1 /* Dead code elimination pass for the GNU compiler.
2 Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
3 Free Software Foundation, Inc.
4 Contributed by Ben Elliston <bje@redhat.com>
5 and Andrew MacLeod <amacleod@redhat.com>
6 Adapted to use control dependence by Steven Bosscher, SUSE Labs.
8 This file is part of GCC.
10 GCC is free software; you can redistribute it and/or modify it
11 under the terms of the GNU General Public License as published by the
12 Free Software Foundation; either version 3, or (at your option) any
15 GCC is distributed in the hope that it will be useful, but WITHOUT
16 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
17 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
20 You should have received a copy of the GNU General Public License
21 along with GCC; see the file COPYING3. If not see
22 <http://www.gnu.org/licenses/>. */
24 /* Dead code elimination.
28 Building an Optimizing Compiler,
29 Robert Morgan, Butterworth-Heinemann, 1998, Section 8.9.
31 Advanced Compiler Design and Implementation,
32 Steven Muchnick, Morgan Kaufmann, 1997, Section 18.10.
34 Dead-code elimination is the removal of statements which have no
35 impact on the program's output. "Dead statements" have no impact
36 on the program's output, while "necessary statements" may have
39 The algorithm consists of three phases:
40 1. Marking as necessary all statements known to be necessary,
41 e.g. most function calls, writing a value to memory, etc;
42 2. Propagating necessary statements, e.g., the statements
43 giving values to operands in necessary statements; and
44 3. Removing dead statements. */
48 #include "coretypes.h"
52 #include "tree-pretty-print.h"
53 #include "gimple-pretty-print.h"
54 #include "basic-block.h"
55 #include "tree-flow.h"
57 #include "tree-dump.h"
58 #include "tree-pass.h"
62 #include "tree-scalar-evolution.h"
64 static struct stmt_stats
72 #define STMT_NECESSARY GF_PLF_1
74 static VEC(gimple
,heap
) *worklist
;
76 /* Vector indicating an SSA name has already been processed and marked
78 static sbitmap processed
;
80 /* Vector indicating that the last statement of a basic block has already
81 been marked as necessary. */
82 static sbitmap last_stmt_necessary
;
84 /* Vector indicating that BB contains statements that are live. */
85 static sbitmap bb_contains_live_stmts
;
87 /* Before we can determine whether a control branch is dead, we need to
88 compute which blocks are control dependent on which edges.
90 We expect each block to be control dependent on very few edges so we
91 use a bitmap for each block recording its edges. An array holds the
92 bitmap. The Ith bit in the bitmap is set if that block is dependent
94 static bitmap
*control_dependence_map
;
96 /* Vector indicating that a basic block has already had all the edges
97 processed that it is control dependent on. */
98 static sbitmap visited_control_parents
;
100 /* TRUE if this pass alters the CFG (by removing control statements).
103 If this pass alters the CFG, then it will arrange for the dominators
105 static bool cfg_altered
;
107 /* Execute code that follows the macro for each edge (given number
108 EDGE_NUMBER within the CODE) for which the block with index N is
109 control dependent. */
110 #define EXECUTE_IF_CONTROL_DEPENDENT(BI, N, EDGE_NUMBER) \
111 EXECUTE_IF_SET_IN_BITMAP (control_dependence_map[(N)], 0, \
115 /* Indicate block BB is control dependent on an edge with index EDGE_INDEX. */
117 set_control_dependence_map_bit (basic_block bb
, int edge_index
)
119 if (bb
== ENTRY_BLOCK_PTR
)
121 gcc_assert (bb
!= EXIT_BLOCK_PTR
);
122 bitmap_set_bit (control_dependence_map
[bb
->index
], edge_index
);
125 /* Clear all control dependences for block BB. */
127 clear_control_dependence_bitmap (basic_block bb
)
129 bitmap_clear (control_dependence_map
[bb
->index
]);
133 /* Find the immediate postdominator PDOM of the specified basic block BLOCK.
134 This function is necessary because some blocks have negative numbers. */
136 static inline basic_block
137 find_pdom (basic_block block
)
139 gcc_assert (block
!= ENTRY_BLOCK_PTR
);
141 if (block
== EXIT_BLOCK_PTR
)
142 return EXIT_BLOCK_PTR
;
145 basic_block bb
= get_immediate_dominator (CDI_POST_DOMINATORS
, block
);
147 return EXIT_BLOCK_PTR
;
153 /* Determine all blocks' control dependences on the given edge with edge_list
154 EL index EDGE_INDEX, ala Morgan, Section 3.6. */
157 find_control_dependence (struct edge_list
*el
, int edge_index
)
159 basic_block current_block
;
160 basic_block ending_block
;
162 gcc_assert (INDEX_EDGE_PRED_BB (el
, edge_index
) != EXIT_BLOCK_PTR
);
164 if (INDEX_EDGE_PRED_BB (el
, edge_index
) == ENTRY_BLOCK_PTR
)
165 ending_block
= single_succ (ENTRY_BLOCK_PTR
);
167 ending_block
= find_pdom (INDEX_EDGE_PRED_BB (el
, edge_index
));
169 for (current_block
= INDEX_EDGE_SUCC_BB (el
, edge_index
);
170 current_block
!= ending_block
&& current_block
!= EXIT_BLOCK_PTR
;
171 current_block
= find_pdom (current_block
))
173 edge e
= INDEX_EDGE (el
, edge_index
);
175 /* For abnormal edges, we don't make current_block control
176 dependent because instructions that throw are always necessary
178 if (e
->flags
& EDGE_ABNORMAL
)
181 set_control_dependence_map_bit (current_block
