1 /* Dead-code elimination pass for the GNU compiler.
2 Copyright (C) 2000 Free Software Foundation, Inc.
3 Written by Jeffrey D. Oldham <oldham@codesourcery.com>.
5 This file is part of GNU CC.
7 GNU CC is free software; you can redistribute it and/or modify it
8 under the terms of the GNU General Public License as published by the
9 Free Software Foundation; either version 2, or (at your option) any
12 GNU CC is distributed in the hope that it will be useful, but WITHOUT
13 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
14 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
17 You should have received a copy of the GNU General Public License
18 along with GNU CC; see the file COPYING. If not, write to the Free
19 Software Foundation, 59 Temple Place - Suite 330, Boston, MA
22 /* Dead-code elimination is the removal of instructions which have no
23 impact on the program's output. "Dead instructions" have no impact
24 on the program's output, while "necessary instructions" may have
27 The algorithm consists of three phases:
28 1) marking as necessary all instructions known to be necessary,
29 e.g., writing a value to memory,
30 2) propagating necessary instructions, e.g., the instructions
31 giving values to operands in necessary instructions, and
32 3) removing dead instructions (except replacing dead conditionals
33 with unconditional jumps).
36 The last step can require adding labels, deleting insns, and
37 modifying basic block structures. Some conditional jumps may be
38 converted to unconditional jumps so the control-flow graph may be
41 Edges from some infinite loops to the exit block can be added to
42 the control-flow graph, but will be removed after this pass is
46 We decided to not simultaneously perform jump optimization and dead
47 loop removal during dead-code elimination. Thus, all jump
48 instructions originally present remain after dead-code elimination
49 but 1) unnecessary conditional jump instructions are changed to
50 unconditional jump instructions and 2) all unconditional jump
54 1) SSA has been performed.
55 2) The basic block and control-flow graph structures are accurate.
56 3) The flow graph permits constructing an edge_list.
57 4) note rtxes should be saved.
60 When replacing unnecessary conditional jumps with unconditional
61 jumps, the control-flow graph is not updated. It should be.
64 Building an Optimizing Compiler
66 Butterworth-Heinemann, 1998
74 #include "hard-reg-set.h"
75 #include "basic-block.h"
77 #include "insn-config.h"
82 /* A map from blocks to the edges on which they are control dependent. */
84 /* An dynamically allocated array. The Nth element corresponds to
85 the block with index N + 2. The Ith bit in the bitmap is set if
86 that block is dependent on the Ith edge. */
88 /* The number of elements in the array. */
90 } control_dependent_block_to_edge_map_s, *control_dependent_block_to_edge_map;
92 /* Local function prototypes. */
93 static control_dependent_block_to_edge_map control_dependent_block_to_edge_map_create
94 PARAMS((size_t num_basic_blocks));
95 static void set_control_dependent_block_to_edge_map_bit
96 PARAMS ((control_dependent_block_to_edge_map c, basic_block bb,
98 static void control_dependent_block_to_edge_map_free
99 PARAMS ((control_dependent_block_to_edge_map c));
100 static void find_all_control_dependences
101 PARAMS ((struct edge_list *el, int *pdom,
102 control_dependent_block_to_edge_map cdbte));
103 static void find_control_dependence
104 PARAMS ((struct edge_list *el, int edge_index, int *pdom,
105 control_dependent_block_to_edge_map cdbte));
106 static basic_block find_pdom
107 PARAMS ((int *pdom, basic_block block));
108 static int inherently_necessary_register_1
109 PARAMS ((rtx *current_rtx, void *data));
