1 /* Dead code elimination pass for the GNU compiler.
2 Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011
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);
303 tree callee = gimple_call_fndecl (stmt);
304 if (callee != NULL_TREE
305 && DECL_BUILT_IN_CLASS (callee) == BUILT_IN_NORMAL)
306 switch (DECL_FUNCTION_CODE (callee))
308 case BUILT_IN_MALLOC:
309 case BUILT_IN_CALLOC:
310 case BUILT_IN_ALLOCA:
315 /* Most, but not all function calls are required. Function calls that
316 produce no result and have no side effects (i.e. const pure
317 functions) are unnecessary. */
318 if (gimple_has_side_effects (stmt))
320 mark_stmt_necessary (stmt, true);
323 if (!gimple_call_lhs (stmt))
329 /* Debug temps without a value are not useful. ??? If we could
330 easily locate the debug temp bind stmt for a use thereof,
331 would could refrain from marking all debug temps here, and
332 mark them only if they're used. */
333 if (!gimple_debug_bind_p (stmt)
334 || gimple_debug_bind_has_value_p (stmt)
335 || TREE_CODE (gimple_debug_bind_get_var (stmt)) != DEBUG_EXPR_DECL)
336 mark_stmt_necessary (stmt, false);
340 gcc_assert (!simple_goto_p (stmt));
341 mark_stmt_necessary (stmt, true);
345 gcc_assert (EDGE_COUNT (gimple_bb (stmt)->succs) == 2);
350 mark_stmt_necessary (stmt, true);
357 /* If the statement has volatile operands, it needs to be preserved.
358 Same for statements that can alter control flow in unpredictable
360 if (gimple_has_volatile_ops (stmt) || is_ctrl_altering_stmt (stmt))
362 mark_stmt_necessary (stmt, true);
366 if (is_hidden_global_store (stmt))
368 mark_stmt_necessary (stmt, true);
376 /* Mark the last statement of BB as necessary. */
379 mark_last_stmt_necessary (basic_block bb)
381 gimple stmt = last_stmt (bb);
383 SET_BIT (last_stmt_necessary, bb->index);
384 SET_BIT (bb_contains_live_stmts, bb->index);
386 /* We actually mark the statement only if it is a control statement. */
387 if (stmt && is_ctrl_stmt (stmt))
388 mark_stmt_necessary (stmt, true);
392 /* Mark control dependent edges of BB as necessary. We have to do this only
393 once for each basic block so we set the appropriate bit after we're done.
395 When IGNORE_SELF is true, ignore BB in the list of control dependences. */
398 mark_control_dependent_edges_necessary (basic_block bb, struct edge_list *el,
402 unsigned edge_number;
403 bool skipped = false;
405 gcc_assert (bb != EXIT_BLOCK_PTR);
407 if (bb == ENTRY_BLOCK_PTR)
410 EXECUTE_IF_CONTROL_DEPENDENT (bi, bb->index, edge_number)
412 basic_block cd_bb = INDEX_EDGE_PRED_BB (el, edge_number);
414 if (ignore_self && cd_bb == bb)
420 if (!TEST_BIT (last_stmt_necessary, cd_bb->index))
421 mark_last_stmt_necessary (cd_bb);
425 SET_BIT (visited_control_parents, bb->index);
429 /* Find obviously necessary statements. These are things like most function
430 calls, and stores to file level variables.
432 If EL is NULL, control statements are conservatively marked as
433 necessary. Otherwise it contains the list of edges used by control
434 dependence analysis. */
437 find_obviously_necessary_stmts (struct edge_list *el)
440 gimple_stmt_iterator gsi;
447 /* PHI nodes are never inherently necessary. */
448 for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
450 phi = gsi_stmt (gsi);
451 gimple_set_plf (phi, STMT_NECESSARY, false);
454 /* Check all statements in the block. */
455 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
457 stmt = gsi_stmt (gsi);
458 gimple_set_plf (stmt, STMT_NECESSARY, false);
459 mark_stmt_if_obviously_necessary (stmt, el != NULL);
463 /* Pure and const functions are finite and thus have no infinite loops in
465 flags = flags_from_decl_or_type (current_function_decl);
466 if ((flags & (ECF_CONST|ECF_PURE)) && !(flags & ECF_LOOPING_CONST_OR_PURE))
469 /* Prevent the empty possibly infinite loops from being removed. */
475 if (mark_irreducible_loops ())
479 FOR_EACH_EDGE (e, ei, bb->succs)
480 if ((e->flags & EDGE_DFS_BACK)
481 && (e->flags & EDGE_IRREDUCIBLE_LOOP))
484 fprintf (dump_file, "Marking back edge of irreducible loop %i->%i\n",
485 e->src->index, e->dest->index);
486 mark_control_dependent_edges_necessary (e->dest, el, false);
490 FOR_EACH_LOOP (li, loop, 0)
491 if (!finite_loop_p (loop))
494 fprintf (dump_file, "can not prove finiteness of loop %i\n", loop->num);
495 mark_control_dependent_edges_necessary (loop->latch, el, false);
502 /* Return true if REF is based on an aliased base, otherwise false. */
505 ref_may_be_aliased (tree ref)
507 gcc_assert (TREE_CODE (ref) != WITH_SIZE_EXPR);
508 while (handled_component_p (ref))
