1 /* Dead code elimination pass for the GNU compiler.
2 Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011
3 Free Software Foundation, Inc.
4 Contributed by Ben Elliston <bje@redhat.com>
5 and Andrew MacLeod <amacleod@redhat.com>
6 Adapted to use control dependence by Steven Bosscher, SUSE Labs.
8 This file is part of GCC.
10 GCC is free software; you can redistribute it and/or modify it
11 under the terms of the GNU General Public License as published by the
12 Free Software Foundation; either version 3, or (at your option) any
15 GCC is distributed in the hope that it will be useful, but WITHOUT
16 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
17 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
20 You should have received a copy of the GNU General Public License
21 along with GCC; see the file COPYING3. If not see
22 <http://www.gnu.org/licenses/>. */
24 /* Dead code elimination.
28 Building an Optimizing Compiler,
29 Robert Morgan, Butterworth-Heinemann, 1998, Section 8.9.
31 Advanced Compiler Design and Implementation,
32 Steven Muchnick, Morgan Kaufmann, 1997, Section 18.10.
34 Dead-code elimination is the removal of statements which have no
35 impact on the program's output. "Dead statements" have no impact
36 on the program's output, while "necessary statements" may have
39 The algorithm consists of three phases:
40 1. Marking as necessary all statements known to be necessary,
41 e.g. most function calls, writing a value to memory, etc;
42 2. Propagating necessary statements, e.g., the statements
43 giving values to operands in necessary statements; and
44 3. Removing dead statements. */
48 #include "coretypes.h"
52 #include "tree-pretty-print.h"
53 #include "gimple-pretty-print.h"
54 #include "basic-block.h"
55 #include "tree-flow.h"
57 #include "tree-dump.h"
58 #include "tree-pass.h"
62 #include "tree-scalar-evolution.h"
64 static struct stmt_stats
72 #define STMT_NECESSARY GF_PLF_1
74 static VEC(gimple
,heap
) *worklist
;
76 /* Vector indicating an SSA name has already been processed and marked
78 static sbitmap processed
;
80 /* Vector indicating that the last statement of a basic block has already
81 been marked as necessary. */
82 static sbitmap last_stmt_necessary
;
84 /* Vector indicating that BB contains statements that are live. */
85 static sbitmap bb_contains_live_stmts
;
87 /* Before we can determine whether a control branch is dead, we need to
88 compute which blocks are control dependent on which edges.
90 We expect each block to be control dependent on very few edges so we
91 use a bitmap for each block recording its edges. An array holds the
92 bitmap. The Ith bit in the bitmap is set if that block is dependent
94 static bitmap
*control_dependence_map
;
96 /* Vector indicating that a basic block has already had all the edges
97 processed that it is control dependent on. */
98 static sbitmap visited_control_parents
;
100 /* TRUE if this pass alters the CFG (by removing control statements).
103 If this pass alters the CFG, then it will arrange for the dominators
105 static bool cfg_altered
;
107 /* Execute code that follows the macro for each edge (given number
108 EDGE_NUMBER within the CODE) for which the block with index N is
109 control dependent. */
110 #define EXECUTE_IF_CONTROL_DEPENDENT(BI, N, EDGE_NUMBER) \
111 EXECUTE_IF_SET_IN_BITMAP (control_dependence_map[(N)], 0, \
115 /* Indicate block BB is control dependent on an edge with index EDGE_INDEX. */
117 set_control_dependence_map_bit (basic_block bb
, int edge_index
)
119 if (bb
== ENTRY_BLOCK_PTR
)
121 gcc_assert (bb
!= EXIT_BLOCK_PTR
);
122 bitmap_set_bit (control_dependence_map
[bb
->index
], edge_index
);
125 /* Clear all control dependences for block BB. */
127 clear_control_dependence_bitmap (basic_block bb
)
129 bitmap_clear (control_dependence_map
[bb
->index
]);
133 /* Find the immediate postdominator PDOM of the specified basic block BLOCK.
134 This function is necessary because some blocks have negative numbers. */
136 static inline basic_block
137 find_pdom (basic_block block
)
139 gcc_assert (block
!= ENTRY_BLOCK_PTR
);
141 if (block
== EXIT_BLOCK_PTR
)
142 return EXIT_BLOCK_PTR
;
145 basic_block bb
= get_immediate_dominator (CDI_POST_DOMINATORS
, block
);
147 return EXIT_BLOCK_PTR
;
153 /* Determine all blocks' control dependences on the given edge with edge_list
154 EL index EDGE_INDEX, ala Morgan, Section 3.6. */
157 find_control_dependence (struct edge_list
*el
, int edge_index
)
159 basic_block current_block
;
160 basic_block ending_block
;
162 gcc_assert (INDEX_EDGE_PRED_BB (el
, edge_index
) != EXIT_BLOCK_PTR
);
164 if (INDEX_EDGE_PRED_BB (el
, edge_index
) == ENTRY_BLOCK_PTR
)
165 ending_block
= single_succ (ENTRY_BLOCK_PTR
);
167 ending_block
= find_pdom (INDEX_EDGE_PRED_BB (el
, edge_index
));
169 for (current_block
= INDEX_EDGE_SUCC_BB (el
, edge_index
);
170 current_block
!= ending_block
&& current_block
!= EXIT_BLOCK_PTR
;
171 current_block
= find_pdom (current_block
))
173 edge e
= INDEX_EDGE (el
, edge_index
);
175 /* For abnormal edges, we don't make current_block control
176 dependent because instructions that throw are always necessary
178 if (e
->flags
& EDGE_ABNORMAL
)
181 set_control_dependence_map_bit (current_block
, edge_index
);
