1 /* Dead code elimination pass for the GNU compiler.
2 Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
3 Free Software Foundation, Inc.
4 Contributed by Ben Elliston <bje@redhat.com>
5 and Andrew MacLeod <amacleod@redhat.com>
6 Adapted to use control dependence by Steven Bosscher, SUSE Labs.
8 This file is part of GCC.
10 GCC is free software; you can redistribute it and/or modify it
11 under the terms of the GNU General Public License as published by the
12 Free Software Foundation; either version 3, or (at your option) any
15 GCC is distributed in the hope that it will be useful, but WITHOUT
16 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
17 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
20 You should have received a copy of the GNU General Public License
21 along with GCC; see the file COPYING3. If not see
22 <http://www.gnu.org/licenses/>. */
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 #include "tree-pretty-print.h"
53 #include "gimple-pretty-print.h"
54 #include "basic-block.h"
55 #include "tree-flow.h"
57 #include "tree-dump.h"
58 #include "tree-pass.h"
62 #include "tree-scalar-evolution.h"
64 static struct stmt_stats
72 #define STMT_NECESSARY GF_PLF_1
74 static VEC(gimple,heap) *worklist;
76 /* Vector indicating an SSA name has already been processed and marked
78 static sbitmap processed;
80 /* Vector indicating that the last statement of a basic block has already
81 been marked as necessary. */
82 static sbitmap last_stmt_necessary;
84 /* Vector indicating that BB contains statements that are live. */
85 static sbitmap bb_contains_live_stmts;
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 that follows the macro for each edge (given number
108 EDGE_NUMBER within the CODE) for which the block with index N is
109 control dependent. */
110 #define EXECUTE_IF_CONTROL_DEPENDENT(BI, N, EDGE_NUMBER) \
111 EXECUTE_IF_SET_IN_BITMAP (control_dependence_map[(N)], 0, \
115 /* Indicate block BB is control dependent on an edge with index EDGE_INDEX. */
117 set_control_dependence_map_bit (basic_block bb, int edge_index)
119 if (bb == ENTRY_BLOCK_PTR)
121 gcc_assert (bb != EXIT_BLOCK_PTR);
122 bitmap_set_bit (control_dependence_map[bb->index], edge_index);
125 /* Clear all control dependences for block BB. */
127 clear_control_dependence_bitmap (basic_block bb)
129 bitmap_clear (control_dependence_map[bb->index]);
133 /* Find the immediate postdominator PDOM of the specified basic block BLOCK.
134 This function is necessary because some blocks have negative numbers. */
136 static inline basic_block
137 find_pdom (basic_block block)
139 gcc_assert (block != ENTRY_BLOCK_PTR);
141 if (block == EXIT_BLOCK_PTR)
142 return EXIT_BLOCK_PTR;
145 basic_block bb = get_immediate_dominator (CDI_POST_DOMINATORS, block);
147 return EXIT_BLOCK_PTR;
153 /* Determine all blocks' control dependences on the given edge with edge_list
154 EL index EDGE_INDEX, ala Morgan, Section 3.6. */
157 find_control_dependence (struct edge_list *el, int edge_index)
159 basic_block current_block;
160 basic_block ending_block;
162 gcc_assert (INDEX_EDGE_PRED_BB (el, edge_index) != EXIT_BLOCK_PTR);
164 if (INDEX_EDGE_PRED_BB (el, edge_index) == ENTRY_BLOCK_PTR)
165 ending_block = single_succ (ENTRY_BLOCK_PTR);
167 ending_block = find_pdom (INDEX_EDGE_PRED_BB (el, edge_index));
169 for (current_block = INDEX_EDGE_SUCC_BB (el, edge_index);
170 current_block != ending_block && current_block != EXIT_BLOCK_PTR;
171 current_block = find_pdom (current_block))
173 edge e = INDEX_EDGE (el, edge_index);
175 /* For abnormal edges, we don't make current_block control
176 dependent because instructions that throw are always necessary
178 if (e->flags & EDGE_ABNORMAL)
181 set_control_dependence_map_bit (current_block, edge_index);
186 /* Record all blocks' control dependences on all edges in the edge
187 list EL, ala Morgan, Section 3.6. */
190 find_all_control_dependences (struct edge_list *el)
194 for (i = 0; i < NUM_EDGES (el); ++i)
195 find_control_dependence (el, i);
198 /* If STMT is not already marked necessary, mark it, and add it to the
199 worklist if ADD_TO_WORKLIST is true. */
202 mark_stmt_necessary (gimple stmt, bool add_to_worklist)
206 if (gimple_plf (stmt, STMT_NECESSARY))
209 if (dump_file && (dump_flags & TDF_DETAILS))
211 fprintf (dump_file, "Marking useful stmt: ");
212 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
213 fprintf (dump_file, "\n");
216 gimple_set_plf (stmt, STMT_NECESSARY, true);
218 VEC_safe_push (gimple, heap, worklist, stmt);
219 if (bb_contains_live_stmts && !is_gimple_debug (stmt))
220 SET_BIT (bb_contains_live_stmts, gimple_bb (stmt)->index);
224 /* Mark the statement defining operand OP as necessary. */
227 mark_operand_necessary (tree op)
234 ver = SSA_NAME_VERSION (op);
235 if (TEST_BIT (processed, ver))
237 stmt = SSA_NAME_DEF_STMT (op);
238 gcc_assert (gimple_nop_p (stmt)
239 || gimple_plf (stmt, STMT_NECESSARY));
242 SET_BIT (processed, ver);
244 stmt = SSA_NAME_DEF_STMT (op);
247 if (gimple_plf (stmt, STMT_NECESSARY) || gimple_nop_p (stmt))
250 if (dump_file && (dump_flags & TDF_DETAILS))
252 fprintf (dump_file, "marking necessary through ");
253 print_generic_expr (dump_file, op, 0);
254 fprintf (dump_file, " stmt ");
255 print_gimple_stmt (dump_file, stmt, 0, 0);
258 gimple_set_plf (stmt, STMT_NECESSARY, true);
259 if (bb_contains_live_stmts)
260 SET_BIT (bb_contains_live_stmts, gimple_bb (stmt)->index);
261 VEC_safe_push (gimple, heap, worklist, stmt);
265 /* Mark STMT as necessary if it obviously is. Add it to the worklist if
266 it can make other statements necessary.
