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);
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_p (stmt)
320 || gimple_debug_bind_has_value_p (stmt)
321 || TREE_CODE (gimple_debug_bind_get_var (stmt)) != DEBUG_EXPR_DECL)
322 mark_stmt_necessary (stmt, false);
326 gcc_assert (!simple_goto_p (stmt));
327 mark_stmt_necessary (stmt, true);
331 gcc_assert (EDGE_COUNT (gimple_bb (stmt)->succs) == 2);
336 mark_stmt_necessary (stmt, true);
343 /* If the statement has volatile operands, it needs to be preserved.
344 Same for statements that can alter control flow in unpredictable
346 if (gimple_has_volatile_ops (stmt) || is_ctrl_altering_stmt (stmt))
348 mark_stmt_necessary (stmt, true);
352 if (is_hidden_global_store (stmt))
354 mark_stmt_necessary (stmt, true);
362 /* Mark the last statement of BB as necessary. */
365 mark_last_stmt_necessary (basic_block bb)
367 gimple stmt = last_stmt (bb);
369 SET_BIT (last_stmt_necessary, bb->index);
370 SET_BIT (bb_contains_live_stmts, bb->index);
372 /* We actually mark the statement only if it is a control statement. */
373 if (stmt && is_ctrl_stmt (stmt))
374 mark_stmt_necessary (stmt, true);
378 /* Mark control dependent edges of BB as necessary. We have to do this only
379 once for each basic block so we set the appropriate bit after we're done.
381 When IGNORE_SELF is true, ignore BB in the list of control dependences. */
384 mark_control_dependent_edges_necessary (basic_block bb, struct edge_list *el,
388 unsigned edge_number;
389 bool skipped = false;
391 gcc_assert (bb != EXIT_BLOCK_PTR);
393 if (bb == ENTRY_BLOCK_PTR)
396 EXECUTE_IF_CONTROL_DEPENDENT (bi, bb->index, edge_number)
398 basic_block cd_bb = INDEX_EDGE_PRED_BB (el, edge_number);
400 if (ignore_self && cd_bb == bb)
406 if (!TEST_BIT (last_stmt_necessary, cd_bb->index))
407 mark_last_stmt_necessary (cd_bb);
411 SET_BIT (visited_control_parents, bb->index);
415 /* Find obviously necessary statements. These are things like most function
416 calls, and stores to file level variables.
418 If EL is NULL, control statements are conservatively marked as
419 necessary. Otherwise it contains the list of edges used by control
420 dependence analysis. */
423 find_obviously_necessary_stmts (struct edge_list *el)
426 gimple_stmt_iterator gsi;
433 /* PHI nodes are never inherently necessary. */
434 for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
436 phi = gsi_stmt (gsi);
437 gimple_set_plf (phi, STMT_NECESSARY, false);
440 /* Check all statements in the block. */
441 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
443 stmt = gsi_stmt (gsi);
444 gimple_set_plf (stmt, STMT_NECESSARY, false);
445 mark_stmt_if_obviously_necessary (stmt, el != NULL);
449 /* Pure and const functions are finite and thus have no infinite loops in
451 flags = flags_from_decl_or_type (current_function_decl);
452 if ((flags & (ECF_CONST|ECF_PURE)) && !(flags & ECF_LOOPING_CONST_OR_PURE))
455 /* Prevent the empty possibly infinite loops from being removed. */
461 if (mark_irreducible_loops ())
465 FOR_EACH_EDGE (e, ei, bb->succs)
466 if ((e->flags & EDGE_DFS_BACK)
467 && (e->flags & EDGE_IRREDUCIBLE_LOOP))
470 fprintf (dump_file, "Marking back edge of irreducible loop %i->%i\n",
471 e->src->index, e->dest->index);
472 mark_control_dependent_edges_necessary (e->dest, el, false);
476 FOR_EACH_LOOP (li, loop, 0)
477 if (!finite_loop_p (loop))
480 fprintf (dump_file, "can not prove finiteness of loop %i\n", loop->num);
481 mark_control_dependent_edges_necessary (loop->latch, el, false);
488 /* Return true if REF is based on an aliased base, otherwise false. */
491 ref_may_be_aliased (tree ref)
493 gcc_assert (TREE_CODE (ref) != WITH_SIZE_EXPR);
494 while (handled_component_p (ref))
495 ref = TREE_OPERAND (ref, 0);
496 if (TREE_CODE (ref) == MEM_REF
497 && TREE_CODE (TREE_OPERAND (ref, 0)) == ADDR_EXPR)
498 ref = TREE_OPERAND (TREE_OPERAND (ref, 0), 0);
499 return !(DECL_P (ref)
500 && !may_be_aliased (ref));