186 /* Record all blocks' control dependences on all edges in the edge
187 list EL, ala Morgan, Section 3.6. */
190 find_all_control_dependences (struct edge_list
*el
)
194 for (i
= 0; i
< NUM_EDGES (el
); ++i
)
195 find_control_dependence (el
, i
);
198 /* If STMT is not already marked necessary, mark it, and add it to the
199 worklist if ADD_TO_WORKLIST is true. */
202 mark_stmt_necessary (gimple stmt
, bool add_to_worklist
)
206 if (gimple_plf (stmt
, STMT_NECESSARY
))
209 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
211 fprintf (dump_file
, "Marking useful stmt: ");
212 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
213 fprintf (dump_file
, "\n");
216 gimple_set_plf (stmt
, STMT_NECESSARY
, true);
218 VEC_safe_push (gimple
, heap
, worklist
, stmt
);
219 if (bb_contains_live_stmts
&& !is_gimple_debug (stmt
))
220 SET_BIT (bb_contains_live_stmts
, gimple_bb (stmt
)->index
);
224 /* Mark the statement defining operand OP as necessary. */
227 mark_operand_necessary (tree op
)
234 ver
= SSA_NAME_VERSION (op
);
235 if (TEST_BIT (processed
, ver
))
237 stmt
= SSA_NAME_DEF_STMT (op
);
238 gcc_assert (gimple_nop_p (stmt
)
239 || gimple_plf (stmt
, STMT_NECESSARY
));
242 SET_BIT (processed
, ver
);
244 stmt
= SSA_NAME_DEF_STMT (op
);
247 if (gimple_plf (stmt
, STMT_NECESSARY
) || gimple_nop_p (stmt
))
250 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
252 fprintf (dump_file
, "marking necessary through ");
253 print_generic_expr (dump_file
, op
, 0);
254 fprintf (dump_file
, " stmt ");
255 print_gimple_stmt (dump_file
, stmt
, 0, 0);
258 gimple_set_plf (stmt
, STMT_NECESSARY
, true);
259 if (bb_contains_live_stmts
)
260 SET_BIT (bb_contains_live_stmts
, gimple_bb (stmt
)->index
);
261 VEC_safe_push (gimple
, heap
, worklist
, stmt
);
265 /* Mark STMT as necessary if it obviously is. Add it to the worklist if
266 it can make other statements necessary.
268 If AGGRESSIVE is false, control statements are conservatively marked as
272 mark_stmt_if_obviously_necessary (gimple stmt
, bool aggressive
)
274 /* With non-call exceptions, we have to assume that all statements could
275 throw. If a statement may throw, it is inherently necessary. */
276 if (cfun
->can_throw_non_call_exceptions
&& stmt_could_throw_p (stmt
))
278 mark_stmt_necessary (stmt
, true);
282 /* Statements that are implicitly live. Most function calls, asm
283 and return statements are required. Labels and GIMPLE_BIND nodes
284 are kept because they are control flow, and we have no way of
285 knowing whether they can be removed. DCE can eliminate all the
286 other statements in a block, and CFG can then remove the block
288 switch (gimple_code (stmt
))
292 mark_stmt_necessary (stmt
, false);
298 mark_stmt_necessary (stmt
, true);
302 /* Most, but not all function calls are required. Function calls that
303 produce no result and have no side effects (i.e. const pure
304 functions) are unnecessary. */
305 if (gimple_has_side_effects (stmt
))
307 mark_stmt_necessary (stmt
, true);
310 if (!gimple_call_lhs (stmt
))
315 /* Debug temps without a value are not useful. ??? If we could
316 easily locate the debug temp bind stmt for a use thereof,
317 would could refrain from marking all debug temps here, and
318 mark them only if they're used. */
319 if (!gimple_debug_bind_p (stmt
)
320 || gimple_debug_bind_has_value_p (stmt
)
321 || TREE_CODE (gimple_debug_bind_get_var (stmt
)) != DEBUG_EXPR_DECL
)
322 mark_stmt_necessary (stmt
, false);
326 gcc_assert (!simple_goto_p (stmt
));
327 mark_stmt_necessary (stmt
, true);
331 gcc_assert (EDGE_COUNT (gimple_bb (stmt
)->succs
) == 2);
336 mark_stmt_necessary (stmt
, true);
343 /* If the statement has volatile operands, it needs to be preserved.
344 Same for statements that can alter control flow in unpredictable
346 if (gimple_has_volatile_ops (stmt
) || is_ctrl_altering_stmt (stmt
))
348 mark_stmt_necessary (stmt
, true);
352 if (is_hidden_global_store (stmt
))
354 mark_stmt_necessary (stmt
, true);
362 /* Mark the last statement of BB as necessary. */
365 mark_last_stmt_necessary (basic_block bb
)
367 gimple stmt
= last_stmt (bb
);
369 SET_BIT (last_stmt_necessary
, bb
->index
);
370 SET_BIT (bb_contains_live_stmts
, bb
->index
);
372 /* We actually mark the statement only if it is a control statement. */
373 if (stmt
&& is_ctrl_stmt (stmt
))
374 mark_stmt_necessary (stmt
, true);
378 /* Mark control dependent edges of BB as necessary. We have to do this only
379 once for each basic block so we set the appropriate bit after we're done.
381 When IGNORE_SELF is true, ignore BB in the list of control dependences. */
384 mark_control_dependent_edges_necessary (basic_block bb
, struct edge_list
*el
,
388 unsigned edge_number
;
389 bool skipped
= false;
391 gcc_assert (bb
!= EXIT_BLOCK_PTR
);
393 if (bb
== ENTRY_BLOCK_PTR
)
396 EXECUTE_IF_CONTROL_DEPENDENT (bi
, bb
->index
, edge_number
)
398 basic_block cd_bb
= INDEX_EDGE_PRED_BB (el
, edge_number
);
400 if (ignore_self
&& cd_bb
== bb
)
406 if (!TEST_BIT (last_stmt_necessary
, cd_bb
->index
))
407 mark_last_stmt_necessary (cd_bb
);
411 SET_BIT (visited_control_parents
, bb
->index
);
415 /* Find obviously necessary statements. These are things like most function
416 calls, and stores to file level variables.