268 If AGGRESSIVE is false, control statements are conservatively marked as
272 mark_stmt_if_obviously_necessary (gimple stmt, bool aggressive)
274 /* With non-call exceptions, we have to assume that all statements could
275 throw. If a statement may throw, it is inherently necessary. */
276 if (cfun->can_throw_non_call_exceptions && stmt_could_throw_p (stmt))
278 mark_stmt_necessary (stmt, true);
282 /* Statements that are implicitly live. Most function calls, asm
283 and return statements are required. Labels and GIMPLE_BIND nodes
284 are kept because they are control flow, and we have no way of
285 knowing whether they can be removed. DCE can eliminate all the
286 other statements in a block, and CFG can then remove the block
288 switch (gimple_code (stmt))
292 mark_stmt_necessary (stmt, false);
298 mark_stmt_necessary (stmt, true);
302 /* Most, but not all function calls are required. Function calls that
303 produce no result and have no side effects (i.e. const pure
304 functions) are unnecessary. */
305 if (gimple_has_side_effects (stmt))
307 mark_stmt_necessary (stmt, true);
310 if (!gimple_call_lhs (stmt))
315 /* Debug temps without a value are not useful. ??? If we could
316 easily locate the debug temp bind stmt for a use thereof,
317 would could refrain from marking all debug temps here, and
318 mark them only if they're used. */
319 if (gimple_debug_bind_has_value_p (stmt)
320 || TREE_CODE (gimple_debug_bind_get_var (stmt)) != DEBUG_EXPR_DECL)
321 mark_stmt_necessary (stmt, false);
325 gcc_assert (!simple_goto_p (stmt));
326 mark_stmt_necessary (stmt, true);
330 gcc_assert (EDGE_COUNT (gimple_bb (stmt)->succs) == 2);
335 mark_stmt_necessary (stmt, true);
342 /* If the statement has volatile operands, it needs to be preserved.
343 Same for statements that can alter control flow in unpredictable
345 if (gimple_has_volatile_ops (stmt) || is_ctrl_altering_stmt (stmt))
347 mark_stmt_necessary (stmt, true);
351 if (is_hidden_global_store (stmt))
353 mark_stmt_necessary (stmt, true);
361 /* Mark the last statement of BB as necessary. */
364 mark_last_stmt_necessary (basic_block bb)
366 gimple stmt = last_stmt (bb);
368 SET_BIT (last_stmt_necessary, bb->index);
369 SET_BIT (bb_contains_live_stmts, bb->index);
371 /* We actually mark the statement only if it is a control statement. */
372 if (stmt && is_ctrl_stmt (stmt))
373 mark_stmt_necessary (stmt, true);
377 /* Mark control dependent edges of BB as necessary. We have to do this only
378 once for each basic block so we set the appropriate bit after we're done.
380 When IGNORE_SELF is true, ignore BB in the list of control dependences. */
383 mark_control_dependent_edges_necessary (basic_block bb, struct edge_list *el,
387 unsigned edge_number;
388 bool skipped = false;
390 gcc_assert (bb != EXIT_BLOCK_PTR);
392 if (bb == ENTRY_BLOCK_PTR)
395 EXECUTE_IF_CONTROL_DEPENDENT (bi, bb->index, edge_number)
397 basic_block cd_bb = INDEX_EDGE_PRED_BB (el, edge_number);
399 if (ignore_self && cd_bb == bb)
405 if (!TEST_BIT (last_stmt_necessary, cd_bb->index))
406 mark_last_stmt_necessary (cd_bb);
410 SET_BIT (visited_control_parents, bb->index);
414 /* Find obviously necessary statements. These are things like most function
415 calls, and stores to file level variables.
