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 last_stmt if a basic block has already been
81 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. */
201 mark_stmt_necessary (gimple stmt
, bool add_to_worklist
)
205 if (gimple_plf (stmt
, STMT_NECESSARY
))
208 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
210 fprintf (dump_file
, "Marking useful stmt: ");
211 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
212 fprintf (dump_file
, "\n");
215 gimple_set_plf (stmt
, STMT_NECESSARY
, true);
217 VEC_safe_push (gimple
, heap
, worklist
, stmt
);
218 if (bb_contains_live_stmts
&& !is_gimple_debug (stmt
))
219 SET_BIT (bb_contains_live_stmts
, gimple_bb (stmt
)->index
);
223 /* Mark the statement defining operand OP as necessary. */
226 mark_operand_necessary (tree op
)
233 ver
= SSA_NAME_VERSION (op
);
234 if (TEST_BIT (processed
, ver
))
236 stmt
= SSA_NAME_DEF_STMT (op
);
237 gcc_assert (gimple_nop_p (stmt
)
238 || gimple_plf (stmt
, STMT_NECESSARY
));
241 SET_BIT (processed
, ver
);
243 stmt
= SSA_NAME_DEF_STMT (op
);
246 if (gimple_plf (stmt
, STMT_NECESSARY
) || gimple_nop_p (stmt
))
249 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
251 fprintf (dump_file
, "marking necessary through ");
252 print_generic_expr (dump_file
, op
, 0);
253 fprintf (dump_file
, " stmt ");
254 print_gimple_stmt (dump_file
, stmt
, 0, 0);
257 gimple_set_plf (stmt
, STMT_NECESSARY
, true);
258 if (bb_contains_live_stmts
)
259 SET_BIT (bb_contains_live_stmts
, gimple_bb (stmt
)->index
);
260 VEC_safe_push (gimple
, heap
, worklist
, stmt
);
264 /* Mark STMT as necessary if it obviously is. Add it to the worklist if
265 it can make other statements necessary.
267 If AGGRESSIVE is false, control statements are conservatively marked as
271 mark_stmt_if_obviously_necessary (gimple stmt
, bool aggressive
)
273 tree lhs
= NULL_TREE
;
275 /* With non-call exceptions, we have to assume that all statements could
276 throw. If a statement may throw, it is inherently necessary. */
277 if (cfun
->can_throw_non_call_exceptions
&& stmt_could_throw_p (stmt
))
279 mark_stmt_necessary (stmt
, true);
283 /* Statements that are implicitly live. Most function calls, asm
284 and return statements are required. Labels and GIMPLE_BIND nodes
285 are kept because they are control flow, and we have no way of
286 knowing whether they can be removed. DCE can eliminate all the
287 other statements in a block, and CFG can then remove the block
289 switch (gimple_code (stmt
))
293 mark_stmt_necessary (stmt
, false);
299 mark_stmt_necessary (stmt
, true);
303 /* Most, but not all function calls are required. Function calls that
304 produce no result and have no side effects (i.e. const pure
305 functions) are unnecessary. */
306 if (gimple_has_side_effects (stmt
))
308 mark_stmt_necessary (stmt
, true);
311 if (!gimple_call_lhs (stmt
))
313 lhs
= gimple_call_lhs (stmt
);
318 lhs
= gimple_assign_lhs (stmt
);
322 /* Debug temps without a value are not useful. ??? If we could
323 easily locate the debug temp bind stmt for a use thereof,
324 would could refrain from marking all debug temps here, and
325 mark them only if they're used. */
326 if (gimple_debug_bind_has_value_p (stmt
)
327 || TREE_CODE (gimple_debug_bind_get_var (stmt
)) != DEBUG_EXPR_DECL
)
328 mark_stmt_necessary (stmt
, false);
332 gcc_assert (!simple_goto_p (stmt
));
333 mark_stmt_necessary (stmt
, true);
337 gcc_assert (EDGE_COUNT (gimple_bb (stmt
)->succs
) == 2);
342 mark_stmt_necessary (stmt
, true);
349 /* If the statement has volatile operands, it needs to be preserved.