418 If EL is NULL, control statements are conservatively marked as
419 necessary. Otherwise it contains the list of edges used by control
420 dependence analysis. */
423 find_obviously_necessary_stmts (struct edge_list
*el
)
426 gimple_stmt_iterator gsi
;
433 /* PHI nodes are never inherently necessary. */
434 for (gsi
= gsi_start_phis (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
436 phi
= gsi_stmt (gsi
);
437 gimple_set_plf (phi
, STMT_NECESSARY
, false);
440 /* Check all statements in the block. */
441 for (gsi
= gsi_start_bb (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
443 stmt
= gsi_stmt (gsi
);
444 gimple_set_plf (stmt
, STMT_NECESSARY
, false);
445 mark_stmt_if_obviously_necessary (stmt
, el
!= NULL
);
449 /* Pure and const functions are finite and thus have no infinite loops in
451 flags
= flags_from_decl_or_type (current_function_decl
);
452 if ((flags
& (ECF_CONST
|ECF_PURE
)) && !(flags
& ECF_LOOPING_CONST_OR_PURE
))
455 /* Prevent the empty possibly infinite loops from being removed. */
461 if (mark_irreducible_loops ())
465 FOR_EACH_EDGE (e
, ei
, bb
->succs
)
466 if ((e
->flags
& EDGE_DFS_BACK
)
467 && (e
->flags
& EDGE_IRREDUCIBLE_LOOP
))
470 fprintf (dump_file
, "Marking back edge of irreducible loop %i->%i\n",
471 e
->src
->index
, e
->dest
->index
);
472 mark_control_dependent_edges_necessary (e
->dest
, el
, false);
476 FOR_EACH_LOOP (li
, loop
, 0)
477 if (!finite_loop_p (loop
))
480 fprintf (dump_file
, "can not prove finiteness of loop %i\n", loop
->num
);
481 mark_control_dependent_edges_necessary (loop
->latch
, el
, false);
488 /* Return true if REF is based on an aliased base, otherwise false. */
491 ref_may_be_aliased (tree ref
)
493 while (handled_component_p (ref
))
494 ref
= TREE_OPERAND (ref
, 0);
495 if (TREE_CODE (ref
) == MEM_REF
496 && TREE_CODE (TREE_OPERAND (ref
, 0)) == ADDR_EXPR
)
497 ref
= TREE_OPERAND (TREE_OPERAND (ref
, 0), 0);
498 return !(DECL_P (ref
)
499 && !may_be_aliased (ref
));
502 static bitmap visited
= NULL
;
503 static unsigned int longest_chain
= 0;
504 static unsigned int total_chain
= 0;
505 static unsigned int nr_walks
= 0;
506 static bool chain_ovfl
= false;
508 /* Worker for the walker that marks reaching definitions of REF,
509 which is based on a non-aliased decl, necessary. It returns
510 true whenever the defining statement of the current VDEF is
511 a kill for REF, as no dominating may-defs are necessary for REF
512 anymore. DATA points to the basic-block that contains the
513 stmt that refers to REF. */
516 mark_aliased_reaching_defs_necessary_1 (ao_ref
*ref
, tree vdef
, void *data
)
518 gimple def_stmt
= SSA_NAME_DEF_STMT (vdef
);
520 /* All stmts we visit are necessary. */
521 mark_operand_necessary (vdef
);
523 /* If the stmt lhs kills ref, then we can stop walking. */
524 if (gimple_has_lhs (def_stmt
)
525 && TREE_CODE (gimple_get_lhs (def_stmt
)) != SSA_NAME
526 /* The assignment is not necessarily carried out if it can throw
527 and we can catch it in the current function where we could inspect
529 ??? We only need to care about the RHS throwing. For aggregate
530 assignments or similar calls and non-call exceptions the LHS
531 might throw as well. */
532 && !stmt_can_throw_internal (def_stmt
))
534 tree base
, lhs
= gimple_get_lhs (def_stmt
);
535 HOST_WIDE_INT size
, offset
, max_size
;
537 base
= get_ref_base_and_extent (lhs
, &offset
, &size
, &max_size
);
538 /* We can get MEM[symbol: sZ, index: D.8862_1] here,
539 so base == refd->base does not always hold. */
540 if (base
== ref
->base
)
542 /* For a must-alias check we need to be able to constrain
543 the accesses properly. */
544 if (size
!= -1 && size
== max_size
545 && ref
->max_size
!= -1)
547 if (offset
<= ref
->offset
548 && offset
+ size
>= ref
->offset
+ ref
->max_size
)
551 /* Or they need to be exactly the same. */
553 /* Make sure there is no induction variable involved
554 in the references (gcc.c-torture/execute/pr42142.c).
