summaryrefslogtreecommitdiff
path: root/gcc/tree-ssa-dce.c
blob: 0c2057104251ddee8bcf7e985c70d932f60938f6 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
/* Dead code elimination pass for the GNU compiler.
   Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008
   Free Software Foundation, Inc.
   Contributed by Ben Elliston <bje@redhat.com>
   and Andrew MacLeod <amacleod@redhat.com>
   Adapted to use control dependence by Steven Bosscher, SUSE Labs.
 
This file is part of GCC.
   
GCC is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 3, or (at your option) any
later version.
   
GCC is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.
   
You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING3.  If not see
<http://www.gnu.org/licenses/>.  */

/* Dead code elimination.

   References:

     Building an Optimizing Compiler,
     Robert Morgan, Butterworth-Heinemann, 1998, Section 8.9.

     Advanced Compiler Design and Implementation,
     Steven Muchnick, Morgan Kaufmann, 1997, Section 18.10.

   Dead-code elimination is the removal of statements which have no
   impact on the program's output.  "Dead statements" have no impact
   on the program's output, while "necessary statements" may have
   impact on the output.

   The algorithm consists of three phases:
   1. Marking as necessary all statements known to be necessary,
      e.g. most function calls, writing a value to memory, etc;
   2. Propagating necessary statements, e.g., the statements
      giving values to operands in necessary statements; and
   3. Removing dead statements.  */

#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "ggc.h"

/* These RTL headers are needed for basic-block.h.  */
#include "rtl.h"
#include "tm_p.h"
#include "hard-reg-set.h"
#include "obstack.h"
#include "basic-block.h"

#include "tree.h"
#include "diagnostic.h"
#include "tree-flow.h"
#include "gimple.h"
#include "tree-dump.h"
#include "tree-pass.h"
#include "timevar.h"
#include "flags.h"
#include "cfgloop.h"
#include "tree-scalar-evolution.h"

static struct stmt_stats
{
  int total;
  int total_phis;
  int removed;
  int removed_phis;
} stats;

#define STMT_NECESSARY GF_PLF_1

static VEC(gimple,heap) *worklist;

/* Vector indicating an SSA name has already been processed and marked
   as necessary.  */
static sbitmap processed;

/* Vector indicating that last_stmt if a basic block has already been
   marked as necessary.  */
static sbitmap last_stmt_necessary;

/* Before we can determine whether a control branch is dead, we need to
   compute which blocks are control dependent on which edges.

   We expect each block to be control dependent on very few edges so we
   use a bitmap for each block recording its edges.  An array holds the
   bitmap.  The Ith bit in the bitmap is set if that block is dependent
   on the Ith edge.  */
static bitmap *control_dependence_map;

/* Vector indicating that a basic block has already had all the edges
   processed that it is control dependent on.  */
static sbitmap visited_control_parents;

/* TRUE if this pass alters the CFG (by removing control statements).
   FALSE otherwise.

   If this pass alters the CFG, then it will arrange for the dominators
   to be recomputed.  */
static bool cfg_altered;

/* Execute code that follows the macro for each edge (given number
   EDGE_NUMBER within the CODE) for which the block with index N is
   control dependent.  */
#define EXECUTE_IF_CONTROL_DEPENDENT(BI, N, EDGE_NUMBER)	\
  EXECUTE_IF_SET_IN_BITMAP (control_dependence_map[(N)], 0,	\
			    (EDGE_NUMBER), (BI))


/* Indicate block BB is control dependent on an edge with index EDGE_INDEX.  */
static inline void
set_control_dependence_map_bit (basic_block bb, int edge_index)
{
  if (bb == ENTRY_BLOCK_PTR)
    return;
  gcc_assert (bb != EXIT_BLOCK_PTR);
  bitmap_set_bit (control_dependence_map[bb->index], edge_index);
}

/* Clear all control dependences for block BB.  */
static inline void
clear_control_dependence_bitmap (basic_block bb)
{
  bitmap_clear (control_dependence_map[bb->index]);
}


/* Find the immediate postdominator PDOM of the specified basic block BLOCK.
   This function is necessary because some blocks have negative numbers.  */

static inline basic_block
find_pdom (basic_block block)
{
  gcc_assert (block != ENTRY_BLOCK_PTR);

  if (block == EXIT_BLOCK_PTR)
    return EXIT_BLOCK_PTR;
  else
    {
      basic_block bb = get_immediate_dominator (CDI_POST_DOMINATORS, block);
      if (! bb)
	return EXIT_BLOCK_PTR;
      return bb;
    }
}


