1 /* Dead code elimination pass for the GNU compiler.
2 Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
3 Free Software Foundation, Inc.
4 Contributed by Ben Elliston <bje@redhat.com>
5 and Andrew MacLeod <amacleod@redhat.com>
6 Adapted to use control dependence by Steven Bosscher, SUSE Labs.
8 This file is part of GCC.
10 GCC is free software; you can redistribute it and/or modify it
11 under the terms of the GNU General Public License as published by the
12 Free Software Foundation; either version 3, or (at your option) any
15 GCC is distributed in the hope that it will be useful, but WITHOUT
16 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
17 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
20 You should have received a copy of the GNU General Public License
21 along with GCC; see the file COPYING3. If not see
22 <http://www.gnu.org/licenses/>. */
24 /* Dead code elimination.
28 Building an Optimizing Compiler,
29 Robert Morgan, Butterworth-Heinemann, 1998, Section 8.9.
31 Advanced Compiler Design and Implementation,
32 Steven Muchnick, Morgan Kaufmann, 1997, Section 18.10.
34 Dead-code elimination is the removal of statements which have no
35 impact on the program's output. "Dead statements" have no impact
36 on the program's output, while "necessary statements" may have
39 The algorithm consists of three phases:
40 1. Marking as necessary all statements known to be necessary,
41 e.g. most function calls, writing a value to memory, etc;
42 2. Propagating necessary statements, e.g., the statements
43 giving values to operands in necessary statements; and
44 3. Removing dead statements. */
48 #include "coretypes.h"
52 #include "tree-pretty-print.h"
53 #include "gimple-pretty-print.h"
54 #include "basic-block.h"
55 #include "tree-flow.h"
57 #include "tree-dump.h"
58 #include "tree-pass.h"
62 #include "tree-scalar-evolution.h"
64 static struct stmt_stats
72 #define STMT_NECESSARY GF_PLF_1
74 static VEC(gimple
,heap
) *worklist
;
76 /* Vector indicating an SSA name has already been processed and marked
78 static sbitmap processed
;
80 /* Vector indicating that the last statement of a basic block has already
81 been marked as necessary. */
82 static sbitmap last_stmt_necessary
;
84 /* Vector indicating that BB contains statements that are live. */
85 static sbitmap bb_contains_live_stmts
;
87 /* Before we can determine whether a control branch is dead, we need to
88 compute which blocks are control dependent on which edges.
90 We expect each block to be control dependent on very few edges so we
91 use a bitmap for each block recording its edges. An array holds the
92 bitmap. The Ith bit in the bitmap is set if that block is dependent
94 static bitmap
*control_dependence_map
;
96 /* Vector indicating that a basic block has already had all the edges
97 processed that it is control dependent on. */
98 static sbitmap visited_control_parents
;
100 /* TRUE if this pass alters the CFG (by removing control statements).
103 If this pass alters the CFG, then it will arrange for the dominators
105 static bool cfg_altered
;
107 /* Execute code that follows the macro for each edge (given number
108 EDGE_NUMBER within the CODE) for which the block with index N is
109 control dependent. */
110 #define EXECUTE_IF_CONTROL_DEPENDENT(BI, N, EDGE_NUMBER) \
111 EXECUTE_IF_SET_IN_BITMAP (control_dependence_map[(N)], 0, \
115 /* Indicate block BB is control dependent on an edge with index EDGE_INDEX. */
117 set_control_dependence_map_bit (basic_block bb
, int edge_index
)
119 if (bb
== ENTRY_BLOCK_PTR
)
121 gcc_assert (bb
!= EXIT_BLOCK_PTR
);
122 bitmap_set_bit (control_dependence_map
[bb
->index
], edge_index
);
125 /* Clear all control dependences for block BB. */
127 clear_control_dependence_bitmap (basic_block bb
)
129 bitmap_clear (control_dependence_map
[bb
->index
]);
133 /* Find the immediate postdominator PDOM of the specified basic block BLOCK.
134 This function is necessary because some blocks have negative numbers. */
136 static inline basic_block
137 find_pdom (basic_block block
)
139 gcc_assert (block
!= ENTRY_BLOCK_PTR
);
141 if (block
== EXIT_BLOCK_PTR
)
142 return EXIT_BLOCK_PTR
;
145 basic_block bb
= get_immediate_dominator (CDI_POST_DOMINATORS
, block
);
147 return EXIT_BLOCK_PTR
;
153 /* Determine all blocks' control dependences on the given edge with edge_list
154 EL index EDGE_INDEX, ala Morgan, Section 3.6. */
157 find_control_dependence (struct edge_list
*el
, int edge_index
)
159 basic_block current_block
;
160 basic_block ending_block
;
162 gcc_assert (INDEX_EDGE_PRED_BB (el
, edge_index
) != EXIT_BLOCK_PTR
);
164 if (INDEX_EDGE_PRED_BB (el
, edge_index
) == ENTRY_BLOCK_PTR
)
165 ending_block
= single_succ (ENTRY_BLOCK_PTR
);
167 ending_block
= find_pdom (INDEX_EDGE_PRED_BB (el
, edge_index
));
169 for (current_block
= INDEX_EDGE_SUCC_BB (el
, edge_index
);
170 current_block
!= ending_block
&& current_block
!= EXIT_BLOCK_PTR
;
171 current_block
= find_pdom (current_block
))
173 edge e
= INDEX_EDGE (el
, edge_index
);
175 /* For abnormal edges, we don't make current_block control
176 dependent because instructions that throw are always necessary
178 if (e
->flags
& EDGE_ABNORMAL
)
181 set_control_dependence_map_bit (current_block
