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 "diagnostic.h"
53 #include "basic-block.h"
54 #include "tree-flow.h"
56 #include "tree-dump.h"
57 #include "tree-pass.h"
61 #include "tree-scalar-evolution.h"
63 static struct stmt_stats
71 #define STMT_NECESSARY GF_PLF_1
73 static VEC(gimple
,heap
) *worklist
;
75 /* Vector indicating an SSA name has already been processed and marked
77 static sbitmap processed
;
79 /* Vector indicating that last_stmt if a basic block has already been
80 marked as necessary. */
81 static sbitmap last_stmt_necessary
;
83 /* Vector indicating that BB contains statements that are live. */
84 static sbitmap bb_contains_live_stmts
;
86 /* Before we can determine whether a control branch is dead, we need to
87 compute which blocks are control dependent on which edges.
89 We expect each block to be control dependent on very few edges so we
90 use a bitmap for each block recording its edges. An array holds the
91 bitmap. The Ith bit in the bitmap is set if that block is dependent
93 static bitmap
*control_dependence_map
;
95 /* Vector indicating that a basic block has already had all the edges
96 processed that it is control dependent on. */
97 static sbitmap visited_control_parents
;
99 /* TRUE if this pass alters the CFG (by removing control statements).
102 If this pass alters the CFG, then it will arrange for the dominators
104 static bool cfg_altered
;
106 /* Execute code that follows the macro for each edge (given number
107 EDGE_NUMBER within the CODE) for which the block with index N is
108 control dependent. */
109 #define EXECUTE_IF_CONTROL_DEPENDENT(BI, N, EDGE_NUMBER) \
110 EXECUTE_IF_SET_IN_BITMAP (control_dependence_map[(N)], 0, \
114 /* Indicate block BB is control dependent on an edge with index EDGE_INDEX. */
116 set_control_dependence_map_bit (basic_block bb
, int edge_index
)
118 if (bb
== ENTRY_BLOCK_PTR
)
120 gcc_assert (bb
!= EXIT_BLOCK_PTR
);
121 bitmap_set_bit (control_dependence_map
[bb
->index
], edge_index
);
124 /* Clear all control dependences for block BB. */
126 clear_control_dependence_bitmap (basic_block bb
)
128 bitmap_clear (control_dependence_map
[bb
->index
]);
132 /* Find the immediate postdominator PDOM of the specified basic block BLOCK.
133 This function is necessary because some blocks have negative numbers. */
135 static inline basic_block
136 find_pdom (basic_block block
)
138 gcc_assert (block
!= ENTRY_BLOCK_PTR
);
140 if (block
== EXIT_BLOCK_PTR
)
141 return EXIT_BLOCK_PTR
;
144 basic_block bb
= get_immediate_dominator (CDI_POST_DOMINATORS
, block
);
146 return EXIT_BLOCK_PTR
;
152 /* Determine all blocks' control dependences on the given edge with edge_list
153 EL index EDGE_INDEX, ala Morgan, Section 3.6. */
156 find_control_dependence (struct edge_list
*el
, int edge_index
)
158 basic_block current_block
;
159 basic_block ending_block
;
161 gcc_assert (INDEX_EDGE_PRED_BB (el
, edge_index
) != EXIT_BLOCK_PTR
);
163 if (INDEX_EDGE_PRED_BB (el
, edge_index
) == ENTRY_BLOCK_PTR
)
164 ending_block
= single_succ (ENTRY_BLOCK_PTR
);
166 ending_block
= find_pdom (INDEX_EDGE_PRED_BB (el
, edge_index
));
168 for (current_block
= INDEX_EDGE_SUCC_BB (el
, edge_index
);
169 current_block
!= ending_block
&& current_block
!= EXIT_BLOCK_PTR
;
170 current_block
= find_pdom (current_block
))
172 edge e
= INDEX_EDGE (el
, edge_index
);
174 /* For abnormal edges, we don't make current_block control
175 dependent because instructions that throw are always necessary
177 if (e
->flags
& EDGE_ABNORMAL
)
180 set_control_dependence_map_bit (current_block
, edge_index
);
185 /* Record all blocks' control dependences on all edges in the edge
186 list EL, ala Morgan, Section 3.6. */
189 find_all_control_dependences (struct edge_list
*el
)
193 for (i
= 0; i
< NUM_EDGES (el
); ++i
)
194 find_control_dependence (el
, i
);
197 /* If STMT is not already marked necessary, mark it, and add it to the
198 worklist if ADD_TO_WORKLIST is true. */
200 mark_stmt_necessary (gimple stmt
, bool add_to_worklist
)
204 if (gimple_plf (stmt
, STMT_NECESSARY
))
207 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
209 fprintf (dump_file
, "Marking useful stmt: ");
210 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
211 fprintf (dump_file
, "\n");
214 gimple_set_plf (stmt
, STMT_NECESSARY
, true);
216 VEC_safe_push (gimple
, heap
, worklist
, stmt
);
217 if (bb_contains_live_stmts
&& !is_gimple_debug (stmt
))
218 SET_BIT (bb_contains_live_stmts
, gimple_bb (stmt
)->index
);
222 /* Mark the statement defining operand OP as necessary. */
225 mark_operand_necessary (tree op
)
232 ver
= SSA_NAME_VERSION (op
);
233 if (TEST_BIT (processed
, ver
))
235 stmt
= SSA_NAME_DEF_STMT (op
);
236 gcc_assert (gimple_nop_p (stmt
)
237 || gimple_plf (stmt
, STMT_NECESSARY
));
240 SET_BIT (processed
, ver
);
242 stmt
= SSA_NAME_DEF_STMT (op
);
245 if (gimple_plf (stmt
, STMT_NECESSARY
) || gimple_nop_p (stmt
))
248 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
250 fprintf (dump_file
, "marking necessary through ");
251 print_generic_expr (dump_file
, op
, 0);
252 fprintf (dump_file
, " stmt ");
253 print_gimple_stmt (dump_file
, stmt
, 0, 0);
256 gimple_set_plf (stmt
, STMT_NECESSARY
, true);
257 if (bb_contains_live_stmts
)
258 SET_BIT (bb_contains_live_stmts
, gimple_bb (stmt
)->index
);
259 VEC_safe_push (gimple
, heap
, worklist
, stmt
);
263 /* Mark STMT as necessary if it obviously is. Add it to the worklist if
264 it can make other statements necessary.