/* Determine all blocks' control dependences on the given edge with edge_list
   EL index EDGE_INDEX, ala Morgan, Section 3.6.  */

static void
find_control_dependence (struct edge_list *el, int edge_index)
{
  basic_block current_block;
  basic_block ending_block;

  gcc_assert (INDEX_EDGE_PRED_BB (el, edge_index) != EXIT_BLOCK_PTR);

  if (INDEX_EDGE_PRED_BB (el, edge_index) == ENTRY_BLOCK_PTR)
    ending_block = single_succ (ENTRY_BLOCK_PTR);
  else
    ending_block = find_pdom (INDEX_EDGE_PRED_BB (el, edge_index));

  for (current_block = INDEX_EDGE_SUCC_BB (el, edge_index);
       current_block != ending_block && current_block != EXIT_BLOCK_PTR;
       current_block = find_pdom (current_block))
    {
      edge e = INDEX_EDGE (el, edge_index);

      /* For abnormal edges, we don't make current_block control
	 dependent because instructions that throw are always necessary
	 anyway.  */
      if (e->flags & EDGE_ABNORMAL)
	continue;

      set_control_dependence_map_bit (current_block, edge_index);
    }
}


/* Record all blocks' control dependences on all edges in the edge
   list EL, ala Morgan, Section 3.6.  */

static void
find_all_control_dependences (struct edge_list *el)
{
  int i;

  for (i = 0; i < NUM_EDGES (el); ++i)
    find_control_dependence (el, i);
}

/* If STMT is not already marked necessary, mark it, and add it to the
   worklist if ADD_TO_WORKLIST is true.  */
static inline void
mark_stmt_necessary (gimple stmt, bool add_to_worklist)
{
  gcc_assert (stmt);

  if (gimple_plf (stmt, STMT_NECESSARY))
    return;

  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "Marking useful stmt: ");
      print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
      fprintf (dump_file, "\n");
    }

  gimple_set_plf (stmt, STMT_NECESSARY, true);
  if (add_to_worklist)
    VEC_safe_push (gimple, heap, worklist, stmt);
}


/* Mark the statement defining operand OP as necessary.  */

static inline void
mark_operand_necessary (tree op)
{
  gimple stmt;
  int ver;

  gcc_assert (op);

  ver = SSA_NAME_VERSION (op);
  if (TEST_BIT (processed, ver))
    {
      stmt = SSA_NAME_DEF_STMT (op);
      gcc_assert (gimple_nop_p (stmt)
		  || gimple_plf (stmt, STMT_NECESSARY));
      return;
    }
  SET_BIT (processed, ver);

  stmt = SSA_NAME_DEF_STMT (op);
  gcc_assert (stmt);

  if (gimple_plf (stmt, STMT_NECESSARY) || gimple_nop_p (stmt))
    return;

  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "marking necessary through ");
      print_generic_expr (dump_file, op, 0);
      fprintf (dump_file, " stmt ");
      print_gimple_stmt (dump_file, stmt, 0, 0);
    }

  gimple_set_plf (stmt, STMT_NECESSARY, true);
  VEC_safe_push (gimple, heap, worklist, stmt);
}


/* Mark STMT as necessary if it obviously is.  Add it to the worklist if
   it can make other statements necessary.

   If AGGRESSIVE is false, control statements are conservatively marked as
   necessary.  */

static void
mark_stmt_if_obviously_necessary (gimple stmt, bool aggressive)
{
  tree lhs = NULL_TREE;
  /* With non-call exceptions, we have to assume that all statements could
     throw.  If a statement may throw, it is inherently necessary.  */
  if (flag_non_call_exceptions
      && stmt_could_throw_p (stmt))
    {
      mark_stmt_necessary (stmt, true);
      return;
    }

  /* Statements that are implicitly live.  Most function calls, asm
     and return statements are required.  Labels and GIMPLE_BIND nodes
     are kept because they are control flow, and we have no way of
     knowing whether they can be removed.  DCE can eliminate all the
     other statements in a block, and CFG can then remove the block
     and labels.  */
  switch (gimple_code (stmt))
    {
    case GIMPLE_PREDICT:
    case GIMPLE_LABEL:
      mark_stmt_necessary (stmt, false);
      return;

    case GIMPLE_ASM:
    case GIMPLE_RESX:
    case GIMPLE_RETURN:
    case GIMPLE_CHANGE_DYNAMIC_TYPE:
      mark_stmt_necessary (stmt, true);
      return;

    case GIMPLE_CALL:
      /* Most, but not all function calls are required.  Function calls that
	 produce no result and have no side effects (i.e. const pure
	 functions) are unnecessary.  */
      if (gimple_has_side_effects (stmt))
	{
	  mark_stmt_necessary (stmt, true);
	  return;
	}
      if (!gimple_call_lhs (stmt))
        return;
      lhs = gimple_call_lhs (stmt);
      /* Fall through */

    case GIMPLE_ASSIGN:
      if (!lhs)
        lhs = gimple_assign_lhs (stmt);
      /* These values are mildly magic bits of the EH runtime.  We can't
	 see the entire lifetime of these values until landing pads are
	 generated.  */
      if (TREE_CODE (lhs) == EXC_PTR_EXPR
	  || TREE_CODE (lhs) == FILTER_EXPR)
	{
	  mark_stmt_necessary (stmt, true);
	  return;
	}
      break;