, edge_index
);
186 /* Record all blocks' control dependences on all edges in the edge
187 list EL, ala Morgan, Section 3.6. */
190 find_all_control_dependences (struct edge_list
*el
)
194 for (i
= 0; i
< NUM_EDGES (el
); ++i
)
195 find_control_dependence (el
, i
);
198 /* If STMT is not already marked necessary, mark it, and add it to the
199 worklist if ADD_TO_WORKLIST is true. */
202 mark_stmt_necessary (gimple stmt
, bool add_to_worklist
)
206 if (gimple_plf (stmt
, STMT_NECESSARY
))
209 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
211 fprintf (dump_file
, "Marking useful stmt: ");
212 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
213 fprintf (dump_file
, "\n");
216 gimple_set_plf (stmt
, STMT_NECESSARY
, true);
218 VEC_safe_push (gimple
, heap
, worklist
, stmt
);
219 if (bb_contains_live_stmts
&& !is_gimple_debug (stmt
))
220 SET_BIT (bb_contains_live_stmts
, gimple_bb (stmt
)->index
);
224 /* Mark the statement defining operand OP as necessary. */
227 mark_operand_necessary (tree op
)
234 ver
= SSA_NAME_VERSION (op
);
235 if (TEST_BIT (processed
, ver
))
237 stmt
= SSA_NAME_DEF_STMT (op
);
238 gcc_assert (gimple_nop_p (stmt
)
239 || gimple_plf (stmt
, STMT_NECESSARY
));
242 SET_BIT (processed
, ver
);
244 stmt
= SSA_NAME_DEF_STMT (op
);
247 if (gimple_plf (stmt
, STMT_NECESSARY
) || gimple_nop_p (stmt
))
250 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
252 fprintf (dump_file
, "marking necessary through ");
253 print_generic_expr (dump_file
, op
, 0);
254 fprintf (dump_file
, " stmt ");
255 print_gimple_stmt (dump_file
, stmt
, 0, 0);
258 gimple_set_plf (stmt
, STMT_NECESSARY
, true);
259 if (bb_contains_live_stmts
)
260 SET_BIT (bb_contains_live_stmts
, gimple_bb (stmt
)->index
);
261 VEC_safe_push (gimple
, heap
, worklist
, stmt
);
265 /* Mark STMT as necessary if it obviously is. Add it to the worklist if
266 it can make other statements necessary.
268 If AGGRESSIVE is false, control statements are conservatively marked as
272 mark_stmt_if_obviously_necessary (gimple stmt
, bool aggressive
)
274 /* With non-call exceptions, we have to assume that all statements could
275 throw. If a statement may throw, it is inherently necessary. */
276 if (cfun
->can_throw_non_call_exceptions
&& stmt_could_throw_p (stmt
))
278 mark_stmt_necessary (stmt
, true);
282 /* Statements that are implicitly live. Most function calls, asm
283 and return statements are required. Labels and GIMPLE_BIND nodes
284 are kept because they are control flow, and we have no way of
285 knowing whether they can be removed. DCE can eliminate all the
286 other statements in a block, and CFG can then remove the block
288 switch (gimple_code (stmt
))
292 mark_stmt_necessary (stmt
, false);
298 mark_stmt_necessary (stmt
, true);
302 /* Most, but not all function calls are required. Function calls that
303 produce no result and have no side effects (i.e. const pure
304 functions) are unnecessary. */
305 if (gimple_has_side_effects (stmt
))
307 mark_stmt_necessary (stmt
, true);
310 if (!gimple_call_lhs (stmt
))
315 /* Debug temps without a value are not useful. ??? If we could
316 easily locate the debug temp bind stmt for a use thereof,
317 would could refrain from marking all debug temps here, and
318 mark them only if they're used. */
319 if (gimple_debug_bind_has_value_p (stmt
)
320 || TREE_CODE (gimple_debug_bind_get_var (stmt
)) != DEBUG_EXPR_DECL
)
321 mark_stmt_necessary (stmt
, false);
325 gcc_assert (!simple_goto_p (stmt
));
326 mark_stmt_necessary (stmt
, true);
330 gcc_assert (EDGE_COUNT (gimple_bb (stmt
)->succs
) == 2);
335 mark_stmt_necessary (stmt
, true);
342 /* If the statement has volatile operands, it needs to be preserved.
343 Same for statements that can alter control flow in unpredictable
345 if (gimple_has_volatile_ops (stmt
) || is_ctrl_altering_stmt (stmt
))
347 mark_stmt_necessary (stmt
, true);
351 if (is_hidden_global_store (stmt
))
353 mark_stmt_necessary (stmt
, true);
361 /* Mark the last statement of BB as necessary. */
364 mark_last_stmt_necessary (basic_block bb
)
366 gimple stmt
= last_stmt (bb
);
368 SET_BIT (last_stmt_necessary
, bb
->index
);
369 SET_BIT (bb_contains_live_stmts
, bb
->index
);
371 /* We actually mark the statement only if it is a control statement. */
372 if (stmt
&& is_ctrl_stmt (stmt
))
373 mark_stmt_necessary (stmt
, true);
377 /* Mark control dependent edges of BB as necessary. We have to do this only
378 once for each basic block so we set the appropriate bit after we're done.
380 When IGNORE_SELF is true, ignore BB in the list of control dependences. */
383 mark_control_dependent_edges_necessary (basic_block bb
, struct edge_list
*el
,
387 unsigned edge_number
;
388 bool skipped
= false;
390 gcc_assert (bb
!= EXIT_BLOCK_PTR
);
392 if (bb
== ENTRY_BLOCK_PTR
)
395 EXECUTE_IF_CONTROL_DEPENDENT (bi
, bb
->index
, edge_number
)
397 basic_block cd_bb
= INDEX_EDGE_PRED_BB (el
, edge_number
);
399 if (ignore_self
&& cd_bb
== bb
)
405 if (!TEST_BIT (last_stmt_necessary
, cd_bb
->index
))
406 mark_last_stmt_necessary (cd_bb
);
410 SET_BIT (visited_control_parents
, bb
->index
);
414 /* Find obviously necessary statements. These are things like most function
415 calls, and stores to file level variables.