417 If EL is NULL, control statements are conservatively marked as
418 necessary. Otherwise it contains the list of edges used by control
419 dependence analysis. */
422 find_obviously_necessary_stmts (struct edge_list *el)
425 gimple_stmt_iterator gsi;
432 /* PHI nodes are never inherently necessary. */
433 for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
435 phi = gsi_stmt (gsi);
436 gimple_set_plf (phi, STMT_NECESSARY, false);
439 /* Check all statements in the block. */
440 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
442 stmt = gsi_stmt (gsi);
443 gimple_set_plf (stmt, STMT_NECESSARY, false);
444 mark_stmt_if_obviously_necessary (stmt, el != NULL);
448 /* Pure and const functions are finite and thus have no infinite loops in
450 flags = flags_from_decl_or_type (current_function_decl);
451 if ((flags & (ECF_CONST|ECF_PURE)) && !(flags & ECF_LOOPING_CONST_OR_PURE))
454 /* Prevent the empty possibly infinite loops from being removed. */
460 if (mark_irreducible_loops ())
464 FOR_EACH_EDGE (e, ei, bb->succs)
465 if ((e->flags & EDGE_DFS_BACK)
466 && (e->flags & EDGE_IRREDUCIBLE_LOOP))
469 fprintf (dump_file, "Marking back edge of irreducible loop %i->%i\n",
470 e->src->index, e->dest->index);
471 mark_control_dependent_edges_necessary (e->dest, el, false);
475 FOR_EACH_LOOP (li, loop, 0)
476 if (!finite_loop_p (loop))
479 fprintf (dump_file, "can not prove finiteness of loop %i\n", loop->num);
480 mark_control_dependent_edges_necessary (loop->latch, el, false);
487 /* Return true if REF is based on an aliased base, otherwise false. */
490 ref_may_be_aliased (tree ref)
492 while (handled_component_p (ref))
493 ref = TREE_OPERAND (ref, 0);
494 if (TREE_CODE (ref) == MEM_REF
495 && TREE_CODE (TREE_OPERAND (ref, 0)) == ADDR_EXPR)
496 ref = TREE_OPERAND (TREE_OPERAND (ref, 0), 0);
497 return !(DECL_P (ref)
498 && !may_be_aliased (ref));
501 static bitmap visited = NULL;
502 static unsigned int longest_chain = 0;
503 static unsigned int total_chain = 0;
504 static unsigned int nr_walks = 0;
505 static bool chain_ovfl = false;
507 /* Worker for the walker that marks reaching definitions of REF,
508 which is based on a non-aliased decl, necessary. It returns
509 true whenever the defining statement of the current VDEF is
510 a kill for REF, as no dominating may-defs are necessary for REF
511 anymore. DATA points to the basic-block that contains the
512 stmt that refers to REF. */
515 mark_aliased_reaching_defs_necessary_1 (ao_ref *ref, tree vdef, void *data)
517 gimple def_stmt = SSA_NAME_DEF_STMT (vdef);
519 /* All stmts we visit are necessary. */
520 mark_operand_necessary (vdef);
522 /* If the stmt lhs kills ref, then we can stop walking. */
523 if (gimple_has_lhs (def_stmt)
524 && TREE_CODE (gimple_get_lhs (def_stmt)) != SSA_NAME)
526 tree base, lhs = gimple_get_lhs (def_stmt);
527 HOST_WIDE_INT size, offset, max_size;
529 base = get_ref_base_and_extent (lhs, &offset, &size, &max_size);
530 /* We can get MEM[symbol: sZ, index: D.8862_1] here,
531 so base == refd->base does not always hold. */
532 if (base == ref->base)
534 /* For a must-alias check we need to be able to constrain
535 the accesses properly. */
536 if (size != -1 && size == max_size
537 && ref->max_size != -1)
539 if (offset <= ref->offset
540 && offset + size >= ref->offset + ref->max_size)
543 /* Or they need to be exactly the same. */
545 /* Make sure there is no induction variable involved
546 in the references (gcc.c-torture/execute/pr42142.c).
547 The simplest way is to check if the kill dominates
549 && dominated_by_p (CDI_DOMINATORS, (basic_block) data,
550 gimple_bb (def_stmt))
551 && operand_equal_p (ref->ref, lhs, 0))
556 /* Otherwise keep walking. */
561 mark_aliased_reaching_defs_necessary (gimple stmt, tree ref)
565 gcc_assert (!chain_ovfl);
566 ao_ref_init (&refd, ref);
567 chain = walk_aliased_vdefs (&refd, gimple_vuse (stmt),
568 mark_aliased_reaching_defs_necessary_1,
569 gimple_bb (stmt), NULL);
570 if (chain > longest_chain)
571 longest_chain = chain;
572 total_chain += chain;
576 /* Worker for the walker that marks reaching definitions of REF, which
577 is not based on a non-aliased decl. For simplicity we need to end
578 up marking all may-defs necessary that are not based on a non-aliased
579 decl. The only job of this walker is to skip may-defs based on
580 a non-aliased decl. */
583 mark_all_reaching_defs_necessary_1 (ao_ref *ref ATTRIBUTE_UNUSED,
584 tree vdef, void *data ATTRIBUTE_UNUSED)
586 gimple def_stmt = SSA_NAME_DEF_STMT (vdef);
588 /* We have to skip already visited (and thus necessary) statements
589 to make the chaining work after we dropped back to simple mode. */
591 && TEST_BIT (processed, SSA_NAME_VERSION (vdef)))
593 gcc_assert (gimple_nop_p (def_stmt)
594 || gimple_plf (def_stmt, STMT_NECESSARY));
598 /* We want to skip stores to non-aliased variables. */
600 && gimple_assign_single_p (def_stmt))
602 tree lhs = gimple_assign_lhs (def_stmt);
603 if (!ref_may_be_aliased (lhs))
607 mark_operand_necessary (vdef);
613 mark_all_reaching_defs_necessary (gimple stmt)
615 walk_aliased_vdefs (NULL, gimple_vuse (stmt),
616 mark_all_reaching_defs_necessary_1, NULL, &visited);
619 /* Return true for PHI nodes with one or identical arguments
622 degenerate_phi_p (gimple phi)
625 tree op = gimple_phi_arg_def (phi, 0);
626 for (i = 1; i < gimple_phi_num_args (phi); i++)
627 if (gimple_phi_arg_def (phi, i) != op)
632 /* Propagate necessity using the operands of necessary statements.