    case GIMPLE_GOTO:
      gcc_assert (!simple_goto_p (stmt));
      mark_stmt_necessary (stmt, true);
      return;

    case GIMPLE_COND:
      gcc_assert (EDGE_COUNT (gimple_bb (stmt)->succs) == 2);
      /* Fall through.  */

    case GIMPLE_SWITCH:
      if (! aggressive)
	mark_stmt_necessary (stmt, true);
      break;

    default:
      break;
    }

  /* If the statement has volatile operands, it needs to be preserved.
     Same for statements that can alter control flow in unpredictable
     ways.  */
  if (gimple_has_volatile_ops (stmt) || is_ctrl_altering_stmt (stmt))
    {
      mark_stmt_necessary (stmt, true);
      return;
    }

  if (is_hidden_global_store (stmt))
    {
      mark_stmt_necessary (stmt, true);
      return;
    }

  return;
}


/* Make corresponding control dependent edges necessary.  We only
   have to do this once for each basic block, so we clear the bitmap
   after we're done.  */
static void
mark_control_dependent_edges_necessary (basic_block bb, struct edge_list *el)
{
  bitmap_iterator bi;
  unsigned edge_number;

  gcc_assert (bb != EXIT_BLOCK_PTR);

  if (bb == ENTRY_BLOCK_PTR)
    return;

  EXECUTE_IF_CONTROL_DEPENDENT (bi, bb->index, edge_number)
    {
      gimple stmt;
      basic_block cd_bb = INDEX_EDGE_PRED_BB (el, edge_number);

      if (TEST_BIT (last_stmt_necessary, cd_bb->index))
	continue;
      SET_BIT (last_stmt_necessary, cd_bb->index);

      stmt = last_stmt (cd_bb);
      if (stmt && is_ctrl_stmt (stmt))
	mark_stmt_necessary (stmt, true);
    }
}


/* Find obviously necessary statements.  These are things like most function
   calls, and stores to file level variables.

   If EL is NULL, control statements are conservatively marked as
   necessary.  Otherwise it contains the list of edges used by control
   dependence analysis.  */

static void
find_obviously_necessary_stmts (struct edge_list *el)
{
  basic_block bb;
  gimple_stmt_iterator gsi;
  edge e;
  gimple phi, stmt;

  FOR_EACH_BB (bb)
    {
      /* PHI nodes are never inherently necessary.  */
      for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
	{
	  phi = gsi_stmt (gsi);
	  gimple_set_plf (phi, STMT_NECESSARY, false);
	}

      /* Check all statements in the block.  */
      for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
	{
	  stmt = gsi_stmt (gsi);
	  gimple_set_plf (stmt, STMT_NECESSARY, false);
	  mark_stmt_if_obviously_necessary (stmt, el != NULL);
	}
    }

  if (el)
    {
      /* Prevent the loops from being removed.  We must keep the infinite loops,
	 and we currently do not have a means to recognize the finite ones.  */
      FOR_EACH_BB (bb)
	{
	  edge_iterator ei;
	  FOR_EACH_EDGE (e, ei, bb->succs)
	    if (e->flags & EDGE_DFS_BACK)
	      mark_control_dependent_edges_necessary (e->dest, el);
	}
    }
}


/* Return true if REF is based on an aliased base, otherwise false.  */

static bool
ref_may_be_aliased (tree ref)
{
  while (handled_component_p (ref))
    ref = TREE_OPERAND (ref, 0);
  return !(DECL_P (ref)
	   && !may_be_aliased (ref));
}

struct ref_data {
  tree base;
  HOST_WIDE_INT size;
  HOST_WIDE_INT offset;
  HOST_WIDE_INT max_size;
};

static bitmap visited = NULL;
static unsigned int longest_chain = 0;
static unsigned int total_chain = 0;
static bool chain_ovfl = false;

/* Worker for the walker that marks reaching definitions of REF,
   which is based on a non-aliased decl, necessary.  It returns
   true whenever the defining statement of the current VDEF is
   a kill for REF, as no dominating may-defs are necessary for REF
   anymore.  DATA points to cached get_ref_base_and_extent data for REF.  */

static bool
mark_aliased_reaching_defs_necessary_1 (tree ref, tree vdef, void *data)
{
  gimple def_stmt = SSA_NAME_DEF_STMT (vdef);
  struct ref_data *refd = (struct ref_data *)data;

  /* All stmts we visit are necessary.  */
  mark_operand_necessary (vdef);