503 static bitmap visited = NULL;
504 static unsigned int longest_chain = 0;
505 static unsigned int total_chain = 0;
506 static unsigned int nr_walks = 0;
507 static bool chain_ovfl = false;
509 /* Worker for the walker that marks reaching definitions of REF,
510 which is based on a non-aliased decl, necessary. It returns
511 true whenever the defining statement of the current VDEF is
512 a kill for REF, as no dominating may-defs are necessary for REF
513 anymore. DATA points to the basic-block that contains the
514 stmt that refers to REF. */
517 mark_aliased_reaching_defs_necessary_1 (ao_ref *ref, tree vdef, void *data)
519 gimple def_stmt = SSA_NAME_DEF_STMT (vdef);
521 /* All stmts we visit are necessary. */
522 mark_operand_necessary (vdef);
524 /* If the stmt lhs kills ref, then we can stop walking. */
525 if (gimple_has_lhs (def_stmt)
526 && TREE_CODE (gimple_get_lhs (def_stmt)) != SSA_NAME
527 /* The assignment is not necessarily carried out if it can throw
528 and we can catch it in the current function where we could inspect
530 ??? We only need to care about the RHS throwing. For aggregate
531 assignments or similar calls and non-call exceptions the LHS
532 might throw as well. */
533 && !stmt_can_throw_internal (def_stmt))
535 tree base, lhs = gimple_get_lhs (def_stmt);
536 HOST_WIDE_INT size, offset, max_size;
538 base = get_ref_base_and_extent (lhs, &offset, &size, &max_size);
539 /* We can get MEM[symbol: sZ, index: D.8862_1] here,
540 so base == refd->base does not always hold. */
541 if (base == ref->base)
543 /* For a must-alias check we need to be able to constrain
544 the accesses properly. */
545 if (size != -1 && size == max_size
546 && ref->max_size != -1)
548 if (offset <= ref->offset
549 && offset + size >= ref->offset + ref->max_size)
552 /* Or they need to be exactly the same. */
554 /* Make sure there is no induction variable involved
555 in the references (gcc.c-torture/execute/pr42142.c).
556 The simplest way is to check if the kill dominates
558 && dominated_by_p (CDI_DOMINATORS, (basic_block) data,
559 gimple_bb (def_stmt))
560 && operand_equal_p (ref->ref, lhs, 0))
565 /* Otherwise keep walking. */
570 mark_aliased_reaching_defs_necessary (gimple stmt, tree ref)
574 gcc_assert (!chain_ovfl);
575 ao_ref_init (&refd, ref);
576 chain = walk_aliased_vdefs (&refd, gimple_vuse (stmt),
577 mark_aliased_reaching_defs_necessary_1,
578 gimple_bb (stmt), NULL);
579 if (chain > longest_chain)
580 longest_chain = chain;
581 total_chain += chain;
585 /* Worker for the walker that marks reaching definitions of REF, which
586 is not based on a non-aliased decl. For simplicity we need to end
587 up marking all may-defs necessary that are not based on a non-aliased
588 decl. The only job of this walker is to skip may-defs based on
589 a non-aliased decl. */
592 mark_all_reaching_defs_necessary_1 (ao_ref *ref ATTRIBUTE_UNUSED,
593 tree vdef, void *data ATTRIBUTE_UNUSED)
595 gimple def_stmt = SSA_NAME_DEF_STMT (vdef);
597 /* We have to skip already visited (and thus necessary) statements
598 to make the chaining work after we dropped back to simple mode. */
600 && TEST_BIT (processed, SSA_NAME_VERSION (vdef)))
602 gcc_assert (gimple_nop_p (def_stmt)
603 || gimple_plf (def_stmt, STMT_NECESSARY));
607 /* We want to skip stores to non-aliased variables. */
609 && gimple_assign_single_p (def_stmt))
611 tree lhs = gimple_assign_lhs (def_stmt);
612 if (!ref_may_be_aliased (lhs))
616 mark_operand_necessary (vdef);
622 mark_all_reaching_defs_necessary (gimple stmt)
624 walk_aliased_vdefs (NULL, gimple_vuse (stmt),
625 mark_all_reaching_defs_necessary_1, NULL, &visited);
628 /* Return true for PHI nodes with one or identical arguments
631 degenerate_phi_p (gimple phi)
634 tree op = gimple_phi_arg_def (phi, 0);
635 for (i = 1; i < gimple_phi_num_args (phi); i++)
636 if (gimple_phi_arg_def (phi, i) != op)
641 /* Propagate necessity using the operands of necessary statements.
