1 /* Dead code elimination pass for the GNU compiler.
2 Copyright (C) 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
3 Contributed by Ben Elliston <bje@redhat.com>
4 and Andrew MacLeod <amacleod@redhat.com>
5 Adapted to use control dependence by Steven Bosscher, SUSE Labs.
7 This file is part of GCC.
9 GCC is free software; you can redistribute it and/or modify it
10 under the terms of the GNU General Public License as published by the
11 Free Software Foundation; either version 2, or (at your option) any
14 GCC is distributed in the hope that it will be useful, but WITHOUT
15 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
16 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
19 You should have received a copy of the GNU General Public License
20 along with GCC; see the file COPYING. If not, write to the Free
21 Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA
24 /* Dead code elimination.
28 Building an Optimizing Compiler,
29 Robert Morgan, Butterworth-Heinemann, 1998, Section 8.9.
31 Advanced Compiler Design and Implementation,
32 Steven Muchnick, Morgan Kaufmann, 1997, Section 18.10.
34 Dead-code elimination is the removal of statements which have no
35 impact on the program's output. "Dead statements" have no impact
36 on the program's output, while "necessary statements" may have
39 The algorithm consists of three phases:
40 1. Marking as necessary all statements known to be necessary,
41 e.g. most function calls, writing a value to memory, etc;
42 2. Propagating necessary statements, e.g., the statements
43 giving values to operands in necessary statements; and
44 3. Removing dead statements. */
48 #include "coretypes.h"
52 /* These RTL headers are needed for basic-block.h. */
55 #include "hard-reg-set.h"
57 #include "basic-block.h"
60 #include "diagnostic.h"
61 #include "tree-flow.h"
62 #include "tree-gimple.h"
63 #include "tree-dump.h"
64 #include "tree-pass.h"
69 static struct stmt_stats
77 static VEC(tree,heap) *worklist;
79 /* Vector indicating an SSA name has already been processed and marked
81 static sbitmap processed;
83 /* Vector indicating that last_stmt if a basic block has already been
84 marked as necessary. */
85 static sbitmap last_stmt_necessary;
87 /* Before we can determine whether a control branch is dead, we need to
88 compute which blocks are control dependent on which edges.
90 We expect each block to be control dependent on very few edges so we
91 use a bitmap for each block recording its edges. An array holds the
92 bitmap. The Ith bit in the bitmap is set if that block is dependent
94 static bitmap *control_dependence_map;
96 /* Vector indicating that a basic block has already had all the edges
97 processed that it is control dependent on. */
98 static sbitmap visited_control_parents;
100 /* TRUE if this pass alters the CFG (by removing control statements).
103 If this pass alters the CFG, then it will arrange for the dominators
105 static bool cfg_altered;
107 /* Execute CODE for each edge (given number EDGE_NUMBER within the CODE)
108 for which the block with index N is control dependent. */
109 #define EXECUTE_IF_CONTROL_DEPENDENT(N, EDGE_NUMBER, CODE) \
111 bitmap_iterator bi; \
113 EXECUTE_IF_SET_IN_BITMAP (control_dependence_map[N], 0, EDGE_NUMBER, bi) \
119 /* Local function prototypes. */
120 static inline void set_control_dependence_map_bit (basic_block, int);
121 static inline void clear_control_dependence_bitmap (basic_block);
122 static void find_all_control_dependences (struct edge_list *);
123 static void find_control_dependence (struct edge_list *, int);
124 static inline basic_block find_pdom (basic_block);
126 static inline void mark_stmt_necessary (tree, bool);
127 static inline void mark_operand_necessary (tree, bool);
