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:
311 case BUILT_IN_ALLOCA_WITH_ALIGN:
316 /* Most, but not all function calls are required. Function calls that
317 produce no result and have no side effects (i.e. const pure
318 functions) are unnecessary. */
319 if (gimple_has_side_effects (stmt))
321 mark_stmt_necessary (stmt, true);
324 if (!gimple_call_lhs (stmt))
330 /* Debug temps without a value are not useful. ??? If we could
331 easily locate the debug temp bind stmt for a use thereof,
332 would could refrain from marking all debug temps here, and
333 mark them only if they're used. */
334 if (!gimple_debug_bind_p (stmt)
335 || gimple_debug_bind_has_value_p (stmt)
336 || TREE_CODE (gimple_debug_bind_get_var (stmt)) != DEBUG_EXPR_DECL)
337 mark_stmt_necessary (stmt, false);
341 gcc_assert (!simple_goto_p (stmt));
342 mark_stmt_necessary (stmt, true);
346 gcc_assert (EDGE_COUNT (gimple_bb (stmt)->succs) == 2);
351 mark_stmt_necessary (stmt, true);
358 /* If the statement has volatile operands, it needs to be preserved.
359 Same for statements that can alter control flow in unpredictable
361 if (gimple_has_volatile_ops (stmt) || is_ctrl_altering_stmt (stmt))
363 mark_stmt_necessary (stmt, true);
367 if (is_hidden_global_store (stmt))
369 mark_stmt_necessary (stmt, true);
377 /* Mark the last statement of BB as necessary. */
380 mark_last_stmt_necessary (basic_block bb)
382 gimple stmt = last_stmt (bb);
384 SET_BIT (last_stmt_necessary, bb->index);
385 SET_BIT (bb_contains_live_stmts, bb->index);
387 /* We actually mark the statement only if it is a control statement. */
388 if (stmt && is_ctrl_stmt (stmt))
389 mark_stmt_necessary (stmt, true);
393 /* Mark control dependent edges of BB as necessary. We have to do this only
394 once for each basic block so we set the appropriate bit after we're done.
396 When IGNORE_SELF is true, ignore BB in the list of control dependences. */
399 mark_control_dependent_edges_necessary (basic_block bb, struct edge_list *el,
403 unsigned edge_number;
404 bool skipped = false;
406 gcc_assert (bb != EXIT_BLOCK_PTR);
408 if (bb == ENTRY_BLOCK_PTR)
411 EXECUTE_IF_CONTROL_DEPENDENT (bi, bb->index, edge_number)
413 basic_block cd_bb = INDEX_EDGE_PRED_BB (el, edge_number);
415 if (ignore_self && cd_bb == bb)
421 if (!TEST_BIT (last_stmt_necessary, cd_bb->index))
422 mark_last_stmt_necessary (cd_bb);
426 SET_BIT (visited_control_parents, bb->index);
430 /* Find obviously necessary statements. These are things like most function
431 calls, and stores to file level variables.
433 If EL is NULL, control statements are conservatively marked as
434 necessary. Otherwise it contains the list of edges used by control
435 dependence analysis. */
438 find_obviously_necessary_stmts (struct edge_list *el)
441 gimple_stmt_iterator gsi;
448 /* PHI nodes are never inherently necessary. */
449 for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
451 phi = gsi_stmt (gsi);
452 gimple_set_plf (phi, STMT_NECESSARY, false);
455 /* Check all statements in the block. */
456 for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
458 stmt = gsi_stmt (gsi);
459 gimple_set_plf (stmt, STMT_NECESSARY, false);
460 mark_stmt_if_obviously_necessary (stmt, el != NULL);
464 /* Pure and const functions are finite and thus have no infinite loops in
466 flags = flags_from_decl_or_type (current_function_decl);
467 if ((flags & (ECF_CONST|ECF_PURE)) && !(flags & ECF_LOOPING_CONST_OR_PURE))
470 /* Prevent the empty possibly infinite loops from being removed. */
476 if (mark_irreducible_loops ())
480 FOR_EACH_EDGE (e, ei, bb->succs)
481 if ((e->flags & EDGE_DFS_BACK)
482 && (e->flags & EDGE_IRREDUCIBLE_LOOP))
485 fprintf (dump_file, "Marking back edge of irreducible loop %i->%i\n",
486 e->src->index, e->dest->index);
487 mark_control_dependent_edges_necessary (e->dest, el, false);
491 FOR_EACH_LOOP (li, loop, 0)
492 if (!finite_loop_p (loop))
495 fprintf (dump_file, "can not prove finiteness of loop %i\n", loop->num);
496 mark_control_dependent_edges_necessary (loop->latch, el, false);
503 /* Return true if REF is based on an aliased base, otherwise false. */
506 ref_may_be_aliased (tree ref)
508 gcc_assert (TREE_CODE (ref) != WITH_SIZE_EXPR);
509 while (handled_component_p (ref))
510 ref = TREE_OPERAND (ref, 0);
511 if (TREE_CODE (ref) == MEM_REF
512 && TREE_CODE (TREE_OPERAND (ref, 0)) == ADDR_EXPR)
513 ref = TREE_OPERAND (TREE_OPERAND (ref, 0), 0);