266 If AGGRESSIVE is false, control statements are conservatively marked as
270 mark_stmt_if_obviously_necessary (gimple stmt
, bool aggressive
)
272 tree lhs
= NULL_TREE
;
273 /* With non-call exceptions, we have to assume that all statements could
274 throw. If a statement may throw, it is inherently necessary. */
275 if (flag_non_call_exceptions
276 && 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
))
312 lhs
= gimple_call_lhs (stmt
);
317 lhs
= gimple_assign_lhs (stmt
);
321 /* Debug temps without a value are not useful. ??? If we could
322 easily locate the debug temp bind stmt for a use thereof,
323 would could refrain from marking all debug temps here, and
324 mark them only if they're used. */
325 if (gimple_debug_bind_has_value_p (stmt
)
326 || TREE_CODE (gimple_debug_bind_get_var (stmt
)) != DEBUG_EXPR_DECL
)
327 mark_stmt_necessary (stmt
, false);
331 gcc_assert (!simple_goto_p (stmt
));
332 mark_stmt_necessary (stmt
, true);
336 gcc_assert (EDGE_COUNT (gimple_bb (stmt
)->succs
) == 2);
341 mark_stmt_necessary (stmt
, true);
348 /* If the statement has volatile operands, it needs to be preserved.
349 Same for statements that can alter control flow in unpredictable
351 if (gimple_has_volatile_ops (stmt
) || is_ctrl_altering_stmt (stmt
))
353 mark_stmt_necessary (stmt
, true);
357 if (is_hidden_global_store (stmt
))
359 mark_stmt_necessary (stmt
, true);
367 /* Make corresponding control dependent edges necessary. We only
368 have to do this once for each basic block, so we clear the bitmap
371 When IGNORE_SELF it true, ignore BB from the list of control dependences. */
373 mark_control_dependent_edges_necessary (basic_block bb
, struct edge_list
*el
, bool ignore_self
)
376 unsigned edge_number
;
377 bool skipped
= false;
379 gcc_assert (bb
!= EXIT_BLOCK_PTR
);
381 if (bb
== ENTRY_BLOCK_PTR
)
384 EXECUTE_IF_CONTROL_DEPENDENT (bi
, bb
->index
, edge_number
)
387 basic_block cd_bb
= INDEX_EDGE_PRED_BB (el
, edge_number
);
389 if (ignore_self
&& cd_bb
== bb
)
395 if (TEST_BIT (last_stmt_necessary
, cd_bb
->index
))
397 SET_BIT (last_stmt_necessary
, cd_bb
->index
);
398 SET_BIT (bb_contains_live_stmts
, cd_bb
->index
);
400 stmt
= last_stmt (cd_bb
);
401 if (stmt
&& is_ctrl_stmt (stmt
))
402 mark_stmt_necessary (stmt
, true);
405 SET_BIT (visited_control_parents
, bb
->index
);
409 /* Find obviously necessary statements. These are things like most function
410 calls, and stores to file level variables.
412 If EL is NULL, control statements are conservatively marked as
413 necessary. Otherwise it contains the list of edges used by control
414 dependence analysis. */
417 find_obviously_necessary_stmts (struct edge_list
*el
)
420 gimple_stmt_iterator gsi
;
426 /* PHI nodes are never inherently necessary. */
427 for (gsi
= gsi_start_phis (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
429 phi
= gsi_stmt (gsi
);
430 gimple_set_plf (phi
, STMT_NECESSARY
, false);
433 /* Check all statements in the block. */
434 for (gsi
= gsi_start_bb (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
436 stmt
= gsi_stmt (gsi
);
437 gimple_set_plf (stmt
, STMT_NECESSARY
, false);
438 mark_stmt_if_obviously_necessary (stmt
, el
!= NULL
);
442 /* Pure and const functions are finite and thus have no infinite loops in
444 if ((TREE_READONLY (current_function_decl
)
445 || DECL_PURE_P (current_function_decl
))
446 && !DECL_LOOPING_CONST_OR_PURE_P (current_function_decl
))
449 /* Prevent the empty possibly infinite loops from being removed. */
455 if (mark_irreducible_loops ())
459 FOR_EACH_EDGE (e
, ei
, bb
->succs
)
460 if ((e
->flags
& EDGE_DFS_BACK
)
461 && (e
->flags
& EDGE_IRREDUCIBLE_LOOP
))
464 fprintf (dump_file
, "Marking back edge of irreducible loop %i->%i\n",
465 e
->src
->index
, e
->dest
->index
);
466 mark_control_dependent_edges_necessary (e
->dest
, el
, false);
470 FOR_EACH_LOOP (li
, loop
, 0)
471 if (!finite_loop_p (loop
))
474 fprintf (dump_file
, "can not prove finiteness of loop %i\n", loop
->num
);
475 mark_control_dependent_edges_necessary (loop
->latch
, el
, false);