417 If EL is NULL, control statements are conservatively marked as
418 necessary. Otherwise it contains the list of edges used by control
419 dependence analysis. */
422 find_obviously_necessary_stmts (struct edge_list
*el
)
425 gimple_stmt_iterator gsi
;
432 /* PHI nodes are never inherently necessary. */
433 for (gsi
= gsi_start_phis (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
435 phi
= gsi_stmt (gsi
);
436 gimple_set_plf (phi
, STMT_NECESSARY
, false);
439 /* Check all statements in the block. */
440 for (gsi
= gsi_start_bb (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
442 stmt
= gsi_stmt (gsi
);
443 gimple_set_plf (stmt
, STMT_NECESSARY
, false);
444 mark_stmt_if_obviously_necessary (stmt
, el
!= NULL
);
448 /* Pure and const functions are finite and thus have no infinite loops in
450 flags
= flags_from_decl_or_type (current_function_decl
);
451 if ((flags
& (ECF_CONST
|ECF_PURE
)) && !(flags
& ECF_LOOPING_CONST_OR_PURE
))
454 /* Prevent the empty possibly infinite loops from being removed. */
460 if (mark_irreducible_loops ())
464 FOR_EACH_EDGE (e
, ei
, bb
->succs
)
465 if ((e
->flags
& EDGE_DFS_BACK
)
466 && (e
->flags
& EDGE_IRREDUCIBLE_LOOP
))
469 fprintf (dump_file
, "Marking back edge of irreducible loop %i->%i\n",
470 e
->src
->index
, e
->dest
->index
);
471 mark_control_dependent_edges_necessary (e
->dest
, el
, false);
475 FOR_EACH_LOOP (li
, loop
, 0)
476 if (!finite_loop_p (loop
))
479 fprintf (dump_file
, "can not prove finiteness of loop %i\n", loop
->num
);
480 mark_control_dependent_edges_necessary (loop
->latch
, el
, false);
487 /* Return true if REF is based on an aliased base, otherwise false. */
490 ref_may_be_aliased (tree ref
)
492 while (handled_component_p (ref
))
493 ref
= TREE_OPERAND (ref
, 0);
494 if (TREE_CODE (ref
) == MEM_REF
495 && TREE_CODE (TREE_OPERAND (ref
, 0)) == ADDR_EXPR
)
496 ref
= TREE_OPERAND (TREE_OPERAND (ref
, 0), 0);
497 return !(DECL_P (ref
)
498 && !may_be_aliased (ref
));
501 static bitmap visited
= NULL
;
502 static unsigned int longest_chain
= 0;
503 static unsigned int total_chain
= 0;
504 static unsigned int nr_walks
= 0;
505 static bool chain_ovfl
= false;
507 /* Worker for the walker that marks reaching definitions of REF,
508 which is based on a non-aliased decl, necessary. It returns
509 true whenever the defining statement of the current VDEF is
510 a kill for REF, as no dominating may-defs are necessary for REF
511 anymore. DATA points to the basic-block that contains the
512 stmt that refers to REF. */
515 mark_aliased_reaching_defs_necessary_1 (ao_ref
*ref
, tree vdef
, void *data
)
517 gimple def_stmt
= SSA_NAME_DEF_STMT (vdef
);
519 /* All stmts we visit are necessary. */
520 mark_operand_necessary (vdef
);
522 /* If the stmt lhs kills ref, then we can stop walking. */
523 if (gimple_has_lhs (def_stmt
)
524 && TREE_CODE (gimple_get_lhs (def_stmt
)) != SSA_NAME
525 /* The assignment is not necessarily carried out if it can throw
526 and we can catch it in the current function where we could inspect
528 ??? We only need to care about the RHS throwing. For aggregate
529 assignments or similar calls and non-call exceptions the LHS
530 might throw as well. */
531 && !stmt_can_throw_internal (def_stmt
))
533 tree base
, lhs
= gimple_get_lhs (def_stmt
);
534 HOST_WIDE_INT size
, offset
, max_size
;
536 base
= get_ref_base_and_extent (lhs
, &offset
, &size
, &max_size
);
537 /* We can get MEM[symbol: sZ, index: D.8862_1] here,
538 so base == refd->base does not always hold. */
539 if (base
== ref
->base
)
541 /* For a must-alias check we need to be able to constrain
542 the accesses properly. */
543 if (size
!= -1 && size
== max_size
544 && ref
->max_size
!= -1)
546 if (offset
<= ref
->offset
547 && offset
+ size
>= ref
->offset
+ ref
->max_size
)
550 /* Or they need to be exactly the same. */
552 /* Make sure there is no induction variable involved
553 in the references (gcc.c-torture/execute/pr42142.c).