350 Same for statements that can alter control flow in unpredictable
352 if (gimple_has_volatile_ops (stmt
) || is_ctrl_altering_stmt (stmt
))
354 mark_stmt_necessary (stmt
, true);
358 if (is_hidden_global_store (stmt
))
360 mark_stmt_necessary (stmt
, true);
368 /* Make corresponding control dependent edges necessary. We only
369 have to do this once for each basic block, so we clear the bitmap
372 When IGNORE_SELF it true, ignore BB from the list of control dependences. */
374 mark_control_dependent_edges_necessary (basic_block bb
, struct edge_list
*el
, bool ignore_self
)
377 unsigned edge_number
;
378 bool skipped
= false;
380 gcc_assert (bb
!= EXIT_BLOCK_PTR
);
382 if (bb
== ENTRY_BLOCK_PTR
)
385 EXECUTE_IF_CONTROL_DEPENDENT (bi
, bb
->index
, edge_number
)
388 basic_block cd_bb
= INDEX_EDGE_PRED_BB (el
, edge_number
);
390 if (ignore_self
&& cd_bb
== bb
)
396 if (TEST_BIT (last_stmt_necessary
, cd_bb
->index
))
398 SET_BIT (last_stmt_necessary
, cd_bb
->index
);
399 SET_BIT (bb_contains_live_stmts
, cd_bb
->index
);
401 stmt
= last_stmt (cd_bb
);
402 if (stmt
&& is_ctrl_stmt (stmt
))
403 mark_stmt_necessary (stmt
, true);
406 SET_BIT (visited_control_parents
, bb
->index
);
410 /* Find obviously necessary statements. These are things like most function
411 calls, and stores to file level variables.
413 If EL is NULL, control statements are conservatively marked as
414 necessary. Otherwise it contains the list of edges used by control
415 dependence analysis. */
418 find_obviously_necessary_stmts (struct edge_list
*el
)
421 gimple_stmt_iterator gsi
;
427 /* PHI nodes are never inherently necessary. */
428 for (gsi
= gsi_start_phis (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
430 phi
= gsi_stmt (gsi
);
431 gimple_set_plf (phi
, STMT_NECESSARY
, false);
434 /* Check all statements in the block. */
435 for (gsi
= gsi_start_bb (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
437 stmt
= gsi_stmt (gsi
);
438 gimple_set_plf (stmt
, STMT_NECESSARY
, false);
439 mark_stmt_if_obviously_necessary (stmt
, el
!= NULL
);
443 /* Pure and const functions are finite and thus have no infinite loops in
445 if ((TREE_READONLY (current_function_decl
)
446 || DECL_PURE_P (current_function_decl
))
447 && !DECL_LOOPING_CONST_OR_PURE_P (current_function_decl
))
450 /* Prevent the empty possibly infinite loops from being removed. */
456 if (mark_irreducible_loops ())
460 FOR_EACH_EDGE (e
, ei
, bb
->succs
)
461 if ((e
->flags
& EDGE_DFS_BACK
)
462 && (e
->flags
& EDGE_IRREDUCIBLE_LOOP
))
465 fprintf (dump_file
, "Marking back edge of irreducible loop %i->%i\n",
466 e
->src
->index
, e
->dest
->index
);
467 mark_control_dependent_edges_necessary (e
->dest
, el
, false);
471 FOR_EACH_LOOP (li
, loop
, 0)
472 if (!finite_loop_p (loop
))
475 fprintf (dump_file
, "can not prove finiteness of loop %i\n", loop
->num
);
476 mark_control_dependent_edges_necessary (loop
->latch
, el
, false);
483 /* Return true if REF is based on an aliased base, otherwise false. */
486 ref_may_be_aliased (tree ref
)
488 while (handled_component_p (ref
))
489 ref
= TREE_OPERAND (ref
, 0);
490 return !(DECL_P (ref
)
491 && !may_be_aliased (ref
));
494 static bitmap visited
= NULL
;
495 static unsigned int longest_chain
= 0;
496 static unsigned int total_chain
= 0;
497 static unsigned int nr_walks
= 0;
498 static bool chain_ovfl
= false;
500 /* Worker for the walker that marks reaching definitions of REF,
501 which is based on a non-aliased decl, necessary. It returns
502 true whenever the defining statement of the current VDEF is
503 a kill for REF, as no dominating may-defs are necessary for REF
504 anymore. DATA points to the basic-block that contains the
505 stmt that refers to REF. */
508 mark_aliased_reaching_defs_necessary_1 (ao_ref
*ref
, tree vdef
, void *data
)
510 gimple def_stmt
= SSA_NAME_DEF_STMT (vdef
);
512 /* All stmts we visit are necessary. */
513 mark_operand_necessary (vdef
);
515 /* If the stmt lhs kills ref, then we can stop walking. */
516 if (gimple_has_lhs (def_stmt
)
517 && TREE_CODE (gimple_get_lhs (def_stmt
)) != SSA_NAME
)
519 tree base
, lhs
= gimple_get_lhs (def_stmt
);
520 HOST_WIDE_INT size
, offset
, max_size
;
522 base
= get_ref_base_and_extent (lhs
, &offset
, &size
, &max_size
);
523 /* We can get MEM[symbol: sZ, index: D.8862_1] here,
524 so base == refd->base does not always hold. */
525 if (base
== ref
->base
)
527 /* For a must-alias check we need to be able to constrain
528 the accesses properly. */
529 if (size
!= -1 && size
== max_size
530 && ref
->max_size
!= -1)
532 if (offset
<= ref
->offset
533 && offset
+ size
>= ref
->offset
+ ref
->max_size
)
536 /* Or they need to be exactly the same. */
538 /* Make sure there is no induction variable involved
539 in the references (gcc.c-torture/execute/pr42142.c).