110 static int inherently_necessary_register
111 PARAMS ((rtx current_rtx));
112 static int find_inherently_necessary
113 PARAMS ((rtx current_rtx));
114 static int propagate_necessity_through_operand
115 PARAMS ((rtx *current_rtx, void *data));
116 static void note_inherently_necessary_set
117 PARAMS ((rtx, rtx, void *));
119 /* Unnecessary insns are indicated using insns' in_struct bit. */
121 /* Indicate INSN is dead-code; returns nothing. */
122 #define KILL_INSN(INSN) INSN_DEAD_CODE_P(INSN) = 1
123 /* Indicate INSN is necessary, i.e., not dead-code; returns nothing. */
124 #define RESURRECT_INSN(INSN) INSN_DEAD_CODE_P(INSN) = 0
125 /* Return nonzero if INSN is unnecessary. */
126 #define UNNECESSARY_P(INSN) INSN_DEAD_CODE_P(INSN)
127 static void mark_all_insn_unnecessary
129 /* Execute CODE with free variable INSN for all unnecessary insns in
130 an unspecified order, producing no output. */
131 #define EXECUTE_IF_UNNECESSARY(INSN, CODE) \
135 for (INSN = get_insns (); INSN != NULL_RTX; INSN = NEXT_INSN (INSN)) \
136 if (INSN_DEAD_CODE_P (INSN)) { \
140 /* Find the label beginning block BB. */
141 static rtx find_block_label
142 PARAMS ((basic_block bb));
143 /* Remove INSN, updating its basic block structure. */
144 static void delete_insn_bb
147 /* Recording which blocks are control dependent on which edges. We
148 expect each block to be control dependent on very few edges so we
149 use a bitmap for each block recording its edges. An array holds
150 the bitmap. Its position 0 entry holds the bitmap for block
151 INVALID_BLOCK+1 so that all blocks, including the entry and exit
152 blocks can participate in the data structure. */
154 /* Create a control_dependent_block_to_edge_map, given the number
155 NUM_BASIC_BLOCKS of non-entry, non-exit basic blocks, e.g.,
156 n_basic_blocks. This memory must be released using
157 control_dependent_block_to_edge_map_free (). */
159 static control_dependent_block_to_edge_map
160 control_dependent_block_to_edge_map_create (num_basic_blocks)
161 size_t num_basic_blocks;
164 control_dependent_block_to_edge_map c
165 = xmalloc (sizeof (control_dependent_block_to_edge_map_s));
166 c->length = num_basic_blocks - (INVALID_BLOCK+1);
167 c->data = xmalloc ((size_t) c->length*sizeof (bitmap));
168 for (i = 0; i < c->length; ++i)
169 c->data[i] = BITMAP_XMALLOC ();
174 /* Indicate block BB is control dependent on an edge with index
175 EDGE_INDEX in the mapping C of blocks to edges on which they are
176 control-dependent. */
179 set_control_dependent_block_to_edge_map_bit (c, bb, edge_index)
180 control_dependent_block_to_edge_map c;
184 if (bb->index - (INVALID_BLOCK+1) >= c->length)
187 bitmap_set_bit (c->data[bb->index - (INVALID_BLOCK+1)],
191 /* Execute CODE for each edge (given number EDGE_NUMBER within the
192 CODE) for which the block containing INSN is control dependent,
193 returning no output. CDBTE is the mapping of blocks to edges on
194 which they are control-dependent. */
196 #define EXECUTE_IF_CONTROL_DEPENDENT(CDBTE, INSN, EDGE_NUMBER, CODE) \
197 EXECUTE_IF_SET_IN_BITMAP \
198 (CDBTE->data[BLOCK_NUM (INSN) - (INVALID_BLOCK+1)], 0, \
201 /* Destroy a control_dependent_block_to_edge_map C. */
204 control_dependent_block_to_edge_map_free (c)
205 control_dependent_block_to_edge_map c;
208 for (i = 0; i < c->length; ++i)
209 BITMAP_XFREE (c->data[i]);
213 /* Record all blocks' control dependences on all edges in the edge
214 list EL, ala Morgan, Section 3.6. The mapping PDOM of blocks to
215 their postdominators are used, and results are stored in CDBTE,
216 which should be empty. */
219 find_all_control_dependences (el, pdom, cdbte)
220 struct edge_list *el;
222 control_dependent_block_to_edge_map cdbte;