514 return !(DECL_P (ref)
515 && !may_be_aliased (ref));
518 static bitmap visited = NULL;
519 static unsigned int longest_chain = 0;
520 static unsigned int total_chain = 0;
521 static unsigned int nr_walks = 0;
522 static bool chain_ovfl = false;
524 /* Worker for the walker that marks reaching definitions of REF,
525 which is based on a non-aliased decl, necessary. It returns
526 true whenever the defining statement of the current VDEF is
527 a kill for REF, as no dominating may-defs are necessary for REF
528 anymore. DATA points to the basic-block that contains the
529 stmt that refers to REF. */
532 mark_aliased_reaching_defs_necessary_1 (ao_ref *ref, tree vdef, void *data)
534 gimple def_stmt = SSA_NAME_DEF_STMT (vdef);
536 /* All stmts we visit are necessary. */
537 mark_operand_necessary (vdef);
539 /* If the stmt lhs kills ref, then we can stop walking. */
540 if (gimple_has_lhs (def_stmt)
541 && TREE_CODE (gimple_get_lhs (def_stmt)) != SSA_NAME
542 /* The assignment is not necessarily carried out if it can throw
543 and we can catch it in the current function where we could inspect
545 ??? We only need to care about the RHS throwing. For aggregate
546 assignments or similar calls and non-call exceptions the LHS
547 might throw as well. */
548 && !stmt_can_throw_internal (def_stmt))
550 tree base, lhs = gimple_get_lhs (def_stmt);
551 HOST_WIDE_INT size, offset, max_size;
553 base = get_ref_base_and_extent (lhs, &offset, &size, &max_size);
554 /* We can get MEM[symbol: sZ, index: D.8862_1] here,
555 so base == refd->base does not always hold. */
556 if (base == ref->base)
558 /* For a must-alias check we need to be able to constrain
559 the accesses properly. */
560 if (size != -1 && size == max_size
561 && ref->max_size != -1)
563 if (offset <= ref->offset
564 && offset + size >= ref->offset + ref->max_size)
567 /* Or they need to be exactly the same. */
569 /* Make sure there is no induction variable involved
570 in the references (gcc.c-torture/execute/pr42142.c).
571 The simplest way is to check if the kill dominates
573 && dominated_by_p (CDI_DOMINATORS, (basic_block) data,
574 gimple_bb (def_stmt))
575 && operand_equal_p (ref->ref, lhs, 0))
580 /* Otherwise keep walking. */
585 mark_aliased_reaching_defs_necessary (gimple stmt, tree ref)
589 gcc_assert (!chain_ovfl);
590 ao_ref_init (&refd, ref);
591 chain = walk_aliased_vdefs (&refd, gimple_vuse (stmt),
592 mark_aliased_reaching_defs_necessary_1,
593 gimple_bb (stmt), NULL);
594 if (chain > longest_chain)
595 longest_chain = chain;
596 total_chain += chain;
600 /* Worker for the walker that marks reaching definitions of REF, which
601 is not based on a non-aliased decl. For simplicity we need to end
602 up marking all may-defs necessary that are not based on a non-aliased
603 decl. The only job of this walker is to skip may-defs based on
604 a non-aliased decl. */
607 mark_all_reaching_defs_necessary_1 (ao_ref *ref ATTRIBUTE_UNUSED,
608 tree vdef, void *data ATTRIBUTE_UNUSED)
610 gimple def_stmt = SSA_NAME_DEF_STMT (vdef);
612 /* We have to skip already visited (and thus necessary) statements
613 to make the chaining work after we dropped back to simple mode. */
615 && TEST_BIT (processed, SSA_NAME_VERSION (vdef)))
617 gcc_assert (gimple_nop_p (def_stmt)
618 || gimple_plf (def_stmt, STMT_NECESSARY));
622 /* We want to skip stores to non-aliased variables. */
624 && gimple_assign_single_p (def_stmt))
626 tree lhs = gimple_assign_lhs (def_stmt);
627 if (!ref_may_be_aliased (lhs))
631 /* We want to skip statments that do not constitute stores but have
632 a virtual definition. */
633 if (is_gimple_call (def_stmt))
635 tree callee = gimple_call_fndecl (def_stmt);
636 if (callee != NULL_TREE
637 && DECL_BUILT_IN_CLASS (callee) == BUILT_IN_NORMAL)
638 switch (DECL_FUNCTION_CODE (callee))
640 case BUILT_IN_MALLOC:
641 case BUILT_IN_CALLOC:
642 case BUILT_IN_ALLOCA:
643 case BUILT_IN_ALLOCA_WITH_ALIGN:
651 mark_operand_necessary (vdef);
657 mark_all_reaching_defs_necessary (gimple stmt)
659 walk_aliased_vdefs (NULL, gimple_vuse (stmt),
660 mark_all_reaching_defs_necessary_1, NULL, &visited);
663 /* Return true for PHI nodes with one or identical arguments
666 degenerate_phi_p (gimple phi)
669 tree op = gimple_phi_arg_def (phi, 0);
670 for (i = 1; i < gimple_phi_num_args (phi); i++)
671 if (gimple_phi_arg_def (phi, i) != op)
676 /* Propagate necessity using the operands of necessary statements.