633 Process the uses on each statement in the worklist, and add all
634 feeding statements which contribute to the calculation of this
635 value to the worklist.
637 In conservative mode, EL is NULL. */
640 propagate_necessity (struct edge_list *el)
643 bool aggressive = (el ? true : false);
645 if (dump_file && (dump_flags & TDF_DETAILS))
646 fprintf (dump_file, "\nProcessing worklist:\n");
648 while (VEC_length (gimple, worklist) > 0)
650 /* Take STMT from worklist. */
651 stmt = VEC_pop (gimple, worklist);
653 if (dump_file && (dump_flags & TDF_DETAILS))
655 fprintf (dump_file, "processing: ");
656 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
657 fprintf (dump_file, "\n");
662 /* Mark the last statement of the basic blocks on which the block
663 containing STMT is control dependent, but only if we haven't
665 basic_block bb = gimple_bb (stmt);
666 if (bb != ENTRY_BLOCK_PTR
667 && !TEST_BIT (visited_control_parents, bb->index))
668 mark_control_dependent_edges_necessary (bb, el, false);
671 if (gimple_code (stmt) == GIMPLE_PHI
672 /* We do not process virtual PHI nodes nor do we track their
674 && is_gimple_reg (gimple_phi_result (stmt)))
676 /* PHI nodes are somewhat special in that each PHI alternative has
677 data and control dependencies. All the statements feeding the
678 PHI node's arguments are always necessary. In aggressive mode,
679 we also consider the control dependent edges leading to the
680 predecessor block associated with each PHI alternative as
684 for (k = 0; k < gimple_phi_num_args (stmt); k++)
686 tree arg = PHI_ARG_DEF (stmt, k);
687 if (TREE_CODE (arg) == SSA_NAME)
688 mark_operand_necessary (arg);
691 /* For PHI operands it matters from where the control flow arrives
692 to the BB. Consider the following example:
702 We need to mark control dependence of the empty basic blocks, since they
703 contains computation of PHI operands.
705 Doing so is too restrictive in the case the predecestor block is in
711 for (i = 0; i<1000; ++i)
717 There is PHI for J in the BB containing return statement.
718 In this case the control dependence of predecestor block (that is
719 within the empty loop) also contains the block determining number
720 of iterations of the block that would prevent removing of empty
723 This scenario can be avoided by splitting critical edges.
724 To save the critical edge splitting pass we identify how the control
725 dependence would look like if the edge was split.
727 Consider the modified CFG created from current CFG by splitting
728 edge B->C. In the postdominance tree of modified CFG, C' is
729 always child of C. There are two cases how chlids of C' can look
734 In this case the only basic block C' is control dependent on is B.
736 2) C' has single child that is B
738 In this case control dependence of C' is same as control
739 dependence of B in original CFG except for block B itself.
740 (since C' postdominate B in modified CFG)
742 Now how to decide what case happens? There are two basic options:
744 a) C postdominate B. Then C immediately postdominate B and
745 case 2 happens iff there is no other way from B to C except
748 There is other way from B to C iff there is succesor of B that
749 is not postdominated by B. Testing this condition is somewhat
750 expensive, because we need to iterate all succesors of B.
751 We are safe to assume that this does not happen: we will mark B
752 as needed when processing the other path from B to C that is
753 conrol dependent on B and marking control dependencies of B
754 itself is harmless because they will be processed anyway after
755 processing control statement in B.
757 b) C does not postdominate B. Always case 1 happens since there is
758 path from C to exit that does not go through B and thus also C'. */
760 if (aggressive && !degenerate_phi_p (stmt))
762 for (k = 0; k < gimple_phi_num_args (stmt); k++)
764 basic_block arg_bb = gimple_phi_arg_edge (stmt, k)->src;
767 != get_immediate_dominator (CDI_POST_DOMINATORS, arg_bb))
769 if (!TEST_BIT (last_stmt_necessary, arg_bb->index))
770 mark_last_stmt_necessary (arg_bb);
772 else if (arg_bb != ENTRY_BLOCK_PTR
773 && !TEST_BIT (visited_control_parents,
775 mark_control_dependent_edges_necessary (arg_bb, el, true);
781 /* Propagate through the operands. Examine all the USE, VUSE and
782 VDEF operands in this statement. Mark all the statements
783 which feed this statement's uses as necessary. */
787 FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
788 mark_operand_necessary (use);
790 use = gimple_vuse (stmt);
794 /* If we dropped to simple mode make all immediately
795 reachable definitions necessary. */
798 mark_all_reaching_defs_necessary (stmt);
802 /* For statements that may load from memory (have a VUSE) we
803 have to mark all reaching (may-)definitions as necessary.