226 for (i = 0; i < NUM_EDGES (el); ++i)
227 find_control_dependence (el, i, pdom, cdbte);
230 /* Determine all blocks' control dependences on the given edge with
231 edge_list EL index EDGE_INDEX, ala Morgan, Section 3.6. The
232 mapping PDOM of blocks to their postdominators are used, and
233 results are stored in CDBTE, which is assumed to be initialized
234 with zeros in each (block b', edge) position. */
237 find_control_dependence (el, edge_index, pdom, cdbte)
238 struct edge_list *el;
241 control_dependent_block_to_edge_map cdbte;
243 basic_block current_block;
244 basic_block ending_block;
246 if (INDEX_EDGE_PRED_BB (el, edge_index) == EXIT_BLOCK_PTR)
249 (INDEX_EDGE_PRED_BB (el, edge_index) == ENTRY_BLOCK_PTR)
251 : find_pdom (pdom, INDEX_EDGE_PRED_BB (el, edge_index));
253 for (current_block = INDEX_EDGE_SUCC_BB (el, edge_index);
254 current_block != ending_block && current_block != EXIT_BLOCK_PTR;
255 current_block = find_pdom (pdom, current_block))
257 set_control_dependent_block_to_edge_map_bit (cdbte,
263 /* Find the immediate postdominator PDOM of the specified basic block
264 BLOCK. This function is necessary because some blocks have
268 find_pdom (pdom, block)
274 if (block->index == INVALID_BLOCK)
277 if (block == ENTRY_BLOCK_PTR)
278 return BASIC_BLOCK (0);
279 else if (block == EXIT_BLOCK_PTR || pdom[block->index] == EXIT_BLOCK)
280 return EXIT_BLOCK_PTR;
282 return BASIC_BLOCK (pdom[block->index]);
285 /* Determine if the given CURRENT_RTX uses a hard register not
286 converted to SSA. Returns nonzero only if it uses such a hard
287 register. DATA is not used.
289 The program counter (PC) is not considered inherently necessary
290 since code should be position-independent and thus not depend on
291 particular PC values. */
294 inherently_necessary_register_1 (current_rtx, data)
296 void *data ATTRIBUTE_UNUSED;
298 rtx x = *current_rtx;
302 switch (GET_CODE (x))
305 /* Do not traverse the rest of the clobber. */
312 if (CONVERT_REGISTER_TO_SSA_P (REGNO (x)) || x == pc_rtx)
323 /* Return nonzero if the insn CURRENT_RTX is inherently necessary. */
326 inherently_necessary_register (current_rtx)
329 return for_each_rtx (¤t_rtx,
330 &inherently_necessary_register_1, NULL);
334 /* Called via note_stores for each store in an insn. Note whether
335 or not a particular store is inherently necessary. Store a
336 nonzero value in inherently_necessary_p if such a storeis found. */
339 note_inherently_necessary_set (dest, set, data)
344 int *inherently_necessary_set_p = (int *)data;
346 while (GET_CODE (dest) == SUBREG
347 || GET_CODE (dest) == STRICT_LOW_PART
348 || GET_CODE (dest) == ZERO_EXTRACT
349 || GET_CODE (dest) == SIGN_EXTRACT)
350 dest = XEXP (dest, 0);
352 if (GET_CODE (dest) == MEM
353 || GET_CODE (dest) == UNSPEC
354 || GET_CODE (dest) == UNSPEC_VOLATILE)
355 *inherently_necessary_set_p = 1;
358 /* Mark X as inherently necessary if appropriate. For example,
359 function calls and storing values into memory are inherently
360 necessary. This function is to be used with for_each_rtx ().
361 Return nonzero iff inherently necessary. */
364 find_inherently_necessary (x)
370 else if (inherently_necessary_register (x))
373 switch (GET_CODE (x))
383 return JUMP_TABLE_DATA_P (x) || computed_jump_p (x) != 0;
387 int inherently_necessary_set = 0;
388 note_stores (PATTERN (x),
389 note_inherently_necessary_set,
390 &inherently_necessary_set);
392 /* If we found an inherently necessary set or an asm
393 instruction, then we consider this insn inherently
395 return (inherently_necessary_set
396 || GET_CODE (PATTERN (x)) == ASM_INPUT
397 || asm_noperands (PATTERN (x)) >= 0);
400 /* Found an impossible insn type. */
406 /* Propagate necessity through REG and SUBREG operands of CURRENT_RTX.