642 Process the uses on each statement in the worklist, and add all
643 feeding statements which contribute to the calculation of this
644 value to the worklist.
646 In conservative mode, EL is NULL. */
649 propagate_necessity (struct edge_list *el)
652 bool aggressive = (el ? true : false);
654 if (dump_file && (dump_flags & TDF_DETAILS))
655 fprintf (dump_file, "\nProcessing worklist:\n");
657 while (VEC_length (gimple, worklist) > 0)
659 /* Take STMT from worklist. */
660 stmt = VEC_pop (gimple, worklist);
662 if (dump_file && (dump_flags & TDF_DETAILS))
664 fprintf (dump_file, "processing: ");
665 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
666 fprintf (dump_file, "\n");
671 /* Mark the last statement of the basic blocks on which the block
672 containing STMT is control dependent, but only if we haven't
674 basic_block bb = gimple_bb (stmt);
675 if (bb != ENTRY_BLOCK_PTR
676 && !TEST_BIT (visited_control_parents, bb->index))
677 mark_control_dependent_edges_necessary (bb, el, false);
680 if (gimple_code (stmt) == GIMPLE_PHI
681 /* We do not process virtual PHI nodes nor do we track their
683 && is_gimple_reg (gimple_phi_result (stmt)))
685 /* PHI nodes are somewhat special in that each PHI alternative has
686 data and control dependencies. All the statements feeding the
687 PHI node's arguments are always necessary. In aggressive mode,
688 we also consider the control dependent edges leading to the
689 predecessor block associated with each PHI alternative as
693 for (k = 0; k < gimple_phi_num_args (stmt); k++)
695 tree arg = PHI_ARG_DEF (stmt, k);
696 if (TREE_CODE (arg) == SSA_NAME)
697 mark_operand_necessary (arg);
700 /* For PHI operands it matters from where the control flow arrives
701 to the BB. Consider the following example:
711 We need to mark control dependence of the empty basic blocks, since they
712 contains computation of PHI operands.
714 Doing so is too restrictive in the case the predecestor block is in
720 for (i = 0; i<1000; ++i)
726 There is PHI for J in the BB containing return statement.
727 In this case the control dependence of predecestor block (that is
728 within the empty loop) also contains the block determining number
729 of iterations of the block that would prevent removing of empty
732 This scenario can be avoided by splitting critical edges.
733 To save the critical edge splitting pass we identify how the control
734 dependence would look like if the edge was split.
736 Consider the modified CFG created from current CFG by splitting
737 edge B->C. In the postdominance tree of modified CFG, C' is
738 always child of C. There are two cases how chlids of C' can look
743 In this case the only basic block C' is control dependent on is B.
745 2) C' has single child that is B
747 In this case control dependence of C' is same as control
748 dependence of B in original CFG except for block B itself.
749 (since C' postdominate B in modified CFG)
751 Now how to decide what case happens? There are two basic options:
753 a) C postdominate B. Then C immediately postdominate B and
754 case 2 happens iff there is no other way from B to C except
757 There is other way from B to C iff there is succesor of B that
758 is not postdominated by B. Testing this condition is somewhat
759 expensive, because we need to iterate all succesors of B.
760 We are safe to assume that this does not happen: we will mark B
761 as needed when processing the other path from B to C that is
762 conrol dependent on B and marking control dependencies of B
763 itself is harmless because they will be processed anyway after
764 processing control statement in B.
766 b) C does not postdominate B. Always case 1 happens since there is
767 path from C to exit that does not go through B and thus also C'. */
769 if (aggressive && !degenerate_phi_p (stmt))
771 for (k = 0; k < gimple_phi_num_args (stmt); k++)
773 basic_block arg_bb = gimple_phi_arg_edge (stmt, k)->src;
776 != get_immediate_dominator (CDI_POST_DOMINATORS, arg_bb))
778 if (!TEST_BIT (last_stmt_necessary, arg_bb->index))
779 mark_last_stmt_necessary (arg_bb);
781 else if (arg_bb != ENTRY_BLOCK_PTR
782 && !TEST_BIT (visited_control_parents,
784 mark_control_dependent_edges_necessary (arg_bb, el, true);
790 /* Propagate through the operands. Examine all the USE, VUSE and
791 VDEF operands in this statement. Mark all the statements
792 which feed this statement's uses as necessary. */
796 FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
797 mark_operand_necessary (use);
799 use = gimple_vuse (stmt);
803 /* If we dropped to simple mode make all immediately
804 reachable definitions necessary. */
807 mark_all_reaching_defs_necessary (stmt);
811 /* For statements that may load from memory (have a VUSE) we
812 have to mark all reaching (may-)definitions as necessary.