677 Process the uses on each statement in the worklist, and add all
678 feeding statements which contribute to the calculation of this
679 value to the worklist.
681 In conservative mode, EL is NULL. */
684 propagate_necessity (struct edge_list *el)
687 bool aggressive = (el ? true : false);
689 if (dump_file && (dump_flags & TDF_DETAILS))
690 fprintf (dump_file, "\nProcessing worklist:\n");
692 while (VEC_length (gimple, worklist) > 0)
694 /* Take STMT from worklist. */
695 stmt = VEC_pop (gimple, worklist);
697 if (dump_file && (dump_flags & TDF_DETAILS))
699 fprintf (dump_file, "processing: ");
700 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
701 fprintf (dump_file, "\n");
706 /* Mark the last statement of the basic blocks on which the block
707 containing STMT is control dependent, but only if we haven't
709 basic_block bb = gimple_bb (stmt);
710 if (bb != ENTRY_BLOCK_PTR
711 && !TEST_BIT (visited_control_parents, bb->index))
712 mark_control_dependent_edges_necessary (bb, el, false);
715 if (gimple_code (stmt) == GIMPLE_PHI
716 /* We do not process virtual PHI nodes nor do we track their
718 && is_gimple_reg (gimple_phi_result (stmt)))
720 /* PHI nodes are somewhat special in that each PHI alternative has
721 data and control dependencies. All the statements feeding the
722 PHI node's arguments are always necessary. In aggressive mode,
723 we also consider the control dependent edges leading to the
724 predecessor block associated with each PHI alternative as
728 for (k = 0; k < gimple_phi_num_args (stmt); k++)
730 tree arg = PHI_ARG_DEF (stmt, k);
731 if (TREE_CODE (arg) == SSA_NAME)
732 mark_operand_necessary (arg);
735 /* For PHI operands it matters from where the control flow arrives
736 to the BB. Consider the following example:
746 We need to mark control dependence of the empty basic blocks, since they
747 contains computation of PHI operands.
749 Doing so is too restrictive in the case the predecestor block is in
755 for (i = 0; i<1000; ++i)
761 There is PHI for J in the BB containing return statement.
762 In this case the control dependence of predecestor block (that is
763 within the empty loop) also contains the block determining number
764 of iterations of the block that would prevent removing of empty
767 This scenario can be avoided by splitting critical edges.
768 To save the critical edge splitting pass we identify how the control
769 dependence would look like if the edge was split.
771 Consider the modified CFG created from current CFG by splitting
772 edge B->C. In the postdominance tree of modified CFG, C' is
773 always child of C. There are two cases how chlids of C' can look
778 In this case the only basic block C' is control dependent on is B.
780 2) C' has single child that is B
782 In this case control dependence of C' is same as control
783 dependence of B in original CFG except for block B itself.
784 (since C' postdominate B in modified CFG)
786 Now how to decide what case happens? There are two basic options:
788 a) C postdominate B. Then C immediately postdominate B and
789 case 2 happens iff there is no other way from B to C except
792 There is other way from B to C iff there is succesor of B that
793 is not postdominated by B. Testing this condition is somewhat
794 expensive, because we need to iterate all succesors of B.
795 We are safe to assume that this does not happen: we will mark B
796 as needed when processing the other path from B to C that is
797 conrol dependent on B and marking control dependencies of B
798 itself is harmless because they will be processed anyway after
799 processing control statement in B.
801 b) C does not postdominate B. Always case 1 happens since there is
802 path from C to exit that does not go through B and thus also C'. */
804 if (aggressive && !degenerate_phi_p (stmt))
806 for (k = 0; k < gimple_phi_num_args (stmt); k++)
808 basic_block arg_bb = gimple_phi_arg_edge (stmt, k)->src;
811 != get_immediate_dominator (CDI_POST_DOMINATORS, arg_bb))
813 if (!TEST_BIT (last_stmt_necessary, arg_bb->index))
814 mark_last_stmt_necessary (arg_bb);
816 else if (arg_bb != ENTRY_BLOCK_PTR
817 && !TEST_BIT (visited_control_parents,
819 mark_control_dependent_edges_necessary (arg_bb, el, true);
825 /* Propagate through the operands. Examine all the USE, VUSE and
826 VDEF operands in this statement. Mark all the statements
827 which feed this statement's uses as necessary. */
831 /* If this is a call to free which is directly fed by an
832 allocation function do not mark that necessary through
833 processing the argument. */
834 if (gimple_call_builtin_p (stmt, BUILT_IN_FREE))
836 tree ptr = gimple_call_arg (stmt, 0);
839 /* If the pointer we free is defined by an allocation
840 function do not add the call to the worklist. */
841 if (TREE_CODE (ptr) == SSA_NAME
842 && is_gimple_call (def_stmt = SSA_NAME_DEF_STMT (ptr))
843 && (def_callee = gimple_call_fndecl (def_stmt))
844 && DECL_BUILT_IN_CLASS (def_callee) == BUILT_IN_NORMAL
845 && (DECL_FUNCTION_CODE (def_callee) == BUILT_IN_MALLOC
846 || DECL_FUNCTION_CODE (def_callee) == BUILT_IN_CALLOC))
850 FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
851 mark_operand_necessary (use);
853 use = gimple_vuse (stmt);
857 /* If we dropped to simple mode make all immediately
858 reachable definitions necessary. */
861 mark_all_reaching_defs_necessary (stmt);
865 /* For statements that may load from memory (have a VUSE) we
866 have to mark all reaching (may-)definitions as necessary.