407 This function is called with for_each_rtx () on necessary
408 instructions. The DATA must be a varray of unprocessed
412 propagate_necessity_through_operand (current_rtx, data)
416 rtx x = *current_rtx;
417 varray_type *unprocessed_instructions = (varray_type *) data;
421 switch ( GET_CODE (x))
424 if (CONVERT_REGISTER_TO_SSA_P (REGNO (x)))
426 rtx insn = VARRAY_RTX (ssa_definition, REGNO (x));
427 if (insn != NULL_RTX && UNNECESSARY_P (insn))
429 RESURRECT_INSN (insn);
430 VARRAY_PUSH_RTX (*unprocessed_instructions, insn);
440 /* Indicate all insns initially assumed to be unnecessary. */
443 mark_all_insn_unnecessary ()
446 for (insn = get_insns (); insn != NULL_RTX; insn = NEXT_INSN (insn))
450 /* Find the label beginning block BB, adding one if necessary. */
453 find_block_label (bb)
461 rtx new_label = emit_label_before (gen_label_rtx (), insn);
462 if (insn == bb->head)
463 bb->head = new_label;
468 /* Remove INSN, updating its basic block structure. */
471 delete_insn_bb (insn)
478 /* Do not actually delete anything that is not an INSN.
480 We can get here because we only consider INSNs as
481 potentially necessary. We leave it to later passes
482 to remove unnecessary notes, unused labels, etc. */
486 bb = BLOCK_FOR_INSN (insn);
489 if (bb->head == bb->end)
491 /* Delete the insn by converting it to a note. */
492 PUT_CODE (insn, NOTE);
493 NOTE_LINE_NUMBER (insn) = NOTE_INSN_DELETED;
496 else if (insn == bb->head)
497 bb->head = NEXT_INSN (insn);
498 else if (insn == bb->end)
499 bb->end = PREV_INSN (insn);
503 /* Perform the dead-code elimination. */
506 ssa_eliminate_dead_code ()
510 /* Necessary instructions with operands to explore. */
511 varray_type unprocessed_instructions;
512 /* Map element (b,e) is nonzero if the block is control dependent on
513 edge. "cdbte" abbreviates control dependent block to edge. */
514 control_dependent_block_to_edge_map cdbte;
515 /* Element I is the immediate postdominator of block I. */
517 struct edge_list *el;
519 int max_insn_uid = get_max_uid ();
521 /* Initialize the data structures. */
522 mark_all_insn_unnecessary ();
523 VARRAY_RTX_INIT (unprocessed_instructions, 64,
524 "unprocessed instructions");
525 cdbte = control_dependent_block_to_edge_map_create (n_basic_blocks);
527 /* Prepare for use of BLOCK_NUM (). */
528 connect_infinite_loops_to_exit ();
529 /* Be careful not to clear the added edges. */
530 compute_bb_for_insn (max_insn_uid);
532 /* Compute control dependence. */
533 pdom = (int *) xmalloc (n_basic_blocks * sizeof (int));
534 for (i = 0; i < n_basic_blocks; ++i)
535 pdom[i] = INVALID_BLOCK;
536 calculate_dominance_info (pdom, NULL, CDI_POST_DOMINATORS);
537 /* Assume there is a path from each node to the exit block. */
538 for (i = 0; i < n_basic_blocks; ++i)
539 if (pdom[i] == INVALID_BLOCK)
540 pdom[i] = EXIT_BLOCK;
541 el = create_edge_list();
542 find_all_control_dependences (el, pdom, cdbte);
544 /* Find inherently necessary instructions. */
545 for (insn = get_insns (); insn != NULL_RTX; insn = NEXT_INSN (insn))
546 if (find_inherently_necessary (insn))
548 RESURRECT_INSN (insn);
549 VARRAY_PUSH_RTX (unprocessed_instructions, insn);
552 /* Propagate necessity using the operands of necessary instructions. */
553 while (VARRAY_ACTIVE_SIZE (unprocessed_instructions) > 0)
555 rtx current_instruction;
558 current_instruction = VARRAY_TOP_RTX (unprocessed_instructions);
559 VARRAY_POP (unprocessed_instructions);
561 /* Make corresponding control dependent edges necessary. */
562 /* Assume the only JUMP_INSN is the block's last insn. It appears
563 that the last instruction of the program need not be a
566 if (INSN_P (current_instruction)
567 && !JUMP_TABLE_DATA_P (current_instruction))
569 /* Notes and labels contain no interesting operands. */
570 EXECUTE_IF_CONTROL_DEPENDENT
571 (cdbte, current_instruction, edge_number,
573 rtx jump_insn = (INDEX_EDGE_PRED_BB (el, edge_number))->end;
574 if (GET_CODE (jump_insn) == JUMP_INSN
575 && UNNECESSARY_P (jump_insn))
577 RESURRECT_INSN (jump_insn);
578 VARRAY_PUSH_RTX (unprocessed_instructions, jump_insn);
582 /* Propagate through the operands. */
583 for_each_rtx (¤t_instruction,
584 &propagate_necessity_through_operand,
585 (PTR) &unprocessed_instructions);
587 /* PHI nodes are somewhat special in that each PHI alternative
588 has data and control dependencies. The data dependencies
589 are handled via propagate_necessity_through_operand. We
590 handle the control dependency here.