540 The simplest way is to check if the kill dominates
542 && dominated_by_p (CDI_DOMINATORS
, (basic_block
) data
,
543 gimple_bb (def_stmt
))
544 && operand_equal_p (ref
->ref
, lhs
, 0))
549 /* Otherwise keep walking. */
554 mark_aliased_reaching_defs_necessary (gimple stmt
, tree ref
)
558 gcc_assert (!chain_ovfl
);
559 ao_ref_init (&refd
, ref
);
560 chain
= walk_aliased_vdefs (&refd
, gimple_vuse (stmt
),
561 mark_aliased_reaching_defs_necessary_1
,
562 gimple_bb (stmt
), NULL
);
563 if (chain
> longest_chain
)
564 longest_chain
= chain
;
565 total_chain
+= chain
;
569 /* Worker for the walker that marks reaching definitions of REF, which
570 is not based on a non-aliased decl. For simplicity we need to end
571 up marking all may-defs necessary that are not based on a non-aliased
572 decl. The only job of this walker is to skip may-defs based on
573 a non-aliased decl. */
576 mark_all_reaching_defs_necessary_1 (ao_ref
*ref ATTRIBUTE_UNUSED
,
577 tree vdef
, void *data ATTRIBUTE_UNUSED
)
579 gimple def_stmt
= SSA_NAME_DEF_STMT (vdef
);
581 /* We have to skip already visited (and thus necessary) statements
582 to make the chaining work after we dropped back to simple mode. */
584 && TEST_BIT (processed
, SSA_NAME_VERSION (vdef
)))
586 gcc_assert (gimple_nop_p (def_stmt
)
587 || gimple_plf (def_stmt
, STMT_NECESSARY
));
591 /* We want to skip stores to non-aliased variables. */
593 && gimple_assign_single_p (def_stmt
))
595 tree lhs
= gimple_assign_lhs (def_stmt
);
596 if (!ref_may_be_aliased (lhs
))
600 mark_operand_necessary (vdef
);
606 mark_all_reaching_defs_necessary (gimple stmt
)
608 walk_aliased_vdefs (NULL
, gimple_vuse (stmt
),
609 mark_all_reaching_defs_necessary_1
, NULL
, &visited
);
612 /* Return true for PHI nodes with one or identical arguments
615 degenerate_phi_p (gimple phi
)
618 tree op
= gimple_phi_arg_def (phi
, 0);
619 for (i
= 1; i
< gimple_phi_num_args (phi
); i
++)
620 if (gimple_phi_arg_def (phi
, i
) != op
)
625 /* Propagate necessity using the operands of necessary statements.
626 Process the uses on each statement in the worklist, and add all
627 feeding statements which contribute to the calculation of this
628 value to the worklist.
630 In conservative mode, EL is NULL. */
633 propagate_necessity (struct edge_list
*el
)
636 bool aggressive
= (el
? true : false);
638 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
639 fprintf (dump_file
, "\nProcessing worklist:\n");
641 while (VEC_length (gimple
, worklist
) > 0)
643 /* Take STMT from worklist. */
644 stmt
= VEC_pop (gimple
, worklist
);
646 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
648 fprintf (dump_file
, "processing: ");
649 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
650 fprintf (dump_file
, "\n");
655 /* Mark the last statements of the basic blocks that the block
656 containing STMT is control dependent on, but only if we haven't
658 basic_block bb
= gimple_bb (stmt
);
659 if (bb
!= ENTRY_BLOCK_PTR
660 && ! TEST_BIT (visited_control_parents
, bb
->index
))
661 mark_control_dependent_edges_necessary (bb
, el
, false);
664 if (gimple_code (stmt
) == GIMPLE_PHI
665 /* We do not process virtual PHI nodes nor do we track their
667 && is_gimple_reg (gimple_phi_result (stmt
)))
669 /* PHI nodes are somewhat special in that each PHI alternative has
670 data and control dependencies. All the statements feeding the
671 PHI node's arguments are always necessary. In aggressive mode,
672 we also consider the control dependent edges leading to the
673 predecessor block associated with each PHI alternative as
677 for (k
= 0; k
< gimple_phi_num_args (stmt
); k
++)
679 tree arg
= PHI_ARG_DEF (stmt
, k
);
680 if (TREE_CODE (arg
) == SSA_NAME
)
681 mark_operand_necessary (arg
);
684 /* For PHI operands it matters from where the control flow arrives
685 to the BB. Consider the following example:
695 We need to mark control dependence of the empty basic blocks, since they
696 contains computation of PHI operands.