129 static void mark_stmt_if_obviously_necessary (tree, bool);
130 static void find_obviously_necessary_stmts (struct edge_list *);
132 static void mark_control_dependent_edges_necessary (basic_block, struct edge_list *);
133 static void propagate_necessity (struct edge_list *);
135 static void eliminate_unnecessary_stmts (void);
136 static void remove_dead_phis (basic_block);
137 static void remove_dead_stmt (block_stmt_iterator *, basic_block);
139 static void print_stats (void);
140 static void tree_dce_init (bool);
141 static void tree_dce_done (bool);
143 /* Indicate block BB is control dependent on an edge with index EDGE_INDEX. */
145 set_control_dependence_map_bit (basic_block bb, int edge_index)
147 if (bb == ENTRY_BLOCK_PTR)
149 gcc_assert (bb != EXIT_BLOCK_PTR);
150 bitmap_set_bit (control_dependence_map[bb->index], edge_index);
153 /* Clear all control dependences for block BB. */
155 void clear_control_dependence_bitmap (basic_block bb)
157 bitmap_clear (control_dependence_map[bb->index]);
160 /* Record all blocks' control dependences on all edges in the edge
161 list EL, ala Morgan, Section 3.6. */
164 find_all_control_dependences (struct edge_list *el)
168 for (i = 0; i < NUM_EDGES (el); ++i)
169 find_control_dependence (el, i);
172 /* Determine all blocks' control dependences on the given edge with edge_list
173 EL index EDGE_INDEX, ala Morgan, Section 3.6. */
176 find_control_dependence (struct edge_list *el, int edge_index)
178 basic_block current_block;
179 basic_block ending_block;
181 gcc_assert (INDEX_EDGE_PRED_BB (el, edge_index) != EXIT_BLOCK_PTR);
183 if (INDEX_EDGE_PRED_BB (el, edge_index) == ENTRY_BLOCK_PTR)
184 ending_block = ENTRY_BLOCK_PTR->next_bb;
186 ending_block = find_pdom (INDEX_EDGE_PRED_BB (el, edge_index));
188 for (current_block = INDEX_EDGE_SUCC_BB (el, edge_index);
189 current_block != ending_block && current_block != EXIT_BLOCK_PTR;
190 current_block = find_pdom (current_block))
192 edge e = INDEX_EDGE (el, edge_index);
194 /* For abnormal edges, we don't make current_block control
195 dependent because instructions that throw are always necessary
197 if (e->flags & EDGE_ABNORMAL)
200 set_control_dependence_map_bit (current_block, edge_index);
204 /* Find the immediate postdominator PDOM of the specified basic block BLOCK.
205 This function is necessary because some blocks have negative numbers. */
207 static inline basic_block
208 find_pdom (basic_block block)
210 gcc_assert (block != ENTRY_BLOCK_PTR);
212 if (block == EXIT_BLOCK_PTR)
213 return EXIT_BLOCK_PTR;
216 basic_block bb = get_immediate_dominator (CDI_POST_DOMINATORS, block);
218 return EXIT_BLOCK_PTR;
223 #define NECESSARY(stmt) stmt->common.asm_written_flag
225 /* If STMT is not already marked necessary, mark it, and add it to the
226 worklist if ADD_TO_WORKLIST is true. */
228 mark_stmt_necessary (tree stmt, bool add_to_worklist)
231 gcc_assert (!DECL_P (stmt));
233 if (NECESSARY (stmt))
236 if (dump_file && (dump_flags & TDF_DETAILS))
238 fprintf (dump_file, "Marking useful stmt: ");
239 print_generic_stmt (dump_file, stmt, TDF_SLIM);
240 fprintf (dump_file, "\n");
243 NECESSARY (stmt) = 1;
245 VEC_safe_push (tree, heap, worklist, stmt);
248 /* Mark the statement defining operand OP as necessary. PHIONLY is true
249 if we should only mark it necessary if it is a phi node. */
252 mark_operand_necessary (tree op, bool phionly)
259 ver = SSA_NAME_VERSION (op);
260 if (TEST_BIT (processed, ver))
262 SET_BIT (processed, ver);
264 stmt = SSA_NAME_DEF_STMT (op);
268 || IS_EMPTY_STMT (stmt)
269 || (phionly && TREE_CODE (stmt) != PHI_NODE))
272 NECESSARY (stmt) = 1;
273 VEC_safe_push (tree, heap, worklist, stmt);
277 /* Mark STMT as necessary if it obviously is. Add it to the worklist if
278 it can make other statements necessary.