813 We partition this task into two cases:
814 1) explicit loads based on decls that are not aliased
815 2) implicit loads (like calls) and explicit loads not
816 based on decls that are not aliased (like indirect
817 references or loads from globals)
818 For 1) we mark all reaching may-defs as necessary, stopping
819 at dominating kills. For 2) we want to mark all dominating
820 references necessary, but non-aliased ones which we handle
821 in 1). By keeping a global visited bitmap for references
822 we walk for 2) we avoid quadratic behavior for those. */
824 if (is_gimple_call (stmt))
826 tree callee = gimple_call_fndecl (stmt);
829 /* Calls to functions that are merely acting as barriers
830 or that only store to memory do not make any previous
832 if (callee != NULL_TREE
833 && DECL_BUILT_IN_CLASS (callee) == BUILT_IN_NORMAL
834 && (DECL_FUNCTION_CODE (callee) == BUILT_IN_MEMSET
835 || DECL_FUNCTION_CODE (callee) == BUILT_IN_MEMSET_CHK
836 || DECL_FUNCTION_CODE (callee) == BUILT_IN_MALLOC
837 || DECL_FUNCTION_CODE (callee) == BUILT_IN_CALLOC
838 || DECL_FUNCTION_CODE (callee) == BUILT_IN_FREE
839 || DECL_FUNCTION_CODE (callee) == BUILT_IN_VA_END
840 || DECL_FUNCTION_CODE (callee) == BUILT_IN_ALLOCA
841 || DECL_FUNCTION_CODE (callee) == BUILT_IN_STACK_SAVE
842 || DECL_FUNCTION_CODE (callee) == BUILT_IN_STACK_RESTORE
843 || DECL_FUNCTION_CODE (callee) == BUILT_IN_ASSUME_ALIGNED))
846 /* Calls implicitly load from memory, their arguments
847 in addition may explicitly perform memory loads. */
848 mark_all_reaching_defs_necessary (stmt);
849 for (i = 0; i < gimple_call_num_args (stmt); ++i)
851 tree arg = gimple_call_arg (stmt, i);
852 if (TREE_CODE (arg) == SSA_NAME
853 || is_gimple_min_invariant (arg))
855 if (TREE_CODE (arg) == WITH_SIZE_EXPR)
856 arg = TREE_OPERAND (arg, 0);
857 if (!ref_may_be_aliased (arg))
858 mark_aliased_reaching_defs_necessary (stmt, arg);
861 else if (gimple_assign_single_p (stmt))
864 bool rhs_aliased = false;
865 /* If this is a load mark things necessary. */
866 rhs = gimple_assign_rhs1 (stmt);
867 if (TREE_CODE (rhs) != SSA_NAME
868 && !is_gimple_min_invariant (rhs))
870 if (!ref_may_be_aliased (rhs))
871 mark_aliased_reaching_defs_necessary (stmt, rhs);
876 mark_all_reaching_defs_necessary (stmt);
878 else if (gimple_code (stmt) == GIMPLE_RETURN)
880 tree rhs = gimple_return_retval (stmt);
881 /* A return statement may perform a load. */
883 && TREE_CODE (rhs) != SSA_NAME
884 && !is_gimple_min_invariant (rhs))
886 if (!ref_may_be_aliased (rhs))
887 mark_aliased_reaching_defs_necessary (stmt, rhs);
889 mark_all_reaching_defs_necessary (stmt);
892 else if (gimple_code (stmt) == GIMPLE_ASM)
895 mark_all_reaching_defs_necessary (stmt);
896 /* Inputs may perform loads. */
897 for (i = 0; i < gimple_asm_ninputs (stmt); ++i)
899 tree op = TREE_VALUE (gimple_asm_input_op (stmt, i));
900 if (TREE_CODE (op) != SSA_NAME
901 && !is_gimple_min_invariant (op)
902 && !ref_may_be_aliased (op))
903 mark_aliased_reaching_defs_necessary (stmt, op);
909 /* If we over-used our alias oracle budget drop to simple
910 mode. The cost metric allows quadratic behavior
911 (number of uses times number of may-defs queries) up to