, edge_index
);
186 /* Record all blocks' control dependences on all edges in the edge
187 list EL, ala Morgan, Section 3.6. */
190 find_all_control_dependences (struct edge_list
*el
)
194 for (i
= 0; i
< NUM_EDGES (el
); ++i
)
195 find_control_dependence (el
, i
);
198 /* If STMT is not already marked necessary, mark it, and add it to the
199 worklist if ADD_TO_WORKLIST is true. */
202 mark_stmt_necessary (gimple stmt
, bool add_to_worklist
)
206 if (gimple_plf (stmt
, STMT_NECESSARY
))
209 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
211 fprintf (dump_file
, "Marking useful stmt: ");
212 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
213 fprintf (dump_file
, "\n");
216 gimple_set_plf (stmt
, STMT_NECESSARY
, true);
218 VEC_safe_push (gimple
, heap
, worklist
, stmt
);
219 if (bb_contains_live_stmts
&& !is_gimple_debug (stmt
))
220 SET_BIT (bb_contains_live_stmts
, gimple_bb (stmt
)->index
);
224 /* Mark the statement defining operand OP as necessary. */
227 mark_operand_necessary (tree op
)
234 ver
= SSA_NAME_VERSION (op
);
235 if (TEST_BIT (processed
, ver
))
237 stmt
= SSA_NAME_DEF_STMT (op
);
238 gcc_assert (gimple_nop_p (stmt
)
239 || gimple_plf (stmt
, STMT_NECESSARY
));
242 SET_BIT (processed
, ver
);
244 stmt
= SSA_NAME_DEF_STMT (op
);
247 if (gimple_plf (stmt
, STMT_NECESSARY
) || gimple_nop_p (stmt
))
250 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
252 fprintf (dump_file
, "marking necessary through ");
253 print_generic_expr (dump_file
, op
, 0);
254 fprintf (dump_file
, " stmt ");
255 print_gimple_stmt (dump_file
, stmt
, 0, 0);
258 gimple_set_plf (stmt
, STMT_NECESSARY
, true);
259 if (bb_contains_live_stmts
)
260 SET_BIT (bb_contains_live_stmts
, gimple_bb (stmt
)->index
);
261 VEC_safe_push (gimple
, heap
, worklist
, stmt
);
265 /* Mark STMT as necessary if it obviously is. Add it to the worklist if
266 it can make other statements necessary.
268 If AGGRESSIVE is false, control statements are conservatively marked as
272 mark_stmt_if_obviously_necessary (gimple stmt
, bool aggressive
)
274 /* With non-call exceptions, we have to assume that all statements could
275 throw. If a statement may throw, it is inherently necessary. */
276 if (cfun
->can_throw_non_call_exceptions
&& stmt_could_throw_p (stmt
))
278 mark_stmt_necessary (stmt
, true);
282 /* Statements that are implicitly live. Most function calls, asm
283 and return statements are required. Labels and GIMPLE_BIND nodes
284 are kept because they are control flow, and we have no way of
285 knowing whether they can be removed. DCE can eliminate all the
286 other statements in a block, and CFG can then remove the block
288 switch (gimple_code (stmt
))
292 mark_stmt_necessary (stmt
, false);
298 mark_stmt_necessary (stmt
, true);
302 /* Most, but not all function calls are required. Function calls that
303 produce no result and have no side effects (i.e. const pure
304 functions) are unnecessary. */
305 if (gimple_has_side_effects (stmt
))
307 mark_stmt_necessary (stmt
, true);
310 if (!gimple_call_lhs (stmt
))
315 /* Debug temps without a value are not useful. ??? If we could
316 easily locate the debug temp bind stmt for a use thereof,
317 would could refrain from marking all debug temps here, and
318 mark them only if they're used. */
319 if (gimple_debug_bind_has_value_p (stmt
)
320 || TREE_CODE (gimple_debug_bind_get_var (stmt
)) != DEBUG_EXPR_DECL
)
321 mark_stmt_necessary (stmt
, false);
325 gcc_assert (!simple_goto_p (stmt
));
326 mark_stmt_necessary (stmt
, true);
330 gcc_assert (EDGE_COUNT (gimple_bb (stmt
)->succs
) == 2);
335 mark_stmt_necessary (stmt
, true);
342 /* If the statement has volatile operands, it needs to be preserved.
343 Same for statements that can alter control flow in unpredictable
345 if (gimple_has_volatile_ops (stmt
) || is_ctrl_altering_stmt (stmt
))
347 mark_stmt_necessary (stmt
, true);
351 if (is_hidden_global_store (stmt
))
353 mark_stmt_necessary (stmt
, true);
361 /* Mark the last statement of BB as necessary. */
364 mark_last_stmt_necessary (basic_block bb
)
366 gimple stmt
= last_stmt (bb
);
368 SET_BIT (last_stmt_necessary
, bb
->index
);
369 SET_BIT (bb_contains_live_stmts
, bb
->index
);
371 /* We actually mark the statement only if it is a control statement. */
372 if (stmt
&& is_ctrl_stmt (stmt
))
373 mark_stmt_necessary (stmt
, true);
377 /* Mark control dependent edges of BB as necessary. We have to do this only
378 once for each basic block so we set the appropriate bit after we're done.
380 When IGNORE_SELF is true, ignore BB in the list of control dependences. */
383 mark_control_dependent_edges_necessary (basic_block bb
, struct edge_list
*el
,
387 unsigned edge_number
;
388 bool skipped
= false;
390 gcc_assert (bb
!= EXIT_BLOCK_PTR
);
392 if (bb
== ENTRY_BLOCK_PTR
)
395 EXECUTE_IF_CONTROL_DEPENDENT (bi
, bb
->index
, edge_number
)
397 basic_block cd_bb
= INDEX_EDGE_PRED_BB (el
, edge_number
);
399 if (ignore_self
&& cd_bb
== bb
)
405 if (!TEST_BIT (last_stmt_necessary
, cd_bb
->index
))
406 mark_last_stmt_necessary (cd_bb
);
410 SET_BIT (visited_control_parents
, bb
->index
);
414 /* Find obviously necessary statements. These are things like most function
415 calls, and stores to file level variables.