482 /* Return true if REF is based on an aliased base, otherwise false. */
485 ref_may_be_aliased (tree ref
)
487 while (handled_component_p (ref
))
488 ref
= TREE_OPERAND (ref
, 0);
489 return !(DECL_P (ref
)
490 && !may_be_aliased (ref
));
493 static bitmap visited
= NULL
;
494 static unsigned int longest_chain
= 0;
495 static unsigned int total_chain
= 0;
496 static unsigned int nr_walks
= 0;
497 static bool chain_ovfl
= false;
499 /* Worker for the walker that marks reaching definitions of REF,
500 which is based on a non-aliased decl, necessary. It returns
501 true whenever the defining statement of the current VDEF is
502 a kill for REF, as no dominating may-defs are necessary for REF
503 anymore. DATA points to the basic-block that contains the
504 stmt that refers to REF. */
507 mark_aliased_reaching_defs_necessary_1 (ao_ref
*ref
, tree vdef
, void *data
)
509 gimple def_stmt
= SSA_NAME_DEF_STMT (vdef
);
511 /* All stmts we visit are necessary. */
512 mark_operand_necessary (vdef
);
514 /* If the stmt lhs kills ref, then we can stop walking. */
515 if (gimple_has_lhs (def_stmt
)
516 && TREE_CODE (gimple_get_lhs (def_stmt
)) != SSA_NAME
)
518 tree base
, lhs
= gimple_get_lhs (def_stmt
);
519 HOST_WIDE_INT size
, offset
, max_size
;
521 base
= get_ref_base_and_extent (lhs
, &offset
, &size
, &max_size
);
522 /* We can get MEM[symbol: sZ, index: D.8862_1] here,
523 so base == refd->base does not always hold. */
524 if (base
== ref
->base
)
526 /* For a must-alias check we need to be able to constrain
527 the accesses properly. */
528 if (size
!= -1 && size
== max_size
529 && ref
->max_size
!= -1)
531 if (offset
<= ref
->offset
532 && offset
+ size
>= ref
->offset
+ ref
->max_size
)
535 /* Or they need to be exactly the same. */
537 /* Make sure there is no induction variable involved
538 in the references (gcc.c-torture/execute/pr42142.c).
539 The simplest way is to check if the kill dominates
541 && dominated_by_p (CDI_DOMINATORS
, (basic_block
) data
,
542 gimple_bb (def_stmt
))
543 && operand_equal_p (ref
->ref
, lhs
, 0))
548 /* Otherwise keep walking. */
553 mark_aliased_reaching_defs_necessary (gimple stmt
, tree ref
)
557 gcc_assert (!chain_ovfl
);
558 ao_ref_init (&refd
, ref
);
559 chain
= walk_aliased_vdefs (&refd
, gimple_vuse (stmt
),
560 mark_aliased_reaching_defs_necessary_1
,
561 gimple_bb (stmt
), NULL
);
562 if (chain
> longest_chain
)
563 longest_chain
= chain
;
564 total_chain
+= chain
;
568 /* Worker for the walker that marks reaching definitions of REF, which
569 is not based on a non-aliased decl. For simplicity we need to end
570 up marking all may-defs necessary that are not based on a non-aliased
571 decl. The only job of this walker is to skip may-defs based on
572 a non-aliased decl. */
575 mark_all_reaching_defs_necessary_1 (ao_ref
*ref ATTRIBUTE_UNUSED
,
576 tree vdef
, void *data ATTRIBUTE_UNUSED
)
578 gimple def_stmt
= SSA_NAME_DEF_STMT (vdef
);
580 /* We have to skip already visited (and thus necessary) statements
581 to make the chaining work after we dropped back to simple mode. */
583 && TEST_BIT (processed
, SSA_NAME_VERSION (vdef
)))
585 gcc_assert (gimple_nop_p (def_stmt
)
586 || gimple_plf (def_stmt
, STMT_NECESSARY
));
590 /* We want to skip stores to non-aliased variables. */
592 && gimple_assign_single_p (def_stmt
))
594 tree lhs
= gimple_assign_lhs (def_stmt
);
595 if (!ref_may_be_aliased (lhs
))
599 mark_operand_necessary (vdef
);
605 mark_all_reaching_defs_necessary (gimple stmt
)
607 walk_aliased_vdefs (NULL
, gimple_vuse (stmt
),
608 mark_all_reaching_defs_necessary_1
, NULL
, &visited
);
611 /* Return true for PHI nodes with one or identical arguments
614 degenerate_phi_p (gimple phi
)
617 tree op
= gimple_phi_arg_def (phi
, 0);
618 for (i
= 1; i
< gimple_phi_num_args (phi
); i
++)
619 if (gimple_phi_arg_def (phi
, i
) != op
)
624 /* Propagate necessity using the operands of necessary statements.
625 Process the uses on each statement in the worklist, and add all
626 feeding statements which contribute to the calculation of this
627 value to the worklist.