554 The simplest way is to check if the kill dominates
556 && dominated_by_p (CDI_DOMINATORS
, (basic_block
) data
,
557 gimple_bb (def_stmt
))
558 && operand_equal_p (ref
->ref
, lhs
, 0))
563 /* Otherwise keep walking. */
568 mark_aliased_reaching_defs_necessary (gimple stmt
, tree ref
)
572 gcc_assert (!chain_ovfl
);
573 ao_ref_init (&refd
, ref
);
574 chain
= walk_aliased_vdefs (&refd
, gimple_vuse (stmt
),
575 mark_aliased_reaching_defs_necessary_1
,
576 gimple_bb (stmt
), NULL
);
577 if (chain
> longest_chain
)
578 longest_chain
= chain
;
579 total_chain
+= chain
;
583 /* Worker for the walker that marks reaching definitions of REF, which
584 is not based on a non-aliased decl. For simplicity we need to end
585 up marking all may-defs necessary that are not based on a non-aliased
586 decl. The only job of this walker is to skip may-defs based on
587 a non-aliased decl. */
590 mark_all_reaching_defs_necessary_1 (ao_ref
*ref ATTRIBUTE_UNUSED
,
591 tree vdef
, void *data ATTRIBUTE_UNUSED
)
593 gimple def_stmt
= SSA_NAME_DEF_STMT (vdef
);
595 /* We have to skip already visited (and thus necessary) statements
596 to make the chaining work after we dropped back to simple mode. */
598 && TEST_BIT (processed
, SSA_NAME_VERSION (vdef
)))
600 gcc_assert (gimple_nop_p (def_stmt
)
601 || gimple_plf (def_stmt
, STMT_NECESSARY
));
605 /* We want to skip stores to non-aliased variables. */
607 && gimple_assign_single_p (def_stmt
))
609 tree lhs
= gimple_assign_lhs (def_stmt
);
610 if (!ref_may_be_aliased (lhs
))
614 mark_operand_necessary (vdef
);
620 mark_all_reaching_defs_necessary (gimple stmt
)
622 walk_aliased_vdefs (NULL
, gimple_vuse (stmt
),
623 mark_all_reaching_defs_necessary_1
, NULL
, &visited
);
626 /* Return true for PHI nodes with one or identical arguments
629 degenerate_phi_p (gimple phi
)
632 tree op
= gimple_phi_arg_def (phi
, 0);
633 for (i
= 1; i
< gimple_phi_num_args (phi
); i
++)
634 if (gimple_phi_arg_def (phi
, i
) != op
)
639 /* Propagate necessity using the operands of necessary statements.
640 Process the uses on each statement in the worklist, and add all
641 feeding statements which contribute to the calculation of this
642 value to the worklist.
644 In conservative mode, EL is NULL. */
647 propagate_necessity (struct edge_list
*el
)
650 bool aggressive
= (el
? true : false);
652 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
653 fprintf (dump_file
, "\nProcessing worklist:\n");
655 while (VEC_length (gimple
, worklist
) > 0)
657 /* Take STMT from worklist. */
658 stmt
= VEC_pop (gimple
, worklist
);
660 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
662 fprintf (dump_file
, "processing: ");
663 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
664 fprintf (dump_file
, "\n");
669 /* Mark the last statement of the basic blocks on which the block
670 containing STMT is control dependent, but only if we haven't
672 basic_block bb
= gimple_bb (stmt
);
673 if (bb
!= ENTRY_BLOCK_PTR
674 && !TEST_BIT (visited_control_parents
, bb
->index
))
675 mark_control_dependent_edges_necessary (bb
, el
, false);
678 if (gimple_code (stmt
) == GIMPLE_PHI
679 /* We do not process virtual PHI nodes nor do we track their
681 && is_gimple_reg (gimple_phi_result (stmt
)))
683 /* PHI nodes are somewhat special in that each PHI alternative has
684 data and control dependencies. All the statements feeding the
685 PHI node's arguments are always necessary. In aggressive mode,
686 we also consider the control dependent edges leading to the
687 predecessor block associated with each PHI alternative as
691 for (k
= 0; k
< gimple_phi_num_args (stmt
); k
++)
693 tree arg
= PHI_ARG_DEF (stmt
, k
);
694 if (TREE_CODE (arg
) == SSA_NAME
)
695 mark_operand_necessary (arg
);
698 /* For PHI operands it matters from where the control flow arrives
699 to the BB. Consider the following example:
709 We need to mark control dependence of the empty basic blocks, since they
710 contains computation of PHI operands.
712 Doing so is too restrictive in the case the predecestor block is in
718 for (i = 0; i<1000; ++i)
724 There is PHI for J in the BB containing return statement.
725 In this case the control dependence of predecestor block (that is
726 within the empty loop) also contains the block determining number
727 of iterations of the block that would prevent removing of empty
730 This scenario can be avoided by splitting critical edges.
731 To save the critical edge splitting pass we identify how the control
732 dependence would look like if the edge was split.
734 Consider the modified CFG created from current CFG by splitting
735 edge B->C. In the postdominance tree of modified CFG, C' is
736 always child of C. There are two cases how chlids of C' can look
741 In this case the only basic block C' is control dependent on is B.
743 2) C' has single child that is B
745 In this case control dependence of C' is same as control
746 dependence of B in original CFG except for block B itself.
747 (since C' postdominate B in modified CFG)
749 Now how to decide what case happens? There are two basic options:
751 a) C postdominate B. Then C immediately postdominate B and
752 case 2 happens iff there is no other way from B to C except
755 There is other way from B to C iff there is succesor of B that
756 is not postdominated by B. Testing this condition is somewhat
757 expensive, because we need to iterate all succesors of B.
758 We are safe to assume that this does not happen: we will mark B
759 as needed when processing the other path from B to C that is
760 conrol dependent on B and marking control dependencies of B
761 itself is harmless because they will be processed anyway after
762 processing control statement in B.
764 b) C does not postdominate B. Always case 1 happens since there is
765 path from C to exit that does not go through B and thus also C'. */
767 if (aggressive
&& !degenerate_phi_p (stmt
))
769 for (k
= 0; k
< gimple_phi_num_args (stmt
); k
++)
771 basic_block arg_bb
= gimple_phi_arg_edge (stmt
, k
)->src
;
774 != get_immediate_dominator (CDI_POST_DOMINATORS
, arg_bb
))
776 if (!TEST_BIT (last_stmt_necessary
, arg_bb
->index
))
777 mark_last_stmt_necessary (arg_bb
);
779 else if (arg_bb
!= ENTRY_BLOCK_PTR
780 && !TEST_BIT (visited_control_parents
,
782 mark_control_dependent_edges_necessary (arg_bb
, el
, true);
788 /* Propagate through the operands. Examine all the USE, VUSE and
789 VDEF operands in this statement. Mark all the statements
790 which feed this statement's uses as necessary. */
794 FOR_EACH_SSA_TREE_OPERAND (use
, stmt
, iter
, SSA_OP_USE
)
795 mark_operand_necessary (use
);
797 use
= gimple_vuse (stmt
);
801 /* If we dropped to simple mode make all immediately
802 reachable definitions necessary. */
805 mark_all_reaching_defs_necessary (stmt
);
809 /* For statements that may load from memory (have a VUSE) we
810 have to mark all reaching (may-)definitions as necessary.