912 a constant maximal number of queries and after that falls back to
913 super-linear complexity. */
914 if (/* Constant but quadratic for small functions. */
915 total_chain > 128 * 128
916 /* Linear in the number of may-defs. */
917 && total_chain > 32 * longest_chain
918 /* Linear in the number of uses. */
919 && total_chain > nr_walks * 32)
923 bitmap_clear (visited);
929 /* Replace all uses of result of PHI by underlying variable and mark it
933 mark_virtual_phi_result_for_renaming (gimple phi)
936 imm_use_iterator iter;
939 tree result_ssa, result_var;
941 if (dump_file && (dump_flags & TDF_DETAILS))
943 fprintf (dump_file, "Marking result for renaming : ");
944 print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
945 fprintf (dump_file, "\n");
948 result_ssa = gimple_phi_result (phi);
949 result_var = SSA_NAME_VAR (result_ssa);
950 FOR_EACH_IMM_USE_STMT (stmt, iter, result_ssa)
952 FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
953 SET_USE (use_p, result_var);
958 mark_sym_for_renaming (result_var);
961 /* Remove dead PHI nodes from block BB. */
964 remove_dead_phis (basic_block bb)
966 bool something_changed = false;
969 gimple_stmt_iterator gsi;
970 phis = phi_nodes (bb);
972 for (gsi = gsi_start (phis); !gsi_end_p (gsi);)
975 phi = gsi_stmt (gsi);
977 /* We do not track necessity of virtual PHI nodes. Instead do
978 very simple dead PHI removal here. */
979 if (!is_gimple_reg (gimple_phi_result (phi)))
981 /* Virtual PHI nodes with one or identical arguments
983 if (degenerate_phi_p (phi))
985 tree vdef = gimple_phi_result (phi);
986 tree vuse = gimple_phi_arg_def (phi, 0);
989 imm_use_iterator iter;
991 FOR_EACH_IMM_USE_STMT (use_stmt, iter, vdef)
992 FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
993 SET_USE (use_p, vuse);
994 if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vdef)
995 && TREE_CODE (vuse) == SSA_NAME)
996 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vuse) = 1;
999 gimple_set_plf (phi, STMT_NECESSARY, true);
1002 if (!gimple_plf (phi, STMT_NECESSARY))
1004 something_changed = true;
1005 if (dump_file && (dump_flags & TDF_DETAILS))
1007 fprintf (dump_file, "Deleting : ");
1008 print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
1009 fprintf (dump_file, "\n");
1012 remove_phi_node (&gsi, true);
1013 stats.removed_phis++;
1019 return something_changed;
1022 /* Forward edge E to respective POST_DOM_BB and update PHIs. */
1025 forward_edge_to_pdom (edge e, basic_block post_dom_bb)
1027 gimple_stmt_iterator gsi;
1031 if (dump_file && (dump_flags & TDF_DETAILS))
1032 fprintf (dump_file, "Redirecting edge %i->%i to %i\n", e->src->index,
1033 e->dest->index, post_dom_bb->index);
1035 e2 = redirect_edge_and_branch (e, post_dom_bb);
1038 /* If edge was already around, no updating is neccesary. */
1042 if (!gimple_seq_empty_p (phi_nodes (post_dom_bb)))
1044 /* We are sure that for every live PHI we are seeing control dependent BB.
1045 This means that we can pick any edge to duplicate PHI args from. */
1046 FOR_EACH_EDGE (e2, ei, post_dom_bb->preds)
1049 for (gsi = gsi_start_phis (post_dom_bb); !gsi_end_p (gsi);)
1051 gimple phi = gsi_stmt (gsi);
1053 source_location locus;
1055 /* PHIs for virtuals have no control dependency relation on them.