804 We partition this task into two cases:
805 1) explicit loads based on decls that are not aliased
806 2) implicit loads (like calls) and explicit loads not
807 based on decls that are not aliased (like indirect
808 references or loads from globals)
809 For 1) we mark all reaching may-defs as necessary, stopping
810 at dominating kills. For 2) we want to mark all dominating
811 references necessary, but non-aliased ones which we handle
812 in 1). By keeping a global visited bitmap for references
813 we walk for 2) we avoid quadratic behavior for those. */
815 if (is_gimple_call (stmt))
817 tree callee = gimple_call_fndecl (stmt);
820 /* Calls to functions that are merely acting as barriers
821 or that only store to memory do not make any previous
823 if (callee != NULL_TREE
824 && DECL_BUILT_IN_CLASS (callee) == BUILT_IN_NORMAL
825 && (DECL_FUNCTION_CODE (callee) == BUILT_IN_MEMSET
826 || DECL_FUNCTION_CODE (callee) == BUILT_IN_MALLOC
827 || DECL_FUNCTION_CODE (callee) == BUILT_IN_FREE))
830 /* Calls implicitly load from memory, their arguments
831 in addition may explicitly perform memory loads. */
832 mark_all_reaching_defs_necessary (stmt);
833 for (i = 0; i < gimple_call_num_args (stmt); ++i)
835 tree arg = gimple_call_arg (stmt, i);
836 if (TREE_CODE (arg) == SSA_NAME
837 || is_gimple_min_invariant (arg))
839 if (!ref_may_be_aliased (arg))
840 mark_aliased_reaching_defs_necessary (stmt, arg);
843 else if (gimple_assign_single_p (stmt))
846 bool rhs_aliased = false;
847 /* If this is a load mark things necessary. */
848 rhs = gimple_assign_rhs1 (stmt);
849 if (TREE_CODE (rhs) != SSA_NAME
850 && !is_gimple_min_invariant (rhs))
852 if (!ref_may_be_aliased (rhs))
853 mark_aliased_reaching_defs_necessary (stmt, rhs);
858 mark_all_reaching_defs_necessary (stmt);
860 else if (gimple_code (stmt) == GIMPLE_RETURN)
862 tree rhs = gimple_return_retval (stmt);
863 /* A return statement may perform a load. */
865 && TREE_CODE (rhs) != SSA_NAME
866 && !is_gimple_min_invariant (rhs))
868 if (!ref_may_be_aliased (rhs))
869 mark_aliased_reaching_defs_necessary (stmt, rhs);
871 mark_all_reaching_defs_necessary (stmt);
874 else if (gimple_code (stmt) == GIMPLE_ASM)
877 mark_all_reaching_defs_necessary (stmt);
878 /* Inputs may perform loads. */
879 for (i = 0; i < gimple_asm_ninputs (stmt); ++i)
881 tree op = TREE_VALUE (gimple_asm_input_op (stmt, i));
882 if (TREE_CODE (op) != SSA_NAME
883 && !is_gimple_min_invariant (op)
884 && !ref_may_be_aliased (op))
885 mark_aliased_reaching_defs_necessary (stmt, op);
891 /* If we over-used our alias oracle budget drop to simple
892 mode. The cost metric allows quadratic behavior
893 (number of uses times number of may-defs queries) up to
894 a constant maximal number of queries and after that falls back to
895 super-linear complexity. */
896 if (/* Constant but quadratic for small functions. */
897 total_chain > 128 * 128
898 /* Linear in the number of may-defs. */
899 && total_chain > 32 * longest_chain
900 /* Linear in the number of uses. */
901 && total_chain > nr_walks * 32)
905 bitmap_clear (visited);
911 /* Replace all uses of result of PHI by underlying variable and mark it
915 mark_virtual_phi_result_for_renaming (gimple phi)
918 imm_use_iterator iter;
921 tree result_ssa, result_var;
923 if (dump_file && (dump_flags & TDF_DETAILS))
925 fprintf (dump_file, "Marking result for renaming : ");
926 print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
927 fprintf (dump_file, "\n");
930 result_ssa = gimple_phi_result (phi);
931 result_var = SSA_NAME_VAR (result_ssa);
932 FOR_EACH_IMM_USE_STMT (stmt, iter, result_ssa)
934 FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
935 SET_USE (use_p, result_var);
940 mark_sym_for_renaming (result_var);
943 /* Remove dead PHI nodes from block BB. */
946 remove_dead_phis (basic_block bb)
948 bool something_changed = false;
951 gimple_stmt_iterator gsi;
952 phis = phi_nodes (bb);
954 for (gsi = gsi_start (phis); !gsi_end_p (gsi);)
957 phi = gsi_stmt (gsi);
959 /* We do not track necessity of virtual PHI nodes. Instead do
960 very simple dead PHI removal here. */
961 if (!is_gimple_reg (gimple_phi_result (phi)))
963 /* Virtual PHI nodes with one or identical arguments
965 if (degenerate_phi_p (phi))
967 tree vdef = gimple_phi_result (phi);
968 tree vuse = gimple_phi_arg_def (phi, 0);
971 imm_use_iterator iter;
973 FOR_EACH_IMM_USE_STMT (use_stmt, iter, vdef)
974 FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
975 SET_USE (use_p, vuse);
976 if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vdef)
977 && TREE_CODE (vuse) == SSA_NAME)
978 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vuse) = 1;
981 gimple_set_plf (phi, STMT_NECESSARY, true);
984 if (!gimple_plf (phi, STMT_NECESSARY))
986 something_changed = true;
987 if (dump_file && (dump_flags & TDF_DETAILS))
989 fprintf (dump_file, "Deleting : ");
990 print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
991 fprintf (dump_file, "\n");
994 remove_phi_node (&gsi, true);
995 stats.removed_phis++;
1001 return something_changed;
1004 /* Forward edge E to respective POST_DOM_BB and update PHIs. */
1007 forward_edge_to_pdom (edge e, basic_block post_dom_bb)
1009 gimple_stmt_iterator gsi;
1013 if (dump_file && (dump_flags & TDF_DETAILS))
1014 fprintf (dump_file, "Redirecting edge %i->%i to %i\n", e->src->index,
1015 e->dest->index, post_dom_bb->index);
1017 e2 = redirect_edge_and_branch (e, post_dom_bb);
1020 /* If edge was already around, no updating is neccesary. */
1024 if (!gimple_seq_empty_p (phi_nodes (post_dom_bb)))
1026 /* We are sure that for every live PHI we are seeing control dependent BB.