417 If EL is NULL, control statements are conservatively marked as
418 necessary. Otherwise it contains the list of edges used by control
419 dependence analysis. */
422 find_obviously_necessary_stmts (struct edge_list
*el
)
425 gimple_stmt_iterator gsi
;
432 /* PHI nodes are never inherently necessary. */
433 for (gsi
= gsi_start_phis (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
435 phi
= gsi_stmt (gsi
);
436 gimple_set_plf (phi
, STMT_NECESSARY
, false);
439 /* Check all statements in the block. */
440 for (gsi
= gsi_start_bb (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
442 stmt
= gsi_stmt (gsi
);
443 gimple_set_plf (stmt
, STMT_NECESSARY
, false);
444 mark_stmt_if_obviously_necessary (stmt
, el
!= NULL
);
448 /* Pure and const functions are finite and thus have no infinite loops in
450 flags
= flags_from_decl_or_type (current_function_decl
);
451 if ((flags
& (ECF_CONST
|ECF_PURE
)) && !(flags
& ECF_LOOPING_CONST_OR_PURE
))
454 /* Prevent the empty possibly infinite loops from being removed. */
460 if (mark_irreducible_loops ())
464 FOR_EACH_EDGE (e
, ei
, bb
->succs
)
465 if ((e
->flags
& EDGE_DFS_BACK
)
466 && (e
->flags
& EDGE_IRREDUCIBLE_LOOP
))
469 fprintf (dump_file
, "Marking back edge of irreducible loop %i->%i\n",
470 e
->src
->index
, e
->dest
->index
);
471 mark_control_dependent_edges_necessary (e
->dest
, el
, false);
475 FOR_EACH_LOOP (li
, loop
, 0)
476 if (!finite_loop_p (loop
))
479 fprintf (dump_file
, "can not prove finiteness of loop %i\n", loop
->num
);
480 mark_control_dependent_edges_necessary (loop
->latch
, el
, false);
487 /* Return true if REF is based on an aliased base, otherwise false. */
490 ref_may_be_aliased (tree ref
)
492 while (handled_component_p (ref
))
493 ref
= TREE_OPERAND (ref
, 0);
494 if (TREE_CODE (ref
) == MEM_REF
495 && TREE_CODE (TREE_OPERAND (ref
, 0)) == ADDR_EXPR
)
496 ref
= TREE_OPERAND (TREE_OPERAND (ref
, 0), 0);
497 return !(DECL_P (ref
)
498 && !may_be_aliased (ref
));
501 static bitmap visited
= NULL
;
502 static unsigned int longest_chain
= 0;
503 static unsigned int total_chain
= 0;
504 static unsigned int nr_walks
= 0;
505 static bool chain_ovfl
= false;
507 /* Worker for the walker that marks reaching definitions of REF,
508 which is based on a non-aliased decl, necessary. It returns
509 true whenever the defining statement of the current VDEF is
510 a kill for REF, as no dominating may-defs are necessary for REF
511 anymore. DATA points to the basic-block that contains the
512 stmt that refers to REF. */
515 mark_aliased_reaching_defs_necessary_1 (ao_ref
*ref
, tree vdef
, void *data
)
517 gimple def_stmt
= SSA_NAME_DEF_STMT (vdef
);
519 /* All stmts we visit are necessary. */
520 mark_operand_necessary (vdef
);
522 /* If the stmt lhs kills ref, then we can stop walking. */
523 if (gimple_has_lhs (def_stmt
)
524 && TREE_CODE (gimple_get_lhs (def_stmt
)) != SSA_NAME
)
526 tree base
, lhs
= gimple_get_lhs (def_stmt
);
527 HOST_WIDE_INT size
, offset
, max_size
;
529 base
= get_ref_base_and_extent (lhs
, &offset
, &size
, &max_size
);
530 /* We can get MEM[symbol: sZ, index: D.8862_1] here,
531 so base == refd->base does not always hold. */
532 if (base
== ref
->base
)
534 /* For a must-alias check we need to be able to constrain
535 the accesses properly. */
536 if (size
!= -1 && size
== max_size
537 && ref
->max_size
!= -1)
539 if (offset
<= ref
->offset
540 && offset
+ size
>= ref
->offset
+ ref
->max_size
)
543 /* Or they need to be exactly the same. */
545 /* Make sure there is no induction variable involved
546 in the references (gcc.c-torture/execute/pr42142.c).
547 The simplest way is to check if the kill dominates
549 && dominated_by_p (CDI_DOMINATORS
, (basic_block
) data
,
550 gimple_bb (def_stmt
))
551 && operand_equal_p (ref
->ref
, lhs
, 0))
556 /* Otherwise keep walking. */
561 mark_aliased_reaching_defs_necessary (gimple stmt
, tree ref
)
565 gcc_assert (!chain_ovfl
);
566 ao_ref_init (&refd
, ref
);
567 chain
= walk_aliased_vdefs (&refd
, gimple_vuse (stmt
),
568 mark_aliased_reaching_defs_necessary_1
,
569 gimple_bb (stmt
), NULL
);
570 if (chain
> longest_chain
)
571 longest_chain
= chain
;
572 total_chain
+= chain
;
576 /* Worker for the walker that marks reaching definitions of REF, which
577 is not based on a non-aliased decl. For simplicity we need to end
578 up marking all may-defs necessary that are not based on a non-aliased
579 decl. The only job of this walker is to skip may-defs based on
580 a non-aliased decl. */
583 mark_all_reaching_defs_necessary_1 (ao_ref
*ref ATTRIBUTE_UNUSED
,
584 tree vdef
, void *data ATTRIBUTE_UNUSED
)
586 gimple def_stmt
= SSA_NAME_DEF_STMT (vdef
);
588 /* We have to skip already visited (and thus necessary) statements
589 to make the chaining work after we dropped back to simple mode. */
591 && TEST_BIT (processed
, SSA_NAME_VERSION (vdef
)))
593 gcc_assert (gimple_nop_p (def_stmt
)
594 || gimple_plf (def_stmt
, STMT_NECESSARY
));
598 /* We want to skip stores to non-aliased variables. */
600 && gimple_assign_single_p (def_stmt
))
602 tree lhs
= gimple_assign_lhs (def_stmt
);
603 if (!ref_may_be_aliased (lhs
))
607 mark_operand_necessary (vdef
);
613 mark_all_reaching_defs_necessary (gimple stmt
)
615 walk_aliased_vdefs (NULL
, gimple_vuse (stmt
),
616 mark_all_reaching_defs_necessary_1
, NULL
, &visited
);
619 /* Return true for PHI nodes with one or identical arguments
622 degenerate_phi_p (gimple phi
)
625 tree op
= gimple_phi_arg_def (phi
, 0);
626 for (i
= 1; i
< gimple_phi_num_args (phi
); i
++)
627 if (gimple_phi_arg_def (phi
, i
) != op
)
632 /* Propagate necessity using the operands of necessary statements.