280 If AGGRESSIVE is false, control statements are conservatively marked as
284 mark_stmt_if_obviously_necessary (tree stmt, bool aggressive)
290 /* With non-call exceptions, we have to assume that all statements could
291 throw. If a statement may throw, it is inherently necessary. */
292 if (flag_non_call_exceptions
293 && tree_could_throw_p (stmt))
295 mark_stmt_necessary (stmt, true);
299 /* Statements that are implicitly live. Most function calls, asm and return
300 statements are required. Labels and BIND_EXPR nodes are kept because
301 they are control flow, and we have no way of knowing whether they can be
302 removed. DCE can eliminate all the other statements in a block, and CFG
303 can then remove the block and labels. */
304 switch (TREE_CODE (stmt))
308 case CASE_LABEL_EXPR:
309 mark_stmt_necessary (stmt, false);
315 mark_stmt_necessary (stmt, true);
319 /* Most, but not all function calls are required. Function calls that
320 produce no result and have no side effects (i.e. const pure
321 functions) are unnecessary. */
322 if (TREE_SIDE_EFFECTS (stmt))
323 mark_stmt_necessary (stmt, true);
327 op = get_call_expr_in (stmt);
328 if (op && TREE_SIDE_EFFECTS (op))
330 mark_stmt_necessary (stmt, true);
334 /* These values are mildly magic bits of the EH runtime. We can't
335 see the entire lifetime of these values until landing pads are
337 if (TREE_CODE (TREE_OPERAND (stmt, 0)) == EXC_PTR_EXPR
338 || TREE_CODE (TREE_OPERAND (stmt, 0)) == FILTER_EXPR)
340 mark_stmt_necessary (stmt, true);
346 gcc_assert (!simple_goto_p (stmt));
347 mark_stmt_necessary (stmt, true);
351 gcc_assert (EDGE_COUNT (bb_for_stmt (stmt)->succs) == 2);
356 mark_stmt_necessary (stmt, true);
363 ann = stmt_ann (stmt);
365 /* If the statement has volatile operands, it needs to be preserved.
366 Same for statements that can alter control flow in unpredictable
368 if (ann->has_volatile_ops || is_ctrl_altering_stmt (stmt))
370 mark_stmt_necessary (stmt, true);
374 FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_DEF)
376 if (is_global_var (SSA_NAME_VAR (def)))
378 mark_stmt_necessary (stmt, true);
382 if (is_hidden_global_store (stmt))
384 mark_stmt_necessary (stmt, true);
391 /* Find obviously necessary statements. These are things like most function
392 calls, and stores to file level variables.
394 If EL is NULL, control statements are conservatively marked as
395 necessary. Otherwise it contains the list of edges used by control
396 dependence analysis. */
399 find_obviously_necessary_stmts (struct edge_list *el)
402 block_stmt_iterator i;
409 /* Check any PHI nodes in the block. */
410 for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
414 /* PHIs for virtual variables do not directly affect code
415 generation and need not be considered inherently necessary
416 regardless of the bits set in their decl.