1027 This means that we can pick any edge to duplicate PHI args from. */
1028 FOR_EACH_EDGE (e2, ei, post_dom_bb->preds)
1031 for (gsi = gsi_start_phis (post_dom_bb); !gsi_end_p (gsi);)
1033 gimple phi = gsi_stmt (gsi);
1035 source_location locus;
1037 /* PHIs for virtuals have no control dependency relation on them.
1038 We are lost here and must force renaming of the symbol. */
1039 if (!is_gimple_reg (gimple_phi_result (phi)))
1041 mark_virtual_phi_result_for_renaming (phi);
1042 remove_phi_node (&gsi, true);
1046 /* Dead PHI do not imply control dependency. */
1047 if (!gimple_plf (phi, STMT_NECESSARY))
1053 op = gimple_phi_arg_def (phi, e2->dest_idx);
1054 locus = gimple_phi_arg_location (phi, e2->dest_idx);
1055 add_phi_arg (phi, op, e, locus);
1056 /* The resulting PHI if not dead can only be degenerate. */
1057 gcc_assert (degenerate_phi_p (phi));
1064 /* Remove dead statement pointed to by iterator I. Receives the basic block BB
1065 containing I so that we don't have to look it up. */
1068 remove_dead_stmt (gimple_stmt_iterator *i, basic_block bb)
1070 gimple stmt = gsi_stmt (*i);
1072 if (dump_file && (dump_flags & TDF_DETAILS))
1074 fprintf (dump_file, "Deleting : ");
1075 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
1076 fprintf (dump_file, "\n");
1081 /* If we have determined that a conditional branch statement contributes
1082 nothing to the program, then we not only remove it, but we also change
1083 the flow graph so that the current block will simply fall-thru to its
1084 immediate post-dominator. The blocks we are circumventing will be
1085 removed by cleanup_tree_cfg if this change in the flow graph makes them
1087 if (is_ctrl_stmt (stmt))
1089 basic_block post_dom_bb;
1093 post_dom_bb = get_immediate_dominator (CDI_POST_DOMINATORS, bb);
1095 e = find_edge (bb, post_dom_bb);
1097 /* If edge is already there, try to use it. This avoids need to update
1098 PHI nodes. Also watch for cases where post dominator does not exists
1099 or is exit block. These can happen for infinite loops as we create
1100 fake edges in the dominator tree. */
1103 else if (! post_dom_bb || post_dom_bb == EXIT_BLOCK_PTR)
1104 e = EDGE_SUCC (bb, 0);
1106 e = forward_edge_to_pdom (EDGE_SUCC (bb, 0), post_dom_bb);
1108 e->probability = REG_BR_PROB_BASE;
1109 e->count = bb->count;
1111 /* The edge is no longer associated with a conditional, so it does
1112 not have TRUE/FALSE flags. */
1113 e->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
1115 /* The lone outgoing edge from BB will be a fallthru edge. */
1116 e->flags |= EDGE_FALLTHRU;
1118 /* Remove the remaining outgoing edges. */
1119 for (ei = ei_start (bb->succs); (e2 = ei_safe_edge (ei)); )
1129 unlink_stmt_vdef (stmt);
1130 gsi_remove (i, true);
1131 release_defs (stmt);
1134 /* Eliminate unnecessary statements. Any instruction not marked as necessary
1135 contributes nothing to the program, and can be deleted. */
1138 eliminate_unnecessary_stmts (void)
1140 bool something_changed = false;
1142 gimple_stmt_iterator gsi, psi;
1145 VEC (basic_block, heap) *h;
1147 if (dump_file && (dump_flags & TDF_DETAILS))
1148 fprintf (dump_file, "\nEliminating unnecessary statements:\n");
1150 clear_special_calls ();
1152 /* Walking basic blocks and statements in reverse order avoids
1153 releasing SSA names before any other DEFs that refer to them are
1154 released. This helps avoid loss of debug information, as we get
1155 a chance to propagate all RHSs of removed SSAs into debug uses,
1156 rather than only the latest ones. E.g., consider:
1162 If we were to release x_3 before a_5, when we reached a_5 and
1163 tried to substitute it into the debug stmt, we'd see x_3 there,
1164 but x_3's DEF, type, etc would have already been disconnected.