509 ref = TREE_OPERAND (ref, 0);
510 if (TREE_CODE (ref) == MEM_REF
511 && TREE_CODE (TREE_OPERAND (ref, 0)) == ADDR_EXPR)
512 ref = TREE_OPERAND (TREE_OPERAND (ref, 0), 0);
513 return !(DECL_P (ref)
514 && !may_be_aliased (ref));
517 static bitmap visited = NULL;
518 static unsigned int longest_chain = 0;
519 static unsigned int total_chain = 0;
520 static unsigned int nr_walks = 0;
521 static bool chain_ovfl = false;
523 /* Worker for the walker that marks reaching definitions of REF,
524 which is based on a non-aliased decl, necessary. It returns
525 true whenever the defining statement of the current VDEF is
526 a kill for REF, as no dominating may-defs are necessary for REF
527 anymore. DATA points to the basic-block that contains the
528 stmt that refers to REF. */
531 mark_aliased_reaching_defs_necessary_1 (ao_ref *ref, tree vdef, void *data)
533 gimple def_stmt = SSA_NAME_DEF_STMT (vdef);
535 /* All stmts we visit are necessary. */
536 mark_operand_necessary (vdef);
538 /* If the stmt lhs kills ref, then we can stop walking. */
539 if (gimple_has_lhs (def_stmt)
540 && TREE_CODE (gimple_get_lhs (def_stmt)) != SSA_NAME
541 /* The assignment is not necessarily carried out if it can throw
542 and we can catch it in the current function where we could inspect
544 ??? We only need to care about the RHS throwing. For aggregate
545 assignments or similar calls and non-call exceptions the LHS
546 might throw as well. */
547 && !stmt_can_throw_internal (def_stmt))
549 tree base, lhs = gimple_get_lhs (def_stmt);
550 HOST_WIDE_INT size, offset, max_size;
552 base = get_ref_base_and_extent (lhs, &offset, &size, &max_size);
553 /* We can get MEM[symbol: sZ, index: D.8862_1] here,
554 so base == refd->base does not always hold. */
555 if (base == ref->base)
557 /* For a must-alias check we need to be able to constrain
558 the accesses properly. */
559 if (size != -1 && size == max_size
560 && ref->max_size != -1)
562 if (offset <= ref->offset
563 && offset + size >= ref->offset + ref->max_size)
566 /* Or they need to be exactly the same. */
568 /* Make sure there is no induction variable involved
569 in the references (gcc.c-torture/execute/pr42142.c).
570 The simplest way is to check if the kill dominates
572 && dominated_by_p (CDI_DOMINATORS, (basic_block) data,
573 gimple_bb (def_stmt))
574 && operand_equal_p (ref->ref, lhs, 0))
579 /* Otherwise keep walking. */
584 mark_aliased_reaching_defs_necessary (gimple stmt, tree ref)
588 gcc_assert (!chain_ovfl);
589 ao_ref_init (&refd, ref);
590 chain = walk_aliased_vdefs (&refd, gimple_vuse (stmt),
591 mark_aliased_reaching_defs_necessary_1,
592 gimple_bb (stmt), NULL);
593 if (chain > longest_chain)
594 longest_chain = chain;
595 total_chain += chain;
599 /* Worker for the walker that marks reaching definitions of REF, which
600 is not based on a non-aliased decl. For simplicity we need to end
601 up marking all may-defs necessary that are not based on a non-aliased
602 decl. The only job of this walker is to skip may-defs based on
603 a non-aliased decl. */
606 mark_all_reaching_defs_necessary_1 (ao_ref *ref ATTRIBUTE_UNUSED,
607 tree vdef, void *data ATTRIBUTE_UNUSED)
609 gimple def_stmt = SSA_NAME_DEF_STMT (vdef);
611 /* We have to skip already visited (and thus necessary) statements
612 to make the chaining work after we dropped back to simple mode. */
614 && TEST_BIT (processed, SSA_NAME_VERSION (vdef)))
616 gcc_assert (gimple_nop_p (def_stmt)
617 || gimple_plf (def_stmt, STMT_NECESSARY));
621 /* We want to skip stores to non-aliased variables. */
623 && gimple_assign_single_p (def_stmt))
625 tree lhs = gimple_assign_lhs (def_stmt);
626 if (!ref_may_be_aliased (lhs))
630 /* We want to skip statments that do not constitute stores but have
631 a virtual definition. */
632 if (is_gimple_call (def_stmt))
634 tree callee = gimple_call_fndecl (def_stmt);
635 if (callee != NULL_TREE
636 && DECL_BUILT_IN_CLASS (callee) == BUILT_IN_NORMAL)
637 switch (DECL_FUNCTION_CODE (callee))
639 case BUILT_IN_MALLOC:
640 case BUILT_IN_CALLOC:
641 case BUILT_IN_ALLOCA:
649 mark_operand_necessary (vdef);
655 mark_all_reaching_defs_necessary (gimple stmt)
657 walk_aliased_vdefs (NULL, gimple_vuse (stmt),
658 mark_all_reaching_defs_necessary_1, NULL, &visited);
661 /* Return true for PHI nodes with one or identical arguments
664 degenerate_phi_p (gimple phi)
667 tree op = gimple_phi_arg_def (phi, 0);
668 for (i = 1; i < gimple_phi_num_args (phi); i++)
669 if (gimple_phi_arg_def (phi, i) != op)
674 /* Propagate necessity using the operands of necessary statements.