555 The simplest way is to check if the kill dominates
557 && dominated_by_p (CDI_DOMINATORS
, (basic_block
) data
,
558 gimple_bb (def_stmt
))
559 && operand_equal_p (ref
->ref
, lhs
, 0))
564 /* Otherwise keep walking. */
569 mark_aliased_reaching_defs_necessary (gimple stmt
, tree ref
)
573 gcc_assert (!chain_ovfl
);
574 ao_ref_init (&refd
, ref
);
575 chain
= walk_aliased_vdefs (&refd
, gimple_vuse (stmt
),
576 mark_aliased_reaching_defs_necessary_1
,
577 gimple_bb (stmt
), NULL
);
578 if (chain
> longest_chain
)
579 longest_chain
= chain
;
580 total_chain
+= chain
;
584 /* Worker for the walker that marks reaching definitions of REF, which
585 is not based on a non-aliased decl. For simplicity we need to end
586 up marking all may-defs necessary that are not based on a non-aliased
587 decl. The only job of this walker is to skip may-defs based on
588 a non-aliased decl. */
591 mark_all_reaching_defs_necessary_1 (ao_ref
*ref ATTRIBUTE_UNUSED
,
592 tree vdef
, void *data ATTRIBUTE_UNUSED
)
594 gimple def_stmt
= SSA_NAME_DEF_STMT (vdef
);
596 /* We have to skip already visited (and thus necessary) statements
597 to make the chaining work after we dropped back to simple mode. */
599 && TEST_BIT (processed
, SSA_NAME_VERSION (vdef
)))
601 gcc_assert (gimple_nop_p (def_stmt
)
602 || gimple_plf (def_stmt
, STMT_NECESSARY
));
606 /* We want to skip stores to non-aliased variables. */
608 && gimple_assign_single_p (def_stmt
))
610 tree lhs
= gimple_assign_lhs (def_stmt
);
611 if (!ref_may_be_aliased (lhs
))
615 mark_operand_necessary (vdef
);
621 mark_all_reaching_defs_necessary (gimple stmt
)
623 walk_aliased_vdefs (NULL
, gimple_vuse (stmt
),
624 mark_all_reaching_defs_necessary_1
, NULL
, &visited
);
627 /* Return true for PHI nodes with one or identical arguments
630 degenerate_phi_p (gimple phi
)
633 tree op
= gimple_phi_arg_def (phi
, 0);
634 for (i
= 1; i
< gimple_phi_num_args (phi
); i
++)
635 if (gimple_phi_arg_def (phi
, i
) != op
)
640 /* Propagate necessity using the operands of necessary statements.
641 Process the uses on each statement in the worklist, and add all
642 feeding statements which contribute to the calculation of this
643 value to the worklist.
645 In conservative mode, EL is NULL. */
648 propagate_necessity (struct edge_list
*el
)
651 bool aggressive
= (el
? true : false);
653 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
654 fprintf (dump_file
, "\nProcessing worklist:\n");
656 while (VEC_length (gimple
, worklist
) > 0)
658 /* Take STMT from worklist. */
659 stmt
= VEC_pop (gimple
, worklist
);
661 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
663 fprintf (dump_file
, "processing: ");
664 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
665 fprintf (dump_file
, "\n");
670 /* Mark the last statement of the basic blocks on which the block
671 containing STMT is control dependent, but only if we haven't
673 basic_block bb
= gimple_bb (stmt
);
674 if (bb
!= ENTRY_BLOCK_PTR
675 && !TEST_BIT (visited_control_parents
, bb
->index
))
676 mark_control_dependent_edges_necessary (bb
, el
, false);
679 if (gimple_code (stmt
) == GIMPLE_PHI
680 /* We do not process virtual PHI nodes nor do we track their
682 && is_gimple_reg (gimple_phi_result (stmt
)))
684 /* PHI nodes are somewhat special in that each PHI alternative has
685 data and control dependencies. All the statements feeding the
686 PHI node's arguments are always necessary. In aggressive mode,
687 we also consider the control dependent edges leading to the
688 predecessor block associated with each PHI alternative as
692 for (k
= 0; k
< gimple_phi_num_args (stmt
); k
++)
694 tree arg
= PHI_ARG_DEF (stmt
, k
);
695 if (TREE_CODE (arg
) == SSA_NAME
)
696 mark_operand_necessary (arg
);
699 /* For PHI operands it matters from where the control flow arrives
700 to the BB. Consider the following example:
710 We need to mark control dependence of the empty basic blocks, since they
711 contains computation of PHI operands.
713 Doing so is too restrictive in the case the predecestor block is in
719 for (i = 0; i<1000; ++i)
725 There is PHI for J in the BB containing return statement.
726 In this case the control dependence of predecestor block (that is
727 within the empty loop) also contains the block determining number
728 of iterations of the block that would prevent removing of empty
731 This scenario can be avoided by splitting critical edges.
732 To save the critical edge splitting pass we identify how the control
733 dependence would look like if the edge was split.
735 Consider the modified CFG created from current CFG by splitting
736 edge B->C. In the postdominance tree of modified CFG, C' is
737 always child of C. There are two cases how chlids of C' can look
742 In this case the only basic block C' is control dependent on is B.
744 2) C' has single child that is B
746 In this case control dependence of C' is same as control
747 dependence of B in original CFG except for block B itself.
748 (since C' postdominate B in modified CFG)
750 Now how to decide what case happens? There are two basic options:
752 a) C postdominate B. Then C immediately postdominate B and
753 case 2 happens iff there is no other way from B to C except
756 There is other way from B to C iff there is succesor of B that
757 is not postdominated by B. Testing this condition is somewhat
758 expensive, because we need to iterate all succesors of B.
759 We are safe to assume that this does not happen: we will mark B
760 as needed when processing the other path from B to C that is
761 conrol dependent on B and marking control dependencies of B
762 itself is harmless because they will be processed anyway after
763 processing control statement in B.
765 b) C does not postdominate B. Always case 1 happens since there is
766 path from C to exit that does not go through B and thus also C'. */
768 if (aggressive
&& !degenerate_phi_p (stmt
))
770 for (k
= 0; k
< gimple_phi_num_args (stmt
); k
++)
772 basic_block arg_bb
= gimple_phi_arg_edge (stmt
, k
)->src
;
775 != get_immediate_dominator (CDI_POST_DOMINATORS
, arg_bb
))
777 if (!TEST_BIT (last_stmt_necessary
, arg_bb
->index
))
778 mark_last_stmt_necessary (arg_bb
);
780 else if (arg_bb
!= ENTRY_BLOCK_PTR
781 && !TEST_BIT (visited_control_parents
,
783 mark_control_dependent_edges_necessary (arg_bb
, el
, true);
789 /* Propagate through the operands. Examine all the USE, VUSE and
790 VDEF operands in this statement. Mark all the statements
791 which feed this statement's uses as necessary. */
795 FOR_EACH_SSA_TREE_OPERAND (use
, stmt
, iter
, SSA_OP_USE
)
796 mark_operand_necessary (use
);
798 use
= gimple_vuse (stmt
);
802 /* If we dropped to simple mode make all immediately
803 reachable definitions necessary. */
806 mark_all_reaching_defs_necessary (stmt
);
810 /* For statements that may load from memory (have a VUSE) we
811 have to mark all reaching (may-)definitions as necessary.