633 Process the uses on each statement in the worklist, and add all
634 feeding statements which contribute to the calculation of this
635 value to the worklist.
637 In conservative mode, EL is NULL. */
640 propagate_necessity (struct edge_list
*el
)
643 bool aggressive
= (el
? true : false);
645 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
646 fprintf (dump_file
, "\nProcessing worklist:\n");
648 while (VEC_length (gimple
, worklist
) > 0)
650 /* Take STMT from worklist. */
651 stmt
= VEC_pop (gimple
, worklist
);
653 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
655 fprintf (dump_file
, "processing: ");
656 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
657 fprintf (dump_file
, "\n");
662 /* Mark the last statement of the basic blocks on which the block
663 containing STMT is control dependent, but only if we haven't
665 basic_block bb
= gimple_bb (stmt
);
666 if (bb
!= ENTRY_BLOCK_PTR
667 && !TEST_BIT (visited_control_parents
, bb
->index
))
668 mark_control_dependent_edges_necessary (bb
, el
, false);
671 if (gimple_code (stmt
) == GIMPLE_PHI
672 /* We do not process virtual PHI nodes nor do we track their
674 && is_gimple_reg (gimple_phi_result (stmt
)))
676 /* PHI nodes are somewhat special in that each PHI alternative has
677 data and control dependencies. All the statements feeding the
678 PHI node's arguments are always necessary. In aggressive mode,
679 we also consider the control dependent edges leading to the
680 predecessor block associated with each PHI alternative as
684 for (k
= 0; k
< gimple_phi_num_args (stmt
); k
++)
686 tree arg
= PHI_ARG_DEF (stmt
, k
);
687 if (TREE_CODE (arg
) == SSA_NAME
)
688 mark_operand_necessary (arg
);
691 /* For PHI operands it matters from where the control flow arrives
692 to the BB. Consider the following example:
702 We need to mark control dependence of the empty basic blocks, since they
703 contains computation of PHI operands.
705 Doing so is too restrictive in the case the predecestor block is in
711 for (i = 0; i<1000; ++i)
717 There is PHI for J in the BB containing return statement.
718 In this case the control dependence of predecestor block (that is
719 within the empty loop) also contains the block determining number
720 of iterations of the block that would prevent removing of empty
723 This scenario can be avoided by splitting critical edges.
724 To save the critical edge splitting pass we identify how the control
725 dependence would look like if the edge was split.
727 Consider the modified CFG created from current CFG by splitting
728 edge B->C. In the postdominance tree of modified CFG, C' is
729 always child of C. There are two cases how chlids of C' can look
734 In this case the only basic block C' is control dependent on is B.
736 2) C' has single child that is B
738 In this case control dependence of C' is same as control
739 dependence of B in original CFG except for block B itself.
740 (since C' postdominate B in modified CFG)
742 Now how to decide what case happens? There are two basic options:
744 a) C postdominate B. Then C immediately postdominate B and
745 case 2 happens iff there is no other way from B to C except
748 There is other way from B to C iff there is succesor of B that
749 is not postdominated by B. Testing this condition is somewhat
750 expensive, because we need to iterate all succesors of B.
751 We are safe to assume that this does not happen: we will mark B
752 as needed when processing the other path from B to C that is
753 conrol dependent on B and marking control dependencies of B
754 itself is harmless because they will be processed anyway after
755 processing control statement in B.
757 b) C does not postdominate B. Always case 1 happens since there is
758 path from C to exit that does not go through B and thus also C'. */
760 if (aggressive
&& !degenerate_phi_p (stmt
))
762 for (k
= 0; k
< gimple_phi_num_args (stmt
); k
++)
764 basic_block arg_bb
= gimple_phi_arg_edge (stmt
, k
)->src
;
767 != get_immediate_dominator (CDI_POST_DOMINATORS
, arg_bb
))
769 if (!TEST_BIT (last_stmt_necessary
, arg_bb
->index
))
770 mark_last_stmt_necessary (arg_bb
);
772 else if (arg_bb
!= ENTRY_BLOCK_PTR
773 && !TEST_BIT (visited_control_parents
,
775 mark_control_dependent_edges_necessary (arg_bb
, el
, true);
781 /* Propagate through the operands. Examine all the USE, VUSE and
782 VDEF operands in this statement. Mark all the statements
783 which feed this statement's uses as necessary. */
787 FOR_EACH_SSA_TREE_OPERAND (use
, stmt
, iter
, SSA_OP_USE
)
788 mark_operand_necessary (use
);
790 use
= gimple_vuse (stmt
);
794 /* If we dropped to simple mode make all immediately
795 reachable definitions necessary. */
798 mark_all_reaching_defs_necessary (stmt
);
802 /* For statements that may load from memory (have a VUSE) we
803 have to mark all reaching (may-)definitions as necessary.