675 Process the uses on each statement in the worklist, and add all
676 feeding statements which contribute to the calculation of this
677 value to the worklist.
679 In conservative mode, EL is NULL. */
682 propagate_necessity (struct edge_list *el)
685 bool aggressive = (el ? true : false);
687 if (dump_file && (dump_flags & TDF_DETAILS))
688 fprintf (dump_file, "\nProcessing worklist:\n");
690 while (VEC_length (gimple, worklist) > 0)
692 /* Take STMT from worklist. */
693 stmt = VEC_pop (gimple, worklist);
695 if (dump_file && (dump_flags & TDF_DETAILS))
697 fprintf (dump_file, "processing: ");
698 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
699 fprintf (dump_file, "\n");
704 /* Mark the last statement of the basic blocks on which the block
705 containing STMT is control dependent, but only if we haven't
707 basic_block bb = gimple_bb (stmt);
708 if (bb != ENTRY_BLOCK_PTR
709 && !TEST_BIT (visited_control_parents, bb->index))
710 mark_control_dependent_edges_necessary (bb, el, false);
713 if (gimple_code (stmt) == GIMPLE_PHI
714 /* We do not process virtual PHI nodes nor do we track their
716 && is_gimple_reg (gimple_phi_result (stmt)))
718 /* PHI nodes are somewhat special in that each PHI alternative has
719 data and control dependencies. All the statements feeding the
720 PHI node's arguments are always necessary. In aggressive mode,
721 we also consider the control dependent edges leading to the
722 predecessor block associated with each PHI alternative as
726 for (k = 0; k < gimple_phi_num_args (stmt); k++)
728 tree arg = PHI_ARG_DEF (stmt, k);
729 if (TREE_CODE (arg) == SSA_NAME)
730 mark_operand_necessary (arg);
733 /* For PHI operands it matters from where the control flow arrives
734 to the BB. Consider the following example:
744 We need to mark control dependence of the empty basic blocks, since they
745 contains computation of PHI operands.
747 Doing so is too restrictive in the case the predecestor block is in
753 for (i = 0; i<1000; ++i)
759 There is PHI for J in the BB containing return statement.
760 In this case the control dependence of predecestor block (that is
761 within the empty loop) also contains the block determining number
762 of iterations of the block that would prevent removing of empty
765 This scenario can be avoided by splitting critical edges.
766 To save the critical edge splitting pass we identify how the control
767 dependence would look like if the edge was split.
769 Consider the modified CFG created from current CFG by splitting
770 edge B->C. In the postdominance tree of modified CFG, C' is
771 always child of C. There are two cases how chlids of C' can look
776 In this case the only basic block C' is control dependent on is B.
778 2) C' has single child that is B
780 In this case control dependence of C' is same as control
781 dependence of B in original CFG except for block B itself.
782 (since C' postdominate B in modified CFG)
784 Now how to decide what case happens? There are two basic options:
786 a) C postdominate B. Then C immediately postdominate B and
787 case 2 happens iff there is no other way from B to C except
790 There is other way from B to C iff there is succesor of B that
791 is not postdominated by B. Testing this condition is somewhat
792 expensive, because we need to iterate all succesors of B.
793 We are safe to assume that this does not happen: we will mark B
794 as needed when processing the other path from B to C that is
795 conrol dependent on B and marking control dependencies of B
796 itself is harmless because they will be processed anyway after
797 processing control statement in B.
