */
265 unlink_stmt_vdef (stmt);
266
267 /* Remove the dead store. */
268 bb = gimple_bb (stmt);
269 if (gsi_remove (gsi, true))
270 bitmap_set_bit (need_eh_cleanup, bb->index);
271
272 /* And release any SSA_NAMEs set in this statement back to the
273 SSA_NAME manager. */
274 release_defs (stmt);
275 }
276 }
277 }
278
279 static void
dse_enter_block(struct dom_walk_data * walk_data ATTRIBUTE_UNUSED,basic_block bb)280 dse_enter_block (struct dom_walk_data *walk_data ATTRIBUTE_UNUSED,
281 basic_block bb)
282 {
283 gimple_stmt_iterator gsi;
284
285 for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
286 {
287 dse_optimize_stmt (&gsi);
288 if (gsi_end_p (gsi))
289 gsi = gsi_last_bb (bb);
290 else
291 gsi_prev (&gsi);
292 }
293 }
294
295 /* Main entry point. */
296
297 static unsigned int
tree_ssa_dse(void)298 tree_ssa_dse (void)
299 {
300 struct dom_walk_data walk_data;
301
302 need_eh_cleanup = BITMAP_ALLOC (NULL);
303
304 renumber_gimple_stmt_uids ();
305
306 /* We might consider making this a property of each pass so that it
307 can be [re]computed on an as-needed basis. Particularly since
308 this pass could be seen as an extension of DCE which needs post
309 dominators. */
310 calculate_dominance_info (CDI_POST_DOMINATORS);
311 calculate_dominance_info (CDI_DOMINATORS);
312
313 /* Dead store elimination is fundamentally a walk of the post-dominator
314 tree and a backwards walk of statements within each block. */
315 walk_data.dom_direction = CDI_POST_DOMINATORS;
316 walk_data.initialize_block_local_data = NULL;
317 walk_data.before_dom_children = dse_enter_block;
318 walk_data.after_dom_children = NULL;
319
320 walk_data.block_local_data_size = 0;
321 walk_data.global_data = NULL;
322
323 /* Initialize the dominator walker. */
324 init_walk_dominator_tree (&walk_data);
325
326 /* Recursively walk the dominator tree. */
327 walk_dominator_tree (&walk_data, EXIT_BLOCK_PTR);
328
329 /* Finalize the dominator walker. */
330 fini_walk_dominator_tree (&walk_data);
331
332 /* Removal of stores may make some EH edges dead. Purge such edges from
333 the CFG as needed. */
334 if (!bitmap_empty_p (need_eh_cleanup))
335 {
336 gimple_purge_all_dead_eh_edges (need_eh_cleanup);
337 cleanup_tree_cfg ();
338 }
339
340 BITMAP_FREE (need_eh_cleanup);
341
342 /* For now, just wipe the post-dominator information. */
343 free_dominance_info (CDI_POST_DOMINATORS);
344 return 0;
345 }
346
347 static bool
gate_dse(void)348 gate_dse (void)
349 {
350 return flag_tree_dse != 0;
351 }
352
353 struct gimple_opt_pass pass_dse =
354 {
355 {
356 GIMPLE_PASS,
357 "dse", /* name */
358 OPTGROUP_NONE, /* optinfo_flags */
359 gate_dse, /* gate */
360 tree_ssa_dse, /* execute */
361 NULL, /* sub */
362 NULL, /* next */
363 0, /* static_pass_number */
364 TV_TREE_DSE, /* tv_id */
365 PROP_cfg | PROP_ssa, /* properties_required */
366 0, /* properties_provided */
367 0, /* properties_destroyed */
368 0, /* todo_flags_start */
369 TODO_ggc_collect
370 | TODO_verify_ssa /* todo_flags_finish */
371 }
372 };
373