  /* If the stmt lhs kills ref, then we can stop walking.  */
  if (gimple_has_lhs (def_stmt)
      && TREE_CODE (gimple_get_lhs (def_stmt)) != SSA_NAME)
    {
      tree base, lhs = gimple_get_lhs (def_stmt);
      HOST_WIDE_INT size, offset, max_size;
      base = get_ref_base_and_extent (lhs, &offset, &size, &max_size);
      /* We can get MEM[symbol: sZ, index: D.8862_1] here,
	 so base == refd->base does not always hold.  */
      if (base == refd->base)
	{
	  /* For a must-alias check we need to be able to constrain
	     the accesses properly.  */
	  if (size != -1 && size == max_size
	      && refd->max_size != -1)
	    {
	      if (offset <= refd->offset
		  && offset + size >= refd->offset + refd->max_size)
		return true;
	    }
	  /* Or they need to be exactly the same.  */
	  else if (operand_equal_p (ref, lhs, 0))
	    return true;
	}
    }

  /* Otherwise keep walking.  */
  return false;
}

static void
mark_aliased_reaching_defs_necessary (gimple stmt, tree ref)
{
  struct ref_data refd;
  unsigned int chain;
  gcc_assert (!chain_ovfl);
  refd.base = get_ref_base_and_extent (ref, &refd.offset, &refd.size,
				       &refd.max_size);
  chain = walk_aliased_vdefs (ref, gimple_vuse (stmt),
			      mark_aliased_reaching_defs_necessary_1,
			      &refd, NULL);
  if (chain > longest_chain)
    longest_chain = chain;
  total_chain += chain;
}

/* Worker for the walker that marks reaching definitions of REF, which
   is not based on a non-aliased decl.  For simplicity we need to end
   up marking all may-defs necessary that are not based on a non-aliased
   decl.  The only job of this walker is to skip may-defs based on
   a non-aliased decl.  */

static bool
mark_all_reaching_defs_necessary_1 (tree ref ATTRIBUTE_UNUSED,
				tree vdef, void *data ATTRIBUTE_UNUSED)
{
  gimple def_stmt = SSA_NAME_DEF_STMT (vdef);

  /* We have to skip already visited (and thus necessary) statements
     to make the chaining work after we dropped back to simple mode.  */
  if (chain_ovfl
      && TEST_BIT (processed, SSA_NAME_VERSION (vdef)))
    {
      gcc_assert (gimple_nop_p (def_stmt)
		  || gimple_plf (def_stmt, STMT_NECESSARY));
      return false;
    }

  /* We want to skip stores to non-aliased variables.  */
  if (!chain_ovfl
      && gimple_assign_single_p (def_stmt))
    {
      tree lhs = gimple_assign_lhs (def_stmt);
      if (!ref_may_be_aliased (lhs))
	return false;
    }

  /* But can stop after the first necessary statement.  */
  mark_operand_necessary (vdef);
  return true;
}

static void
mark_all_reaching_defs_necessary (gimple stmt)
{
  walk_aliased_vdefs (NULL, gimple_vuse (stmt),
		      mark_all_reaching_defs_necessary_1, NULL, &visited);
}

/* Propagate necessity using the operands of necessary statements.
   Process the uses on each statement in the worklist, and add all
   feeding statements which contribute to the calculation of this
   value to the worklist. 

   In conservative mode, EL is NULL.  */

static void
propagate_necessity (struct edge_list *el)
{
  gimple stmt;
  bool aggressive = (el ? true : false); 

  if (dump_file && (dump_flags & TDF_DETAILS))
    fprintf (dump_file, "\nProcessing worklist:\n");

  while (VEC_length (gimple, worklist) > 0)
    {
      /* Take STMT from worklist.  */
      stmt = VEC_pop (gimple, worklist);

      if (dump_file && (dump_flags & TDF_DETAILS))
	{
	  fprintf (dump_file, "processing: ");
	  print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
	  fprintf (dump_file, "\n");
	}

      if (aggressive)
	{
	  /* Mark the last statements of the basic blocks that the block
	     containing STMT is control dependent on, but only if we haven't
	     already done so.  */
	  basic_block bb = gimple_bb (stmt);
	  if (bb != ENTRY_BLOCK_PTR
	      && ! TEST_BIT (visited_control_parents, bb->index))
	    {
	      SET_BIT (visited_control_parents, bb->index);
	      mark_control_dependent_edges_necessary (bb, el);
	    }
	}

      if (gimple_code (stmt) == GIMPLE_PHI
	  /* We do not process virtual PHI nodes nor do we track their
	     necessity.  */
	  && is_gimple_reg (gimple_phi_result (stmt)))
	{
	  /* PHI nodes are somewhat special in that each PHI alternative has
	     data and control dependencies.  All the statements feeding the
	     PHI node's arguments are always necessary.  In aggressive mode,
	     we also consider the control dependent edges leading to the
	     predecessor block associated with each PHI alternative as
	     necessary.  */
	  size_t k;