1056 We are lost here and must force renaming of the symbol. */
1057 if (!is_gimple_reg (gimple_phi_result (phi)))
1059 mark_virtual_phi_result_for_renaming (phi);
1060 remove_phi_node (&gsi, true);
1064 /* Dead PHI do not imply control dependency. */
1065 if (!gimple_plf (phi, STMT_NECESSARY))
1071 op = gimple_phi_arg_def (phi, e2->dest_idx);
1072 locus = gimple_phi_arg_location (phi, e2->dest_idx);
1073 add_phi_arg (phi, op, e, locus);
1074 /* The resulting PHI if not dead can only be degenerate. */
1075 gcc_assert (degenerate_phi_p (phi));
1082 /* Remove dead statement pointed to by iterator I. Receives the basic block BB
1083 containing I so that we don't have to look it up. */
1086 remove_dead_stmt (gimple_stmt_iterator *i, basic_block bb)
1088 gimple stmt = gsi_stmt (*i);
1090 if (dump_file && (dump_flags & TDF_DETAILS))
1092 fprintf (dump_file, "Deleting : ");
1093 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
1094 fprintf (dump_file, "\n");
1099 /* If we have determined that a conditional branch statement contributes
1100 nothing to the program, then we not only remove it, but we also change
1101 the flow graph so that the current block will simply fall-thru to its
1102 immediate post-dominator. The blocks we are circumventing will be
1103 removed by cleanup_tree_cfg if this change in the flow graph makes them
1105 if (is_ctrl_stmt (stmt))
1107 basic_block post_dom_bb;
1111 post_dom_bb = get_immediate_dominator (CDI_POST_DOMINATORS, bb);
1113 e = find_edge (bb, post_dom_bb);
1115 /* If edge is already there, try to use it. This avoids need to update
1116 PHI nodes. Also watch for cases where post dominator does not exists
1117 or is exit block. These can happen for infinite loops as we create
1118 fake edges in the dominator tree. */
1121 else if (! post_dom_bb || post_dom_bb == EXIT_BLOCK_PTR)
1122 e = EDGE_SUCC (bb, 0);
1124 e = forward_edge_to_pdom (EDGE_SUCC (bb, 0), post_dom_bb);
1126 e->probability = REG_BR_PROB_BASE;
1127 e->count = bb->count;
1129 /* The edge is no longer associated with a conditional, so it does
1130 not have TRUE/FALSE flags. */
1131 e->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
1133 /* The lone outgoing edge from BB will be a fallthru edge. */
1134 e->flags |= EDGE_FALLTHRU;
1136 /* Remove the remaining outgoing edges. */
1137 for (ei = ei_start (bb->succs); (e2 = ei_safe_edge (ei)); )
1147 unlink_stmt_vdef (stmt);
1148 gsi_remove (i, true);
1149 release_defs (stmt);
1152 /* Eliminate unnecessary statements. Any instruction not marked as necessary
1153 contributes nothing to the program, and can be deleted. */
1156 eliminate_unnecessary_stmts (void)
1158 bool something_changed = false;
1160 gimple_stmt_iterator gsi, psi;
1163 VEC (basic_block, heap) *h;
1165 if (dump_file && (dump_flags & TDF_DETAILS))
1166 fprintf (dump_file, "\nEliminating unnecessary statements:\n");
1168 clear_special_calls ();
1170 /* Walking basic blocks and statements in reverse order avoids
1171 releasing SSA names before any other DEFs that refer to them are
1172 released. This helps avoid loss of debug information, as we get
1173 a chance to propagate all RHSs of removed SSAs into debug uses,
1174 rather than only the latest ones. E.g., consider:
1180 If we were to release x_3 before a_5, when we reached a_5 and
1181 tried to substitute it into the debug stmt, we'd see x_3 there,
1182 but x_3's DEF, type, etc would have already been disconnected.
1183 By going backwards, the debug stmt first changes to:
1185 # DEBUG a => x_3 - b_4
1189 # DEBUG a => y_1 + z_2 - b_4
1192 gcc_assert (dom_info_available_p (CDI_DOMINATORS));
1193 h = get_all_dominated_blocks (CDI_DOMINATORS, single_succ (ENTRY_BLOCK_PTR));
1195 while (VEC_length (basic_block, h))
1197 bb = VEC_pop (basic_block, h);
1199 /* Remove dead statements. */
1200 for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi = psi)
1202 stmt = gsi_stmt (gsi);
1209 /* If GSI is not necessary then remove it. */
1210 if (!gimple_plf (stmt, STMT_NECESSARY))
1212 if (!is_gimple_debug (stmt))
1213 something_changed = true;
1214 remove_dead_stmt (&gsi, bb);
1216 else if (is_gimple_call (stmt))
1218 call = gimple_call_fndecl (stmt);
1223 /* When LHS of var = call (); is dead, simplify it into
1224 call (); saving one operand. */
1225 name = gimple_call_lhs (stmt);
1226 if (name && TREE_CODE (name) == SSA_NAME
1227 && !TEST_BIT (processed, SSA_NAME_VERSION (name)))
1229 something_changed = true;
1230 if (dump_file && (dump_flags & TDF_DETAILS))
1232 fprintf (dump_file, "Deleting LHS of call: ");
1233 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
1234 fprintf (dump_file, "\n");
1237 gimple_call_set_lhs (stmt, NULL_TREE);
1238 maybe_clean_or_replace_eh_stmt (stmt, stmt);
1240 release_ssa_name (name);
1242 notice_special_calls (stmt);
1248 VEC_free (basic_block, heap, h);
1250 /* Since we don't track liveness of virtual PHI nodes, it is possible that we
1251 rendered some PHI nodes unreachable while they are still in use.