812 We partition this task into two cases:
813 1) explicit loads based on decls that are not aliased
814 2) implicit loads (like calls) and explicit loads not
815 based on decls that are not aliased (like indirect
816 references or loads from globals)
817 For 1) we mark all reaching may-defs as necessary, stopping
818 at dominating kills. For 2) we want to mark all dominating
819 references necessary, but non-aliased ones which we handle
820 in 1). By keeping a global visited bitmap for references
821 we walk for 2) we avoid quadratic behavior for those. */
823 if (is_gimple_call (stmt
))
825 tree callee
= gimple_call_fndecl (stmt
);
828 /* Calls to functions that are merely acting as barriers
829 or that only store to memory do not make any previous
831 if (callee
!= NULL_TREE
832 && DECL_BUILT_IN_CLASS (callee
) == BUILT_IN_NORMAL
833 && (DECL_FUNCTION_CODE (callee
) == BUILT_IN_MEMSET
834 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_MALLOC
835 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_FREE
836 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_ALLOCA
837 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_STACK_SAVE
838 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_STACK_RESTORE
))
841 /* Calls implicitly load from memory, their arguments
842 in addition may explicitly perform memory loads. */
843 mark_all_reaching_defs_necessary (stmt
);
844 for (i
= 0; i
< gimple_call_num_args (stmt
); ++i
)
846 tree arg
= gimple_call_arg (stmt
, i
);
847 if (TREE_CODE (arg
) == SSA_NAME
848 || is_gimple_min_invariant (arg
))
850 if (!ref_may_be_aliased (arg
))
851 mark_aliased_reaching_defs_necessary (stmt
, arg
);
854 else if (gimple_assign_single_p (stmt
))
857 bool rhs_aliased
= false;
858 /* If this is a load mark things necessary. */
859 rhs
= gimple_assign_rhs1 (stmt
);
860 if (TREE_CODE (rhs
) != SSA_NAME
861 && !is_gimple_min_invariant (rhs
))
863 if (!ref_may_be_aliased (rhs
))
864 mark_aliased_reaching_defs_necessary (stmt
, rhs
);
869 mark_all_reaching_defs_necessary (stmt
);
871 else if (gimple_code (stmt
) == GIMPLE_RETURN
)
873 tree rhs
= gimple_return_retval (stmt
);
874 /* A return statement may perform a load. */
876 && TREE_CODE (rhs
) != SSA_NAME
877 && !is_gimple_min_invariant (rhs
))
879 if (!ref_may_be_aliased (rhs
))
880 mark_aliased_reaching_defs_necessary (stmt
, rhs
);
882 mark_all_reaching_defs_necessary (stmt
);
885 else if (gimple_code (stmt
) == GIMPLE_ASM
)
888 mark_all_reaching_defs_necessary (stmt
);
889 /* Inputs may perform loads. */
890 for (i
= 0; i
< gimple_asm_ninputs (stmt
); ++i
)
892 tree op
= TREE_VALUE (gimple_asm_input_op (stmt
, i
));
893 if (TREE_CODE (op
) != SSA_NAME
894 && !is_gimple_min_invariant (op
)
895 && !ref_may_be_aliased (op
))
896 mark_aliased_reaching_defs_necessary (stmt
, op
);
902 /* If we over-used our alias oracle budget drop to simple
903 mode. The cost metric allows quadratic behavior
904 (number of uses times number of may-defs queries) up to
905 a constant maximal number of queries and after that falls back to
906 super-linear complexity. */
907 if (/* Constant but quadratic for small functions. */
908 total_chain
> 128 * 128
909 /* Linear in the number of may-defs. */
910 && total_chain
> 32 * longest_chain
911 /* Linear in the number of uses. */
912 && total_chain
> nr_walks
* 32)
916 bitmap_clear (visited
);
922 /* Replace all uses of result of PHI by underlying variable and mark it
926 mark_virtual_phi_result_for_renaming (gimple phi
)
929 imm_use_iterator iter
;
932 tree result_ssa
, result_var
;
934 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
936 fprintf (dump_file
, "Marking result for renaming : ");
937 print_gimple_stmt (dump_file
, phi
, 0, TDF_SLIM
);
938 fprintf (dump_file
, "\n");
941 result_ssa
= gimple_phi_result (phi
);
942 result_var
= SSA_NAME_VAR (result_ssa
);
943 FOR_EACH_IMM_USE_STMT (stmt
, iter
, result_ssa
)
945 FOR_EACH_IMM_USE_ON_STMT (use_p
, iter
)
946 SET_USE (use_p
, result_var
);
951 mark_sym_for_renaming (result_var
);
954 /* Remove dead PHI nodes from block BB. */
957 remove_dead_phis (basic_block bb
)
959 bool something_changed
= false;
962 gimple_stmt_iterator gsi
;
963 phis
= phi_nodes (bb
);
965 for (gsi
= gsi_start (phis
); !gsi_end_p (gsi
);)
968 phi
= gsi_stmt (gsi
);
970 /* We do not track necessity of virtual PHI nodes. Instead do
971 very simple dead PHI removal here. */
972 if (!is_gimple_reg (gimple_phi_result (phi
)))
974 /* Virtual PHI nodes with one or identical arguments
976 if (degenerate_phi_p (phi
))
978 tree vdef
= gimple_phi_result (phi
);
979 tree vuse
= gimple_phi_arg_def (phi
, 0);
982 imm_use_iterator iter
;
984 FOR_EACH_IMM_USE_STMT (use_stmt
, iter
, vdef
)
985 FOR_EACH_IMM_USE_ON_STMT (use_p
, iter
)
986 SET_USE (use_p
, vuse
);
987 if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vdef
)
988 && TREE_CODE (vuse
) == SSA_NAME
)
989 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vuse
) = 1;
992 gimple_set_plf (phi
, STMT_NECESSARY
, true);
995 if (!gimple_plf (phi
, STMT_NECESSARY
))
997 something_changed
= true;
998 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1000 fprintf (dump_file
, "Deleting : ");
1001 print_gimple_stmt (dump_file
, phi
, 0, TDF_SLIM
);
1002 fprintf (dump_file
, "\n");
1005 remove_phi_node (&gsi
, true);
1006 stats
.removed_phis
++;
1012 return something_changed
;
1015 /* Forward edge E to respective POST_DOM_BB and update PHIs. */
1018 forward_edge_to_pdom (edge e
, basic_block post_dom_bb
)
1020 gimple_stmt_iterator gsi
;
1024 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1025 fprintf (dump_file
, "Redirecting edge %i->%i to %i\n", e
->src
->index
,
1026 e
->dest
->index
, post_dom_bb
->index
);
1028 e2
= redirect_edge_and_branch (e
, post_dom_bb
);
1031 /* If edge was already around, no updating is neccesary. */
1035 if (!gimple_seq_empty_p (phi_nodes (post_dom_bb
)))
1037 /* We are sure that for every live PHI we are seeing control dependent BB.