698 Doing so is too restrictive in the case the predecestor block is in
704 for (i = 0; i<1000; ++i)
710 There is PHI for J in the BB containing return statement.
711 In this case the control dependence of predecestor block (that is
712 within the empty loop) also contains the block determining number
713 of iterations of the block that would prevent removing of empty
716 This scenario can be avoided by splitting critical edges.
717 To save the critical edge splitting pass we identify how the control
718 dependence would look like if the edge was split.
720 Consider the modified CFG created from current CFG by splitting
721 edge B->C. In the postdominance tree of modified CFG, C' is
722 always child of C. There are two cases how chlids of C' can look
727 In this case the only basic block C' is control dependent on is B.
729 2) C' has single child that is B
731 In this case control dependence of C' is same as control
732 dependence of B in original CFG except for block B itself.
733 (since C' postdominate B in modified CFG)
735 Now how to decide what case happens? There are two basic options:
737 a) C postdominate B. Then C immediately postdominate B and
738 case 2 happens iff there is no other way from B to C except
741 There is other way from B to C iff there is succesor of B that
742 is not postdominated by B. Testing this condition is somewhat
743 expensive, because we need to iterate all succesors of B.
744 We are safe to assume that this does not happen: we will mark B
745 as needed when processing the other path from B to C that is
746 conrol dependent on B and marking control dependencies of B
747 itself is harmless because they will be processed anyway after
748 processing control statement in B.
750 b) C does not postdominate B. Always case 1 happens since there is
751 path from C to exit that does not go through B and thus also C'. */
753 if (aggressive
&& !degenerate_phi_p (stmt
))
755 for (k
= 0; k
< gimple_phi_num_args (stmt
); k
++)
757 basic_block arg_bb
= gimple_phi_arg_edge (stmt
, k
)->src
;
760 != get_immediate_dominator (CDI_POST_DOMINATORS
, arg_bb
))
762 if (!TEST_BIT (last_stmt_necessary
, arg_bb
->index
))
765 SET_BIT (last_stmt_necessary
, arg_bb
->index
);
766 SET_BIT (bb_contains_live_stmts
, arg_bb
->index
);
768 stmt2
= last_stmt (arg_bb
);
769 if (stmt2
&& is_ctrl_stmt (stmt2
))
770 mark_stmt_necessary (stmt2
, true);
773 else if (arg_bb
!= ENTRY_BLOCK_PTR
774 && ! TEST_BIT (visited_control_parents
, arg_bb
->index
))
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
))
830 /* Calls implicitly load from memory, their arguments
831 in addition may explicitly perform memory loads. */
832 mark_all_reaching_defs_necessary (stmt
);
833 for (i
= 0; i
< gimple_call_num_args (stmt
); ++i
)
835 tree arg
= gimple_call_arg (stmt
, i
);
836 if (TREE_CODE (arg
) == SSA_NAME
837 || is_gimple_min_invariant (arg
))
839 if (!ref_may_be_aliased (arg
))
840 mark_aliased_reaching_defs_necessary (stmt
, arg
);
843 else if (gimple_assign_single_p (stmt
))
846 bool rhs_aliased
= false;
847 /* If this is a load mark things necessary. */
848 rhs
= gimple_assign_rhs1 (stmt
);
849 if (TREE_CODE (rhs
) != SSA_NAME
850 && !is_gimple_min_invariant (rhs
))
852 if (!ref_may_be_aliased (rhs
))
853 mark_aliased_reaching_defs_necessary (stmt
, rhs
);
858 mark_all_reaching_defs_necessary (stmt
);
860 else if (gimple_code (stmt
) == GIMPLE_RETURN
)
862 tree rhs
= gimple_return_retval (stmt
);
863 /* A return statement may perform a load. */
864 if (TREE_CODE (rhs
) != SSA_NAME
865 && !is_gimple_min_invariant (rhs
))
867 if (!ref_may_be_aliased (rhs
))
868 mark_aliased_reaching_defs_necessary (stmt
, rhs
);
870 mark_all_reaching_defs_necessary (stmt
);
873 else if (gimple_code (stmt
) == GIMPLE_ASM
)
876 mark_all_reaching_defs_necessary (stmt