867 We partition this task into two cases:
868 1) explicit loads based on decls that are not aliased
869 2) implicit loads (like calls) and explicit loads not
870 based on decls that are not aliased (like indirect
871 references or loads from globals)
872 For 1) we mark all reaching may-defs as necessary, stopping
873 at dominating kills. For 2) we want to mark all dominating
874 references necessary, but non-aliased ones which we handle
875 in 1). By keeping a global visited bitmap for references
876 we walk for 2) we avoid quadratic behavior for those. */
878 if (is_gimple_call (stmt))
880 tree callee = gimple_call_fndecl (stmt);
883 /* Calls to functions that are merely acting as barriers
884 or that only store to memory do not make any previous
886 if (callee != NULL_TREE
887 && DECL_BUILT_IN_CLASS (callee) == BUILT_IN_NORMAL
888 && (DECL_FUNCTION_CODE (callee) == BUILT_IN_MEMSET
889 || DECL_FUNCTION_CODE (callee) == BUILT_IN_MEMSET_CHK
890 || DECL_FUNCTION_CODE (callee) == BUILT_IN_MALLOC
891 || DECL_FUNCTION_CODE (callee) == BUILT_IN_CALLOC
892 || DECL_FUNCTION_CODE (callee) == BUILT_IN_FREE
893 || DECL_FUNCTION_CODE (callee) == BUILT_IN_VA_END
894 || DECL_FUNCTION_CODE (callee) == BUILT_IN_ALLOCA
895 || (DECL_FUNCTION_CODE (callee)
896 == BUILT_IN_ALLOCA_WITH_ALIGN)
897 || DECL_FUNCTION_CODE (callee) == BUILT_IN_STACK_SAVE
898 || DECL_FUNCTION_CODE (callee) == BUILT_IN_STACK_RESTORE
899 || DECL_FUNCTION_CODE (callee) == BUILT_IN_ASSUME_ALIGNED))
902 /* Calls implicitly load from memory, their arguments
903 in addition may explicitly perform memory loads. */
904 mark_all_reaching_defs_necessary (stmt);
905 for (i = 0; i < gimple_call_num_args (stmt); ++i)
907 tree arg = gimple_call_arg (stmt, i);
908 if (TREE_CODE (arg) == SSA_NAME
909 || is_gimple_min_invariant (arg))
911 if (TREE_CODE (arg) == WITH_SIZE_EXPR)
912 arg = TREE_OPERAND (arg, 0);
913 if (!ref_may_be_aliased (arg))
914 mark_aliased_reaching_defs_necessary (stmt, arg);
917 else if (gimple_assign_single_p (stmt))
920 bool rhs_aliased = false;
921 /* If this is a load mark things necessary. */
922 rhs = gimple_assign_rhs1 (stmt);
923 if (TREE_CODE (rhs) != SSA_NAME
924 && !is_gimple_min_invariant (rhs))
926 if (!ref_may_be_aliased (rhs))
927 mark_aliased_reaching_defs_necessary (stmt, rhs);
932 mark_all_reaching_defs_necessary (stmt);
934 else if (gimple_code (stmt) == GIMPLE_RETURN)
936 tree rhs = gimple_return_retval (stmt);
937 /* A return statement may perform a load. */
939 && TREE_CODE (rhs) != SSA_NAME
940 && !is_gimple_min_invariant (rhs))
942 if (!ref_may_be_aliased (rhs))
943 mark_aliased_reaching_defs_necessary (stmt, rhs);
945 mark_all_reaching_defs_necessary (stmt);
948 else if (gimple_code (stmt) == GIMPLE_ASM)
951 mark_all_reaching_defs_necessary (stmt);
952 /* Inputs may perform loads. */
953 for (i = 0; i < gimple_asm_ninputs (stmt); ++i)
955 tree op = TREE_VALUE (gimple_asm_input_op (stmt, i));
956 if (TREE_CODE (op) != SSA_NAME
957 && !is_gimple_min_invariant (op)
958 && !ref_may_be_aliased (op))
959 mark_aliased_reaching_defs_necessary (stmt, op);
965 /* If we over-used our alias oracle budget drop to simple
966 mode. The cost metric allows quadratic behavior
967 (number of uses times number of may-defs queries) up to