804 We partition this task into two cases:
805 1) explicit loads based on decls that are not aliased
806 2) implicit loads (like calls) and explicit loads not
807 based on decls that are not aliased (like indirect
808 references or loads from globals)
809 For 1) we mark all reaching may-defs as necessary, stopping
810 at dominating kills. For 2) we want to mark all dominating
811 references necessary, but non-aliased ones which we handle
812 in 1). By keeping a global visited bitmap for references
813 we walk for 2) we avoid quadratic behavior for those. */
815 if (is_gimple_call (stmt
))
817 tree callee
= gimple_call_fndecl (stmt
);
820 /* Calls to functions that are merely acting as barriers
821 or that only store to memory do not make any previous
823 if (callee
!= NULL_TREE
824 && DECL_BUILT_IN_CLASS (callee
) == BUILT_IN_NORMAL
825 && (DECL_FUNCTION_CODE (callee
) == BUILT_IN_MEMSET
826 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_MALLOC
827 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_FREE
828 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_ALLOCA
829 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_STACK_SAVE
830 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_STACK_RESTORE
))
833 /* Calls implicitly load from memory, their arguments
834 in addition may explicitly perform memory loads. */
835 mark_all_reaching_defs_necessary (stmt
);
836 for (i
= 0; i
< gimple_call_num_args (stmt
); ++i
)
838 tree arg
= gimple_call_arg (stmt
, i
);
839 if (TREE_CODE (arg
) == SSA_NAME
840 || is_gimple_min_invariant (arg
))
842 if (!ref_may_be_aliased (arg
))
843 mark_aliased_reaching_defs_necessary (stmt
, arg
);
846 else if (gimple_assign_single_p (stmt
))
849 bool rhs_aliased
= false;
850 /* If this is a load mark things necessary. */
851 rhs
= gimple_assign_rhs1 (stmt
);
852 if (TREE_CODE (rhs
) != SSA_NAME
853 && !is_gimple_min_invariant (rhs
))
855 if (!ref_may_be_aliased (rhs
))
856 mark_aliased_reaching_defs_necessary (stmt
, rhs
);
861 mark_all_reaching_defs_necessary (stmt
);
863 else if (gimple_code (stmt
) == GIMPLE_RETURN
)
865 tree rhs
= gimple_return_retval (stmt
);
866 /* A return statement may perform a load. */
868 && TREE_CODE (rhs
) != SSA_NAME
869 && !is_gimple_min_invariant (rhs
))
871 if (!ref_may_be_aliased (rhs
))
872 mark_aliased_reaching_defs_necessary (stmt
, rhs
);
874 mark_all_reaching_defs_necessary (stmt
);
877 else if (gimple_code (stmt
) == GIMPLE_ASM
)
880 mark_all_reaching_defs_necessary (stmt
);
881 /* Inputs may perform loads. */
882 for (i
= 0; i
< gimple_asm_ninputs (stmt
); ++i
)
884 tree op
= TREE_VALUE (gimple_asm_input_op (stmt
, i
));
885 if (TREE_CODE (op
) != SSA_NAME
886 && !is_gimple_min_invariant (op
)
887 && !ref_may_be_aliased (op
))
888 mark_aliased_reaching_defs_necessary (stmt
, op
);
894 /* If we over-used our alias oracle budget drop to simple
895 mode. The cost metric allows quadratic behavior
896 (number of uses times number of may-defs queries) up to
897 a constant maximal number of queries and after that falls back to
898 super-linear complexity. */
899 if (/* Constant but quadratic for small functions. */
900 total_chain
> 128 * 128
901 /* Linear in the number of may-defs. */
902 && total_chain
> 32 * longest_chain
903 /* Linear in the number of uses. */
904 && total_chain
> nr_walks
* 32)
908 bitmap_clear (visited
);
914 /* Replace all uses of result of PHI by underlying variable and mark it
918 mark_virtual_phi_result_for_renaming (gimple phi
)
921 imm_use_iterator iter
;
924 tree result_ssa
, result_var
;
926 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
928 fprintf (dump_file
, "Marking result for renaming : ");
929 print_gimple_stmt (dump_file
, phi
, 0, TDF_SLIM
);
930 fprintf (dump_file
, "\n");
933 result_ssa
= gimple_phi_result (phi
);
934 result_var
= SSA_NAME_VAR (result_ssa
);
935 FOR_EACH_IMM_USE_STMT (stmt
, iter
, result_ssa
)
937 FOR_EACH_IMM_USE_ON_STMT (use_p
, iter
)
938 SET_USE (use_p
, result_var
);
943 mark_sym_for_renaming (result_var
);
946 /* Remove dead PHI nodes from block BB. */
949 remove_dead_phis (basic_block bb
)
951 bool something_changed
= false;
954 gimple_stmt_iterator gsi
;
955 phis
= phi_nodes (bb
);
957 for (gsi
= gsi_start (phis
); !gsi_end_p (gsi
);)
960 phi
= gsi_stmt (gsi
);
962 /* We do not track necessity of virtual PHI nodes. Instead do
963 very simple dead PHI removal here. */
964 if (!is_gimple_reg (gimple_phi_result (phi
)))
966 /* Virtual PHI nodes with one or identical arguments
968 if (degenerate_phi_p (phi
))
970 tree vdef
= gimple_phi_result (phi
);
971 tree vuse
= gimple_phi_arg_def (phi
, 0);
974 imm_use_iterator iter
;
976 FOR_EACH_IMM_USE_STMT (use_stmt
, iter
, vdef
)
977 FOR_EACH_IMM_USE_ON_STMT (use_p
, iter
)
978 SET_USE (use_p
, vuse
);
979 if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vdef
)
980 && TREE_CODE (vuse
) == SSA_NAME
)
981 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vuse
) = 1;
984 gimple_set_plf (phi
, STMT_NECESSARY
, true);
987 if (!gimple_plf (phi
, STMT_NECESSARY
))
989 something_changed
= true;
990 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
992 fprintf (dump_file
, "Deleting : ");
993 print_gimple_stmt (dump_file
, phi
, 0, TDF_SLIM
);
994 fprintf (dump_file
, "\n");
997 remove_phi_node (&gsi
, true);
998 stats
.removed_phis
++;
1004 return something_changed
;
1007 /* Forward edge E to respective POST_DOM_BB and update PHIs. */
1010 forward_edge_to_pdom (edge e
, basic_block post_dom_bb
)
1012 gimple_stmt_iterator gsi
;
1016 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1017 fprintf (dump_file
, "Redirecting edge %i->%i to %i\n", e
->src
->index
,
1018 e
->dest
->index
, post_dom_bb
->index
);
1020 e2
= redirect_edge_and_branch (e
, post_dom_bb
);
1023 /* If edge was already around, no updating is neccesary. */
1027 if (!gimple_seq_empty_p (phi_nodes (post_dom_bb
)))
1029 /* We are sure that for every live PHI we are seeing control dependent BB.