418 Thus, we only need to mark PHIs for real variables which
419 need their result preserved as being inherently necessary. */
420 if (is_gimple_reg (PHI_RESULT (phi))
421 && is_global_var (SSA_NAME_VAR (PHI_RESULT (phi))))
422 mark_stmt_necessary (phi, true);
425 /* Check all statements in the block. */
426 for (i = bsi_start (bb); ! bsi_end_p (i); bsi_next (&i))
428 tree stmt = bsi_stmt (i);
429 NECESSARY (stmt) = 0;
430 mark_stmt_if_obviously_necessary (stmt, el != NULL);
436 /* Prevent the loops from being removed. We must keep the infinite loops,
437 and we currently do not have a means to recognize the finite ones. */
441 FOR_EACH_EDGE (e, ei, bb->succs)
442 if (e->flags & EDGE_DFS_BACK)
443 mark_control_dependent_edges_necessary (e->dest, el);
448 /* Make corresponding control dependent edges necessary. We only
449 have to do this once for each basic block, so we clear the bitmap
452 mark_control_dependent_edges_necessary (basic_block bb, struct edge_list *el)
454 unsigned edge_number;
456 gcc_assert (bb != EXIT_BLOCK_PTR);
458 if (bb == ENTRY_BLOCK_PTR)
461 EXECUTE_IF_CONTROL_DEPENDENT (bb->index, edge_number,
464 basic_block cd_bb = INDEX_EDGE_PRED_BB (el, edge_number);
466 if (TEST_BIT (last_stmt_necessary, cd_bb->index))
468 SET_BIT (last_stmt_necessary, cd_bb->index);
470 t = last_stmt (cd_bb);
471 if (t && is_ctrl_stmt (t))
472 mark_stmt_necessary (t, true);
476 /* Propagate necessity using the operands of necessary statements. Process
477 the uses on each statement in the worklist, and add all feeding statements
478 which contribute to the calculation of this value to the worklist.
480 In conservative mode, EL is NULL. */
483 propagate_necessity (struct edge_list *el)
486 bool aggressive = (el ? true : false);
488 if (dump_file && (dump_flags & TDF_DETAILS))
489 fprintf (dump_file, "\nProcessing worklist:\n");
491 while (VEC_length (tree, worklist) > 0)
493 /* Take `i' from worklist. */
494 i = VEC_pop (tree, worklist);
496 if (dump_file && (dump_flags & TDF_DETAILS))
498 fprintf (dump_file, "processing: ");
499 print_generic_stmt (dump_file, i, TDF_SLIM);
500 fprintf (dump_file, "\n");
505 /* Mark the last statements of the basic blocks that the block
506 containing `i' is control dependent on, but only if we haven't
508 basic_block bb = bb_for_stmt (i);
509 if (bb != ENTRY_BLOCK_PTR
510 && ! TEST_BIT (visited_control_parents, bb->index))
512 SET_BIT (visited_control_parents, bb->index);
513 mark_control_dependent_edges_necessary (bb, el);
517 if (TREE_CODE (i) == PHI_NODE)
519 /* PHI nodes are somewhat special in that each PHI alternative has
520 data and control dependencies. All the statements feeding the
521 PHI node's arguments are always necessary. In aggressive mode,
522 we also consider the control dependent edges leading to the
523 predecessor block associated with each PHI alternative as
526 for (k = 0; k < PHI_NUM_ARGS (i); k++)
528 tree arg = PHI_ARG_DEF (i, k);
529 if (TREE_CODE (arg) == SSA_NAME)
530 mark_operand_necessary (arg, false);
535 for (k = 0; k < PHI_NUM_ARGS (i); k++)
537 basic_block arg_bb = PHI_ARG_EDGE (i, k)->src;
538 if (arg_bb != ENTRY_BLOCK_PTR
539 && ! TEST_BIT (visited_control_parents, arg_bb->index))
541 SET_BIT (visited_control_parents, arg_bb->index);
542 mark_control_dependent_edges_necessary (arg_bb, el);
549 /* Propagate through the operands. Examine all the USE, VUSE and
550 V_MAY_DEF operands in this statement. Mark all the statements
551 which feed this statement's uses as necessary. */
555 /* The operands of V_MAY_DEF expressions are also needed as they
556 represent potential definitions that may reach this
557 statement (V_MAY_DEF operands allow us to follow def-def
560 FOR_EACH_SSA_TREE_OPERAND (use, i, iter, SSA_OP_ALL_USES)
561 mark_operand_necessary (use, false);
567 /* Propagate necessity around virtual phi nodes used in kill operands.