	  for (k = 0; k < gimple_phi_num_args (stmt); k++)
            {
	      tree arg = PHI_ARG_DEF (stmt, k);
	      if (TREE_CODE (arg) == SSA_NAME)
		mark_operand_necessary (arg);
	    }

	  if (aggressive)
	    {
	      for (k = 0; k < gimple_phi_num_args (stmt); k++)
		{
		  basic_block arg_bb = gimple_phi_arg_edge (stmt, k)->src;
		  if (arg_bb != ENTRY_BLOCK_PTR
		      && ! TEST_BIT (visited_control_parents, arg_bb->index))
		    {
		      SET_BIT (visited_control_parents, arg_bb->index);
		      mark_control_dependent_edges_necessary (arg_bb, el);
		    }
		}
	    }
	}
      else
	{
	  /* Propagate through the operands.  Examine all the USE, VUSE and
	     VDEF operands in this statement.  Mark all the statements 
	     which feed this statement's uses as necessary.  */
	  ssa_op_iter iter;
	  tree use;

	  FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
	    mark_operand_necessary (use);

	  use = gimple_vuse (stmt);
	  if (!use)
	    continue;

	  /* If we dropped to simple mode make all immediately
	     reachable definitions necessary.  */
	  if (chain_ovfl)
	    {
	      mark_all_reaching_defs_necessary (stmt);
	      continue;
	    }

	  /* For statements that may load from memory (have a VUSE) we
	     have to mark all reaching (may-)definitions as necessary.
	     We partition this task into two cases:
	      1) explicit loads based on decls that are not aliased
	      2) implicit loads (like calls) and explicit loads not
	         based on decls that are not aliased (like indirect
		 references or loads from globals)
	     For 1) we mark all reaching may-defs as necessary, stopping
	     at dominating kills.  For 2) we want to mark all dominating
	     references necessary, but non-aliased ones which we handle
	     in 1).  Instead of doing so for each load we rely on the
	     worklist to eventually reach all dominating references and
	     instead just mark the immediately dominating references
	     as necessary (but skipping non-aliased ones).  */

	  if (is_gimple_call (stmt))
	    {
	      unsigned i;

	      /* Calls implicitly load from memory, their arguments
	         in addition may explicitly perform memory loads.
		 This also ensures propagation for case 2 for stores.  */
	      mark_all_reaching_defs_necessary (stmt);
	      for (i = 0; i < gimple_call_num_args (stmt); ++i)
		{
		  tree arg = gimple_call_arg (stmt, i);
		  if (TREE_CODE (arg) == SSA_NAME
		      || is_gimple_min_invariant (arg))
		    continue;
		  if (!ref_may_be_aliased (arg))
		    mark_aliased_reaching_defs_necessary (stmt, arg);
		}
	    }
	  else if (gimple_assign_single_p (stmt))
	    {
	      tree lhs, rhs;
	      bool rhs_aliased = false;
	      /* If this is a load mark things necessary.  */
	      rhs = gimple_assign_rhs1 (stmt);
	      if (TREE_CODE (rhs) != SSA_NAME
		  && !is_gimple_min_invariant (rhs))
		{
		  if (!ref_may_be_aliased (rhs))
		    mark_aliased_reaching_defs_necessary (stmt, rhs);
		  else
		    rhs_aliased = true;
		}
	      /* If this is an aliased store, mark things necessary.
		 This is where we make sure to propagate for case 2.  */
	      lhs = gimple_assign_lhs (stmt);
	      if (rhs_aliased
		  || (TREE_CODE (lhs) != SSA_NAME
		      && ref_may_be_aliased (lhs)))
		mark_all_reaching_defs_necessary (stmt);
	    }
	  else if (gimple_code (stmt) == GIMPLE_RETURN)
	    {
	      tree rhs = gimple_return_retval (stmt);
	      /* A return statement may perform a load.  */
	      if (TREE_CODE (rhs) != SSA_NAME
		  && !is_gimple_min_invariant (rhs))
		{
		  if (!ref_may_be_aliased (rhs))
		    mark_aliased_reaching_defs_necessary (stmt, rhs);
		  else
		    mark_all_reaching_defs_necessary (stmt);
		}
	    }
	  else if (gimple_code (stmt) == GIMPLE_ASM)
	    {
	      unsigned i;
	      mark_all_reaching_defs_necessary (stmt);
	      /* Inputs may perform loads.  */
	      for (i = 0; i < gimple_asm_ninputs (stmt); ++i)
		{
		  tree op = TREE_VALUE (gimple_asm_input_op (stmt, i));
		  if (TREE_CODE (op) != SSA_NAME
		      && !is_gimple_min_invariant (op)
		      && !ref_may_be_aliased (op))
		    mark_aliased_reaching_defs_necessary (stmt, op);
		}
	    }
	  else
	    gcc_unreachable ();