);
877 /* Inputs may perform loads. */
878 for (i
= 0; i
< gimple_asm_ninputs (stmt
); ++i
)
880 tree op
= TREE_VALUE (gimple_asm_input_op (stmt
, i
));
881 if (TREE_CODE (op
) != SSA_NAME
882 && !is_gimple_min_invariant (op
)
883 && !ref_may_be_aliased (op
))
884 mark_aliased_reaching_defs_necessary (stmt
, op
);
890 /* If we over-used our alias oracle budget drop to simple
891 mode. The cost metric allows quadratic behavior
892 (number of uses times number of may-defs queries) up to
893 a constant maximal number of queries and after that falls back to
894 super-linear complexity. */
895 if (/* Constant but quadratic for small functions. */
896 total_chain
> 128 * 128
897 /* Linear in the number of may-defs. */
898 && total_chain
> 32 * longest_chain
899 /* Linear in the number of uses. */
900 && total_chain
> nr_walks
* 32)
904 bitmap_clear (visited
);
910 /* Replace all uses of result of PHI by underlying variable and mark it
914 mark_virtual_phi_result_for_renaming (gimple phi
)
917 imm_use_iterator iter
;
920 tree result_ssa
, result_var
;
922 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
924 fprintf (dump_file
, "Marking result for renaming : ");
925 print_gimple_stmt (dump_file
, phi
, 0, TDF_SLIM
);
926 fprintf (dump_file
, "\n");
929 result_ssa
= gimple_phi_result (phi
);
930 result_var
= SSA_NAME_VAR (result_ssa
);
931 FOR_EACH_IMM_USE_STMT (stmt
, iter
, result_ssa
)
933 FOR_EACH_IMM_USE_ON_STMT (use_p
, iter
)
934 SET_USE (use_p
, result_var
);
939 mark_sym_for_renaming (result_var
);
942 /* Remove dead PHI nodes from block BB. */
945 remove_dead_phis (basic_block bb
)
947 bool something_changed
= false;
950 gimple_stmt_iterator gsi
;
951 phis
= phi_nodes (bb
);
953 for (gsi
= gsi_start (phis
); !gsi_end_p (gsi
);)
956 phi
= gsi_stmt (gsi
);
958 /* We do not track necessity of virtual PHI nodes. Instead do
959 very simple dead PHI removal here. */
960 if (!is_gimple_reg (gimple_phi_result (phi
)))
962 /* Virtual PHI nodes with one or identical arguments
964 if (degenerate_phi_p (phi
))
966 tree vdef
= gimple_phi_result (phi
);
967 tree vuse
= gimple_phi_arg_def (phi
, 0);
970 imm_use_iterator iter
;
972 FOR_EACH_IMM_USE_STMT (use_stmt
, iter
, vdef
)
973 FOR_EACH_IMM_USE_ON_STMT (use_p
, iter
)
974 SET_USE (use_p
, vuse
);
975 if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vdef
)
976 && TREE_CODE (vuse
) == SSA_NAME
)
977 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vuse
) = 1;
980 gimple_set_plf (phi
, STMT_NECESSARY
, true);
983 if (!gimple_plf (phi
, STMT_NECESSARY
))
985 something_changed
= true;
986 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
988 fprintf (dump_file
, "Deleting : ");
989 print_gimple_stmt (dump_file
, phi
, 0, TDF_SLIM
);
990 fprintf (dump_file
, "\n");
993 remove_phi_node (&gsi
, true);
994 stats
.removed_phis
++;
1000 return something_changed
;
1003 /* Forward edge E to respective POST_DOM_BB and update PHIs. */
1006 forward_edge_to_pdom (edge e
, basic_block post_dom_bb
)
1008 gimple_stmt_iterator gsi
;
1012 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1013 fprintf (dump_file
, "Redirecting edge %i->%i to %i\n", e
->src
->index
,
1014 e
->dest
->index
, post_dom_bb
->index
);
1016 e2
= redirect_edge_and_branch (e
, post_dom_bb
);
1019 /* If edge was already around, no updating is neccesary. */
1023 if (!gimple_seq_empty_p (phi_nodes (post_dom_bb
)))
1025 /* We are sure that for every live PHI we are seeing control dependent BB.
1026 This means that we can pick any edge to duplicate PHI args from. */
1027 FOR_EACH_EDGE (e2
, ei
, post_dom_bb
->preds
)
1030 for (gsi
= gsi_start_phis (post_dom_bb
); !gsi_end_p (gsi
);)
1032 gimple phi
= gsi_stmt (gsi
);
1034 source_location locus
;
1036 /* PHIs for virtuals have no control dependency relation on them.