799 b) C does not postdominate B. Always case 1 happens since there is
800 path from C to exit that does not go through B and thus also C'. */
802 if (aggressive && !degenerate_phi_p (stmt))
804 for (k = 0; k < gimple_phi_num_args (stmt); k++)
806 basic_block arg_bb = gimple_phi_arg_edge (stmt, k)->src;
809 != get_immediate_dominator (CDI_POST_DOMINATORS, arg_bb))
811 if (!TEST_BIT (last_stmt_necessary, arg_bb->index))
812 mark_last_stmt_necessary (arg_bb);
814 else if (arg_bb != ENTRY_BLOCK_PTR
815 && !TEST_BIT (visited_control_parents,
817 mark_control_dependent_edges_necessary (arg_bb, el, true);
823 /* Propagate through the operands. Examine all the USE, VUSE and
824 VDEF operands in this statement. Mark all the statements
825 which feed this statement's uses as necessary. */
829 /* If this is a call to free which is directly fed by an
830 allocation function do not mark that necessary through
831 processing the argument. */
832 if (gimple_call_builtin_p (stmt, BUILT_IN_FREE))
834 tree ptr = gimple_call_arg (stmt, 0);
837 /* If the pointer we free is defined by an allocation
838 function do not add the call to the worklist. */
839 if (TREE_CODE (ptr) == SSA_NAME
840 && is_gimple_call (def_stmt = SSA_NAME_DEF_STMT (ptr))
841 && (def_callee = gimple_call_fndecl (def_stmt))
842 && DECL_BUILT_IN_CLASS (def_callee) == BUILT_IN_NORMAL
843 && (DECL_FUNCTION_CODE (def_callee) == BUILT_IN_MALLOC
844 || DECL_FUNCTION_CODE (def_callee) == BUILT_IN_CALLOC))
848 FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
849 mark_operand_necessary (use);
851 use = gimple_vuse (stmt);
855 /* If we dropped to simple mode make all immediately
856 reachable definitions necessary. */
859 mark_all_reaching_defs_necessary (stmt);
863 /* For statements that may load from memory (have a VUSE) we
864 have to mark all reaching (may-)definitions as necessary.
865 We partition this task into two cases:
866 1) explicit loads based on decls that are not aliased
867 2) implicit loads (like calls) and explicit loads not
868 based on decls that are not aliased (like indirect
869 references or loads from globals)
870 For 1) we mark all reaching may-defs as necessary, stopping
871 at dominating kills. For 2) we want to mark all dominating
872 references necessary, but non-aliased ones which we handle
873 in 1). By keeping a global visited bitmap for references
874 we walk for 2) we avoid quadratic behavior for those. */
876 if (is_gimple_call (stmt))
878 tree callee = gimple_call_fndecl (stmt);
881 /* Calls to functions that are merely acting as barriers
882 or that only store to memory do not make any previous
884 if (callee != NULL_TREE
885 && DECL_BUILT_IN_CLASS (callee) == BUILT_IN_NORMAL
886 && (DECL_FUNCTION_CODE (callee) == BUILT_IN_MEMSET
887 || DECL_FUNCTION_CODE (callee) == BUILT_IN_MEMSET_CHK
888 || DECL_FUNCTION_CODE (callee) == BUILT_IN_MALLOC
889 || DECL_FUNCTION_CODE (callee) == BUILT_IN_CALLOC
890 || DECL_FUNCTION_CODE (callee) == BUILT_IN_FREE
891 || DECL_FUNCTION_CODE (callee) == BUILT_IN_VA_END
892 || DECL_FUNCTION_CODE (callee) == BUILT_IN_ALLOCA
893 || DECL_FUNCTION_CODE (callee) == BUILT_IN_STACK_SAVE
894 || DECL_FUNCTION_CODE (callee) == BUILT_IN_STACK_RESTORE
895 || DECL_FUNCTION_CODE (callee) == BUILT_IN_ASSUME_ALIGNED))
898 /* Calls implicitly load from memory, their arguments
899 in addition may explicitly perform memory loads. */
900 mark_all_reaching_defs_necessary (stmt);
901 for (i = 0; i < gimple_call_num_args (stmt); ++i)
903 tree arg = gimple_call_arg (stmt, i);
904 if (TREE_CODE (arg) == SSA_NAME
905 || is_gimple_min_invariant (arg))
907 if (TREE_CODE (arg) == WITH_SIZE_EXPR)
908 arg = TREE_OPERAND (arg, 0);
909 if (!ref_may_be_aliased (arg))
910 mark_aliased_reaching_defs_necessary (stmt, arg);
913 else if (gimple_assign_single_p (stmt))
916 bool rhs_aliased = false;
917 /* If this is a load mark things necessary. */
918 rhs = gimple_assign_rhs1 (stmt);
919 if (TREE_CODE (rhs) != SSA_NAME
920 && !is_gimple_min_invariant (rhs))
922 if (!ref_may_be_aliased (rhs))
923 mark_aliased_reaching_defs_necessary (stmt, rhs);
928 mark_all_reaching_defs_necessary (stmt);
930 else if (gimple_code (stmt) == GIMPLE_RETURN)