	  /* If we over-used our alias oracle budget drop to simple
	     mode.  The cost metric allows quadratic behavior up to
	     a constant maximal chain and after that falls back to
	     super-linear complexity.  */
	  if (longest_chain > 256
	      && total_chain > 256 * longest_chain)
	    {
	      chain_ovfl = true;
	      if (visited)
		bitmap_clear (visited);
	    }
	}
    }
}


/* Remove dead PHI nodes from block BB.  */

static bool
remove_dead_phis (basic_block bb)
{
  bool something_changed = false;
  gimple_seq phis;
  gimple phi;
  gimple_stmt_iterator gsi;
  phis = phi_nodes (bb);

  for (gsi = gsi_start (phis); !gsi_end_p (gsi);)
    {
      stats.total_phis++;
      phi = gsi_stmt (gsi);

      /* We do not track necessity of virtual PHI nodes.  Instead do
         very simple dead PHI removal here.  */
      if (!is_gimple_reg (gimple_phi_result (phi)))
	{
	  unsigned i;
	  tree vuse;

	  /* Virtual PHI nodes with one or identical arguments
	     can be removed.  */
	  vuse = gimple_phi_arg_def (phi, 0);
	  for (i = 1; i < gimple_phi_num_args (phi); ++i)
	    {
	      if (gimple_phi_arg_def (phi, i) != vuse)
		{
		  vuse = NULL_TREE;
		  break;
		}
	    }
	  if (vuse != NULL_TREE)
	    {
	      tree vdef = gimple_phi_result (phi);
	      use_operand_p use_p;
	      imm_use_iterator iter;
	      gimple use_stmt;
	      FOR_EACH_IMM_USE_STMT (use_stmt, iter, vdef)
		FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
		  SET_USE (use_p, vuse);
	      if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vdef))
		SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vuse) = 1;
	    }
	  else
	    gimple_set_plf (phi, STMT_NECESSARY, true);
	}

      if (!gimple_plf (phi, STMT_NECESSARY))
	{
	  something_changed = true;
	  if (dump_file && (dump_flags & TDF_DETAILS))
	    {
	      fprintf (dump_file, "Deleting : ");
	      print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
	      fprintf (dump_file, "\n");
	    }

	  remove_phi_node (&gsi, true);
	  stats.removed_phis++;
	  continue;
	}

      gsi_next (&gsi);
    }
  return something_changed;
}


/* Remove dead statement pointed to by iterator I.  Receives the basic block BB
   containing I so that we don't have to look it up.  */

static void
remove_dead_stmt (gimple_stmt_iterator *i, basic_block bb)
{
  gimple stmt = gsi_stmt (*i);

  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "Deleting : ");
      print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
      fprintf (dump_file, "\n");
    }

  stats.removed++;

  /* If we have determined that a conditional branch statement contributes
     nothing to the program, then we not only remove it, but we also change
     the flow graph so that the current block will simply fall-thru to its
     immediate post-dominator.  The blocks we are circumventing will be
     removed by cleanup_tree_cfg if this change in the flow graph makes them
     unreachable.  */
  if (is_ctrl_stmt (stmt))
    {
      basic_block post_dom_bb;

      /* The post dominance info has to be up-to-date.  */
      gcc_assert (dom_info_state (CDI_POST_DOMINATORS) == DOM_OK);
      /* Get the immediate post dominator of bb.  */
      post_dom_bb = get_immediate_dominator (CDI_POST_DOMINATORS, bb);

      /* There are three particularly problematical cases.

	 1. Blocks that do not have an immediate post dominator.  This
	    can happen with infinite loops.

	 2. Blocks that are only post dominated by the exit block.  These
	    can also happen for infinite loops as we create fake edges
	    in the dominator tree.

	 3. If the post dominator has PHI nodes we may be able to compute
	    the right PHI args for them.

	 In each of these cases we must remove the control statement
	 as it may reference SSA_NAMEs which are going to be removed and
	 we remove all but one outgoing edge from the block.  */
      if (! post_dom_bb
	  || post_dom_bb == EXIT_BLOCK_PTR
	  || phi_nodes (post_dom_bb))
	;
      else
	{
	  /* Redirect the first edge out of BB to reach POST_DOM_BB.  */
	  redirect_edge_and_branch (EDGE_SUCC (bb, 0), post_dom_bb);
	  PENDING_STMT (EDGE_SUCC (bb, 0)) = NULL;

	  /* It is not sufficient to set cfg_altered below during edge
	     removal, in case BB has two successors and one of them
	     is POST_DOM_BB.  */
	  cfg_altered = true;
	}
      EDGE_SUCC (bb, 0)->probability = REG_BR_PROB_BASE;
      EDGE_SUCC (bb, 0)->count = bb->count;

      /* The edge is no longer associated with a conditional, so it does
	 not have TRUE/FALSE flags.  */
      EDGE_SUCC (bb, 0)->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);

      /* The lone outgoing edge from BB will be a fallthru edge.  */
      EDGE_SUCC (bb, 0)->flags |= EDGE_FALLTHRU;