1252 Mark them for renaming. */
1255 basic_block prev_bb;
1257 find_unreachable_blocks ();
1259 /* Delete all unreachable basic blocks in reverse dominator order. */
1260 for (bb = EXIT_BLOCK_PTR->prev_bb; bb != ENTRY_BLOCK_PTR; bb = prev_bb)
1262 prev_bb = bb->prev_bb;
1264 if (!TEST_BIT (bb_contains_live_stmts, bb->index)
1265 || !(bb->flags & BB_REACHABLE))
1267 for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
1268 if (!is_gimple_reg (gimple_phi_result (gsi_stmt (gsi))))
1271 imm_use_iterator iter;
1273 FOR_EACH_IMM_USE_STMT (stmt, iter, gimple_phi_result (gsi_stmt (gsi)))
1275 if (!(gimple_bb (stmt)->flags & BB_REACHABLE))
1277 if (gimple_code (stmt) == GIMPLE_PHI
1278 || gimple_plf (stmt, STMT_NECESSARY))
1281 BREAK_FROM_IMM_USE_STMT (iter);
1285 mark_virtual_phi_result_for_renaming (gsi_stmt (gsi));
1288 if (!(bb->flags & BB_REACHABLE))
1290 /* Speed up the removal of blocks that don't
1291 dominate others. Walking backwards, this should
1292 be the common case. ??? Do we need to recompute
1293 dominators because of cfg_altered? */
1294 if (!MAY_HAVE_DEBUG_STMTS
1295 || !first_dom_son (CDI_DOMINATORS, bb))
1296 delete_basic_block (bb);
1299 h = get_all_dominated_blocks (CDI_DOMINATORS, bb);
1301 while (VEC_length (basic_block, h))
1303 bb = VEC_pop (basic_block, h);
1304 prev_bb = bb->prev_bb;
1305 /* Rearrangements to the CFG may have failed
1306 to update the dominators tree, so that
1307 formerly-dominated blocks are now
1308 otherwise reachable. */
1309 if (!!(bb->flags & BB_REACHABLE))
1311 delete_basic_block (bb);
1314 VEC_free (basic_block, heap, h);
1322 /* Remove dead PHI nodes. */
1323 something_changed |= remove_dead_phis (bb);
1326 return something_changed;
1330 /* Print out removed statement statistics. */
1337 percg = ((float) stats.removed / (float) stats.total) * 100;
1338 fprintf (dump_file, "Removed %d of %d statements (%d%%)\n",
1339 stats.removed, stats.total, (int) percg);
1341 if (stats.total_phis == 0)
1344 percg = ((float) stats.removed_phis / (float) stats.total_phis) * 100;
1346 fprintf (dump_file, "Removed %d of %d PHI nodes (%d%%)\n",
1347 stats.removed_phis, stats.total_phis, (int) percg);
1350 /* Initialization for this pass. Set up the used data structures. */
1353 tree_dce_init (bool aggressive)
1355 memset ((void *) &stats, 0, sizeof (stats));
1361 control_dependence_map = XNEWVEC (bitmap, last_basic_block);
1362 for (i = 0; i < last_basic_block; ++i)
1363 control_dependence_map[i] = BITMAP_ALLOC (NULL);
1365 last_stmt_necessary = sbitmap_alloc (last_basic_block);
1366 sbitmap_zero (last_stmt_necessary);
1367 bb_contains_live_stmts = sbitmap_alloc (last_basic_block);
1368 sbitmap_zero (bb_contains_live_stmts);
1371 processed = sbitmap_alloc (num_ssa_names + 1);
1372 sbitmap_zero (processed);
1374 worklist = VEC_alloc (gimple, heap, 64);
1375 cfg_altered = false;
1378 /* Cleanup after this pass. */
1381 tree_dce_done (bool aggressive)
1387 for (i = 0; i < last_basic_block; ++i)
1388 BITMAP_FREE (control_dependence_map[i]);
1389 free (control_dependence_map);
1391 sbitmap_free (visited_control_parents);
1392 sbitmap_free (last_stmt_necessary);
1393 sbitmap_free (bb_contains_live_stmts);
1394 bb_contains_live_stmts = NULL;
1397 sbitmap_free (processed);
1399 VEC_free (gimple, heap, worklist);
1402 /* Main routine to eliminate dead code.
1404 AGGRESSIVE controls the aggressiveness of the algorithm.
1405 In conservative mode, we ignore control dependence and simply declare
1406 all but the most trivially dead branches necessary. This mode is fast.
1407 In aggressive mode, control dependences are taken into account, which
1408 results in more dead code elimination, but at the cost of some time.