629 In conservative mode, EL is NULL. */
632 propagate_necessity (struct edge_list
*el
)
635 bool aggressive
= (el
? true : false);
637 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
638 fprintf (dump_file
, "\nProcessing worklist:\n");
640 while (VEC_length (gimple
, worklist
) > 0)
642 /* Take STMT from worklist. */
643 stmt
= VEC_pop (gimple
, worklist
);
645 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
647 fprintf (dump_file
, "processing: ");
648 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
649 fprintf (dump_file
, "\n");
654 /* Mark the last statements of the basic blocks that the block
655 containing STMT is control dependent on, but only if we haven't
657 basic_block bb
= gimple_bb (stmt
);
658 if (bb
!= ENTRY_BLOCK_PTR
659 && ! TEST_BIT (visited_control_parents
, bb
->index
))
660 mark_control_dependent_edges_necessary (bb
, el
, false);
663 if (gimple_code (stmt
) == GIMPLE_PHI
664 /* We do not process virtual PHI nodes nor do we track their
666 && is_gimple_reg (gimple_phi_result (stmt
)))
668 /* PHI nodes are somewhat special in that each PHI alternative has
669 data and control dependencies. All the statements feeding the
670 PHI node's arguments are always necessary. In aggressive mode,
671 we also consider the control dependent edges leading to the
672 predecessor block associated with each PHI alternative as
676 for (k
= 0; k
< gimple_phi_num_args (stmt
); k
++)
678 tree arg
= PHI_ARG_DEF (stmt
, k
);
679 if (TREE_CODE (arg
) == SSA_NAME
)
680 mark_operand_necessary (arg
);
683 /* For PHI operands it matters from where the control flow arrives
684 to the BB. Consider the following example:
694 We need to mark control dependence of the empty basic blocks, since they
695 contains computation of PHI operands.
697 Doing so is too restrictive in the case the predecestor block is in
703 for (i = 0; i<1000; ++i)
709 There is PHI for J in the BB containing return statement.
710 In this case the control dependence of predecestor block (that is
711 within the empty loop) also contains the block determining number
712 of iterations of the block that would prevent removing of empty
715 This scenario can be avoided by splitting critical edges.
716 To save the critical edge splitting pass we identify how the control
717 dependence would look like if the edge was split.
719 Consider the modified CFG created from current CFG by splitting
720 edge B->C. In the postdominance tree of modified CFG, C' is
721 always child of C. There are two cases how chlids of C' can look
726 In this case the only basic block C' is control dependent on is B.
728 2) C' has single child that is B
730 In this case control dependence of C' is same as control
731 dependence of B in original CFG except for block B itself.
732 (since C' postdominate B in modified CFG)
734 Now how to decide what case happens? There are two basic options:
736 a) C postdominate B. Then C immediately postdominate B and
737 case 2 happens iff there is no other way from B to C except
740 There is other way from B to C iff there is succesor of B that
741 is not postdominated by B. Testing this condition is somewhat
742 expensive, because we need to iterate all succesors of B.
743 We are safe to assume that this does not happen: we will mark B
744 as needed when processing the other path from B to C that is
745 conrol dependent on B and marking control dependencies of B
746 itself is harmless because they will be processed anyway after
747 processing control statement in B.
749 b) C does not postdominate B. Always case 1 happens since there is
750 path from C to exit that does not go through B and thus also C'. */
752 if (aggressive
&& !degenerate_phi_p (stmt
))
754 for (k
= 0; k
< gimple_phi_num_args (stmt
); k
++)
756 basic_block arg_bb
= gimple_phi_arg_edge (stmt
, k
)->src
;
759 != get_immediate_dominator (CDI_POST_DOMINATORS
, arg_bb
))
761 if (!TEST_BIT (last_stmt_necessary
, arg_bb
->index
))
764 SET_BIT (last_stmt_necessary
, arg_bb
->index
);
765 SET_BIT (bb_contains_live_stmts
, arg_bb
->index
);
767 stmt2
= last_stmt (arg_bb
);
768 if (stmt2
&& is_ctrl_stmt (stmt2
))
769 mark_stmt_necessary (stmt2
, true);
772 else if (arg_bb
!= ENTRY_BLOCK_PTR
773 && ! TEST_BIT (visited_control_parents
, arg_bb
->index
))
774 mark_control_dependent_edges_necessary (arg_bb
, el
, true);
780 /* Propagate through the operands. Examine all the USE, VUSE and
781 VDEF operands in this statement. Mark all the statements
782 which feed this statement's uses as necessary. */
786 FOR_EACH_SSA_TREE_OPERAND (use
, stmt
, iter
, SSA_OP_USE
)
787 mark_operand_necessary (use
);
789 use
= gimple_vuse (stmt
);
793 /* If we dropped to simple mode make all immediately
794 reachable definitions necessary. */
797 mark_all_reaching_defs_necessary (stmt
);
801 /* For statements that may load from memory (have a VUSE) we
802 have to mark all reaching (may-)definitions as necessary.