568 The reason this isn't done during propagate_necessity is because we don't
569 want to keep phis around that are just there for must-defs, unless we
570 absolutely have to. After we've rewritten the reaching definitions to be
571 correct in the previous part of the fixup routine, we can simply propagate
572 around the information about which of these virtual phi nodes are really
573 used, and set the NECESSARY flag accordingly.
574 Note that we do the minimum here to ensure that we keep alive the phis that
575 are actually used in the corrected SSA form. In particular, some of these
576 phis may now have all of the same operand, and will be deleted by some
580 mark_really_necessary_kill_operand_phis (void)
585 /* Seed the worklist with the new virtual phi arguments and virtual
589 block_stmt_iterator bsi;
592 for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
594 if (!is_gimple_reg (PHI_RESULT (phi)) && NECESSARY (phi))
596 for (i = 0; i < PHI_NUM_ARGS (phi); i++)
597 mark_operand_necessary (PHI_ARG_DEF (phi, i), true);
601 for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
603 tree stmt = bsi_stmt (bsi);
605 if (NECESSARY (stmt))
609 FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter,
610 SSA_OP_VIRTUAL_USES | SSA_OP_VIRTUAL_KILLS)
612 tree use = USE_FROM_PTR (use_p);
613 mark_operand_necessary (use, true);
619 /* Mark all virtual phis still in use as necessary, and all of their
620 arguments that are phis as necessary. */
621 while (VEC_length (tree, worklist) > 0)
623 tree use = VEC_pop (tree, worklist);
625 for (i = 0; i < PHI_NUM_ARGS (use); i++)
626 mark_operand_necessary (PHI_ARG_DEF (use, i), true);
633 /* Eliminate unnecessary statements. Any instruction not marked as necessary
634 contributes nothing to the program, and can be deleted. */
637 eliminate_unnecessary_stmts (void)
640 block_stmt_iterator i;
642 if (dump_file && (dump_flags & TDF_DETAILS))
643 fprintf (dump_file, "\nEliminating unnecessary statements:\n");
645 clear_special_calls ();
648 /* Remove dead PHI nodes. */
649 remove_dead_phis (bb);
654 /* Remove dead statements. */
655 for (i = bsi_start (bb); ! bsi_end_p (i) ; )
657 tree t = bsi_stmt (i);
661 /* If `i' is not necessary then remove it. */
663 remove_dead_stmt (&i, bb);
666 tree call = get_call_expr_in (t);
668 notice_special_calls (call);
675 /* Remove dead PHI nodes from block BB. */
678 remove_dead_phis (basic_block bb)
683 phi = phi_nodes (bb);
688 if (! NECESSARY (phi))
690 tree next = PHI_CHAIN (phi);
692 if (dump_file && (dump_flags & TDF_DETAILS))
694 fprintf (dump_file, "Deleting : ");
695 print_generic_stmt (dump_file, phi, TDF_SLIM);
696 fprintf (dump_file, "\n");
699 remove_phi_node (phi, prev);
700 stats.removed_phis++;
706 phi = PHI_CHAIN (phi);
711 /* Remove dead statement pointed to by iterator I. Receives the basic block BB
712 containing I so that we don't have to look it up. */
715 remove_dead_stmt (block_stmt_iterator *i, basic_block bb)
717 tree t = bsi_stmt (*i);
722 if (dump_file && (dump_flags & TDF_DETAILS))
724 fprintf (dump_file, "Deleting : ");
725 print_generic_stmt (dump_file, t, TDF_SLIM);
726 fprintf (dump_file, "\n");
731 /* If we have determined that a conditional branch statement contributes
732 nothing to the program, then we not only remove it, but we also change
733 the flow graph so that the current block will simply fall-thru to its
734 immediate post-dominator. The blocks we are circumventing will be
735 removed by cleaup_tree_cfg if this change in the flow graph makes them
737 if (is_ctrl_stmt (t))
739 basic_block post_dom_bb;