811 We partition this task into two cases:
812 1) explicit loads based on decls that are not aliased
813 2) implicit loads (like calls) and explicit loads not
814 based on decls that are not aliased (like indirect
815 references or loads from globals)
816 For 1) we mark all reaching may-defs as necessary, stopping
817 at dominating kills. For 2) we want to mark all dominating
818 references necessary, but non-aliased ones which we handle
819 in 1). By keeping a global visited bitmap for references
820 we walk for 2) we avoid quadratic behavior for those. */
822 if (is_gimple_call (stmt
))
824 tree callee
= gimple_call_fndecl (stmt
);
827 /* Calls to functions that are merely acting as barriers
828 or that only store to memory do not make any previous
830 if (callee
!= NULL_TREE
831 && DECL_BUILT_IN_CLASS (callee
) == BUILT_IN_NORMAL
832 && (DECL_FUNCTION_CODE (callee
) == BUILT_IN_MEMSET
833 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_MALLOC
834 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_FREE
835 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_ALLOCA
836 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_STACK_SAVE
837 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_STACK_RESTORE
))
840 /* Calls implicitly load from memory, their arguments
841 in addition may explicitly perform memory loads. */
842 mark_all_reaching_defs_necessary (stmt
);
843 for (i
= 0; i
< gimple_call_num_args (stmt
); ++i
)
845 tree arg
= gimple_call_arg (stmt
, i
);
846 if (TREE_CODE (arg
) == SSA_NAME
847 || is_gimple_min_invariant (arg
))
849 if (!ref_may_be_aliased (arg
))
850 mark_aliased_reaching_defs_necessary (stmt
, arg
);
853 else if (gimple_assign_single_p (stmt
))
856 bool rhs_aliased
= false;
857 /* If this is a load mark things necessary. */
858 rhs
= gimple_assign_rhs1 (stmt
);
859 if (TREE_CODE (rhs
) != SSA_NAME
860 && !is_gimple_min_invariant (rhs
))
862 if (!ref_may_be_aliased (rhs
))
863 mark_aliased_reaching_defs_necessary (stmt
, rhs
);
868 mark_all_reaching_defs_necessary (stmt
);
870 else if (gimple_code (stmt
) == GIMPLE_RETURN
)
872 tree rhs
= gimple_return_retval (stmt
);
873 /* A return statement may perform a load. */
875 && TREE_CODE (rhs
) != SSA_NAME
876 && !is_gimple_min_invariant (rhs
))
878 if (!ref_may_be_aliased (rhs
))
879 mark_aliased_reaching_defs_necessary (stmt
, rhs
);
881 mark_all_reaching_defs_necessary (stmt
);
884 else if (gimple_code (stmt
) == GIMPLE_ASM
)
887 mark_all_reaching_defs_necessary (stmt
);
888 /* Inputs may perform loads. */
889 for (i
= 0; i
< gimple_asm_ninputs (stmt
); ++i
)
891 tree op
= TREE_VALUE (gimple_asm_input_op (stmt
, i
));
892 if (TREE_CODE (op
) != SSA_NAME
893 && !is_gimple_min_invariant (op
)
894 && !ref_may_be_aliased (op
))
895 mark_aliased_reaching_defs_necessary (stmt
, op
);
901 /* If we over-used our alias oracle budget drop to simple
902 mode. The cost metric allows quadratic behavior
903 (number of uses times number of may-defs queries) up to
904 a constant maximal number of queries and after that falls back to
905 super-linear complexity. */
906 if (/* Constant but quadratic for small functions. */
907 total_chain
> 128 * 128
908 /* Linear in the number of may-defs. */
909 && total_chain
> 32 * longest_chain
910 /* Linear in the number of uses. */
911 && total_chain
> nr_walks
* 32)
915 bitmap_clear (visited
);
921 /* Replace all uses of result of PHI by underlying variable and mark it
925 mark_virtual_phi_result_for_renaming (gimple phi
)
928 imm_use_iterator iter
;
931 tree result_ssa
, result_var
;
933 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
935 fprintf (dump_file
, "Marking result for renaming : ");
936 print_gimple_stmt (dump_file
, phi
, 0, TDF_SLIM
);
937 fprintf (dump_file
, "\n");
940 result_ssa
= gimple_phi_result (phi
);
941 result_var
= SSA_NAME_VAR (result_ssa
);
942 FOR_EACH_IMM_USE_STMT (stmt
, iter
, result_ssa
)
944 FOR_EACH_IMM_USE_ON_STMT (use_p
, iter
)
945 SET_USE (use_p
, result_var
);
950 mark_sym_for_renaming (result_var
);
953 /* Remove dead PHI nodes from block BB. */
956 remove_dead_phis (basic_block bb
)
958 bool something_changed
= false;
961 gimple_stmt_iterator gsi
;
962 phis
= phi_nodes (bb
);
964 for (gsi
= gsi_start (phis
); !gsi_end_p (gsi
);)
967 phi
= gsi_stmt (gsi
);
969 /* We do not track necessity of virtual PHI nodes. Instead do
970 very simple dead PHI removal here. */
971 if (!is_gimple_reg (gimple_phi_result (phi
)))
973 /* Virtual PHI nodes with one or identical arguments
975 if (degenerate_phi_p (phi
))
977 tree vdef
= gimple_phi_result (phi
);
978 tree vuse
= gimple_phi_arg_def (phi
, 0);
981 imm_use_iterator iter
;
983 FOR_EACH_IMM_USE_STMT (use_stmt
, iter
, vdef
)
984 FOR_EACH_IMM_USE_ON_STMT (use_p
, iter
)
985 SET_USE (use_p
, vuse
);
986 if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vdef
)
987 && TREE_CODE (vuse
) == SSA_NAME
)
988 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vuse
) = 1;
991 gimple_set_plf (phi
, STMT_NECESSARY
, true);
994 if (!gimple_plf (phi
, STMT_NECESSARY
))
996 something_changed
= true;
997 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
999 fprintf (dump_file
, "Deleting : ");
1000 print_gimple_stmt (dump_file
, phi
, 0, TDF_SLIM
);
1001 fprintf (dump_file
, "\n");
1004 remove_phi_node (&gsi
, true);
1005 stats
.removed_phis
++;
1011 return something_changed
;
1014 /* Forward edge E to respective POST_DOM_BB and update PHIs. */
1017 forward_edge_to_pdom (edge e
, basic_block post_dom_bb
)
1019 gimple_stmt_iterator gsi
;
1023 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1024 fprintf (dump_file
, "Redirecting edge %i->%i to %i\n", e
->src
->index
,
1025 e
->dest
->index
, post_dom_bb
->index
);
1027 e2
= redirect_edge_and_branch (e
, post_dom_bb
);
1030 /* If edge was already around, no updating is neccesary. */
1034 if (!gimple_seq_empty_p (phi_nodes (post_dom_bb
)))
1036 /* We are sure that for every live PHI we are seeing control dependent BB.