932 tree rhs = gimple_return_retval (stmt);
933 /* A return statement may perform a load. */
935 && TREE_CODE (rhs) != SSA_NAME
936 && !is_gimple_min_invariant (rhs))
938 if (!ref_may_be_aliased (rhs))
939 mark_aliased_reaching_defs_necessary (stmt, rhs);
941 mark_all_reaching_defs_necessary (stmt);
944 else if (gimple_code (stmt) == GIMPLE_ASM)
947 mark_all_reaching_defs_necessary (stmt);
948 /* Inputs may perform loads. */
949 for (i = 0; i < gimple_asm_ninputs (stmt); ++i)
951 tree op = TREE_VALUE (gimple_asm_input_op (stmt, i));
952 if (TREE_CODE (op) != SSA_NAME
953 && !is_gimple_min_invariant (op)
954 && !ref_may_be_aliased (op))
955 mark_aliased_reaching_defs_necessary (stmt, op);
961 /* If we over-used our alias oracle budget drop to simple
962 mode. The cost metric allows quadratic behavior
963 (number of uses times number of may-defs queries) up to
964 a constant maximal number of queries and after that falls back to
965 super-linear complexity. */
966 if (/* Constant but quadratic for small functions. */
967 total_chain > 128 * 128
968 /* Linear in the number of may-defs. */
969 && total_chain > 32 * longest_chain
970 /* Linear in the number of uses. */
971 && total_chain > nr_walks * 32)
975 bitmap_clear (visited);
981 /* Replace all uses of result of PHI by underlying variable and mark it
985 mark_virtual_phi_result_for_renaming (gimple phi)
988 imm_use_iterator iter;
991 tree result_ssa, result_var;
993 if (dump_file && (dump_flags & TDF_DETAILS))
995 fprintf (dump_file, "Marking result for renaming : ");
996 print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
997 fprintf (dump_file, "\n");
1000 result_ssa = gimple_phi_result (phi);
1001 result_var = SSA_NAME_VAR (result_ssa);
1002 FOR_EACH_IMM_USE_STMT (stmt, iter, result_ssa)
1004 FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
1005 SET_USE (use_p, result_var);
1010 mark_sym_for_renaming (result_var);
1013 /* Remove dead PHI nodes from block BB. */
1016 remove_dead_phis (basic_block bb)
1018 bool something_changed = false;
1021 gimple_stmt_iterator gsi;
1022 phis = phi_nodes (bb);
1024 for (gsi = gsi_start (phis); !gsi_end_p (gsi);)
1027 phi = gsi_stmt (gsi);
1029 /* We do not track necessity of virtual PHI nodes. Instead do
1030 very simple dead PHI removal here. */
1031 if (!is_gimple_reg (gimple_phi_result (phi)))
1033 /* Virtual PHI nodes with one or identical arguments
1035 if (degenerate_phi_p (phi))
1037 tree vdef = gimple_phi_result (phi);
1038 tree vuse = gimple_phi_arg_def (phi, 0);
1040 use_operand_p use_p;
1041 imm_use_iterator iter;
1043 FOR_EACH_IMM_USE_STMT (use_stmt, iter, vdef)
1044 FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
1045 SET_USE (use_p, vuse);
1046 if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vdef)
1047 && TREE_CODE (vuse) == SSA_NAME)
1048 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vuse) = 1;
1051 gimple_set_plf (phi, STMT_NECESSARY, true);
1054 if (!gimple_plf (phi, STMT_NECESSARY))
1056 something_changed = true;
1057 if (dump_file && (dump_flags & TDF_DETAILS))
1059 fprintf (dump_file, "Deleting : ");
1060 print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
1061 fprintf (dump_file, "\n");
1064 remove_phi_node (&gsi, true);
1065 stats.removed_phis++;
1071 return something_changed;
1074 /* Forward edge E to respective POST_DOM_BB and update PHIs. */
1077 forward_edge_to_pdom (edge e, basic_block post_dom_bb)
1079 gimple_stmt_iterator gsi;
1083 if (dump_file && (dump_flags & TDF_DETAILS))
1084 fprintf (dump_file, "Redirecting edge %i->%i to %i\n", e->src->index,
1085 e->dest->index, post_dom_bb->index);
1087 e2 = redirect_edge_and_branch (e, post_dom_bb);
1090 /* If edge was already around, no updating is neccesary. */
1094 if (!gimple_seq_empty_p (phi_nodes (post_dom_bb)))
1096 /* We are sure that for every live PHI we are seeing control dependent BB.
1097 This means that we can pick any edge to duplicate PHI args from. */
1098 FOR_EACH_EDGE (e2, ei, post_dom_bb->preds)
1101 for (gsi = gsi_start_phis (post_dom_bb); !gsi_end_p (gsi);)
1103 gimple phi = gsi_stmt (gsi);
1105 source_location locus;
1107 /* PHIs for virtuals have no control dependency relation on them.