1038 This means that we can pick any edge to duplicate PHI args from. */
1039 FOR_EACH_EDGE (e2
, ei
, post_dom_bb
->preds
)
1042 for (gsi
= gsi_start_phis (post_dom_bb
); !gsi_end_p (gsi
);)
1044 gimple phi
= gsi_stmt (gsi
);
1046 source_location locus
;
1048 /* PHIs for virtuals have no control dependency relation on them.
1049 We are lost here and must force renaming of the symbol. */
1050 if (!is_gimple_reg (gimple_phi_result (phi
)))
1052 mark_virtual_phi_result_for_renaming (phi
);
1053 remove_phi_node (&gsi
, true);
1057 /* Dead PHI do not imply control dependency. */
1058 if (!gimple_plf (phi
, STMT_NECESSARY
))
1064 op
= gimple_phi_arg_def (phi
, e2
->dest_idx
);
1065 locus
= gimple_phi_arg_location (phi
, e2
->dest_idx
);
1066 add_phi_arg (phi
, op
, e
, locus
);
1067 /* The resulting PHI if not dead can only be degenerate. */
1068 gcc_assert (degenerate_phi_p (phi
));
1075 /* Remove dead statement pointed to by iterator I. Receives the basic block BB
1076 containing I so that we don't have to look it up. */
1079 remove_dead_stmt (gimple_stmt_iterator
*i
, basic_block bb
)
1081 gimple stmt
= gsi_stmt (*i
);
1083 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1085 fprintf (dump_file
, "Deleting : ");
1086 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
1087 fprintf (dump_file
, "\n");
1092 /* If we have determined that a conditional branch statement contributes
1093 nothing to the program, then we not only remove it, but we also change
1094 the flow graph so that the current block will simply fall-thru to its
1095 immediate post-dominator. The blocks we are circumventing will be
1096 removed by cleanup_tree_cfg if this change in the flow graph makes them
1098 if (is_ctrl_stmt (stmt
))
1100 basic_block post_dom_bb
;
1104 post_dom_bb
= get_immediate_dominator (CDI_POST_DOMINATORS
, bb
);
1106 e
= find_edge (bb
, post_dom_bb
);
1108 /* If edge is already there, try to use it. This avoids need to update
1109 PHI nodes. Also watch for cases where post dominator does not exists
1110 or is exit block. These can happen for infinite loops as we create
1111 fake edges in the dominator tree. */
1114 else if (! post_dom_bb
|| post_dom_bb
== EXIT_BLOCK_PTR
)
1115 e
= EDGE_SUCC (bb
, 0);
1117 e
= forward_edge_to_pdom (EDGE_SUCC (bb
, 0), post_dom_bb
);
1119 e
->probability
= REG_BR_PROB_BASE
;
1120 e
->count
= bb
->count
;
1122 /* The edge is no longer associated with a conditional, so it does
1123 not have TRUE/FALSE flags. */
1124 e
->flags
&= ~(EDGE_TRUE_VALUE
| EDGE_FALSE_VALUE
);
1126 /* The lone outgoing edge from BB will be a fallthru edge. */
1127 e
->flags
|= EDGE_FALLTHRU
;
1129 /* Remove the remaining outgoing edges. */
1130 for (ei
= ei_start (bb
->succs
); (e2
= ei_safe_edge (ei
)); )
1140 unlink_stmt_vdef (stmt
);
1141 gsi_remove (i
, true);
1142 release_defs (stmt
);
1145 /* Eliminate unnecessary statements. Any instruction not marked as necessary
1146 contributes nothing to the program, and can be deleted. */
1149 eliminate_unnecessary_stmts (void)
1151 bool something_changed
= false;
1153 gimple_stmt_iterator gsi
, psi
;
1156 VEC (basic_block
, heap
) *h
;
1158 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1159 fprintf (dump_file
, "\nEliminating unnecessary statements:\n");
1161 clear_special_calls ();
1163 /* Walking basic blocks and statements in reverse order avoids
1164 releasing SSA names before any other DEFs that refer to them are
1165 released. This helps avoid loss of debug information, as we get
1166 a chance to propagate all RHSs of removed SSAs into debug uses,
1167 rather than only the latest ones. E.g., consider:
1173 If we were to release x_3 before a_5, when we reached a_5 and
1174 tried to substitute it into the debug stmt, we'd see x_3 there,
1175 but x_3's DEF, type, etc would have already been disconnected.