1037 This means that we can pick any edge to duplicate PHI args from. */
1038 FOR_EACH_EDGE (e2
, ei
, post_dom_bb
->preds
)
1041 for (gsi
= gsi_start_phis (post_dom_bb
); !gsi_end_p (gsi
);)
1043 gimple phi
= gsi_stmt (gsi
);
1045 source_location locus
;
1047 /* PHIs for virtuals have no control dependency relation on them.
1048 We are lost here and must force renaming of the symbol. */
1049 if (!is_gimple_reg (gimple_phi_result (phi
)))
1051 mark_virtual_phi_result_for_renaming (phi
);
1052 remove_phi_node (&gsi
, true);
1056 /* Dead PHI do not imply control dependency. */
1057 if (!gimple_plf (phi
, STMT_NECESSARY
))
1063 op
= gimple_phi_arg_def (phi
, e2
->dest_idx
);
1064 locus
= gimple_phi_arg_location (phi
, e2
->dest_idx
);
1065 add_phi_arg (phi
, op
, e
, locus
);
1066 /* The resulting PHI if not dead can only be degenerate. */
1067 gcc_assert (degenerate_phi_p (phi
));
1074 /* Remove dead statement pointed to by iterator I. Receives the basic block BB
1075 containing I so that we don't have to look it up. */
1078 remove_dead_stmt (gimple_stmt_iterator
*i
, basic_block bb
)
1080 gimple stmt
= gsi_stmt (*i
);
1082 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1084 fprintf (dump_file
, "Deleting : ");
1085 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
1086 fprintf (dump_file
, "\n");
1091 /* If we have determined that a conditional branch statement contributes
1092 nothing to the program, then we not only remove it, but we also change
1093 the flow graph so that the current block will simply fall-thru to its
1094 immediate post-dominator. The blocks we are circumventing will be
1095 removed by cleanup_tree_cfg if this change in the flow graph makes them
1097 if (is_ctrl_stmt (stmt
))
1099 basic_block post_dom_bb
;
1103 post_dom_bb
= get_immediate_dominator (CDI_POST_DOMINATORS
, bb
);
1105 e
= find_edge (bb
, post_dom_bb
);
1107 /* If edge is already there, try to use it. This avoids need to update
1108 PHI nodes. Also watch for cases where post dominator does not exists
1109 or is exit block. These can happen for infinite loops as we create
1110 fake edges in the dominator tree. */
1113 else if (! post_dom_bb
|| post_dom_bb
== EXIT_BLOCK_PTR
)
1114 e
= EDGE_SUCC (bb
, 0);
1116 e
= forward_edge_to_pdom (EDGE_SUCC (bb
, 0), post_dom_bb
);
1118 e
->probability
= REG_BR_PROB_BASE
;
1119 e
->count
= bb
->count
;
1121 /* The edge is no longer associated with a conditional, so it does
1122 not have TRUE/FALSE flags. */
1123 e
->flags
&= ~(EDGE_TRUE_VALUE
| EDGE_FALSE_VALUE
);
1125 /* The lone outgoing edge from BB will be a fallthru edge. */
1126 e
->flags
|= EDGE_FALLTHRU
;
1128 /* Remove the remaining outgoing edges. */
1129 for (ei
= ei_start (bb
->succs
); (e2
= ei_safe_edge (ei
)); )
1139 unlink_stmt_vdef (stmt
);
1140 gsi_remove (i
, true);
1141 release_defs (stmt
);
1144 /* Eliminate unnecessary statements. Any instruction not marked as necessary
1145 contributes nothing to the program, and can be deleted. */
1148 eliminate_unnecessary_stmts (void)
1150 bool something_changed
= false;
1152 gimple_stmt_iterator gsi
, psi
;
1155 VEC (basic_block
, heap
) *h
;
1157 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1158 fprintf (dump_file
, "\nEliminating unnecessary statements:\n");
1160 clear_special_calls ();
1162 /* Walking basic blocks and statements in reverse order avoids
1163 releasing SSA names before any other DEFs that refer to them are
1164 released. This helps avoid loss of debug information, as we get
1165 a chance to propagate all RHSs of removed SSAs into debug uses,
1166 rather than only the latest ones. E.g., consider:
1172 If we were to release x_3 before a_5, when we reached a_5 and
1173 tried to substitute it into the debug stmt, we'd see x_3 there,
1174 but x_3's DEF, type, etc would have already been disconnected.