968 a constant maximal number of queries and after that falls back to
969 super-linear complexity. */
970 if (/* Constant but quadratic for small functions. */
971 total_chain > 128 * 128
972 /* Linear in the number of may-defs. */
973 && total_chain > 32 * longest_chain
974 /* Linear in the number of uses. */
975 && total_chain > nr_walks * 32)
979 bitmap_clear (visited);
985 /* Replace all uses of result of PHI by underlying variable and mark it
989 mark_virtual_phi_result_for_renaming (gimple phi)
992 imm_use_iterator iter;
995 tree result_ssa, result_var;
997 if (dump_file && (dump_flags & TDF_DETAILS))
999 fprintf (dump_file, "Marking result for renaming : ");
1000 print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
1001 fprintf (dump_file, "\n");
1004 result_ssa = gimple_phi_result (phi);
1005 result_var = SSA_NAME_VAR (result_ssa);
1006 FOR_EACH_IMM_USE_STMT (stmt, iter, result_ssa)
1008 FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
1009 SET_USE (use_p, result_var);
1014 mark_sym_for_renaming (result_var);
1017 /* Remove dead PHI nodes from block BB. */
1020 remove_dead_phis (basic_block bb)
1022 bool something_changed = false;
1025 gimple_stmt_iterator gsi;
1026 phis = phi_nodes (bb);
1028 for (gsi = gsi_start (phis); !gsi_end_p (gsi);)
1031 phi = gsi_stmt (gsi);
1033 /* We do not track necessity of virtual PHI nodes. Instead do
1034 very simple dead PHI removal here. */
1035 if (!is_gimple_reg (gimple_phi_result (phi)))
1037 /* Virtual PHI nodes with one or identical arguments
1039 if (degenerate_phi_p (phi))
1041 tree vdef = gimple_phi_result (phi);
1042 tree vuse = gimple_phi_arg_def (phi, 0);
1044 use_operand_p use_p;
1045 imm_use_iterator iter;
1047 FOR_EACH_IMM_USE_STMT (use_stmt, iter, vdef)
1048 FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
1049 SET_USE (use_p, vuse);
1050 if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vdef)
1051 && TREE_CODE (vuse) == SSA_NAME)
1052 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vuse) = 1;
1055 gimple_set_plf (phi, STMT_NECESSARY, true);
1058 if (!gimple_plf (phi, STMT_NECESSARY))
1060 something_changed = true;
1061 if (dump_file && (dump_flags & TDF_DETAILS))
1063 fprintf (dump_file, "Deleting : ");
1064 print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
1065 fprintf (dump_file, "\n");
1068 remove_phi_node (&gsi, true);
1069 stats.removed_phis++;
1075 return something_changed;
1078 /* Forward edge E to respective POST_DOM_BB and update PHIs. */
1081 forward_edge_to_pdom (edge e, basic_block post_dom_bb)
1083 gimple_stmt_iterator gsi;
1087 if (dump_file && (dump_flags & TDF_DETAILS))
1088 fprintf (dump_file, "Redirecting edge %i->%i to %i\n", e->src->index,
1089 e->dest->index, post_dom_bb->index);
1091 e2 = redirect_edge_and_branch (e, post_dom_bb);
1094 /* If edge was already around, no updating is neccesary. */
1098 if (!gimple_seq_empty_p (phi_nodes (post_dom_bb)))
1100 /* We are sure that for every live PHI we are seeing control dependent BB.
1101 This means that we can pick any edge to duplicate PHI args from. */
1102 FOR_EACH_EDGE (e2, ei, post_dom_bb->preds)
1105 for (gsi = gsi_start_phis (post_dom_bb); !gsi_end_p (gsi);)
1107 gimple phi = gsi_stmt (gsi);
1109 source_location locus;
1111 /* PHIs for virtuals have no control dependency relation on them.