1108 We are lost here and must force renaming of the symbol. */
1109 if (!is_gimple_reg (gimple_phi_result (phi)))
1111 mark_virtual_phi_result_for_renaming (phi);
1112 remove_phi_node (&gsi, true);
1116 /* Dead PHI do not imply control dependency. */
1117 if (!gimple_plf (phi, STMT_NECESSARY))
1123 op = gimple_phi_arg_def (phi, e2->dest_idx);
1124 locus = gimple_phi_arg_location (phi, e2->dest_idx);
1125 add_phi_arg (phi, op, e, locus);
1126 /* The resulting PHI if not dead can only be degenerate. */
1127 gcc_assert (degenerate_phi_p (phi));
1134 /* Remove dead statement pointed to by iterator I. Receives the basic block BB
1135 containing I so that we don't have to look it up. */
1138 remove_dead_stmt (gimple_stmt_iterator *i, basic_block bb)
1140 gimple stmt = gsi_stmt (*i);
1142 if (dump_file && (dump_flags & TDF_DETAILS))
1144 fprintf (dump_file, "Deleting : ");
1145 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
1146 fprintf (dump_file, "\n");
1151 /* If we have determined that a conditional branch statement contributes
1152 nothing to the program, then we not only remove it, but we also change
1153 the flow graph so that the current block will simply fall-thru to its
1154 immediate post-dominator. The blocks we are circumventing will be
1155 removed by cleanup_tree_cfg if this change in the flow graph makes them
1157 if (is_ctrl_stmt (stmt))
1159 basic_block post_dom_bb;
1163 post_dom_bb = get_immediate_dominator (CDI_POST_DOMINATORS, bb);
1165 e = find_edge (bb, post_dom_bb);
1167 /* If edge is already there, try to use it. This avoids need to update
1168 PHI nodes. Also watch for cases where post dominator does not exists
1169 or is exit block. These can happen for infinite loops as we create
1170 fake edges in the dominator tree. */
1173 else if (! post_dom_bb || post_dom_bb == EXIT_BLOCK_PTR)
1174 e = EDGE_SUCC (bb, 0);
1176 e = forward_edge_to_pdom (EDGE_SUCC (bb, 0), post_dom_bb);
1178 e->probability = REG_BR_PROB_BASE;
1179 e->count = bb->count;
1181 /* The edge is no longer associated with a conditional, so it does
1182 not have TRUE/FALSE flags. */
1183 e->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
1185 /* The lone outgoing edge from BB will be a fallthru edge. */
1186 e->flags |= EDGE_FALLTHRU;
1188 /* Remove the remaining outgoing edges. */
1189 for (ei = ei_start (bb->succs); (e2 = ei_safe_edge (ei)); )
1199 unlink_stmt_vdef (stmt);
1200 gsi_remove (i, true);
1201 release_defs (stmt);
1204 /* Eliminate unnecessary statements. Any instruction not marked as necessary
1205 contributes nothing to the program, and can be deleted. */
1208 eliminate_unnecessary_stmts (void)
1210 bool something_changed = false;
1212 gimple_stmt_iterator gsi, psi;
1215 VEC (basic_block, heap) *h;
1217 if (dump_file && (dump_flags & TDF_DETAILS))
1218 fprintf (dump_file, "\nEliminating unnecessary statements:\n");
1220 clear_special_calls ();
1222 /* Walking basic blocks and statements in reverse order avoids
1223 releasing SSA names before any other DEFs that refer to them are
1224 released. This helps avoid loss of debug information, as we get
1225 a chance to propagate all RHSs of removed SSAs into debug uses,
1226 rather than only the latest ones. E.g., consider:
1232 If we were to release x_3 before a_5, when we reached a_5 and
1233 tried to substitute it into the debug stmt, we'd see x_3 there,
1234 but x_3's DEF, type, etc would have already been disconnected.
1235 By going backwards, the debug stmt first changes to:
1237 # DEBUG a => x_3 - b_4
1241 # DEBUG a => y_1 + z_2 - b_4
1244 gcc_assert (dom_info_available_p (CDI_DOMINATORS));
1245 h = get_all_dominated_blocks (CDI_DOMINATORS, single_succ (ENTRY_BLOCK_PTR));
1247 while (VEC_length (basic_block, h))
1249 bb = VEC_pop (basic_block, h);
1251 /* Remove dead statements. */
1252 for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi = psi)
1254 stmt = gsi_stmt (gsi);
1261 /* We can mark a call to free as not necessary if the
1262 defining statement of its argument is an allocation
1263 function and that is not necessary itself. */
1264 if (gimple_call_builtin_p (stmt, BUILT_IN_FREE))
1266 tree ptr = gimple_call_arg (stmt, 0);
1269 if (TREE_CODE (ptr) != SSA_NAME)
1271 def_stmt = SSA_NAME_DEF_STMT (ptr);
1272 if (!is_gimple_call (def_stmt)
1273 || gimple_plf (def_stmt, STMT_NECESSARY))
1275 callee2 = gimple_call_fndecl (def_stmt);
1276 if (callee2 == NULL_TREE
1277 || DECL_BUILT_IN_CLASS (callee2) != BUILT_IN_NORMAL
1278 || (DECL_FUNCTION_CODE (callee2) != BUILT_IN_MALLOC
1279 && DECL_FUNCTION_CODE (callee2) != BUILT_IN_CALLOC))
1281 gimple_set_plf (stmt, STMT_NECESSARY, false);
1284 /* If GSI is not necessary then remove it. */
1285 if (!gimple_plf (stmt, STMT_NECESSARY))
1287 if (!is_gimple_debug (stmt))
1288 something_changed = true;
1289 remove_dead_stmt (&gsi, bb);
1291 else if (is_gimple_call (stmt))
1293 call = gimple_call_fndecl (stmt);
1298 /* When LHS of var = call (); is dead, simplify it into
1299 call (); saving one operand. */
1300 name = gimple_call_lhs (stmt);
1301 if (name && TREE_CODE (name) == SSA_NAME
1302 && !TEST_BIT (processed, SSA_NAME_VERSION (name)))
1304 something_changed = true;
1305 if (dump_file && (dump_flags & TDF_DETAILS))
1307 fprintf (dump_file, "Deleting LHS of call: ");
1308 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
1309 fprintf (dump_file, "\n");
1312 gimple_call_set_lhs (stmt, NULL_TREE);
1313 maybe_clean_or_replace_eh_stmt (stmt, stmt);
1315 release_ssa_name (name);
1317 notice_special_calls (stmt);
1323 VEC_free (basic_block, heap, h);
1325 /* Since we don't track liveness of virtual PHI nodes, it is possible that we
1326 rendered some PHI nodes unreachable while they are still in use.