1165 By going backwards, the debug stmt first changes to:
1167 # DEBUG a => x_3 - b_4
1171 # DEBUG a => y_1 + z_2 - b_4
1174 gcc_assert (dom_info_available_p (CDI_DOMINATORS));
1175 h = get_all_dominated_blocks (CDI_DOMINATORS, single_succ (ENTRY_BLOCK_PTR));
1177 while (VEC_length (basic_block, h))
1179 bb = VEC_pop (basic_block, h);
1181 /* Remove dead statements. */
1182 for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi = psi)
1184 stmt = gsi_stmt (gsi);
1191 /* If GSI is not necessary then remove it. */
1192 if (!gimple_plf (stmt, STMT_NECESSARY))
1194 if (!is_gimple_debug (stmt))
1195 something_changed = true;
1196 remove_dead_stmt (&gsi, bb);
1198 else if (is_gimple_call (stmt))
1200 call = gimple_call_fndecl (stmt);
1205 /* When LHS of var = call (); is dead, simplify it into
1206 call (); saving one operand. */
1207 name = gimple_call_lhs (stmt);
1208 if (name && TREE_CODE (name) == SSA_NAME
1209 && !TEST_BIT (processed, SSA_NAME_VERSION (name)))
1211 something_changed = true;
1212 if (dump_file && (dump_flags & TDF_DETAILS))
1214 fprintf (dump_file, "Deleting LHS of call: ");
1215 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
1216 fprintf (dump_file, "\n");
1219 gimple_call_set_lhs (stmt, NULL_TREE);
1220 maybe_clean_or_replace_eh_stmt (stmt, stmt);
1222 release_ssa_name (name);
1224 notice_special_calls (stmt);
1230 VEC_free (basic_block, heap, h);
1232 /* Since we don't track liveness of virtual PHI nodes, it is possible that we
1233 rendered some PHI nodes unreachable while they are still in use.
1234 Mark them for renaming. */
1237 basic_block prev_bb;
1239 find_unreachable_blocks ();
1241 /* Delete all unreachable basic blocks in reverse dominator order. */
1242 for (bb = EXIT_BLOCK_PTR->prev_bb; bb != ENTRY_BLOCK_PTR; bb = prev_bb)
1244 prev_bb = bb->prev_bb;
1246 if (!TEST_BIT (bb_contains_live_stmts, bb->index)
1247 || !(bb->flags & BB_REACHABLE))
1249 for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
1250 if (!is_gimple_reg (gimple_phi_result (gsi_stmt (gsi))))
1253 imm_use_iterator iter;
1255 FOR_EACH_IMM_USE_STMT (stmt, iter, gimple_phi_result (gsi_stmt (gsi)))
1257 if (!(gimple_bb (stmt)->flags & BB_REACHABLE))
1259 if (gimple_code (stmt) == GIMPLE_PHI
1260 || gimple_plf (stmt, STMT_NECESSARY))
1263 BREAK_FROM_IMM_USE_STMT (iter);
1267 mark_virtual_phi_result_for_renaming (gsi_stmt (gsi));
1270 if (!(bb->flags & BB_REACHABLE))
1272 /* Speed up the removal of blocks that don't
1273 dominate others. Walking backwards, this should
1274 be the common case. ??? Do we need to recompute
1275 dominators because of cfg_altered? */
1276 if (!MAY_HAVE_DEBUG_STMTS
1277 || !first_dom_son (CDI_DOMINATORS, bb))
1278 delete_basic_block (bb);
1281 h = get_all_dominated_blocks (CDI_DOMINATORS, bb);
1283 while (VEC_length (basic_block, h))
1285 bb = VEC_pop (basic_block, h);
1286 prev_bb = bb->prev_bb;
1287 /* Rearrangements to the CFG may have failed
1288 to update the dominators tree, so that
1289 formerly-dominated blocks are now
1290 otherwise reachable. */
1291 if (!!(bb->flags & BB_REACHABLE))
1293 delete_basic_block (bb);
1296 VEC_free (basic_block, heap, h);
1304 /* Remove dead PHI nodes. */
1305 something_changed |= remove_dead_phis (bb);
1308 return something_changed;
1312 /* Print out removed statement statistics. */
1319 percg = ((float) stats.removed / (float) stats.total) * 100;
1320 fprintf (dump_file, "Removed %d of %d statements (%d%%)\n",
1321 stats.removed, stats.total, (int) percg);
1323 if (stats.total_phis == 0)
1326 percg = ((float) stats.removed_phis / (float) stats.total_phis) * 100;
1328 fprintf (dump_file, "Removed %d of %d PHI nodes (%d%%)\n",
1329 stats.removed_phis, stats.total_phis, (int) percg);
1332 /* Initialization for this pass. Set up the used data structures. */
1335 tree_dce_init (bool aggressive)
1337 memset ((void *) &stats, 0, sizeof (stats));
1343 control_dependence_map = XNEWVEC (bitmap, last_basic_block);
1344 for (i = 0; i < last_basic_block; ++i)
1345 control_dependence_map[i] = BITMAP_ALLOC (NULL);
1347 last_stmt_necessary = sbitmap_alloc (last_basic_block);
1348 sbitmap_zero (last_stmt_necessary);
1349 bb_contains_live_stmts = sbitmap_alloc (last_basic_block);
1350 sbitmap_zero (bb_contains_live_stmts);
1353 processed = sbitmap_alloc (num_ssa_names + 1);
1354 sbitmap_zero (processed);
1356 worklist = VEC_alloc (gimple, heap, 64);
1357 cfg_altered = false;
1360 /* Cleanup after this pass. */
1363 tree_dce_done (bool aggressive)
1369 for (i = 0; i < last_basic_block; ++i)
1370 BITMAP_FREE (control_dependence_map[i]);
1371 free (control_dependence_map);
1373 sbitmap_free (visited_control_parents);
1374 sbitmap_free (last_stmt_necessary);
1375 sbitmap_free (bb_contains_live_stmts);
1376 bb_contains_live_stmts = NULL;
1379 sbitmap_free (processed);
1381 VEC_free (gimple, heap, worklist);
1384 /* Main routine to eliminate dead code.