1030 This means that we can pick any edge to duplicate PHI args from. */
1031 FOR_EACH_EDGE (e2
, ei
, post_dom_bb
->preds
)
1034 for (gsi
= gsi_start_phis (post_dom_bb
); !gsi_end_p (gsi
);)
1036 gimple phi
= gsi_stmt (gsi
);
1038 source_location locus
;
1040 /* PHIs for virtuals have no control dependency relation on them.
1041 We are lost here and must force renaming of the symbol. */
1042 if (!is_gimple_reg (gimple_phi_result (phi
)))
1044 mark_virtual_phi_result_for_renaming (phi
);
1045 remove_phi_node (&gsi
, true);
1049 /* Dead PHI do not imply control dependency. */
1050 if (!gimple_plf (phi
, STMT_NECESSARY
))
1056 op
= gimple_phi_arg_def (phi
, e2
->dest_idx
);
1057 locus
= gimple_phi_arg_location (phi
, e2
->dest_idx
);
1058 add_phi_arg (phi
, op
, e
, locus
);
1059 /* The resulting PHI if not dead can only be degenerate. */
1060 gcc_assert (degenerate_phi_p (phi
));
1067 /* Remove dead statement pointed to by iterator I. Receives the basic block BB
1068 containing I so that we don't have to look it up. */
1071 remove_dead_stmt (gimple_stmt_iterator
*i
, basic_block bb
)
1073 gimple stmt
= gsi_stmt (*i
);
1075 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1077 fprintf (dump_file
, "Deleting : ");
1078 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
1079 fprintf (dump_file
, "\n");
1084 /* If we have determined that a conditional branch statement contributes
1085 nothing to the program, then we not only remove it, but we also change
1086 the flow graph so that the current block will simply fall-thru to its
1087 immediate post-dominator. The blocks we are circumventing will be
1088 removed by cleanup_tree_cfg if this change in the flow graph makes them
1090 if (is_ctrl_stmt (stmt
))
1092 basic_block post_dom_bb
;
1096 post_dom_bb
= get_immediate_dominator (CDI_POST_DOMINATORS
, bb
);
1098 e
= find_edge (bb
, post_dom_bb
);
1100 /* If edge is already there, try to use it. This avoids need to update
1101 PHI nodes. Also watch for cases where post dominator does not exists
1102 or is exit block. These can happen for infinite loops as we create
1103 fake edges in the dominator tree. */
1106 else if (! post_dom_bb
|| post_dom_bb
== EXIT_BLOCK_PTR
)
1107 e
= EDGE_SUCC (bb
, 0);
1109 e
= forward_edge_to_pdom (EDGE_SUCC (bb
, 0), post_dom_bb
);
1111 e
->probability
= REG_BR_PROB_BASE
;
1112 e
->count
= bb
->count
;
1114 /* The edge is no longer associated with a conditional, so it does
1115 not have TRUE/FALSE flags. */
1116 e
->flags
&= ~(EDGE_TRUE_VALUE
| EDGE_FALSE_VALUE
);
1118 /* The lone outgoing edge from BB will be a fallthru edge. */
1119 e
->flags
|= EDGE_FALLTHRU
;
1121 /* Remove the remaining outgoing edges. */
1122 for (ei
= ei_start (bb
->succs
); (e2
= ei_safe_edge (ei
)); )
1132 unlink_stmt_vdef (stmt
);
1133 gsi_remove (i
, true);
1134 release_defs (stmt
);
1137 /* Eliminate unnecessary statements. Any instruction not marked as necessary
1138 contributes nothing to the program, and can be deleted. */
1141 eliminate_unnecessary_stmts (void)
1143 bool something_changed
= false;
1145 gimple_stmt_iterator gsi
, psi
;
1148 VEC (basic_block
, heap
) *h
;
1150 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1151 fprintf (dump_file
, "\nEliminating unnecessary statements:\n");
1153 clear_special_calls ();
1155 /* Walking basic blocks and statements in reverse order avoids
1156 releasing SSA names before any other DEFs that refer to them are
1157 released. This helps avoid loss of debug information, as we get
1158 a chance to propagate all RHSs of removed SSAs into debug uses,
1159 rather than only the latest ones. E.g., consider:
1165 If we were to release x_3 before a_5, when we reached a_5 and
1166 tried to substitute it into the debug stmt, we'd see x_3 there,
1167 but x_3's DEF, type, etc would have already been disconnected.