1410 FIXME: Aggressive mode before PRE doesn't work currently because
1411 the dominance info is not invalidated after DCE1. This is
1412 not an issue right now because we only run aggressive DCE
1413 as the last tree SSA pass, but keep this in mind when you
1414 start experimenting with pass ordering. */
1417 perform_tree_ssa_dce (bool aggressive)
1419 struct edge_list *el = NULL;
1420 bool something_changed = 0;
1422 calculate_dominance_info (CDI_DOMINATORS);
1424 /* Preheaders are needed for SCEV to work.
1425 Simple lateches and recorded exits improve chances that loop will
1426 proved to be finite in testcases such as in loop-15.c and loop-24.c */
1428 loop_optimizer_init (LOOPS_NORMAL
1429 | LOOPS_HAVE_RECORDED_EXITS);
1431 tree_dce_init (aggressive);
1435 /* Compute control dependence. */
1436 timevar_push (TV_CONTROL_DEPENDENCES);
1437 calculate_dominance_info (CDI_POST_DOMINATORS);
1438 el = create_edge_list ();
1439 find_all_control_dependences (el);
1440 timevar_pop (TV_CONTROL_DEPENDENCES);
1442 visited_control_parents = sbitmap_alloc (last_basic_block);
1443 sbitmap_zero (visited_control_parents);
1445 mark_dfs_back_edges ();
1448 find_obviously_necessary_stmts (el);
1451 loop_optimizer_finalize ();
1457 visited = BITMAP_ALLOC (NULL);
1458 propagate_necessity (el);
1459 BITMAP_FREE (visited);
1461 something_changed |= eliminate_unnecessary_stmts ();
1462 something_changed |= cfg_altered;
1464 /* We do not update postdominators, so free them unconditionally. */
1465 free_dominance_info (CDI_POST_DOMINATORS);
1467 /* If we removed paths in the CFG, then we need to update
1468 dominators as well. I haven't investigated the possibility
1469 of incrementally updating dominators. */
1471 free_dominance_info (CDI_DOMINATORS);
1473 statistics_counter_event (cfun, "Statements deleted", stats.removed);
1474 statistics_counter_event (cfun, "PHI nodes deleted", stats.removed_phis);
1476 /* Debugging dumps. */
1477 if (dump_file && (dump_flags & (TDF_STATS|TDF_DETAILS)))
1480 tree_dce_done (aggressive);
1482 free_edge_list (el);
1484 if (something_changed)
1485 return (TODO_update_ssa | TODO_cleanup_cfg | TODO_ggc_collect
1486 | TODO_remove_unused_locals);
1491 /* Pass entry points. */
1495 return perform_tree_ssa_dce (/*aggressive=*/false);
1499 tree_ssa_dce_loop (void)
1502 todo = perform_tree_ssa_dce (/*aggressive=*/false);
1505 free_numbers_of_iterations_estimates ();
1512 tree_ssa_cd_dce (void)
1514 return perform_tree_ssa_dce (/*aggressive=*/optimize >= 2);
1520 return flag_tree_dce != 0;
1523 struct gimple_opt_pass pass_dce =
1528 gate_dce, /* gate */
1529 tree_ssa_dce, /* execute */
1532 0, /* static_pass_number */
1533 TV_TREE_DCE, /* tv_id */
1534 PROP_cfg | PROP_ssa, /* properties_required */
1535 0, /* properties_provided */
1536 0, /* properties_destroyed */
1537 0, /* todo_flags_start */
1538 TODO_verify_ssa /* todo_flags_finish */
1542 struct gimple_opt_pass pass_dce_loop =
1546 "dceloop", /* name */
1547 gate_dce, /* gate */
1548 tree_ssa_dce_loop, /* execute */
1551 0, /* static_pass_number */
1552 TV_TREE_DCE, /* tv_id */
1553 PROP_cfg | PROP_ssa, /* properties_required */
1554 0, /* properties_provided */
1555 0, /* properties_destroyed */
1556 0, /* todo_flags_start */
1557 TODO_verify_ssa /* todo_flags_finish */
1561 struct gimple_opt_pass pass_cd_dce =
1566 gate_dce, /* gate */
1567 tree_ssa_cd_dce, /* execute */
1570 0, /* static_pass_number */
1571 TV_TREE_CD_DCE, /* tv_id */
1572 PROP_cfg | PROP_ssa, /* properties_required */
1573 0, /* properties_provided */
1574 0, /* properties_destroyed */
1575 0, /* todo_flags_start */
1577 | TODO_verify_flow /* todo_flags_finish */