      /* Remove the remaining the outgoing edges.  */
      while (!single_succ_p (bb))
	{
	  /* FIXME.  When we remove the edge, we modify the CFG, which
	     in turn modifies the dominator and post-dominator tree.
	     Is it safe to postpone recomputing the dominator and
	     post-dominator tree until the end of this pass given that
	     the post-dominators are used above?  */
	  cfg_altered = true;
          remove_edge (EDGE_SUCC (bb, 1));
	}
    }

  unlink_stmt_vdef (stmt);
  gsi_remove (i, true);  
  release_defs (stmt); 
}


/* Eliminate unnecessary statements. Any instruction not marked as necessary
   contributes nothing to the program, and can be deleted.  */

static bool
eliminate_unnecessary_stmts (void)
{
  bool something_changed = false;
  basic_block bb;
  gimple_stmt_iterator gsi;
  gimple stmt;
  tree call;

  if (dump_file && (dump_flags & TDF_DETAILS))
    fprintf (dump_file, "\nEliminating unnecessary statements:\n");

  clear_special_calls ();

  FOR_EACH_BB (bb)
    {
      /* Remove dead statements.  */
      for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi);)
	{
	  stmt = gsi_stmt (gsi);

	  stats.total++;

	  /* If GSI is not necessary then remove it.  */
	  if (!gimple_plf (stmt, STMT_NECESSARY))
	    {
	      remove_dead_stmt (&gsi, bb);
	      something_changed = true;
	    }
	  else if (is_gimple_call (stmt))
	    {
	      call = gimple_call_fndecl (stmt);
	      if (call)
		{
		  tree name;

		  /* When LHS of var = call (); is dead, simplify it into
		     call (); saving one operand.  */
		  name = gimple_call_lhs (stmt);
		  if (name && TREE_CODE (name) == SSA_NAME
		           && !TEST_BIT (processed, SSA_NAME_VERSION (name)))
		    {
		      something_changed = true;
		      if (dump_file && (dump_flags & TDF_DETAILS))
			{
			  fprintf (dump_file, "Deleting LHS of call: ");
			  print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
			  fprintf (dump_file, "\n");
			}
		      
		      push_stmt_changes (gsi_stmt_ptr (&gsi));
		      gimple_call_set_lhs (stmt, NULL_TREE);
		      maybe_clean_or_replace_eh_stmt (stmt, stmt);
		      pop_stmt_changes (gsi_stmt_ptr (&gsi));
		      release_ssa_name (name);
		    }
		  notice_special_calls (stmt);
		}
	      gsi_next (&gsi);
	    }
	  else
	    {
	      gsi_next (&gsi);
	    }
	}
    }

  FOR_EACH_BB (bb)
    {
      /* Remove dead PHI nodes.  */
      something_changed |= remove_dead_phis (bb);
    }

  return something_changed;
}


/* Print out removed statement statistics.  */

static void
print_stats (void)
{
  float percg;

  percg = ((float) stats.removed / (float) stats.total) * 100;
  fprintf (dump_file, "Removed %d of %d statements (%d%%)\n",
	   stats.removed, stats.total, (int) percg);

  if (stats.total_phis == 0)
    percg = 0;
  else
    percg = ((float) stats.removed_phis / (float) stats.total_phis) * 100;

  fprintf (dump_file, "Removed %d of %d PHI nodes (%d%%)\n",
	   stats.removed_phis, stats.total_phis, (int) percg);
}

/* Initialization for this pass.  Set up the used data structures.  */

static void
tree_dce_init (bool aggressive)
{
  memset ((void *) &stats, 0, sizeof (stats));

  if (aggressive)
    {
      int i;

      control_dependence_map = XNEWVEC (bitmap, last_basic_block);
      for (i = 0; i < last_basic_block; ++i)
	control_dependence_map[i] = BITMAP_ALLOC (NULL);

      last_stmt_necessary = sbitmap_alloc (last_basic_block);
      sbitmap_zero (last_stmt_necessary);
    }

  processed = sbitmap_alloc (num_ssa_names + 1);
  sbitmap_zero (processed);

  worklist = VEC_alloc (gimple, heap, 64);
  cfg_altered = false;
}

/* Cleanup after this pass.  */

static void
tree_dce_done (bool aggressive)
{
  if (aggressive)
    {
      int i;

      for (i = 0; i < last_basic_block; ++i)
	BITMAP_FREE (control_dependence_map[i]);
      free (control_dependence_map);

      sbitmap_free (visited_control_parents);
      sbitmap_free (last_stmt_necessary);
    }

  sbitmap_free (processed);

  VEC_free (gimple, heap, worklist);
}

/* Main routine to eliminate dead code.

   AGGRESSIVE controls the aggressiveness of the algorithm.
   In conservative mode, we ignore control dependence and simply declare
   all but the most trivially dead branches necessary.  This mode is fast.
   In aggressive mode, control dependences are taken into account, which
   results in more dead code elimination, but at the cost of some time.