1175 By going backwards, the debug stmt first changes to:
1177 # DEBUG a => x_3 - b_4
1181 # DEBUG a => y_1 + z_2 - b_4
1184 gcc_assert (dom_info_available_p (CDI_DOMINATORS
));
1185 h
= get_all_dominated_blocks (CDI_DOMINATORS
, single_succ (ENTRY_BLOCK_PTR
));
1187 while (VEC_length (basic_block
, h
))
1189 bb
= VEC_pop (basic_block
, h
);
1191 /* Remove dead statements. */
1192 for (gsi
= gsi_last_bb (bb
); !gsi_end_p (gsi
); gsi
= psi
)
1194 stmt
= gsi_stmt (gsi
);
1201 /* If GSI is not necessary then remove it. */
1202 if (!gimple_plf (stmt
, STMT_NECESSARY
))
1204 if (!is_gimple_debug (stmt
))
1205 something_changed
= true;
1206 remove_dead_stmt (&gsi
, bb
);
1208 else if (is_gimple_call (stmt
))
1210 call
= gimple_call_fndecl (stmt
);
1215 /* When LHS of var = call (); is dead, simplify it into
1216 call (); saving one operand. */
1217 name
= gimple_call_lhs (stmt
);
1218 if (name
&& TREE_CODE (name
) == SSA_NAME
1219 && !TEST_BIT (processed
, SSA_NAME_VERSION (name
)))
1221 something_changed
= true;
1222 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1224 fprintf (dump_file
, "Deleting LHS of call: ");
1225 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
1226 fprintf (dump_file
, "\n");
1229 gimple_call_set_lhs (stmt
, NULL_TREE
);
1230 maybe_clean_or_replace_eh_stmt (stmt
, stmt
);
1232 release_ssa_name (name
);
1234 notice_special_calls (stmt
);
1240 VEC_free (basic_block
, heap
, h
);
1242 /* Since we don't track liveness of virtual PHI nodes, it is possible that we
1243 rendered some PHI nodes unreachable while they are still in use.
1244 Mark them for renaming. */
1247 basic_block prev_bb
;
1249 find_unreachable_blocks ();
1251 /* Delete all unreachable basic blocks in reverse dominator order. */
1252 for (bb
= EXIT_BLOCK_PTR
->prev_bb
; bb
!= ENTRY_BLOCK_PTR
; bb
= prev_bb
)
1254 prev_bb
= bb
->prev_bb
;
1256 if (!TEST_BIT (bb_contains_live_stmts
, bb
->index
)
1257 || !(bb
->flags
& BB_REACHABLE
))
1259 for (gsi
= gsi_start_phis (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
1260 if (!is_gimple_reg (gimple_phi_result (gsi_stmt (gsi
))))
1263 imm_use_iterator iter
;
1265 FOR_EACH_IMM_USE_STMT (stmt
, iter
, gimple_phi_result (gsi_stmt (gsi
)))
1267 if (!(gimple_bb (stmt
)->flags
& BB_REACHABLE
))
1269 if (gimple_code (stmt
) == GIMPLE_PHI
1270 || gimple_plf (stmt
, STMT_NECESSARY
))
1273 BREAK_FROM_IMM_USE_STMT (iter
);
1277 mark_virtual_phi_result_for_renaming (gsi_stmt (gsi
));
1280 if (!(bb
->flags
& BB_REACHABLE
))
1282 /* Speed up the removal of blocks that don't
1283 dominate others. Walking backwards, this should
1284 be the common case. ??? Do we need to recompute
1285 dominators because of cfg_altered? */
1286 if (!MAY_HAVE_DEBUG_STMTS
1287 || !first_dom_son (CDI_DOMINATORS
, bb
))
1288 delete_basic_block (bb
);
1291 h
= get_all_dominated_blocks (CDI_DOMINATORS
, bb
);
1293 while (VEC_length (basic_block
, h
))
1295 bb
= VEC_pop (basic_block
, h
);
1296 prev_bb
= bb
->prev_bb
;
1297 /* Rearrangements to the CFG may have failed
1298 to update the dominators tree, so that
1299 formerly-dominated blocks are now
1300 otherwise reachable. */
1301 if (!!(bb
->flags
& BB_REACHABLE
))
1303 delete_basic_block (bb
);
1306 VEC_free (basic_block
, heap
, h
);
1314 /* Remove dead PHI nodes. */
1315 something_changed
|= remove_dead_phis (bb
);
1318 return something_changed
;
1322 /* Print out removed statement statistics. */
1329 percg
= ((float) stats
.removed
/ (float) stats
.total
) * 100;
1330 fprintf (dump_file
, "Removed %d of %d statements (%d%%)\n",
1331 stats
.removed
, stats
.total
, (int) percg
);
1333 if (stats
.total_phis
== 0)
1336 percg
= ((float) stats
.removed_phis
/ (float) stats
.total_phis
) * 100;
1338 fprintf (dump_file
, "Removed %d of %d PHI nodes (%d%%)\n",
1339 stats
.removed_phis
, stats
.total_phis
, (int) percg
);
1342 /* Initialization for this pass. Set up the used data structures. */
1345 tree_dce_init (bool aggressive
)
1347 memset ((void *) &stats
, 0, sizeof (stats
));
1353 control_dependence_map
= XNEWVEC (bitmap
, last_basic_block
);
1354 for (i
= 0; i
< last_basic_block
; ++i
)
1355 control_dependence_map
[i
] = BITMAP_ALLOC (NULL
);
1357 last_stmt_necessary
= sbitmap_alloc (last_basic_block
);
1358 sbitmap_zero (last_stmt_necessary
);
1359 bb_contains_live_stmts
= sbitmap_alloc (last_basic_block
);
1360 sbitmap_zero (bb_contains_live_stmts
);
1363 processed
= sbitmap_alloc (num_ssa_names
+ 1);
1364 sbitmap_zero (processed
);
1366 worklist
= VEC_alloc (gimple
, heap
, 64);
1367 cfg_altered
= false;
1370 /* Cleanup after this pass. */
1373 tree_dce_done (bool aggressive
)
1379 for (i
= 0; i
< last_basic_block
; ++i
)
1380 BITMAP_FREE (control_dependence_map
[i
]);
1381 free (control_dependence_map
);
1383 sbitmap_free (visited_control_parents
);
1384 sbitmap_free (last_stmt_necessary
);
1385 sbitmap_free (bb_contains_live_stmts
);
1386 bb_contains_live_stmts
= NULL
;
1389 sbitmap_free (processed
);
1391 VEC_free (gimple
, heap
, worklist
);
1394 /* Main routine to eliminate dead code.
