1 /* Dead store elimination
2 Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010
3 Free Software Foundation, Inc.
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)
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/>. */
23 #include "coretypes.h"
28 #include "basic-block.h"
30 #include "gimple-pretty-print.h"
31 #include "tree-flow.h"
32 #include "tree-pass.h"
33 #include "tree-dump.h"
36 #include "langhooks.h"
38 /* This file implements dead store elimination.
40 A dead store is a store into a memory location which will later be
41 overwritten by another store without any intervening loads. In this
42 case the earlier store can be deleted.
44 In our SSA + virtual operand world we use immediate uses of virtual
45 operands to detect dead stores. If a store's virtual definition
46 is used precisely once by a later store to the same location which
47 post dominates the first store, then the first store is dead.
49 The single use of the store's virtual definition ensures that
50 there are no intervening aliased loads and the requirement that
51 the second load post dominate the first ensures that if the earlier
52 store executes, then the later stores will execute before the function
55 It may help to think of this as first moving the earlier store to
56 the point immediately before the later store. Again, the single
57 use of the virtual definition and the post-dominance relationship
58 ensure that such movement would be safe. Clearly if there are
59 back to back stores, then the second is redundant.
61 Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"
62 may also help in understanding this code since it discusses the
63 relationship between dead store and redundant load elimination. In
64 fact, they are the same transformation applied to different views of
68 /* Bitmap of blocks that have had EH statements cleaned. We should
69 remove their dead edges eventually. */
70 static bitmap need_eh_cleanup
;
72 static bool gate_dse (void);
73 static unsigned int tree_ssa_dse (void);
74 static void dse_enter_block (struct dom_walk_data
*, basic_block
);
77 /* A helper of dse_optimize_stmt.
78 Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that
79 may prove STMT to be dead.
80 Return TRUE if the above conditions are met, otherwise FALSE. */
83 dse_possible_dead_store_p (gimple stmt
, gimple
*use_stmt
)
90 /* Find the first dominated statement that clobbers (part of) the
91 memory stmt stores to with no intermediate statement that may use
92 part of the memory stmt stores. That is, find a store that may
93 prove stmt to be a dead store. */
102 /* Limit stmt walking to be linear in the number of possibly
107 if (gimple_code (temp
) == GIMPLE_PHI
)
108 defvar
= PHI_RESULT (temp
);
110 defvar
= gimple_vdef (temp
);
112 FOR_EACH_IMM_USE_STMT (use_stmt
, ui
, defvar
)
116 /* If we ever reach our DSE candidate stmt again fail. We
117 cannot handle dead stores in loops. */
118 if (use_stmt
== stmt
)
121 BREAK_FROM_IMM_USE_STMT (ui
);
123 /* In simple cases we can look through PHI nodes, but we
124 have to be careful with loops and with memory references
125 containing operands that are also operands of PHI nodes.
126 See gcc.c-torture/execute/20051110-*.c. */
127 else if (gimple_code (use_stmt
) == GIMPLE_PHI
)
130 /* Make sure we are not in a loop latch block. */
131 || gimple_bb (stmt
) == gimple_bb (use_stmt
)
132 || dominated_by_p (CDI_DOMINATORS
,
133 gimple_bb (stmt
), gimple_bb (use_stmt
))
134 /* We can look through PHIs to regions post-dominating
135 the DSE candidate stmt. */
136 || !dominated_by_p (CDI_POST_DOMINATORS
,
137 gimple_bb (stmt
), gimple_bb (use_stmt
)))
140 BREAK_FROM_IMM_USE_STMT (ui
);
144 /* If the statement is a use the store is not dead. */
145 else if (ref_maybe_used_by_stmt_p (use_stmt
,
146 gimple_assign_lhs (stmt
)))
149 BREAK_FROM_IMM_USE_STMT (ui
);
151 /* If this is a store, remember it or bail out if we have
152 multiple ones (the will be in different CFG parts then). */
153 else if (gimple_vdef (use_stmt
))
158 BREAK_FROM_IMM_USE_STMT (ui
);
167 /* If we didn't find any definition this means the store is dead
168 if it isn't a store to global reachable memory. In this case
169 just pretend the stmt makes itself dead. Otherwise fail. */
172 if (stmt_may_clobber_global_p (stmt
))
179 /* We deliberately stop on clobbering statements and not only on
180 killing ones to make walking cheaper. Otherwise we can just
181 continue walking until both stores have equal reference trees. */
182 while (!stmt_may_clobber_ref_p (temp
, gimple_assign_lhs (stmt
)));
190 /* Attempt to eliminate dead stores in the statement referenced by BSI.
192 A dead store is a store into a memory location which will later be
193 overwritten by another store without any intervening loads. In this
194 case the earlier store can be deleted.