803 We partition this task into two cases:
804 1) explicit loads based on decls that are not aliased
805 2) implicit loads (like calls) and explicit loads not
806 based on decls that are not aliased (like indirect
807 references or loads from globals)
808 For 1) we mark all reaching may-defs as necessary, stopping
809 at dominating kills. For 2) we want to mark all dominating
810 references necessary, but non-aliased ones which we handle
811 in 1). By keeping a global visited bitmap for references
812 we walk for 2) we avoid quadratic behavior for those. */
814 if (is_gimple_call (stmt
))
816 tree callee
= gimple_call_fndecl (stmt
);
819 /* Calls to functions that are merely acting as barriers
820 or that only store to memory do not make any previous
822 if (callee
!= NULL_TREE
823 && DECL_BUILT_IN_CLASS (callee
) == BUILT_IN_NORMAL
824 && (DECL_FUNCTION_CODE (callee
) == BUILT_IN_MEMSET
825 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_MALLOC
826 || DECL_FUNCTION_CODE (callee
) == BUILT_IN_FREE
))
829 /* Calls implicitly load from memory, their arguments
830 in addition may explicitly perform memory loads. */
831 mark_all_reaching_defs_necessary (stmt
);
832 for (i
= 0; i
< gimple_call_num_args (stmt
); ++i
)
834 tree arg
= gimple_call_arg (stmt
, i
);
835 if (TREE_CODE (arg
) == SSA_NAME
836 || is_gimple_min_invariant (arg
))
838 if (!ref_may_be_aliased (arg
))
839 mark_aliased_reaching_defs_necessary (stmt
, arg
);
842 else if (gimple_assign_single_p (stmt
))
845 bool rhs_aliased
= false;
846 /* If this is a load mark things necessary. */
847 rhs
= gimple_assign_rhs1 (stmt
);
848 if (TREE_CODE (rhs
) != SSA_NAME
849 && !is_gimple_min_invariant (rhs
))
851 if (!ref_may_be_aliased (rhs
))
852 mark_aliased_reaching_defs_necessary (stmt
, rhs
);
857 mark_all_reaching_defs_necessary (stmt
);
859 else if (gimple_code (stmt
) == GIMPLE_RETURN
)
861 tree rhs
= gimple_return_retval (stmt
);
862 /* A return statement may perform a load. */
863 if (TREE_CODE (rhs
) != SSA_NAME
864 && !is_gimple_min_invariant (rhs
))
866 if (!ref_may_be_aliased (rhs
))
867 mark_aliased_reaching_defs_necessary (stmt
, rhs
);
869 mark_all_reaching_defs_necessary (stmt
);
872 else if (gimple_code (stmt
) == GIMPLE_ASM
)
875 mark_all_reaching_defs_necessary (stmt
);
876 /* Inputs may perform loads. */
877 for (i
= 0; i
< gimple_asm_ninputs (stmt
); ++i
)
879 tree op
= TREE_VALUE (gimple_asm_input_op (stmt
, i
));
880 if (TREE_CODE (op
) != SSA_NAME
881 && !is_gimple_min_invariant (op
)
882 && !ref_may_be_aliased (op
))
883 mark_aliased_reaching_defs_necessary (stmt
, op
);
889 /* If we over-used our alias oracle budget drop to simple
890 mode. The cost metric allows quadratic behavior
891 (number of uses times number of may-defs queries) up to
892 a constant maximal number of queries and after that falls back to
893 super-linear complexity. */
894 if (/* Constant but quadratic for small functions. */
895 total_chain
> 128 * 128
896 /* Linear in the number of may-defs. */
897 && total_chain
> 32 * longest_chain
898 /* Linear in the number of uses. */
899 && total_chain
> nr_walks
* 32)
903 bitmap_clear (visited
);
909 /* Replace all uses of result of PHI by underlying variable and mark it
913 mark_virtual_phi_result_for_renaming (gimple phi
)
916 imm_use_iterator iter
;
919 tree result_ssa
, result_var
;
921 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
923 fprintf (dump_file
, "Marking result for renaming : ");
924 print_gimple_stmt (dump_file
, phi
, 0, TDF_SLIM
);
925 fprintf (dump_file
, "\n");
928 result_ssa
= gimple_phi_result (phi
);
929 result_var
= SSA_NAME_VAR (result_ssa
);
930 FOR_EACH_IMM_USE_STMT (stmt
, iter
, result_ssa
)
932 FOR_EACH_IMM_USE_ON_STMT (use_p
, iter
)
933 SET_USE (use_p
, result_var
);
938 mark_sym_for_renaming (result_var
);
941 /* Remove dead PHI nodes from block BB. */
944 remove_dead_phis (basic_block bb
)
946 bool something_changed
= false;
949 gimple_stmt_iterator gsi
;
950 phis
= phi_nodes (bb
);
952 for (gsi
= gsi_start (phis
); !gsi_end_p (gsi
);)
955 phi
= gsi_stmt (gsi
);
957 /* We do not track necessity of virtual PHI nodes. Instead do
958 very simple dead PHI removal here. */
959 if (!is_gimple_reg (gimple_phi_result (phi
)))
961 /* Virtual PHI nodes with one or identical arguments
963 if (degenerate_phi_p (phi
))
965 tree vdef
= gimple_phi_result (phi
);
966 tree vuse
= gimple_phi_arg_def (phi
, 0);
969 imm_use_iterator iter
;
971 FOR_EACH_IMM_USE_STMT (use_stmt
, iter
, vdef
)
972 FOR_EACH_IMM_USE_ON_STMT (use_p
, iter
)
973 SET_USE (use_p
, vuse
);
974 if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vdef
)
975 && TREE_CODE (vuse
) == SSA_NAME
)
976 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vuse
) = 1;
979 gimple_set_plf (phi
, STMT_NECESSARY
, true);
982 if (!gimple_plf (phi
, STMT_NECESSARY
))
984 something_changed
= true;
985 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
987 fprintf (dump_file
, "Deleting : ");
988 print_gimple_stmt (dump_file
, phi
, 0, TDF_SLIM
);
989 fprintf (dump_file
, "\n");
992 remove_phi_node (&gsi
, true);
993 stats
.removed_phis
++;
999 return something_changed
;
1002 /* Forward edge E to respective POST_DOM_BB and update PHIs. */
1005 forward_edge_to_pdom (edge e
, basic_block post_dom_bb
)
1007 gimple_stmt_iterator gsi
;
1011 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1012 fprintf (dump_file
, "Redirecting edge %i->%i to %i\n", e
->src
->index
,
1013 e
->dest
->index
, post_dom_bb
->index
);
1015 e2
= redirect_edge_and_branch (e
, post_dom_bb
);
1018 /* If edge was already around, no updating is neccesary. */
1022 if (!gimple_seq_empty_p (phi_nodes (post_dom_bb
)))
1024 /* We are sure that for every live PHI we are seeing control dependent BB.