1396 AGGRESSIVE controls the aggressiveness of the algorithm.
1397 In conservative mode, we ignore control dependence and simply declare
1398 all but the most trivially dead branches necessary. This mode is fast.
1399 In aggressive mode, control dependences are taken into account, which
1400 results in more dead code elimination, but at the cost of some time.
1402 FIXME: Aggressive mode before PRE doesn't work currently because
1403 the dominance info is not invalidated after DCE1. This is
1404 not an issue right now because we only run aggressive DCE
1405 as the last tree SSA pass, but keep this in mind when you
1406 start experimenting with pass ordering. */
1409 perform_tree_ssa_dce (bool aggressive
)
1411 struct edge_list
*el
= NULL
;
1412 bool something_changed
= 0;
1414 calculate_dominance_info (CDI_DOMINATORS
);
1416 /* Preheaders are needed for SCEV to work.
1417 Simple lateches and recorded exits improve chances that loop will
1418 proved to be finite in testcases such as in loop-15.c and loop-24.c */
1420 loop_optimizer_init (LOOPS_NORMAL
1421 | LOOPS_HAVE_RECORDED_EXITS
);
1423 tree_dce_init (aggressive
);
1427 /* Compute control dependence. */
1428 timevar_push (TV_CONTROL_DEPENDENCES
);
1429 calculate_dominance_info (CDI_POST_DOMINATORS
);
1430 el
= create_edge_list ();
1431 find_all_control_dependences (el
);
1432 timevar_pop (TV_CONTROL_DEPENDENCES
);
1434 visited_control_parents
= sbitmap_alloc (last_basic_block
);
1435 sbitmap_zero (visited_control_parents
);
1437 mark_dfs_back_edges ();
1440 find_obviously_necessary_stmts (el
);
1443 loop_optimizer_finalize ();
1449 visited
= BITMAP_ALLOC (NULL
);
1450 propagate_necessity (el
);
1451 BITMAP_FREE (visited
);
1453 something_changed
|= eliminate_unnecessary_stmts ();
1454 something_changed
|= cfg_altered
;
1456 /* We do not update postdominators, so free them unconditionally. */
1457 free_dominance_info (CDI_POST_DOMINATORS
);
1459 /* If we removed paths in the CFG, then we need to update
1460 dominators as well. I haven't investigated the possibility
1461 of incrementally updating dominators. */
1463 free_dominance_info (CDI_DOMINATORS
);
1465 statistics_counter_event (cfun
, "Statements deleted", stats
.removed
);
1466 statistics_counter_event (cfun
, "PHI nodes deleted", stats
.removed_phis
);
1468 /* Debugging dumps. */
1469 if (dump_file
&& (dump_flags
& (TDF_STATS
|TDF_DETAILS
)))
1472 tree_dce_done (aggressive
);
1474 free_edge_list (el
);
1476 if (something_changed
)
1477 return (TODO_update_ssa
| TODO_cleanup_cfg
| TODO_ggc_collect
1478 | TODO_remove_unused_locals
);
1483 /* Pass entry points. */
1487 return perform_tree_ssa_dce (/*aggressive=*/false);
1491 tree_ssa_dce_loop (void)
1494 todo
= perform_tree_ssa_dce (/*aggressive=*/false);
1497 free_numbers_of_iterations_estimates ();
1504 tree_ssa_cd_dce (void)
1506 return perform_tree_ssa_dce (/*aggressive=*/optimize
>= 2);
1512 return flag_tree_dce
!= 0;
1515 struct gimple_opt_pass pass_dce
=
1520 gate_dce
, /* gate */
1521 tree_ssa_dce
, /* execute */
1524 0, /* static_pass_number */
1525 TV_TREE_DCE
, /* tv_id */
1526 PROP_cfg
| PROP_ssa
, /* properties_required */
1527 0, /* properties_provided */
1528 0, /* properties_destroyed */
1529 0, /* todo_flags_start */
1530 TODO_verify_ssa
/* todo_flags_finish */
1534 struct gimple_opt_pass pass_dce_loop
=
1538 "dceloop", /* name */
1539 gate_dce
, /* gate */
1540 tree_ssa_dce_loop
, /* execute */
1543 0, /* static_pass_number */
1544 TV_TREE_DCE
, /* tv_id */
1545 PROP_cfg
| PROP_ssa
, /* properties_required */
1546 0, /* properties_provided */
1547 0, /* properties_destroyed */
1548 0, /* todo_flags_start */
1549 TODO_verify_ssa
/* todo_flags_finish */
1553 struct gimple_opt_pass pass_cd_dce
=
1558 gate_dce
, /* gate */
1559 tree_ssa_cd_dce
, /* execute */
1562 0, /* static_pass_number */
1563 TV_TREE_CD_DCE
, /* tv_id */
1564 PROP_cfg
| PROP_ssa
, /* properties_required */
1565 0, /* properties_provided */
1566 0, /* properties_destroyed */
1567 0, /* todo_flags_start */
1569 | TODO_verify_flow
/* todo_flags_finish */