741 /* The post dominance info has to be up-to-date. */
742 gcc_assert (dom_computed[CDI_POST_DOMINATORS] == DOM_OK);
743 /* Get the immediate post dominator of bb. */
744 post_dom_bb = get_immediate_dominator (CDI_POST_DOMINATORS, bb);
745 /* Some blocks don't have an immediate post dominator. This can happen
746 for example with infinite loops. Removing an infinite loop is an
747 inappropriate transformation anyway... */
754 /* If the post dominator block has PHI nodes, we might be unable
755 to compute the right PHI args for them. Since the control
756 statement is unnecessary, all edges can be regarded as
757 equivalent, but we have to get rid of the condition, since it
758 might reference a variable that was determined to be
759 unnecessary and thus removed. */
760 if (phi_nodes (post_dom_bb))
761 post_dom_bb = EDGE_SUCC (bb, 0)->dest;
764 /* Redirect the first edge out of BB to reach POST_DOM_BB. */
765 redirect_edge_and_branch (EDGE_SUCC (bb, 0), post_dom_bb);
766 PENDING_STMT (EDGE_SUCC (bb, 0)) = NULL;
768 EDGE_SUCC (bb, 0)->probability = REG_BR_PROB_BASE;
769 EDGE_SUCC (bb, 0)->count = bb->count;
771 /* The edge is no longer associated with a conditional, so it does
772 not have TRUE/FALSE flags. */
773 EDGE_SUCC (bb, 0)->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
775 /* If the edge reaches any block other than the exit, then it is a
776 fallthru edge; if it reaches the exit, then it is not a fallthru
778 if (post_dom_bb != EXIT_BLOCK_PTR)
779 EDGE_SUCC (bb, 0)->flags |= EDGE_FALLTHRU;
781 EDGE_SUCC (bb, 0)->flags &= ~EDGE_FALLTHRU;
783 /* Remove the remaining the outgoing edges. */
784 while (!single_succ_p (bb))
786 /* FIXME. When we remove the edge, we modify the CFG, which
787 in turn modifies the dominator and post-dominator tree.
788 Is it safe to postpone recomputing the dominator and
789 post-dominator tree until the end of this pass given that
790 the post-dominators are used above? */
792 remove_edge (EDGE_SUCC (bb, 1));
796 FOR_EACH_SSA_DEF_OPERAND (def_p, t, iter, SSA_OP_VIRTUAL_DEFS)
798 tree def = DEF_FROM_PTR (def_p);
799 mark_sym_for_renaming (SSA_NAME_VAR (def));
805 /* Print out removed statement statistics. */
810 if (dump_file && (dump_flags & (TDF_STATS|TDF_DETAILS)))
814 percg = ((float) stats.removed / (float) stats.total) * 100;
815 fprintf (dump_file, "Removed %d of %d statements (%d%%)\n",
816 stats.removed, stats.total, (int) percg);
818 if (stats.total_phis == 0)
821 percg = ((float) stats.removed_phis / (float) stats.total_phis) * 100;
823 fprintf (dump_file, "Removed %d of %d PHI nodes (%d%%)\n",
824 stats.removed_phis, stats.total_phis, (int) percg);
828 /* Initialization for this pass. Set up the used data structures. */
831 tree_dce_init (bool aggressive)
833 memset ((void *) &stats, 0, sizeof (stats));
839 control_dependence_map
840 = xmalloc (last_basic_block * sizeof (bitmap));
841 for (i = 0; i < last_basic_block; ++i)
842 control_dependence_map[i] = BITMAP_ALLOC (NULL);
844 last_stmt_necessary = sbitmap_alloc (last_basic_block);
845 sbitmap_zero (last_stmt_necessary);
848 processed = sbitmap_alloc (num_ssa_names + 1);
849 sbitmap_zero (processed);
851 worklist = VEC_alloc (tree, heap, 64);
855 /* Cleanup after this pass. */
858 tree_dce_done (bool aggressive)
864 for (i = 0; i < last_basic_block; ++i)
865 BITMAP_FREE (control_dependence_map[i]);
866 free (control_dependence_map);
868 sbitmap_free (visited_control_parents);
869 sbitmap_free (last_stmt_necessary);
872 sbitmap_free (processed);
874 VEC_free (tree, heap, worklist);
877 /* Main routine to eliminate dead code.