1386 AGGRESSIVE controls the aggressiveness of the algorithm.
1387 In conservative mode, we ignore control dependence and simply declare
1388 all but the most trivially dead branches necessary. This mode is fast.
1389 In aggressive mode, control dependences are taken into account, which
1390 results in more dead code elimination, but at the cost of some time.
1392 FIXME: Aggressive mode before PRE doesn't work currently because
1393 the dominance info is not invalidated after DCE1. This is
1394 not an issue right now because we only run aggressive DCE
1395 as the last tree SSA pass, but keep this in mind when you
1396 start experimenting with pass ordering. */
1399 perform_tree_ssa_dce (bool aggressive)
1401 struct edge_list *el = NULL;
1402 bool something_changed = 0;
1404 calculate_dominance_info (CDI_DOMINATORS);
1406 /* Preheaders are needed for SCEV to work.
1407 Simple lateches and recorded exits improve chances that loop will
1408 proved to be finite in testcases such as in loop-15.c and loop-24.c */
1410 loop_optimizer_init (LOOPS_NORMAL
1411 | LOOPS_HAVE_RECORDED_EXITS);
1413 tree_dce_init (aggressive);
1417 /* Compute control dependence. */
1418 timevar_push (TV_CONTROL_DEPENDENCES);
1419 calculate_dominance_info (CDI_POST_DOMINATORS);
1420 el = create_edge_list ();
1421 find_all_control_dependences (el);
1422 timevar_pop (TV_CONTROL_DEPENDENCES);
1424 visited_control_parents = sbitmap_alloc (last_basic_block);
1425 sbitmap_zero (visited_control_parents);
1427 mark_dfs_back_edges ();
1430 find_obviously_necessary_stmts (el);
1433 loop_optimizer_finalize ();
1439 visited = BITMAP_ALLOC (NULL);
1440 propagate_necessity (el);
1441 BITMAP_FREE (visited);
1443 something_changed |= eliminate_unnecessary_stmts ();
1444 something_changed |= cfg_altered;
1446 /* We do not update postdominators, so free them unconditionally. */
1447 free_dominance_info (CDI_POST_DOMINATORS);
1449 /* If we removed paths in the CFG, then we need to update
1450 dominators as well. I haven't investigated the possibility
1451 of incrementally updating dominators. */
1453 free_dominance_info (CDI_DOMINATORS);
1455 statistics_counter_event (cfun, "Statements deleted", stats.removed);
1456 statistics_counter_event (cfun, "PHI nodes deleted", stats.removed_phis);
1458 /* Debugging dumps. */
1459 if (dump_file && (dump_flags & (TDF_STATS|TDF_DETAILS)))
1462 tree_dce_done (aggressive);
1464 free_edge_list (el);
1466 if (something_changed)
1467 return (TODO_update_ssa | TODO_cleanup_cfg | TODO_ggc_collect
1468 | TODO_remove_unused_locals);
1473 /* Pass entry points. */
1477 return perform_tree_ssa_dce (/*aggressive=*/false);
1481 tree_ssa_dce_loop (void)
1484 todo = perform_tree_ssa_dce (/*aggressive=*/false);
1487 free_numbers_of_iterations_estimates ();
1494 tree_ssa_cd_dce (void)
1496 return perform_tree_ssa_dce (/*aggressive=*/optimize >= 2);
1502 return flag_tree_dce != 0;
1505 struct gimple_opt_pass pass_dce =
1510 gate_dce, /* gate */
1511 tree_ssa_dce, /* execute */
1514 0, /* static_pass_number */
1515 TV_TREE_DCE, /* tv_id */
1516 PROP_cfg | PROP_ssa, /* properties_required */
1517 0, /* properties_provided */
1518 0, /* properties_destroyed */
1519 0, /* todo_flags_start */
1520 TODO_dump_func | TODO_verify_ssa /* todo_flags_finish */
1524 struct gimple_opt_pass pass_dce_loop =
1528 "dceloop", /* name */
1529 gate_dce, /* gate */
1530 tree_ssa_dce_loop, /* execute */
1533 0, /* static_pass_number */
1534 TV_TREE_DCE, /* tv_id */
1535 PROP_cfg | PROP_ssa, /* properties_required */
1536 0, /* properties_provided */
1537 0, /* properties_destroyed */
1538 0, /* todo_flags_start */
1539 TODO_dump_func | TODO_verify_ssa /* todo_flags_finish */
1543 struct gimple_opt_pass pass_cd_dce =
1548 gate_dce, /* gate */
1549 tree_ssa_cd_dce, /* execute */
1552 0, /* static_pass_number */
1553 TV_TREE_CD_DCE, /* tv_id */
1554 PROP_cfg | PROP_ssa, /* properties_required */
1555 0, /* properties_provided */
1556 0, /* properties_destroyed */
1557 0, /* todo_flags_start */
1558 TODO_dump_func | TODO_verify_ssa
1559 | TODO_verify_flow /* todo_flags_finish */