   FIXME: Aggressive mode before PRE doesn't work currently because
	  the dominance info is not invalidated after DCE1.  This is
	  not an issue right now because we only run aggressive DCE
	  as the last tree SSA pass, but keep this in mind when you
	  start experimenting with pass ordering.  */

static unsigned int
perform_tree_ssa_dce (bool aggressive)
{
  struct edge_list *el = NULL;
  bool something_changed = 0;

  tree_dce_init (aggressive);

  if (aggressive)
    {
      /* Compute control dependence.  */
      timevar_push (TV_CONTROL_DEPENDENCES);
      calculate_dominance_info (CDI_POST_DOMINATORS);
      el = create_edge_list ();
      find_all_control_dependences (el);
      timevar_pop (TV_CONTROL_DEPENDENCES);

      visited_control_parents = sbitmap_alloc (last_basic_block);
      sbitmap_zero (visited_control_parents);

      mark_dfs_back_edges ();
    }

  find_obviously_necessary_stmts (el);

  longest_chain = 0;
  total_chain = 0;
  chain_ovfl = false;
  propagate_necessity (el);
  BITMAP_FREE (visited);

  something_changed |= eliminate_unnecessary_stmts ();
  something_changed |= cfg_altered;

  /* We do not update postdominators, so free them unconditionally.  */
  free_dominance_info (CDI_POST_DOMINATORS);

  /* If we removed paths in the CFG, then we need to update
     dominators as well.  I haven't investigated the possibility
     of incrementally updating dominators.  */
  if (cfg_altered)
    free_dominance_info (CDI_DOMINATORS);

  statistics_counter_event (cfun, "Statements deleted", stats.removed);
  statistics_counter_event (cfun, "PHI nodes deleted", stats.removed_phis);

  /* Debugging dumps.  */
  if (dump_file && (dump_flags & (TDF_STATS|TDF_DETAILS)))
    print_stats ();

  tree_dce_done (aggressive);

  free_edge_list (el);

  if (something_changed)
    return (TODO_update_ssa | TODO_cleanup_cfg | TODO_ggc_collect 
	    | TODO_remove_unused_locals);
  else
    return 0;
}

/* Pass entry points.  */
static unsigned int
tree_ssa_dce (void)
{
  return perform_tree_ssa_dce (/*aggressive=*/false);
}

static unsigned int
tree_ssa_dce_loop (void)
{
  unsigned int todo;
  todo = perform_tree_ssa_dce (/*aggressive=*/false);
  if (todo)
    {
      free_numbers_of_iterations_estimates ();
      scev_reset ();
    }
  return todo;
}

static unsigned int
tree_ssa_cd_dce (void)
{
  return perform_tree_ssa_dce (/*aggressive=*/optimize >= 2);
}

static bool
gate_dce (void)
{
  return flag_tree_dce != 0;
}

struct gimple_opt_pass pass_dce =
{
 {
  GIMPLE_PASS,
  "dce",				/* name */
  gate_dce,				/* gate */
  tree_ssa_dce,				/* execute */
  NULL,					/* sub */
  NULL,					/* next */
  0,					/* static_pass_number */
  TV_TREE_DCE,				/* tv_id */
  PROP_cfg | PROP_ssa,			/* properties_required */
  0,					/* properties_provided */
  0,					/* properties_destroyed */
  0,					/* todo_flags_start */
  TODO_dump_func | TODO_verify_ssa	/* todo_flags_finish */
 }
};

struct gimple_opt_pass pass_dce_loop =
{
 {
  GIMPLE_PASS,
  "dceloop",				/* name */
  gate_dce,				/* gate */
  tree_ssa_dce_loop,			/* execute */
  NULL,					/* sub */
  NULL,					/* next */
  0,					/* static_pass_number */
  TV_TREE_DCE,				/* tv_id */
  PROP_cfg | PROP_ssa,			/* properties_required */
  0,					/* properties_provided */
  0,					/* properties_destroyed */
  0,					/* todo_flags_start */
  TODO_dump_func | TODO_verify_ssa	/* todo_flags_finish */
 }
};

struct gimple_opt_pass pass_cd_dce =
{
 {
  GIMPLE_PASS,
  "cddce",				/* name */
  gate_dce,				/* gate */
  tree_ssa_cd_dce,			/* execute */
  NULL,					/* sub */
  NULL,					/* next */
  0,					/* static_pass_number */
  TV_TREE_CD_DCE,			/* tv_id */
  PROP_cfg | PROP_ssa,			/* properties_required */
  0,					/* properties_provided */
  0,					/* properties_destroyed */
  0,					/* todo_flags_start */
  TODO_dump_func | TODO_verify_ssa
  | TODO_verify_flow			/* todo_flags_finish */
 }
};