1037 We are lost here and must force renaming of the symbol. */
1038 if (!is_gimple_reg (gimple_phi_result (phi
)))
1040 mark_virtual_phi_result_for_renaming (phi
);
1041 remove_phi_node (&gsi
, true);
1045 /* Dead PHI do not imply control dependency. */
1046 if (!gimple_plf (phi
, STMT_NECESSARY
))
1052 op
= gimple_phi_arg_def (phi
, e2
->dest_idx
);
1053 locus
= gimple_phi_arg_location (phi
, e2
->dest_idx
);
1054 add_phi_arg (phi
, op
, e
, locus
);
1055 /* The resulting PHI if not dead can only be degenerate. */
1056 gcc_assert (degenerate_phi_p (phi
));
1063 /* Remove dead statement pointed to by iterator I. Receives the basic block BB
1064 containing I so that we don't have to look it up. */
1067 remove_dead_stmt (gimple_stmt_iterator
*i
, basic_block bb
)
1069 gimple stmt
= gsi_stmt (*i
);
1071 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1073 fprintf (dump_file
, "Deleting : ");
1074 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
1075 fprintf (dump_file
, "\n");
1080 /* If we have determined that a conditional branch statement contributes
1081 nothing to the program, then we not only remove it, but we also change
1082 the flow graph so that the current block will simply fall-thru to its
1083 immediate post-dominator. The blocks we are circumventing will be
1084 removed by cleanup_tree_cfg if this change in the flow graph makes them
1086 if (is_ctrl_stmt (stmt
))
1088 basic_block post_dom_bb
;
1092 post_dom_bb
= get_immediate_dominator (CDI_POST_DOMINATORS
, bb
);
1094 e
= find_edge (bb
, post_dom_bb
);
1096 /* If edge is already there, try to use it. This avoids need to update
1097 PHI nodes. Also watch for cases where post dominator does not exists
1098 or is exit block. These can happen for infinite loops as we create
1099 fake edges in the dominator tree. */
1102 else if (! post_dom_bb
|| post_dom_bb
== EXIT_BLOCK_PTR
)
1103 e
= EDGE_SUCC (bb
, 0);
1105 e
= forward_edge_to_pdom (EDGE_SUCC (bb
, 0), post_dom_bb
);
1107 e
->probability
= REG_BR_PROB_BASE
;
1108 e
->count
= bb
->count
;
1110 /* The edge is no longer associated with a conditional, so it does
1111 not have TRUE/FALSE flags. */
1112 e
->flags
&= ~(EDGE_TRUE_VALUE
| EDGE_FALSE_VALUE
);
1114 /* The lone outgoing edge from BB will be a fallthru edge. */
1115 e
->flags
|= EDGE_FALLTHRU
;
1117 /* Remove the remaining outgoing edges. */
1118 for (ei
= ei_start (bb
->succs
); (e2
= ei_safe_edge (ei
)); )
1128 unlink_stmt_vdef (stmt
);
1129 gsi_remove (i
, true);
1130 release_defs (stmt
);
1133 /* Eliminate unnecessary statements. Any instruction not marked as necessary
1134 contributes nothing to the program, and can be deleted. */
1137 eliminate_unnecessary_stmts (void)
1139 bool something_changed
= false;
1141 gimple_stmt_iterator gsi
, psi
;
1144 VEC (basic_block
, heap
) *h
;
1146 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1147 fprintf (dump_file
, "\nEliminating unnecessary statements:\n");
1149 clear_special_calls ();
1151 /* Walking basic blocks and statements in reverse order avoids
1152 releasing SSA names before any other DEFs that refer to them are
1153 released. This helps avoid loss of debug information, as we get
1154 a chance to propagate all RHSs of removed SSAs into debug uses,
1155 rather than only the latest ones. E.g., consider:
1161 If we were to release x_3 before a_5, when we reached a_5 and
1162 tried to substitute it into the debug stmt, we'd see x_3 there,
1163 but x_3's DEF, type, etc would have already been disconnected.
1164 By going backwards, the debug stmt first changes to:
1166 # DEBUG a => x_3 - b_4
1170 # DEBUG a => y_1 + z_2 - b_4
1173 gcc_assert (dom_info_available_p (CDI_DOMINATORS
));
1174 h
= get_all_dominated_blocks (CDI_DOMINATORS
, single_succ (ENTRY_BLOCK_PTR
));
1176 while (VEC_length (basic_block
, h
))
1178 bb
= VEC_pop (basic_block
, h
);
1180 /* Remove dead statements. */
1181 for (gsi
= gsi_last_bb (bb
); !gsi_end_p (gsi
); gsi
= psi
)
1183 stmt
= gsi_stmt (gsi
);
1190 /* If GSI is not necessary then remove it. */
1191 if (!gimple_plf (stmt
, STMT_NECESSARY
))
1193 if (!is_gimple_debug (stmt
))
1194 something_changed
= true;
1195 remove_dead_stmt (&gsi
, bb
);
1197 else if (is_gimple_call (stmt
))
1199 call
= gimple_call_fndecl (stmt
);
1204 /* When LHS of var = call (); is dead, simplify it into
1205 call (); saving one operand. */
1206 name
= gimple_call_lhs (stmt
);
1207 if (name
&& TREE_CODE (name
) == SSA_NAME
1208 && !TEST_BIT (processed
, SSA_NAME_VERSION (name
)))
1210 something_changed
= true;
1211 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1213 fprintf (dump_file
, "Deleting LHS of call: ");
1214 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
1215 fprintf (dump_file
, "\n");
1218 gimple_call_set_lhs (stmt
, NULL_TREE
);
1219 maybe_clean_or_replace_eh_stmt (stmt
, stmt
);
1221 release_ssa_name (name
);
1223 notice_special_calls (stmt
);
1229 VEC_free (basic_block
, heap
, h
);
1231 /* Since we don't track liveness of virtual PHI nodes, it is possible that we
1232 rendered some PHI nodes unreachable while they are still in use.