1176 By going backwards, the debug stmt first changes to:
1178 # DEBUG a => x_3 - b_4
1182 # DEBUG a => y_1 + z_2 - b_4
1185 gcc_assert (dom_info_available_p (CDI_DOMINATORS
));
1186 h
= get_all_dominated_blocks (CDI_DOMINATORS
, single_succ (ENTRY_BLOCK_PTR
));
1188 while (VEC_length (basic_block
, h
))
1190 bb
= VEC_pop (basic_block
, h
);
1192 /* Remove dead statements. */
1193 for (gsi
= gsi_last_bb (bb
); !gsi_end_p (gsi
); gsi
= psi
)
1195 stmt
= gsi_stmt (gsi
);
1202 /* If GSI is not necessary then remove it. */
1203 if (!gimple_plf (stmt
, STMT_NECESSARY
))
1205 if (!is_gimple_debug (stmt
))
1206 something_changed
= true;
1207 remove_dead_stmt (&gsi
, bb
);
1209 else if (is_gimple_call (stmt
))
1211 call
= gimple_call_fndecl (stmt
);
1216 /* When LHS of var = call (); is dead, simplify it into
1217 call (); saving one operand. */
1218 name
= gimple_call_lhs (stmt
);
1219 if (name
&& TREE_CODE (name
) == SSA_NAME
1220 && !TEST_BIT (processed
, SSA_NAME_VERSION (name
)))
1222 something_changed
= true;
1223 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1225 fprintf (dump_file
, "Deleting LHS of call: ");
1226 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
1227 fprintf (dump_file
, "\n");
1230 gimple_call_set_lhs (stmt
, NULL_TREE
);
1231 maybe_clean_or_replace_eh_stmt (stmt
, stmt
);
1233 release_ssa_name (name
);
1235 notice_special_calls (stmt
);
1241 VEC_free (basic_block
, heap
, h
);
1243 /* Since we don't track liveness of virtual PHI nodes, it is possible that we
1244 rendered some PHI nodes unreachable while they are still in use.
1245 Mark them for renaming. */
1248 basic_block prev_bb
;
1250 find_unreachable_blocks ();
1252 /* Delete all unreachable basic blocks in reverse dominator order. */
1253 for (bb
= EXIT_BLOCK_PTR
->prev_bb
; bb
!= ENTRY_BLOCK_PTR
; bb
= prev_bb
)
1255 prev_bb
= bb
->prev_bb
;
1257 if (!TEST_BIT (bb_contains_live_stmts
, bb
->index
)
1258 || !(bb
->flags
& BB_REACHABLE
))
1260 for (gsi
= gsi_start_phis (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
1261 if (!is_gimple_reg (gimple_phi_result (gsi_stmt (gsi
))))
1264 imm_use_iterator iter
;
1266 FOR_EACH_IMM_USE_STMT (stmt
, iter
, gimple_phi_result (gsi_stmt (gsi
)))
1268 if (!(gimple_bb (stmt
)->flags
& BB_REACHABLE
))
1270 if (gimple_code (stmt
) == GIMPLE_PHI
1271 || gimple_plf (stmt
, STMT_NECESSARY
))
1274 BREAK_FROM_IMM_USE_STMT (iter
);
1278 mark_virtual_phi_result_for_renaming (gsi_stmt (gsi
));
1281 if (!(bb
->flags
& BB_REACHABLE
))
1283 /* Speed up the removal of blocks that don't
1284 dominate others. Walking backwards, this should
1285 be the common case. ??? Do we need to recompute
1286 dominators because of cfg_altered? */
1287 if (!MAY_HAVE_DEBUG_STMTS
1288 || !first_dom_son (CDI_DOMINATORS
, bb
))
1289 delete_basic_block (bb
);
1292 h
= get_all_dominated_blocks (CDI_DOMINATORS
, bb
);
1294 while (VEC_length (basic_block
, h
))
1296 bb
= VEC_pop (basic_block
, h
);
1297 prev_bb
= bb
->prev_bb
;
1298 /* Rearrangements to the CFG may have failed
1299 to update the dominators tree, so that
1300 formerly-dominated blocks are now
1301 otherwise reachable. */
1302 if (!!(bb
->flags
& BB_REACHABLE
))
1304 delete_basic_block (bb
);
1307 VEC_free (basic_block
, heap
, h
);
1315 /* Remove dead PHI nodes. */
1316 something_changed
|= remove_dead_phis (bb
);
1319 return something_changed
;
1323 /* Print out removed statement statistics. */
1330 percg
= ((float) stats
.removed
/ (float) stats
.total
) * 100;
1331 fprintf (dump_file
, "Removed %d of %d statements (%d%%)\n",
1332 stats
.removed
, stats
.total
, (int) percg
);
1334 if (stats
.total_phis
== 0)
1337 percg
= ((float) stats
.removed_phis
/ (float) stats
.total_phis
) * 100;
1339 fprintf (dump_file
, "Removed %d of %d PHI nodes (%d%%)\n",
1340 stats
.removed_phis
, stats
.total_phis
, (int) percg
);
1343 /* Initialization for this pass. Set up the used data structures. */
1346 tree_dce_init (bool aggressive
)
1348 memset ((void *) &stats
, 0, sizeof (stats
));
1354 control_dependence_map
= XNEWVEC (bitmap
, last_basic_block
);
1355 for (i
= 0; i
< last_basic_block
; ++i
)
1356 control_dependence_map
[i
] = BITMAP_ALLOC (NULL
);
1358 last_stmt_necessary
= sbitmap_alloc (last_basic_block
);
1359 sbitmap_zero (last_stmt_necessary
);
1360 bb_contains_live_stmts
= sbitmap_alloc (last_basic_block
);
1361 sbitmap_zero (bb_contains_live_stmts
);
1364 processed
= sbitmap_alloc (num_ssa_names
+ 1);
1365 sbitmap_zero (processed
);
1367 worklist
= VEC_alloc (gimple
, heap
, 64);
1368 cfg_altered
= false;
1371 /* Cleanup after this pass. */
1374 tree_dce_done (bool aggressive
)
1380 for (i
= 0; i
< last_basic_block
; ++i
)
1381 BITMAP_FREE (control_dependence_map
[i
]);
1382 free (control_dependence_map
);
1384 sbitmap_free (visited_control_parents
);
1385 sbitmap_free (last_stmt_necessary
);
1386 sbitmap_free (bb_contains_live_stmts
);
1387 bb_contains_live_stmts
= NULL
;
1390 sbitmap_free (processed
);
1392 VEC_free (gimple
, heap
, worklist
);
1395 /* Main routine to eliminate dead code.