196 In our SSA + virtual operand world we use immediate uses of virtual
197 operands to detect dead stores. If a store's virtual definition
198 is used precisely once by a later store to the same location which
199 post dominates the first store, then the first store is dead. */
202 dse_optimize_stmt (gimple_stmt_iterator
*gsi
)
204 gimple stmt
= gsi_stmt (*gsi
);
206 /* If this statement has no virtual defs, then there is nothing
208 if (!gimple_vdef (stmt
))
211 /* We know we have virtual definitions. If this is a GIMPLE_ASSIGN
212 that's not also a function call, then record it into our table. */
213 if (is_gimple_call (stmt
) && gimple_call_fndecl (stmt
))
216 if (gimple_has_volatile_ops (stmt
))
219 if (is_gimple_assign (stmt
))
223 if (!dse_possible_dead_store_p (stmt
, &use_stmt
))
226 /* If we have precisely one immediate use at this point and the
227 stores are to the same memory location or there is a chain of
228 virtual uses from stmt and the stmt which stores to that same
229 memory location, then we may have found redundant store. */
230 if ((gimple_has_lhs (use_stmt
)
231 && (operand_equal_p (gimple_assign_lhs (stmt
),
232 gimple_get_lhs (use_stmt
), 0)))
233 || stmt_kills_ref_p (use_stmt
, gimple_assign_lhs (stmt
)))
237 /* If use_stmt is or might be a nop assignment, e.g. for
238 struct { ... } S a, b, *p; ...
241 b = a; b = *p; where p might be &b,
243 *p = a; *p = b; where p might be &b,
245 *p = *u; *p = *v; where p might be v, then USE_STMT
246 acts as a use as well as definition, so store in STMT
249 && ref_maybe_used_by_stmt_p (use_stmt
, gimple_assign_lhs (stmt
)))
252 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
254 fprintf (dump_file
, " Deleted dead store '");
255 print_gimple_stmt (dump_file
, gsi_stmt (*gsi
), dump_flags
, 0);
256 fprintf (dump_file
, "'\n");
259 /* Then we need to fix the operand of the consuming stmt. */
260 unlink_stmt_vdef (stmt
);
262 /* Remove the dead store. */
263 bb
= gimple_bb (stmt
);
264 if (gsi_remove (gsi
, true))
265 bitmap_set_bit (need_eh_cleanup
, bb
->index
);
267 /* And release any SSA_NAMEs set in this statement back to the
275 dse_enter_block (struct dom_walk_data
*walk_data ATTRIBUTE_UNUSED
,
278 gimple_stmt_iterator gsi
;
280 for (gsi
= gsi_last_bb (bb
); !gsi_end_p (gsi
);)
282 dse_optimize_stmt (&gsi
);
284 gsi
= gsi_last_bb (bb
);
290 /* Main entry point. */
295 struct dom_walk_data walk_data
;
297 need_eh_cleanup
= BITMAP_ALLOC (NULL
);
299 renumber_gimple_stmt_uids ();
301 /* We might consider making this a property of each pass so that it
302 can be [re]computed on an as-needed basis. Particularly since
303 this pass could be seen as an extension of DCE which needs post
305 calculate_dominance_info (CDI_POST_DOMINATORS
);
306 calculate_dominance_info (CDI_DOMINATORS
);
308 /* Dead store elimination is fundamentally a walk of the post-dominator
309 tree and a backwards walk of statements within each block. */
310 walk_data
.dom_direction
= CDI_POST_DOMINATORS
;
311 walk_data
.initialize_block_local_data
= NULL
;
312 walk_data
.before_dom_children
= dse_enter_block
;
313 walk_data
.after_dom_children
= NULL
;
315 walk_data
.block_local_data_size
= 0;
316 walk_data
.global_data
= NULL
;
318 /* Initialize the dominator walker. */
319 init_walk_dominator_tree (&walk_data
);
321 /* Recursively walk the dominator tree. */
322 walk_dominator_tree (&walk_data
, EXIT_BLOCK_PTR
);
324 /* Finalize the dominator walker. */
325 fini_walk_dominator_tree (&walk_data
);
327 /* Removal of stores may make some EH edges dead. Purge such edges from
328 the CFG as needed. */
329 if (!bitmap_empty_p (need_eh_cleanup
))
331 gimple_purge_all_dead_eh_edges (need_eh_cleanup
);
335 BITMAP_FREE (need_eh_cleanup
);
337 /* For now, just wipe the post-dominator information. */
338 free_dominance_info (CDI_POST_DOMINATORS
);
345 return flag_tree_dse
!= 0;
348 struct gimple_opt_pass pass_dse
=
354 tree_ssa_dse
, /* execute */
357 0, /* static_pass_number */
358 TV_TREE_DSE
, /* tv_id */
359 PROP_cfg
| PROP_ssa
, /* properties_required */
360 0, /* properties_provided */
361 0, /* properties_destroyed */
362 0, /* todo_flags_start */
364 | TODO_verify_ssa
/* todo_flags_finish */