879 AGGRESSIVE controls the aggressiveness of the algorithm.
880 In conservative mode, we ignore control dependence and simply declare
881 all but the most trivially dead branches necessary. This mode is fast.
882 In aggressive mode, control dependences are taken into account, which
883 results in more dead code elimination, but at the cost of some time.
885 FIXME: Aggressive mode before PRE doesn't work currently because
886 the dominance info is not invalidated after DCE1. This is
887 not an issue right now because we only run aggressive DCE
888 as the last tree SSA pass, but keep this in mind when you
889 start experimenting with pass ordering. */
892 perform_tree_ssa_dce (bool aggressive)
894 struct edge_list *el = NULL;
896 tree_dce_init (aggressive);
900 /* Compute control dependence. */
901 timevar_push (TV_CONTROL_DEPENDENCES);
902 calculate_dominance_info (CDI_POST_DOMINATORS);
903 el = create_edge_list ();
904 find_all_control_dependences (el);
905 timevar_pop (TV_CONTROL_DEPENDENCES);
907 visited_control_parents = sbitmap_alloc (last_basic_block);
908 sbitmap_zero (visited_control_parents);
910 mark_dfs_back_edges ();
913 find_obviously_necessary_stmts (el);
915 propagate_necessity (el);
917 mark_really_necessary_kill_operand_phis ();
918 eliminate_unnecessary_stmts ();
921 free_dominance_info (CDI_POST_DOMINATORS);
923 /* If we removed paths in the CFG, then we need to update
924 dominators as well. I haven't investigated the possibility
925 of incrementally updating dominators. */
927 free_dominance_info (CDI_DOMINATORS);
929 /* Debugging dumps. */
933 tree_dce_done (aggressive);
938 /* Pass entry points. */
942 perform_tree_ssa_dce (/*aggressive=*/false);
946 tree_ssa_dce_loop (void)
948 perform_tree_ssa_dce (/*aggressive=*/false);
949 free_numbers_of_iterations_estimates (current_loops);
953 tree_ssa_cd_dce (void)
955 perform_tree_ssa_dce (/*aggressive=*/optimize >= 2);
961 return flag_tree_dce != 0;
964 struct tree_opt_pass pass_dce =
968 tree_ssa_dce, /* execute */
971 0, /* static_pass_number */
972 TV_TREE_DCE, /* tv_id */
973 PROP_cfg | PROP_ssa | PROP_alias, /* properties_required */
974 0, /* properties_provided */
975 0, /* properties_destroyed */
976 0, /* todo_flags_start */
978 | TODO_update_ssa_no_phi
981 | TODO_verify_ssa, /* todo_flags_finish */
985 struct tree_opt_pass pass_dce_loop =
987 "dceloop", /* name */
989 tree_ssa_dce_loop, /* execute */
992 0, /* static_pass_number */
993 TV_TREE_DCE, /* tv_id */
994 PROP_cfg | PROP_ssa | PROP_alias, /* properties_required */
995 0, /* properties_provided */
996 0, /* properties_destroyed */
997 0, /* todo_flags_start */
999 | TODO_update_ssa_no_phi
1001 | TODO_verify_ssa, /* todo_flags_finish */
1005 struct tree_opt_pass pass_cd_dce =
1008 gate_dce, /* gate */
1009 tree_ssa_cd_dce, /* execute */
1012 0, /* static_pass_number */
1013 TV_TREE_CD_DCE, /* tv_id */
1014 PROP_cfg | PROP_ssa | PROP_alias, /* properties_required */
1015 0, /* properties_provided */
1016 0, /* properties_destroyed */
1017 0, /* todo_flags_start */
1019 | TODO_update_ssa_no_phi
1023 | TODO_verify_flow, /* todo_flags_finish */