1397 AGGRESSIVE controls the aggressiveness of the algorithm.
1398 In conservative mode, we ignore control dependence and simply declare
1399 all but the most trivially dead branches necessary. This mode is fast.
1400 In aggressive mode, control dependences are taken into account, which
1401 results in more dead code elimination, but at the cost of some time.
1403 FIXME: Aggressive mode before PRE doesn't work currently because
1404 the dominance info is not invalidated after DCE1. This is
1405 not an issue right now because we only run aggressive DCE
1406 as the last tree SSA pass, but keep this in mind when you
1407 start experimenting with pass ordering. */
1410 perform_tree_ssa_dce (bool aggressive
)
1412 struct edge_list
*el
= NULL
;
1413 bool something_changed
= 0;
1415 calculate_dominance_info (CDI_DOMINATORS
);
1417 /* Preheaders are needed for SCEV to work.
1418 Simple lateches and recorded exits improve chances that loop will
1419 proved to be finite in testcases such as in loop-15.c and loop-24.c */
1421 loop_optimizer_init (LOOPS_NORMAL
1422 | LOOPS_HAVE_RECORDED_EXITS
);
1424 tree_dce_init (aggressive
);
1428 /* Compute control dependence. */
1429 timevar_push (TV_CONTROL_DEPENDENCES
);
1430 calculate_dominance_info (CDI_POST_DOMINATORS
);
1431 el
= create_edge_list ();
1432 find_all_control_dependences (el
);
1433 timevar_pop (TV_CONTROL_DEPENDENCES
);
1435 visited_control_parents
= sbitmap_alloc (last_basic_block
);
1436 sbitmap_zero (visited_control_parents
);
1438 mark_dfs_back_edges ();
1441 find_obviously_necessary_stmts (el
);
1444 loop_optimizer_finalize ();
1450 visited
= BITMAP_ALLOC (NULL
);
1451 propagate_necessity (el
);
1452 BITMAP_FREE (visited
);
1454 something_changed
|= eliminate_unnecessary_stmts ();
1455 something_changed
|= cfg_altered
;
1457 /* We do not update postdominators, so free them unconditionally. */
1458 free_dominance_info (CDI_POST_DOMINATORS
);
1460 /* If we removed paths in the CFG, then we need to update
1461 dominators as well. I haven't investigated the possibility
1462 of incrementally updating dominators. */
1464 free_dominance_info (CDI_DOMINATORS
);
1466 statistics_counter_event (cfun
, "Statements deleted", stats
.removed
);
1467 statistics_counter_event (cfun
, "PHI nodes deleted", stats
.removed_phis
);
1469 /* Debugging dumps. */
1470 if (dump_file
&& (dump_flags
& (TDF_STATS
|TDF_DETAILS
)))
1473 tree_dce_done (aggressive
);
1475 free_edge_list (el
);
1477 if (something_changed
)
1478 return (TODO_update_ssa
| TODO_cleanup_cfg
| TODO_ggc_collect
1479 | TODO_remove_unused_locals
);
1484 /* Pass entry points. */
1488 return perform_tree_ssa_dce (/*aggressive=*/false);
1492 tree_ssa_dce_loop (void)
1495 todo
= perform_tree_ssa_dce (/*aggressive=*/false);
1498 free_numbers_of_iterations_estimates ();
1505 tree_ssa_cd_dce (void)
1507 return perform_tree_ssa_dce (/*aggressive=*/optimize
>= 2);
1513 return flag_tree_dce
!= 0;
1516 struct gimple_opt_pass pass_dce
=
1521 gate_dce
, /* gate */
1522 tree_ssa_dce
, /* execute */
1525 0, /* static_pass_number */
1526 TV_TREE_DCE
, /* tv_id */
1527 PROP_cfg
| PROP_ssa
, /* properties_required */
1528 0, /* properties_provided */
1529 0, /* properties_destroyed */
1530 0, /* todo_flags_start */
1531 TODO_verify_ssa
/* todo_flags_finish */
1535 struct gimple_opt_pass pass_dce_loop
=
1539 "dceloop", /* name */
1540 gate_dce
, /* gate */
1541 tree_ssa_dce_loop
, /* execute */
1544 0, /* static_pass_number */
1545 TV_TREE_DCE
, /* tv_id */
1546 PROP_cfg
| PROP_ssa
, /* properties_required */
1547 0, /* properties_provided */
1548 0, /* properties_destroyed */
1549 0, /* todo_flags_start */
1550 TODO_verify_ssa
/* todo_flags_finish */
1554 struct gimple_opt_pass pass_cd_dce
=
1559 gate_dce
, /* gate */
1560 tree_ssa_cd_dce
, /* execute */
1563 0, /* static_pass_number */
1564 TV_TREE_CD_DCE
, /* tv_id */
1565 PROP_cfg
| PROP_ssa
, /* properties_required */
1566 0, /* properties_provided */
1567 0, /* properties_destroyed */
1568 0, /* todo_flags_start */
1570 | TODO_verify_flow
/* todo_flags_finish */