1112 We are lost here and must force renaming of the symbol. */
1113 if (!is_gimple_reg (gimple_phi_result (phi)))
1115 mark_virtual_phi_result_for_renaming (phi);
1116 remove_phi_node (&gsi, true);
1120 /* Dead PHI do not imply control dependency. */
1121 if (!gimple_plf (phi, STMT_NECESSARY))
1127 op = gimple_phi_arg_def (phi, e2->dest_idx);
1128 locus = gimple_phi_arg_location (phi, e2->dest_idx);
1129 add_phi_arg (phi, op, e, locus);
1130 /* The resulting PHI if not dead can only be degenerate. */
1131 gcc_assert (degenerate_phi_p (phi));
1138 /* Remove dead statement pointed to by iterator I. Receives the basic block BB
1139 containing I so that we don't have to look it up. */
1142 remove_dead_stmt (gimple_stmt_iterator *i, basic_block bb)
1144 gimple stmt = gsi_stmt (*i);
1146 if (dump_file && (dump_flags & TDF_DETAILS))
1148 fprintf (dump_file, "Deleting : ");
1149 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
1150 fprintf (dump_file, "\n");
1155 /* If we have determined that a conditional branch statement contributes
1156 nothing to the program, then we not only remove it, but we also change
1157 the flow graph so that the current block will simply fall-thru to its
1158 immediate post-dominator. The blocks we are circumventing will be
1159 removed by cleanup_tree_cfg if this change in the flow graph makes them
1161 if (is_ctrl_stmt (stmt))
1163 basic_block post_dom_bb;
1167 post_dom_bb = get_immediate_dominator (CDI_POST_DOMINATORS, bb);
1169 e = find_edge (bb, post_dom_bb);
1171 /* If edge is already there, try to use it. This avoids need to update
1172 PHI nodes. Also watch for cases where post dominator does not exists
1173 or is exit block. These can happen for infinite loops as we create
1174 fake edges in the dominator tree. */
1177 else if (! post_dom_bb || post_dom_bb == EXIT_BLOCK_PTR)
1178 e = EDGE_SUCC (bb, 0);
1180 e = forward_edge_to_pdom (EDGE_SUCC (bb, 0), post_dom_bb);
1182 e->probability = REG_BR_PROB_BASE;
1183 e->count = bb->count;
1185 /* The edge is no longer associated with a conditional, so it does
1186 not have TRUE/FALSE flags. */
1187 e->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
1189 /* The lone outgoing edge from BB will be a fallthru edge. */
1190 e->flags |= EDGE_FALLTHRU;
1192 /* Remove the remaining outgoing edges. */
1193 for (ei = ei_start (bb->succs); (e2 = ei_safe_edge (ei)); )
1203 unlink_stmt_vdef (stmt);
1204 gsi_remove (i, true);
1205 release_defs (stmt);
1208 /* Eliminate unnecessary statements. Any instruction not marked as necessary
1209 contributes nothing to the program, and can be deleted. */
1212 eliminate_unnecessary_stmts (void)
1214 bool something_changed = false;
1216 gimple_stmt_iterator gsi, psi;
1219 VEC (basic_block, heap) *h;
1221 if (dump_file && (dump_flags & TDF_DETAILS))
1222 fprintf (dump_file, "\nEliminating unnecessary statements:\n");
1224 clear_special_calls ();
1226 /* Walking basic blocks and statements in reverse order avoids
1227 releasing SSA names before any other DEFs that refer to them are
1228 released. This helps avoid loss of debug information, as we get
1229 a chance to propagate all RHSs of removed SSAs into debug uses,
1230 rather than only the latest ones. E.g., consider:
1236 If we were to release x_3 before a_5, when we reached a_5 and
1237 tried to substitute it into the debug stmt, we'd see x_3 there,
1238 but x_3's DEF, type, etc would have already been disconnected.
1239 By going backwards, the debug stmt first changes to:
1241 # DEBUG a => x_3 - b_4
1245 # DEBUG a => y_1 + z_2 - b_4
1248 gcc_assert (dom_info_available_p (CDI_DOMINATORS));
1249 h = get_all_dominated_blocks (CDI_DOMINATORS, single_succ (ENTRY_BLOCK_PTR));
1251 while (VEC_length (basic_block, h))
1253 bb = VEC_pop (basic_block, h);
1255 /* Remove dead statements. */
1256 for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi = psi)
1258 stmt = gsi_stmt (gsi);
1265 /* We can mark a call to free as not necessary if the
1266 defining statement of its argument is an allocation
1267 function and that is not necessary itself. */
1268 if (gimple_call_builtin_p (stmt, BUILT_IN_FREE))
1270 tree ptr = gimple_call_arg (stmt, 0);
1273 if (TREE_CODE (ptr) != SSA_NAME)
1275 def_stmt = SSA_NAME_DEF_STMT (ptr);
1276 if (!is_gimple_call (def_stmt)
1277 || gimple_plf (def_stmt, STMT_NECESSARY))
1279 callee2 = gimple_call_fndecl (def_stmt);
1280 if (callee2 == NULL_TREE
1281 || DECL_BUILT_IN_CLASS (callee2) != BUILT_IN_NORMAL
1282 || (DECL_FUNCTION_CODE (callee2) != BUILT_IN_MALLOC
1283 && DECL_FUNCTION_CODE (callee2) != BUILT_IN_CALLOC))
1285 gimple_set_plf (stmt, STMT_NECESSARY, false);
1288 /* If GSI is not necessary then remove it. */
1289 if (!gimple_plf (stmt, STMT_NECESSARY))
1291 if (!is_gimple_debug (stmt))
1292 something_changed = true;
1293 remove_dead_stmt (&gsi, bb);
1295 else if (is_gimple_call (stmt))
1297 call = gimple_call_fndecl (stmt);
1302 /* When LHS of var = call (); is dead, simplify it into
1303 call (); saving one operand. */
1304 name = gimple_call_lhs (stmt);
1305 if (name && TREE_CODE (name) == SSA_NAME
1306 && !TEST_BIT (processed, SSA_NAME_VERSION (name)))
1308 something_changed = true;
1309 if (dump_file && (dump_flags & TDF_DETAILS))
1311 fprintf (dump_file, "Deleting LHS of call: ");
1312 print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
1313 fprintf (dump_file, "\n");
1316 gimple_call_set_lhs (stmt, NULL_TREE);
1317 maybe_clean_or_replace_eh_stmt (stmt, stmt);
1319 release_ssa_name (name);
1321 notice_special_calls (stmt);
1327 VEC_free (basic_block, heap, h);
1329 /* Since we don't track liveness of virtual PHI nodes, it is possible that we
1330 rendered some PHI nodes unreachable while they are still in use.