1233 Mark them for renaming. */
1236 basic_block prev_bb
;
1238 find_unreachable_blocks ();
1240 /* Delete all unreachable basic blocks in reverse dominator order. */
1241 for (bb
= EXIT_BLOCK_PTR
->prev_bb
; bb
!= ENTRY_BLOCK_PTR
; bb
= prev_bb
)
1243 prev_bb
= bb
->prev_bb
;
1245 if (!TEST_BIT (bb_contains_live_stmts
, bb
->index
)
1246 || !(bb
->flags
& BB_REACHABLE
))
1248 for (gsi
= gsi_start_phis (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
1249 if (!is_gimple_reg (gimple_phi_result (gsi_stmt (gsi
))))
1252 imm_use_iterator iter
;
1254 FOR_EACH_IMM_USE_STMT (stmt
, iter
, gimple_phi_result (gsi_stmt (gsi
)))
1256 if (!(gimple_bb (stmt
)->flags
& BB_REACHABLE
))
1258 if (gimple_code (stmt
) == GIMPLE_PHI
1259 || gimple_plf (stmt
, STMT_NECESSARY
))
1262 BREAK_FROM_IMM_USE_STMT (iter
);
1266 mark_virtual_phi_result_for_renaming (gsi_stmt (gsi
));
1269 if (!(bb
->flags
& BB_REACHABLE
))
1271 /* Speed up the removal of blocks that don't
1272 dominate others. Walking backwards, this should
1273 be the common case. ??? Do we need to recompute
1274 dominators because of cfg_altered? */
1275 if (!MAY_HAVE_DEBUG_STMTS
1276 || !first_dom_son (CDI_DOMINATORS
, bb
))
1277 delete_basic_block (bb
);
1280 h
= get_all_dominated_blocks (CDI_DOMINATORS
, bb
);
1282 while (VEC_length (basic_block
, h
))
1284 bb
= VEC_pop (basic_block
, h
);
1285 prev_bb
= bb
->prev_bb
;
1286 /* Rearrangements to the CFG may have failed
1287 to update the dominators tree, so that
1288 formerly-dominated blocks are now
1289 otherwise reachable. */
1290 if (!!(bb
->flags
& BB_REACHABLE
))
1292 delete_basic_block (bb
);
1295 VEC_free (basic_block
, heap
, h
);
1303 /* Remove dead PHI nodes. */
1304 something_changed
|= remove_dead_phis (bb
);
1307 return something_changed
;
1311 /* Print out removed statement statistics. */
1318 percg
= ((float) stats
.removed
/ (float) stats
.total
) * 100;
1319 fprintf (dump_file
, "Removed %d of %d statements (%d%%)\n",
1320 stats
.removed
, stats
.total
, (int) percg
);
1322 if (stats
.total_phis
== 0)
1325 percg
= ((float) stats
.removed_phis
/ (float) stats
.total_phis
) * 100;
1327 fprintf (dump_file
, "Removed %d of %d PHI nodes (%d%%)\n",
1328 stats
.removed_phis
, stats
.total_phis
, (int) percg
);
1331 /* Initialization for this pass. Set up the used data structures. */
1334 tree_dce_init (bool aggressive
)
1336 memset ((void *) &stats
, 0, sizeof (stats
));
1342 control_dependence_map
= XNEWVEC (bitmap
, last_basic_block
);
1343 for (i
= 0; i
< last_basic_block
; ++i
)
1344 control_dependence_map
[i
] = BITMAP_ALLOC (NULL
);
1346 last_stmt_necessary
= sbitmap_alloc (last_basic_block
);
1347 sbitmap_zero (last_stmt_necessary
);
1348 bb_contains_live_stmts
= sbitmap_alloc (last_basic_block
);
1349 sbitmap_zero (bb_contains_live_stmts
);
1352 processed
= sbitmap_alloc (num_ssa_names
+ 1);
1353 sbitmap_zero (processed
);
1355 worklist
= VEC_alloc (gimple
, heap
, 64);
1356 cfg_altered
= false;
1359 /* Cleanup after this pass. */
1362 tree_dce_done (bool aggressive
)
1368 for (i
= 0; i
< last_basic_block
; ++i
)
1369 BITMAP_FREE (control_dependence_map
[i
]);
1370 free (control_dependence_map
);
1372 sbitmap_free (visited_control_parents
);
1373 sbitmap_free (last_stmt_necessary
);
1374 sbitmap_free (bb_contains_live_stmts
);
1375 bb_contains_live_stmts
= NULL
;
1378 sbitmap_free (processed
);
1380 VEC_free (gimple
, heap
, worklist
);
1383 /* Main routine to eliminate dead code.
1385 AGGRESSIVE controls the aggressiveness of the algorithm.
1386 In conservative mode, we ignore control dependence and simply declare
1387 all but the most trivially dead branches necessary. This mode is fast.