592 We consider the control dependent edges leading to the
593 predecessor block associated with each PHI alternative
595 if (PHI_NODE_P (current_instruction))
597 rtvec phi_vec = XVEC (SET_SRC (PATTERN (current_instruction)), 0);
598 int num_elem = GET_NUM_ELEM (phi_vec);
601 for (v = num_elem - 2; v >= 0; v -= 2)
605 bb = BASIC_BLOCK (INTVAL (RTVEC_ELT (phi_vec, v + 1)));
606 EXECUTE_IF_CONTROL_DEPENDENT
607 (cdbte, bb->end, edge_number,
611 jump_insn = (INDEX_EDGE_PRED_BB (el, edge_number))->end;
612 if (((GET_CODE (jump_insn) == JUMP_INSN))
613 && UNNECESSARY_P (jump_insn))
615 RESURRECT_INSN (jump_insn);
616 VARRAY_PUSH_RTX (unprocessed_instructions, jump_insn);
625 /* Remove the unnecessary instructions. */
626 EXECUTE_IF_UNNECESSARY (insn,
628 if (any_condjump_p (insn))
630 /* Convert unnecessary conditional insn to an unconditional
631 jump to immediate postdominator block. */
632 rtx old_label = JUMP_LABEL (insn);
633 int pdom_block_number =
634 find_pdom (pdom, BLOCK_FOR_INSN (insn))->index;
636 /* Prevent the conditional jump's label from being deleted so
637 we do not have to modify the basic block structure. */
638 ++LABEL_NUSES (old_label);
640 if (pdom_block_number != EXIT_BLOCK
641 && pdom_block_number != INVALID_BLOCK)
643 rtx lbl = find_block_label (BASIC_BLOCK (pdom_block_number));
644 rtx new_jump = emit_jump_insn_before (gen_jump (lbl), insn);
646 /* Let jump know that label is in use. */
647 JUMP_LABEL (new_jump) = lbl;
650 delete_insn_bb (insn);
652 /* A conditional branch is unnecessary if and only if any
653 block control-dependent on it is unnecessary. Thus,
654 any phi nodes in these unnecessary blocks are also
655 removed and these nodes need not be updated. */
657 /* A barrier must follow any unconditional jump. Barriers
658 are not in basic blocks so this must occur after
659 deleting the conditional jump. */
660 emit_barrier_after (new_jump);
663 /* The block drops off the end of the function and the
664 ending conditional jump is not needed. */
665 delete_insn_bb (insn);
667 else if (!JUMP_P (insn))
668 delete_insn_bb (insn);
671 /* Remove fake edges from the CFG. */
672 remove_fake_edges ();
674 /* Release allocated memory. */
675 for (insn = get_insns (); insn != NULL_RTX; insn = NEXT_INSN (insn))
676 RESURRECT_INSN (insn);
677 if (VARRAY_ACTIVE_SIZE (unprocessed_instructions) != 0)
679 VARRAY_FREE (unprocessed_instructions);
680 control_dependent_block_to_edge_map_free (cdbte);