1025 This means that we can pick any edge to duplicate PHI args from. */
1026 FOR_EACH_EDGE (e2
, ei
, post_dom_bb
->preds
)
1029 for (gsi
= gsi_start_phis (post_dom_bb
); !gsi_end_p (gsi
);)
1031 gimple phi
= gsi_stmt (gsi
);
1033 source_location locus
;
1035 /* PHIs for virtuals have no control dependency relation on them.
1036 We are lost here and must force renaming of the symbol. */
1037 if (!is_gimple_reg (gimple_phi_result (phi
)))
1039 mark_virtual_phi_result_for_renaming (phi
);
1040 remove_phi_node (&gsi
, true);
1044 /* Dead PHI do not imply control dependency. */
1045 if (!gimple_plf (phi
, STMT_NECESSARY
))
1051 op
= gimple_phi_arg_def (phi
, e2
->dest_idx
);
1052 locus
= gimple_phi_arg_location (phi
, e2
->dest_idx
);
1053 add_phi_arg (phi
, op
, e
, locus
);
1054 /* The resulting PHI if not dead can only be degenerate. */
1055 gcc_assert (degenerate_phi_p (phi
));
1062 /* Remove dead statement pointed to by iterator I. Receives the basic block BB
1063 containing I so that we don't have to look it up. */
1066 remove_dead_stmt (gimple_stmt_iterator
*i
, basic_block bb
)
1068 gimple stmt
= gsi_stmt (*i
);
1070 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1072 fprintf (dump_file
, "Deleting : ");
1073 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
1074 fprintf (dump_file
, "\n");
1079 /* If we have determined that a conditional branch statement contributes
1080 nothing to the program, then we not only remove it, but we also change
1081 the flow graph so that the current block will simply fall-thru to its
1082 immediate post-dominator. The blocks we are circumventing will be
1083 removed by cleanup_tree_cfg if this change in the flow graph makes them
1085 if (is_ctrl_stmt (stmt
))
1087 basic_block post_dom_bb
;
1091 post_dom_bb
= get_immediate_dominator (CDI_POST_DOMINATORS
, bb
);
1093 e
= find_edge (bb
, post_dom_bb
);
1095 /* If edge is already there, try to use it. This avoids need to update
1096 PHI nodes. Also watch for cases where post dominator does not exists
1097 or is exit block. These can happen for infinite loops as we create
1098 fake edges in the dominator tree. */
1101 else if (! post_dom_bb
|| post_dom_bb
== EXIT_BLOCK_PTR
)
1102 e
= EDGE_SUCC (bb
, 0);
1104 e
= forward_edge_to_pdom (EDGE_SUCC (bb
, 0), post_dom_bb
);
1106 e
->probability
= REG_BR_PROB_BASE
;
1107 e
->count
= bb
->count
;
1109 /* The edge is no longer associated with a conditional, so it does
1110 not have TRUE/FALSE flags. */
1111 e
->flags
&= ~(EDGE_TRUE_VALUE
| EDGE_FALSE_VALUE
);
1113 /* The lone outgoing edge from BB will be a fallthru edge. */
1114 e
->flags
|= EDGE_FALLTHRU
;
1116 /* Remove the remaining outgoing edges. */
1117 for (ei
= ei_start (bb
->succs
); (e2
= ei_safe_edge (ei
)); )
1127 unlink_stmt_vdef (stmt
);
1128 gsi_remove (i
, true);
1129 release_defs (stmt
);
1132 /* Eliminate unnecessary statements. Any instruction not marked as necessary
1133 contributes nothing to the program, and can be deleted. */
1136 eliminate_unnecessary_stmts (void)
1138 bool something_changed
= false;
1140 gimple_stmt_iterator gsi
, psi
;
1143 VEC (basic_block
, heap
) *h
;
1145 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1146 fprintf (dump_file
, "\nEliminating unnecessary statements:\n");
1148 clear_special_calls ();
1150 /* Walking basic blocks and statements in reverse order avoids
1151 releasing SSA names before any other DEFs that refer to them are
1152 released. This helps avoid loss of debug information, as we get
1153 a chance to propagate all RHSs of removed SSAs into debug uses,
1154 rather than only the latest ones. E.g., consider:
1160 If we were to release x_3 before a_5, when we reached a_5 and
1161 tried to substitute it into the debug stmt, we'd see x_3 there,
1162 but x_3's DEF, type, etc would have already been disconnected.