1327 Mark them for renaming. */
1330 basic_block prev_bb;
1332 find_unreachable_blocks ();
1334 /* Delete all unreachable basic blocks in reverse dominator order. */
1335 for (bb = EXIT_BLOCK_PTR->prev_bb; bb != ENTRY_BLOCK_PTR; bb = prev_bb)
1337 prev_bb = bb->prev_bb;
1339 if (!TEST_BIT (bb_contains_live_stmts, bb->index)
1340 || !(bb->flags & BB_REACHABLE))
1342 for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
1343 if (!is_gimple_reg (gimple_phi_result (gsi_stmt (gsi))))
1346 imm_use_iterator iter;
1348 FOR_EACH_IMM_USE_STMT (stmt, iter, gimple_phi_result (gsi_stmt (gsi)))
1350 if (!(gimple_bb (stmt)->flags & BB_REACHABLE))
1352 if (gimple_code (stmt) == GIMPLE_PHI
1353 || gimple_plf (stmt, STMT_NECESSARY))
1356 BREAK_FROM_IMM_USE_STMT (iter);
1360 mark_virtual_phi_result_for_renaming (gsi_stmt (gsi));
1363 if (!(bb->flags & BB_REACHABLE))
1365 /* Speed up the removal of blocks that don't
1366 dominate others. Walking backwards, this should
1367 be the common case. ??? Do we need to recompute
1368 dominators because of cfg_altered? */
1369 if (!MAY_HAVE_DEBUG_STMTS
1370 || !first_dom_son (CDI_DOMINATORS, bb))
1371 delete_basic_block (bb);
1374 h = get_all_dominated_blocks (CDI_DOMINATORS, bb);
1376 while (VEC_length (basic_block, h))
1378 bb = VEC_pop (basic_block, h);
1379 prev_bb = bb->prev_bb;
1380 /* Rearrangements to the CFG may have failed
1381 to update the dominators tree, so that
1382 formerly-dominated blocks are now
1383 otherwise reachable. */
1384 if (!!(bb->flags & BB_REACHABLE))
1386 delete_basic_block (bb);
1389 VEC_free (basic_block, heap, h);
1397 /* Remove dead PHI nodes. */
1398 something_changed |= remove_dead_phis (bb);
1401 return something_changed;
1405 /* Print out removed statement statistics. */
1412 percg = ((float) stats.removed / (float) stats.total) * 100;
1413 fprintf (dump_file, "Removed %d of %d statements (%d%%)\n",
1414 stats.removed, stats.total, (int) percg);
1416 if (stats.total_phis == 0)
1419 percg = ((float) stats.removed_phis / (float) stats.total_phis) * 100;
1421 fprintf (dump_file, "Removed %d of %d PHI nodes (%d%%)\n",
1422 stats.removed_phis, stats.total_phis, (int) percg);
1425 /* Initialization for this pass. Set up the used data structures. */
1428 tree_dce_init (bool aggressive)
1430 memset ((void *) &stats, 0, sizeof (stats));
1436 control_dependence_map = XNEWVEC (bitmap, last_basic_block);
1437 for (i = 0; i < last_basic_block; ++i)
1438 control_dependence_map[i] = BITMAP_ALLOC (NULL);
1440 last_stmt_necessary = sbitmap_alloc (last_basic_block);
1441 sbitmap_zero (last_stmt_necessary);
1442 bb_contains_live_stmts = sbitmap_alloc (last_basic_block);
1443 sbitmap_zero (bb_contains_live_stmts);
1446 processed = sbitmap_alloc (num_ssa_names + 1);
1447 sbitmap_zero (processed);
1449 worklist = VEC_alloc (gimple, heap, 64);
1450 cfg_altered = false;
1453 /* Cleanup after this pass. */
1456 tree_dce_done (bool aggressive)
1462 for (i = 0; i < last_basic_block; ++i)
1463 BITMAP_FREE (control_dependence_map[i]);
1464 free (control_dependence_map);
1466 sbitmap_free (visited_control_parents);
1467 sbitmap_free (last_stmt_necessary);
1468 sbitmap_free (bb_contains_live_stmts);
1469 bb_contains_live_stmts = NULL;
1472 sbitmap_free (processed);
1474 VEC_free (gimple, heap, worklist);
1477 /* Main routine to eliminate dead code.