1168 By going backwards, the debug stmt first changes to:
1170 # DEBUG a => x_3 - b_4
1174 # DEBUG a => y_1 + z_2 - b_4
1177 gcc_assert (dom_info_available_p (CDI_DOMINATORS
));
1178 h
= get_all_dominated_blocks (CDI_DOMINATORS
, single_succ (ENTRY_BLOCK_PTR
));
1180 while (VEC_length (basic_block
, h
))
1182 bb
= VEC_pop (basic_block
, h
);
1184 /* Remove dead statements. */
1185 for (gsi
= gsi_last_bb (bb
); !gsi_end_p (gsi
); gsi
= psi
)
1187 stmt
= gsi_stmt (gsi
);
1194 /* If GSI is not necessary then remove it. */
1195 if (!gimple_plf (stmt
, STMT_NECESSARY
))
1197 if (!is_gimple_debug (stmt
))
1198 something_changed
= true;
1199 remove_dead_stmt (&gsi
, bb
);
1201 else if (is_gimple_call (stmt
))
1203 call
= gimple_call_fndecl (stmt
);
1208 /* When LHS of var = call (); is dead, simplify it into
1209 call (); saving one operand. */
1210 name
= gimple_call_lhs (stmt
);
1211 if (name
&& TREE_CODE (name
) == SSA_NAME
1212 && !TEST_BIT (processed
, SSA_NAME_VERSION (name
)))
1214 something_changed
= true;
1215 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1217 fprintf (dump_file
, "Deleting LHS of call: ");
1218 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
1219 fprintf (dump_file
, "\n");
1222 gimple_call_set_lhs (stmt
, NULL_TREE
);
1223 maybe_clean_or_replace_eh_stmt (stmt
, stmt
);
1225 release_ssa_name (name
);
1227 notice_special_calls (stmt
);
1233 VEC_free (basic_block
, heap
, h
);
1235 /* Since we don't track liveness of virtual PHI nodes, it is possible that we
1236 rendered some PHI nodes unreachable while they are still in use.
1237 Mark them for renaming. */
1240 basic_block prev_bb
;
1242 find_unreachable_blocks ();
1244 /* Delete all unreachable basic blocks in reverse dominator order. */
1245 for (bb
= EXIT_BLOCK_PTR
->prev_bb
; bb
!= ENTRY_BLOCK_PTR
; bb
= prev_bb
)
1247 prev_bb
= bb
->prev_bb
;
1249 if (!TEST_BIT (bb_contains_live_stmts
, bb
->index
)
1250 || !(bb
->flags
& BB_REACHABLE
))
1252 for (gsi
= gsi_start_phis (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
1253 if (!is_gimple_reg (gimple_phi_result (gsi_stmt (gsi
))))
1256 imm_use_iterator iter
;
1258 FOR_EACH_IMM_USE_STMT (stmt
, iter
, gimple_phi_result (gsi_stmt (gsi
)))
1260 if (!(gimple_bb (stmt
)->flags
& BB_REACHABLE
))
1262 if (gimple_code (stmt
) == GIMPLE_PHI
1263 || gimple_plf (stmt
, STMT_NECESSARY
))
1266 BREAK_FROM_IMM_USE_STMT (iter
);
1270 mark_virtual_phi_result_for_renaming (gsi_stmt (gsi
));
1273 if (!(bb
->flags
& BB_REACHABLE
))
1275 /* Speed up the removal of blocks that don't
1276 dominate others. Walking backwards, this should
1277 be the common case. ??? Do we need to recompute
1278 dominators because of cfg_altered? */
1279 if (!MAY_HAVE_DEBUG_STMTS
1280 || !first_dom_son (CDI_DOMINATORS
, bb
))
1281 delete_basic_block (bb
);
1284 h
= get_all_dominated_blocks (CDI_DOMINATORS
, bb
);
1286 while (VEC_length (basic_block
, h
))
1288 bb
= VEC_pop (basic_block
, h
);
1289 prev_bb
= bb
->prev_bb
;
1290 /* Rearrangements to the CFG may have failed
1291 to update the dominators tree, so that
1292 formerly-dominated blocks are now
1293 otherwise reachable. */
1294 if (!!(bb
->flags
& BB_REACHABLE
))
1296 delete_basic_block (bb
);
1299 VEC_free (basic_block
, heap
, h
);
1307 /* Remove dead PHI nodes. */
1308 something_changed
|= remove_dead_phis (bb
);
1311 return something_changed
;
1315 /* Print out removed statement statistics. */
1322 percg
= ((float) stats
.removed
/ (float) stats
.total
) * 100;
1323 fprintf (dump_file
, "Removed %d of %d statements (%d%%)\n",
1324 stats
.removed
, stats
.total
, (int) percg
);
1326 if (stats
.total_phis
== 0)
1329 percg
= ((float) stats
.removed_phis
/ (float) stats
.total_phis
) * 100;
1331 fprintf (dump_file
, "Removed %d of %d PHI nodes (%d%%)\n",
1332 stats
.removed_phis
, stats
.total_phis
, (int) percg
);
1335 /* Initialization for this pass. Set up the used data structures. */
1338 tree_dce_init (bool aggressive
)
1340 memset ((void *) &stats
, 0, sizeof (stats
));
1346 control_dependence_map
= XNEWVEC (bitmap
, last_basic_block
);
1347 for (i
= 0; i
< last_basic_block
; ++i
)
1348 control_dependence_map
[i
] = BITMAP_ALLOC (NULL
);
1350 last_stmt_necessary
= sbitmap_alloc (last_basic_block
);
1351 sbitmap_zero (last_stmt_necessary
);
1352 bb_contains_live_stmts
= sbitmap_alloc (last_basic_block
);
1353 sbitmap_zero (bb_contains_live_stmts
);
1356 processed
= sbitmap_alloc (num_ssa_names
+ 1);
1357 sbitmap_zero (processed
);
1359 worklist
= VEC_alloc (gimple
, heap
, 64);
1360 cfg_altered
= false;
1363 /* Cleanup after this pass. */
1366 tree_dce_done (bool aggressive
)
1372 for (i
= 0; i
< last_basic_block
; ++i
)
1373 BITMAP_FREE (control_dependence_map
[i
]);
1374 free (control_dependence_map
);
1376 sbitmap_free (visited_control_parents
);
1377 sbitmap_free (last_stmt_necessary
);
1378 sbitmap_free (bb_contains_live_stmts
);
1379 bb_contains_live_stmts
= NULL
;
1382 sbitmap_free (processed
);
1384 VEC_free (gimple
, heap
, worklist
);
1387 /* Main routine to eliminate dead code.