1163 By going backwards, the debug stmt first changes to:
1165 # DEBUG a => x_3 - b_4
1169 # DEBUG a => y_1 + z_2 - b_4
1172 gcc_assert (dom_info_available_p (CDI_DOMINATORS
));
1173 h
= get_all_dominated_blocks (CDI_DOMINATORS
, single_succ (ENTRY_BLOCK_PTR
));
1175 while (VEC_length (basic_block
, h
))
1177 bb
= VEC_pop (basic_block
, h
);
1179 /* Remove dead statements. */
1180 for (gsi
= gsi_last_bb (bb
); !gsi_end_p (gsi
); gsi
= psi
)
1182 stmt
= gsi_stmt (gsi
);
1189 /* If GSI is not necessary then remove it. */
1190 if (!gimple_plf (stmt
, STMT_NECESSARY
))
1192 if (!is_gimple_debug (stmt
))
1193 something_changed
= true;
1194 remove_dead_stmt (&gsi
, bb
);
1196 else if (is_gimple_call (stmt
))
1198 call
= gimple_call_fndecl (stmt
);
1203 /* When LHS of var = call (); is dead, simplify it into
1204 call (); saving one operand. */
1205 name
= gimple_call_lhs (stmt
);
1206 if (name
&& TREE_CODE (name
) == SSA_NAME
1207 && !TEST_BIT (processed
, SSA_NAME_VERSION (name
)))
1209 something_changed
= true;
1210 if (dump_file
&& (dump_flags
& TDF_DETAILS
))
1212 fprintf (dump_file
, "Deleting LHS of call: ");
1213 print_gimple_stmt (dump_file
, stmt
, 0, TDF_SLIM
);
1214 fprintf (dump_file
, "\n");
1217 gimple_call_set_lhs (stmt
, NULL_TREE
);
1218 maybe_clean_or_replace_eh_stmt (stmt
, stmt
);
1220 release_ssa_name (name
);
1222 notice_special_calls (stmt
);
1228 VEC_free (basic_block
, heap
, h
);
1230 /* Since we don't track liveness of virtual PHI nodes, it is possible that we
1231 rendered some PHI nodes unreachable while they are still in use.
1232 Mark them for renaming. */
1235 basic_block prev_bb
;
1237 find_unreachable_blocks ();
1239 /* Delete all unreachable basic blocks in reverse dominator order. */
1240 for (bb
= EXIT_BLOCK_PTR
->prev_bb
; bb
!= ENTRY_BLOCK_PTR
; bb
= prev_bb
)
1242 prev_bb
= bb
->prev_bb
;
1244 if (!TEST_BIT (bb_contains_live_stmts
, bb
->index
)
1245 || !(bb
->flags
& BB_REACHABLE
))
1247 for (gsi
= gsi_start_phis (bb
); !gsi_end_p (gsi
); gsi_next (&gsi
))
1248 if (!is_gimple_reg (gimple_phi_result (gsi_stmt (gsi
))))
1251 imm_use_iterator iter
;
1253 FOR_EACH_IMM_USE_STMT (stmt
, iter
, gimple_phi_result (gsi_stmt (gsi
)))
1255 if (!(gimple_bb (stmt
)->flags
& BB_REACHABLE
))
1257 if (gimple_code (stmt
) == GIMPLE_PHI
1258 || gimple_plf (stmt
, STMT_NECESSARY
))
1261 BREAK_FROM_IMM_USE_STMT (iter
);
1265 mark_virtual_phi_result_for_renaming (gsi_stmt (gsi
));
1268 if (!(bb
->flags
& BB_REACHABLE
))
1270 /* Speed up the removal of blocks that don't
1271 dominate others. Walking backwards, this should
1272 be the common case. ??? Do we need to recompute
1273 dominators because of cfg_altered? */
1274 if (!MAY_HAVE_DEBUG_STMTS
1275 || !first_dom_son (CDI_DOMINATORS
, bb
))
1276 delete_basic_block (bb
);
1279 h
= get_all_dominated_blocks (CDI_DOMINATORS
, bb
);
1281 while (VEC_length (basic_block
, h
))
1283 bb
= VEC_pop (basic_block
, h
);
1284 prev_bb
= bb
->prev_bb
;
1285 /* Rearrangements to the CFG may have failed
1286 to update the dominators tree, so that
1287 formerly-dominated blocks are now
1288 otherwise reachable. */
1289 if (!!(bb
->flags
& BB_REACHABLE
))
1291 delete_basic_block (bb
);
1294 VEC_free (basic_block
, heap
, h
);
1302 /* Remove dead PHI nodes. */
1303 something_changed
|= remove_dead_phis (bb
);
1306 return something_changed
;
1310 /* Print out removed statement statistics. */
1317 percg
= ((float) stats
.removed
/ (float) stats
.total
) * 100;
1318 fprintf (dump_file
, "Removed %d of %d statements (%d%%)\n",
1319 stats
.removed
, stats
.total
, (int) percg
);
1321 if (stats
.total_phis
== 0)
1324 percg
= ((float) stats
.removed_phis
/ (float) stats
.total_phis
) * 100;
1326 fprintf (dump_file
, "Removed %d of %d PHI nodes (%d%%)\n",
1327 stats
.removed_phis
, stats
.total_phis
, (int) percg
);
1330 /* Initialization for this pass. Set up the used data structures. */
1333 tree_dce_init (bool aggressive
)
1335 memset ((void *) &stats
, 0, sizeof (stats
));
1341 control_dependence_map
= XNEWVEC (bitmap
, last_basic_block
);
1342 for (i
= 0; i
< last_basic_block
; ++i
)
1343 control_dependence_map
[i
] = BITMAP_ALLOC (NULL
);
1345 last_stmt_necessary
= sbitmap_alloc (last_basic_block
);
1346 sbitmap_zero (last_stmt_necessary
);
1347 bb_contains_live_stmts
= sbitmap_alloc (last_basic_block
);
1348 sbitmap_zero (bb_contains_live_stmts
);
1351 processed
= sbitmap_alloc (num_ssa_names
+ 1);
1352 sbitmap_zero (processed
);
1354 worklist
= VEC_alloc (gimple
, heap
, 64);
1355 cfg_altered
= false;
1358 /* Cleanup after this pass. */
1361 tree_dce_done (bool aggressive
)
1367 for (i
= 0; i
< last_basic_block
; ++i
)
1368 BITMAP_FREE (control_dependence_map
[i
]);
1369 free (control_dependence_map
);
1371 sbitmap_free (visited_control_parents
);
1372 sbitmap_free (last_stmt_necessary
);
1373 sbitmap_free (bb_contains_live_stmts
);
1374 bb_contains_live_stmts
= NULL
;
1377 sbitmap_free (processed
);
1379 VEC_free (gimple
, heap
, worklist
);
1382 /* Main routine to eliminate dead code.