1331 Mark them for renaming. */
1334 basic_block prev_bb;
1336 find_unreachable_blocks ();
1338 /* Delete all unreachable basic blocks in reverse dominator order. */
1339 for (bb = EXIT_BLOCK_PTR->prev_bb; bb != ENTRY_BLOCK_PTR; bb = prev_bb)
1341 prev_bb = bb->prev_bb;
1343 if (!TEST_BIT (bb_contains_live_stmts, bb->index)
1344 || !(bb->flags & BB_REACHABLE))
1346 for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
1347 if (!is_gimple_reg (gimple_phi_result (gsi_stmt (gsi))))
1350 imm_use_iterator iter;
1352 FOR_EACH_IMM_USE_STMT (stmt, iter, gimple_phi_result (gsi_stmt (gsi)))
1354 if (!(gimple_bb (stmt)->flags & BB_REACHABLE))
1356 if (gimple_code (stmt) == GIMPLE_PHI
1357 || gimple_plf (stmt, STMT_NECESSARY))
1360 BREAK_FROM_IMM_USE_STMT (iter);
1364 mark_virtual_phi_result_for_renaming (gsi_stmt (gsi));
1367 if (!(bb->flags & BB_REACHABLE))
1369 /* Speed up the removal of blocks that don't
1370 dominate others. Walking backwards, this should
1371 be the common case. ??? Do we need to recompute
1372 dominators because of cfg_altered? */
1373 if (!MAY_HAVE_DEBUG_STMTS
1374 || !first_dom_son (CDI_DOMINATORS, bb))
1375 delete_basic_block (bb);
1378 h = get_all_dominated_blocks (CDI_DOMINATORS, bb);
1380 while (VEC_length (basic_block, h))
1382 bb = VEC_pop (basic_block, h);
1383 prev_bb = bb->prev_bb;
1384 /* Rearrangements to the CFG may have failed
1385 to update the dominators tree, so that
1386 formerly-dominated blocks are now
1387 otherwise reachable. */
1388 if (!!(bb->flags & BB_REACHABLE))
1390 delete_basic_block (bb);
1393 VEC_free (basic_block, heap, h);
1401 /* Remove dead PHI nodes. */
1402 something_changed |= remove_dead_phis (bb);
1405 return something_changed;
1409 /* Print out removed statement statistics. */
1416 percg = ((float) stats.removed / (float) stats.total) * 100;
1417 fprintf (dump_file, "Removed %d of %d statements (%d%%)\n",
1418 stats.removed, stats.total, (int) percg);
1420 if (stats.total_phis == 0)
1423 percg = ((float) stats.removed_phis / (float) stats.total_phis) * 100;
1425 fprintf (dump_file, "Removed %d of %d PHI nodes (%d%%)\n",
1426 stats.removed_phis, stats.total_phis, (int) percg);
1429 /* Initialization for this pass. Set up the used data structures. */
1432 tree_dce_init (bool aggressive)
1434 memset ((void *) &stats, 0, sizeof (stats));
1440 control_dependence_map = XNEWVEC (bitmap, last_basic_block);
1441 for (i = 0; i < last_basic_block; ++i)
1442 control_dependence_map[i] = BITMAP_ALLOC (NULL);
1444 last_stmt_necessary = sbitmap_alloc (last_basic_block);
1445 sbitmap_zero (last_stmt_necessary);
1446 bb_contains_live_stmts = sbitmap_alloc (last_basic_block);
1447 sbitmap_zero (bb_contains_live_stmts);
1450 processed = sbitmap_alloc (num_ssa_names + 1);
1451 sbitmap_zero (processed);
1453 worklist = VEC_alloc (gimple, heap, 64);
1454 cfg_altered = false;
1457 /* Cleanup after this pass. */
1460 tree_dce_done (bool aggressive)
1466 for (i = 0; i < last_basic_block; ++i)
1467 BITMAP_FREE (control_dependence_map[i]);
1468 free (control_dependence_map);
1470 sbitmap_free (visited_control_parents);
1471 sbitmap_free (last_stmt_necessary);
1472 sbitmap_free (bb_contains_live_stmts);
1473 bb_contains_live_stmts = NULL;
1476 sbitmap_free (processed);
1478 VEC_free (gimple, heap, worklist);
1481 /* Main routine to eliminate dead code.