1388 In aggressive mode, control dependences are taken into account, which
1389 results in more dead code elimination, but at the cost of some time.
1391 FIXME: Aggressive mode before PRE doesn't work currently because
1392 the dominance info is not invalidated after DCE1. This is
1393 not an issue right now because we only run aggressive DCE
1394 as the last tree SSA pass, but keep this in mind when you
1395 start experimenting with pass ordering. */
1398 perform_tree_ssa_dce (bool aggressive
)
1400 struct edge_list
*el
= NULL
;
1401 bool something_changed
= 0;
1403 /* Preheaders are needed for SCEV to work.
1404 Simple lateches and recorded exits improve chances that loop will
1405 proved to be finite in testcases such as in loop-15.c and loop-24.c */
1407 loop_optimizer_init (LOOPS_NORMAL
1408 | LOOPS_HAVE_RECORDED_EXITS
);
1410 tree_dce_init (aggressive
);
1414 /* Compute control dependence. */
1415 timevar_push (TV_CONTROL_DEPENDENCES
);
1416 calculate_dominance_info (CDI_POST_DOMINATORS
);
1417 el
= create_edge_list ();
1418 find_all_control_dependences (el
);
1419 timevar_pop (TV_CONTROL_DEPENDENCES
);
1421 visited_control_parents
= sbitmap_alloc (last_basic_block
);
1422 sbitmap_zero (visited_control_parents
);
1424 mark_dfs_back_edges ();
1427 find_obviously_necessary_stmts (el
);
1430 loop_optimizer_finalize ();
1436 visited
= BITMAP_ALLOC (NULL
);
1437 propagate_necessity (el
);
1438 BITMAP_FREE (visited
);
1440 something_changed
|= eliminate_unnecessary_stmts ();
1441 something_changed
|= cfg_altered
;
1443 /* We do not update postdominators, so free them unconditionally. */
1444 free_dominance_info (CDI_POST_DOMINATORS
);
1446 /* If we removed paths in the CFG, then we need to update
1447 dominators as well. I haven't investigated the possibility
1448 of incrementally updating dominators. */
1450 free_dominance_info (CDI_DOMINATORS
);
1452 statistics_counter_event (cfun
, "Statements deleted", stats
.removed
);
1453 statistics_counter_event (cfun
, "PHI nodes deleted", stats
.removed_phis
);
1455 /* Debugging dumps. */
1456 if (dump_file
&& (dump_flags
& (TDF_STATS
|TDF_DETAILS
)))
1459 tree_dce_done (aggressive
);
1461 free_edge_list (el
);
1463 if (something_changed
)
1464 return (TODO_update_ssa
| TODO_cleanup_cfg
| TODO_ggc_collect
1465 | TODO_remove_unused_locals
);
1470 /* Pass entry points. */
1474 return perform_tree_ssa_dce (/*aggressive=*/false);
1478 tree_ssa_dce_loop (void)
1481 todo
= perform_tree_ssa_dce (/*aggressive=*/false);
1484 free_numbers_of_iterations_estimates ();
1491 tree_ssa_cd_dce (void)
1493 return perform_tree_ssa_dce (/*aggressive=*/optimize
>= 2);
1499 return flag_tree_dce
!= 0;
1502 struct gimple_opt_pass pass_dce
=
1507 gate_dce
, /* gate */
1508 tree_ssa_dce
, /* execute */
1511 0, /* static_pass_number */
1512 TV_TREE_DCE
, /* tv_id */
1513 PROP_cfg
| PROP_ssa
, /* properties_required */
1514 0, /* properties_provided */
1515 0, /* properties_destroyed */
1516 0, /* todo_flags_start */
1517 TODO_dump_func
| TODO_verify_ssa
/* todo_flags_finish */
1521 struct gimple_opt_pass pass_dce_loop
=
1525 "dceloop", /* name */
1526 gate_dce
, /* gate */
1527 tree_ssa_dce_loop
, /* execute */
1530 0, /* static_pass_number */
1531 TV_TREE_DCE
, /* tv_id */
1532 PROP_cfg
| PROP_ssa
, /* properties_required */
1533 0, /* properties_provided */
1534 0, /* properties_destroyed */
1535 0, /* todo_flags_start */
1536 TODO_dump_func
| TODO_verify_ssa
/* todo_flags_finish */
1540 struct gimple_opt_pass pass_cd_dce
=
1545 gate_dce
, /* gate */
1546 tree_ssa_cd_dce
, /* execute */
1549 0, /* static_pass_number */
1550 TV_TREE_CD_DCE
, /* tv_id */
1551 PROP_cfg
| PROP_ssa
, /* properties_required */
1552 0, /* properties_provided */
1553 0, /* properties_destroyed */
1554 0, /* todo_flags_start */
1555 TODO_dump_func
| TODO_verify_ssa
1556 | TODO_verify_flow
/* todo_flags_finish */