1384 AGGRESSIVE controls the aggressiveness of the algorithm.
1385 In conservative mode, we ignore control dependence and simply declare
1386 all but the most trivially dead branches necessary. This mode is fast.
1387 In aggressive mode, control dependences are taken into account, which
1388 results in more dead code elimination, but at the cost of some time.
1390 FIXME: Aggressive mode before PRE doesn't work currently because
1391 the dominance info is not invalidated after DCE1. This is
1392 not an issue right now because we only run aggressive DCE
1393 as the last tree SSA pass, but keep this in mind when you
1394 start experimenting with pass ordering. */
1397 perform_tree_ssa_dce (bool aggressive
)
1399 struct edge_list
*el
= NULL
;
1400 bool something_changed
= 0;
1402 /* Preheaders are needed for SCEV to work.
1403 Simple lateches and recorded exits improve chances that loop will
1404 proved to be finite in testcases such as in loop-15.c and loop-24.c */
1406 loop_optimizer_init (LOOPS_NORMAL
1407 | LOOPS_HAVE_RECORDED_EXITS
);
1409 tree_dce_init (aggressive
);
1413 /* Compute control dependence. */
1414 timevar_push (TV_CONTROL_DEPENDENCES
);
1415 calculate_dominance_info (CDI_POST_DOMINATORS
);
1416 el
= create_edge_list ();
1417 find_all_control_dependences (el
);
1418 timevar_pop (TV_CONTROL_DEPENDENCES
);
1420 visited_control_parents
= sbitmap_alloc (last_basic_block
);
1421 sbitmap_zero (visited_control_parents
);
1423 mark_dfs_back_edges ();
1426 find_obviously_necessary_stmts (el
);
1429 loop_optimizer_finalize ();
1435 visited
= BITMAP_ALLOC (NULL
);
1436 propagate_necessity (el
);
1437 BITMAP_FREE (visited
);
1439 something_changed
|= eliminate_unnecessary_stmts ();
1440 something_changed
|= cfg_altered
;
1442 /* We do not update postdominators, so free them unconditionally. */
1443 free_dominance_info (CDI_POST_DOMINATORS
);
1445 /* If we removed paths in the CFG, then we need to update
1446 dominators as well. I haven't investigated the possibility
1447 of incrementally updating dominators. */
1449 free_dominance_info (CDI_DOMINATORS
);
1451 statistics_counter_event (cfun
, "Statements deleted", stats
.removed
);
1452 statistics_counter_event (cfun
, "PHI nodes deleted", stats
.removed_phis
);
1454 /* Debugging dumps. */
1455 if (dump_file
&& (dump_flags
& (TDF_STATS
|TDF_DETAILS
)))
1458 tree_dce_done (aggressive
);
1460 free_edge_list (el
);
1462 if (something_changed
)
1463 return (TODO_update_ssa
| TODO_cleanup_cfg
| TODO_ggc_collect
1464 | TODO_remove_unused_locals
);
1469 /* Pass entry points. */
1473 return perform_tree_ssa_dce (/*aggressive=*/false);
1477 tree_ssa_dce_loop (void)
1480 todo
= perform_tree_ssa_dce (/*aggressive=*/false);
1483 free_numbers_of_iterations_estimates ();
1490 tree_ssa_cd_dce (void)
1492 return perform_tree_ssa_dce (/*aggressive=*/optimize
>= 2);
1498 return flag_tree_dce
!= 0;
1501 struct gimple_opt_pass pass_dce
=
1506 gate_dce
, /* gate */
1507 tree_ssa_dce
, /* execute */
1510 0, /* static_pass_number */
1511 TV_TREE_DCE
, /* tv_id */
1512 PROP_cfg
| PROP_ssa
, /* properties_required */
1513 0, /* properties_provided */
1514 0, /* properties_destroyed */
1515 0, /* todo_flags_start */
1516 TODO_dump_func
| TODO_verify_ssa
/* todo_flags_finish */
1520 struct gimple_opt_pass pass_dce_loop
=
1524 "dceloop", /* name */
1525 gate_dce
, /* gate */
1526 tree_ssa_dce_loop
, /* execute */
1529 0, /* static_pass_number */
1530 TV_TREE_DCE
, /* tv_id */
1531 PROP_cfg
| PROP_ssa
, /* properties_required */
1532 0, /* properties_provided */
1533 0, /* properties_destroyed */
1534 0, /* todo_flags_start */
1535 TODO_dump_func
| TODO_verify_ssa
/* todo_flags_finish */
1539 struct gimple_opt_pass pass_cd_dce
=
1544 gate_dce
, /* gate */
1545 tree_ssa_cd_dce
, /* execute */
1548 0, /* static_pass_number */
1549 TV_TREE_CD_DCE
, /* tv_id */
1550 PROP_cfg
| PROP_ssa
, /* properties_required */
1551 0, /* properties_provided */
1552 0, /* properties_destroyed */
1553 0, /* todo_flags_start */
1554 TODO_dump_func
| TODO_verify_ssa
1555 | TODO_verify_flow
/* todo_flags_finish */