1483 AGGRESSIVE controls the aggressiveness of the algorithm.
1484 In conservative mode, we ignore control dependence and simply declare
1485 all but the most trivially dead branches necessary. This mode is fast.
1486 In aggressive mode, control dependences are taken into account, which
1487 results in more dead code elimination, but at the cost of some time.
1489 FIXME: Aggressive mode before PRE doesn't work currently because
1490 the dominance info is not invalidated after DCE1. This is
1491 not an issue right now because we only run aggressive DCE
1492 as the last tree SSA pass, but keep this in mind when you
1493 start experimenting with pass ordering. */
1496 perform_tree_ssa_dce (bool aggressive)
1498 struct edge_list *el = NULL;
1499 bool something_changed = 0;
1501 calculate_dominance_info (CDI_DOMINATORS);
1503 /* Preheaders are needed for SCEV to work.
1504 Simple lateches and recorded exits improve chances that loop will
1505 proved to be finite in testcases such as in loop-15.c and loop-24.c */
1507 loop_optimizer_init (LOOPS_NORMAL
1508 | LOOPS_HAVE_RECORDED_EXITS);
1510 tree_dce_init (aggressive);
1514 /* Compute control dependence. */
1515 timevar_push (TV_CONTROL_DEPENDENCES);
1516 calculate_dominance_info (CDI_POST_DOMINATORS);
1517 el = create_edge_list ();
1518 find_all_control_dependences (el);
1519 timevar_pop (TV_CONTROL_DEPENDENCES);
1521 visited_control_parents = sbitmap_alloc (last_basic_block);
1522 sbitmap_zero (visited_control_parents);
1524 mark_dfs_back_edges ();
1527 find_obviously_necessary_stmts (el);
1530 loop_optimizer_finalize ();
1536 visited = BITMAP_ALLOC (NULL);
1537 propagate_necessity (el);
1538 BITMAP_FREE (visited);
1540 something_changed |= eliminate_unnecessary_stmts ();
1541 something_changed |= cfg_altered;
1543 /* We do not update postdominators, so free them unconditionally. */
1544 free_dominance_info (CDI_POST_DOMINATORS);
1546 /* If we removed paths in the CFG, then we need to update
1547 dominators as well. I haven't investigated the possibility
1548 of incrementally updating dominators. */
1550 free_dominance_info (CDI_DOMINATORS);
1552 statistics_counter_event (cfun, "Statements deleted", stats.removed);
1553 statistics_counter_event (cfun, "PHI nodes deleted", stats.removed_phis);
1555 /* Debugging dumps. */
1556 if (dump_file && (dump_flags & (TDF_STATS|TDF_DETAILS)))
1559 tree_dce_done (aggressive);
1561 free_edge_list (el);
1563 if (something_changed)
1564 return (TODO_update_ssa | TODO_cleanup_cfg | TODO_ggc_collect
1565 | TODO_remove_unused_locals);
1570 /* Pass entry points. */
1574 return perform_tree_ssa_dce (/*aggressive=*/false);
1578 tree_ssa_dce_loop (void)
1581 todo = perform_tree_ssa_dce (/*aggressive=*/false);
1584 free_numbers_of_iterations_estimates ();
1591 tree_ssa_cd_dce (void)
1593 return perform_tree_ssa_dce (/*aggressive=*/optimize >= 2);
1599 return flag_tree_dce != 0;
1602 struct gimple_opt_pass pass_dce =
1607 gate_dce, /* gate */
1608 tree_ssa_dce, /* execute */
1611 0, /* static_pass_number */
1612 TV_TREE_DCE, /* tv_id */
1613 PROP_cfg | PROP_ssa, /* properties_required */
1614 0, /* properties_provided */
1615 0, /* properties_destroyed */
1616 0, /* todo_flags_start */
1617 TODO_verify_ssa /* todo_flags_finish */
1621 struct gimple_opt_pass pass_dce_loop =
1625 "dceloop", /* name */
1626 gate_dce, /* gate */
1627 tree_ssa_dce_loop, /* execute */
1630 0, /* static_pass_number */
1631 TV_TREE_DCE, /* tv_id */
1632 PROP_cfg | PROP_ssa, /* properties_required */
1633 0, /* properties_provided */
1634 0, /* properties_destroyed */
1635 0, /* todo_flags_start */
1636 TODO_verify_ssa /* todo_flags_finish */
1640 struct gimple_opt_pass pass_cd_dce =
1645 gate_dce, /* gate */
1646 tree_ssa_cd_dce, /* execute */
1649 0, /* static_pass_number */
1650 TV_TREE_CD_DCE, /* tv_id */
1651 PROP_cfg | PROP_ssa, /* properties_required */
1652 0, /* properties_provided */
1653 0, /* properties_destroyed */
1654 0, /* todo_flags_start */
1656 | TODO_verify_flow /* todo_flags_finish */