1389 AGGRESSIVE controls the aggressiveness of the algorithm.
1390 In conservative mode, we ignore control dependence and simply declare
1391 all but the most trivially dead branches necessary. This mode is fast.
1392 In aggressive mode, control dependences are taken into account, which
1393 results in more dead code elimination, but at the cost of some time.
1395 FIXME: Aggressive mode before PRE doesn't work currently because
1396 the dominance info is not invalidated after DCE1. This is
1397 not an issue right now because we only run aggressive DCE
1398 as the last tree SSA pass, but keep this in mind when you
1399 start experimenting with pass ordering. */
1402 perform_tree_ssa_dce (bool aggressive
)
1404 struct edge_list
*el
= NULL
;
1405 bool something_changed
= 0;
1407 calculate_dominance_info (CDI_DOMINATORS
);
1409 /* Preheaders are needed for SCEV to work.
1410 Simple lateches and recorded exits improve chances that loop will
1411 proved to be finite in testcases such as in loop-15.c and loop-24.c */
1413 loop_optimizer_init (LOOPS_NORMAL
1414 | LOOPS_HAVE_RECORDED_EXITS
);
1416 tree_dce_init (aggressive
);
1420 /* Compute control dependence. */
1421 timevar_push (TV_CONTROL_DEPENDENCES
);
1422 calculate_dominance_info (CDI_POST_DOMINATORS
);
1423 el
= create_edge_list ();
1424 find_all_control_dependences (el
);
1425 timevar_pop (TV_CONTROL_DEPENDENCES
);
1427 visited_control_parents
= sbitmap_alloc (last_basic_block
);
1428 sbitmap_zero (visited_control_parents
);
1430 mark_dfs_back_edges ();
1433 find_obviously_necessary_stmts (el
);
1436 loop_optimizer_finalize ();
1442 visited
= BITMAP_ALLOC (NULL
);
1443 propagate_necessity (el
);
1444 BITMAP_FREE (visited
);
1446 something_changed
|= eliminate_unnecessary_stmts ();
1447 something_changed
|= cfg_altered
;
1449 /* We do not update postdominators, so free them unconditionally. */
1450 free_dominance_info (CDI_POST_DOMINATORS
);
1452 /* If we removed paths in the CFG, then we need to update
1453 dominators as well. I haven't investigated the possibility
1454 of incrementally updating dominators. */
1456 free_dominance_info (CDI_DOMINATORS
);
1458 statistics_counter_event (cfun
, "Statements deleted", stats
.removed
);
1459 statistics_counter_event (cfun
, "PHI nodes deleted", stats
.removed_phis
);
1461 /* Debugging dumps. */
1462 if (dump_file
&& (dump_flags
& (TDF_STATS
|TDF_DETAILS
)))
1465 tree_dce_done (aggressive
);
1467 free_edge_list (el
);
1469 if (something_changed
)
1470 return (TODO_update_ssa
| TODO_cleanup_cfg
| TODO_ggc_collect
1471 | TODO_remove_unused_locals
);
1476 /* Pass entry points. */
1480 return perform_tree_ssa_dce (/*aggressive=*/false);
1484 tree_ssa_dce_loop (void)
1487 todo
= perform_tree_ssa_dce (/*aggressive=*/false);
1490 free_numbers_of_iterations_estimates ();
1497 tree_ssa_cd_dce (void)
1499 return perform_tree_ssa_dce (/*aggressive=*/optimize
>= 2);
1505 return flag_tree_dce
!= 0;
1508 struct gimple_opt_pass pass_dce
=
1513 gate_dce
, /* gate */
1514 tree_ssa_dce
, /* execute */
1517 0, /* static_pass_number */
1518 TV_TREE_DCE
, /* tv_id */
1519 PROP_cfg
| PROP_ssa
, /* properties_required */
1520 0, /* properties_provided */
1521 0, /* properties_destroyed */
1522 0, /* todo_flags_start */
1523 TODO_dump_func
| TODO_verify_ssa
/* todo_flags_finish */
1527 struct gimple_opt_pass pass_dce_loop
=
1531 "dceloop", /* name */
1532 gate_dce
, /* gate */
1533 tree_ssa_dce_loop
, /* execute */
1536 0, /* static_pass_number */
1537 TV_TREE_DCE
, /* tv_id */
1538 PROP_cfg
| PROP_ssa
, /* properties_required */
1539 0, /* properties_provided */
1540 0, /* properties_destroyed */
1541 0, /* todo_flags_start */
1542 TODO_dump_func
| TODO_verify_ssa
/* todo_flags_finish */
1546 struct gimple_opt_pass pass_cd_dce
=
1551 gate_dce
, /* gate */
1552 tree_ssa_cd_dce
, /* execute */
1555 0, /* static_pass_number */
1556 TV_TREE_CD_DCE
, /* tv_id */
1557 PROP_cfg
| PROP_ssa
, /* properties_required */
1558 0, /* properties_provided */
1559 0, /* properties_destroyed */
1560 0, /* todo_flags_start */
1561 TODO_dump_func
| TODO_verify_ssa
1562 | TODO_verify_flow
/* todo_flags_finish */