1479 AGGRESSIVE controls the aggressiveness of the algorithm.
1480 In conservative mode, we ignore control dependence and simply declare
1481 all but the most trivially dead branches necessary. This mode is fast.
1482 In aggressive mode, control dependences are taken into account, which
1483 results in more dead code elimination, but at the cost of some time.
1485 FIXME: Aggressive mode before PRE doesn't work currently because
1486 the dominance info is not invalidated after DCE1. This is
1487 not an issue right now because we only run aggressive DCE
1488 as the last tree SSA pass, but keep this in mind when you
1489 start experimenting with pass ordering. */
1492 perform_tree_ssa_dce (bool aggressive)
1494 struct edge_list *el = NULL;
1495 bool something_changed = 0;
1497 calculate_dominance_info (CDI_DOMINATORS);
1499 /* Preheaders are needed for SCEV to work.
1500 Simple lateches and recorded exits improve chances that loop will
1501 proved to be finite in testcases such as in loop-15.c and loop-24.c */
1503 loop_optimizer_init (LOOPS_NORMAL
1504 | LOOPS_HAVE_RECORDED_EXITS);
1506 tree_dce_init (aggressive);
1510 /* Compute control dependence. */
1511 timevar_push (TV_CONTROL_DEPENDENCES);
1512 calculate_dominance_info (CDI_POST_DOMINATORS);
1513 el = create_edge_list ();
1514 find_all_control_dependences (el);
1515 timevar_pop (TV_CONTROL_DEPENDENCES);
1517 visited_control_parents = sbitmap_alloc (last_basic_block);
1518 sbitmap_zero (visited_control_parents);
1520 mark_dfs_back_edges ();
1523 find_obviously_necessary_stmts (el);
1526 loop_optimizer_finalize ();
1532 visited = BITMAP_ALLOC (NULL);
1533 propagate_necessity (el);
1534 BITMAP_FREE (visited);
1536 something_changed |= eliminate_unnecessary_stmts ();
1537 something_changed |= cfg_altered;
1539 /* We do not update postdominators, so free them unconditionally. */
1540 free_dominance_info (CDI_POST_DOMINATORS);
1542 /* If we removed paths in the CFG, then we need to update
1543 dominators as well. I haven't investigated the possibility
1544 of incrementally updating dominators. */
1546 free_dominance_info (CDI_DOMINATORS);
1548 statistics_counter_event (cfun, "Statements deleted", stats.removed);
1549 statistics_counter_event (cfun, "PHI nodes deleted", stats.removed_phis);
1551 /* Debugging dumps. */
1552 if (dump_file && (dump_flags & (TDF_STATS|TDF_DETAILS)))
1555 tree_dce_done (aggressive);
1557 free_edge_list (el);
1559 if (something_changed)
1560 return (TODO_update_ssa | TODO_cleanup_cfg | TODO_ggc_collect
1561 | TODO_remove_unused_locals);
1566 /* Pass entry points. */
1570 return perform_tree_ssa_dce (/*aggressive=*/false);
1574 tree_ssa_dce_loop (void)
1577 todo = perform_tree_ssa_dce (/*aggressive=*/false);
1580 free_numbers_of_iterations_estimates ();
1587 tree_ssa_cd_dce (void)
1589 return perform_tree_ssa_dce (/*aggressive=*/optimize >= 2);
1595 return flag_tree_dce != 0;
1598 struct gimple_opt_pass pass_dce =
1603 gate_dce, /* gate */
1604 tree_ssa_dce, /* execute */
1607 0, /* static_pass_number */
1608 TV_TREE_DCE, /* tv_id */
1609 PROP_cfg | PROP_ssa, /* properties_required */
1610 0, /* properties_provided */
1611 0, /* properties_destroyed */
1612 0, /* todo_flags_start */
1613 TODO_verify_ssa /* todo_flags_finish */
1617 struct gimple_opt_pass pass_dce_loop =
1621 "dceloop", /* name */
1622 gate_dce, /* gate */
1623 tree_ssa_dce_loop, /* execute */
1626 0, /* static_pass_number */
1627 TV_TREE_DCE, /* tv_id */
1628 PROP_cfg | PROP_ssa, /* properties_required */
1629 0, /* properties_provided */
1630 0, /* properties_destroyed */
1631 0, /* todo_flags_start */
1632 TODO_verify_ssa /* todo_flags_finish */
1636 struct gimple_opt_pass pass_cd_dce =
1641 gate_dce, /* gate */
1642 tree_ssa_cd_dce, /* execute */
1645 0, /* static_pass_number */
1646 TV_TREE_CD_DCE, /* tv_id */
1647 PROP_cfg | PROP_ssa, /* properties_required */
1648 0, /* properties_provided */
1649 0, /* properties_destroyed */
1650 0, /* todo_flags_start */
1652 | TODO_verify_flow /* todo_flags_finish */