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[official-gcc.git] / gcc / tree-ssa-sink.c
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1 /* Code sinking for trees
2 Copyright (C) 2001-2017 Free Software Foundation, Inc.
3 Contributed by Daniel Berlin <dan@dberlin.org>
5 This file is part of GCC.
7 GCC is free software; you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation; either version 3, or (at your option)
10 any later version.
12 GCC is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with GCC; see the file COPYING3. If not see
19 <http://www.gnu.org/licenses/>. */
21 #include "config.h"
22 #include "system.h"
23 #include "coretypes.h"
24 #include "backend.h"
25 #include "tree.h"
26 #include "gimple.h"
27 #include "cfghooks.h"
28 #include "tree-pass.h"
29 #include "ssa.h"
30 #include "gimple-pretty-print.h"
31 #include "fold-const.h"
32 #include "stor-layout.h"
33 #include "cfganal.h"
34 #include "gimple-iterator.h"
35 #include "tree-cfg.h"
36 #include "cfgloop.h"
37 #include "params.h"
39 /* TODO:
40 1. Sinking store only using scalar promotion (IE without moving the RHS):
42 *q = p;
43 p = p + 1;
44 if (something)
45 *q = <not p>;
46 else
47 y = *q;
50 should become
51 sinktemp = p;
52 p = p + 1;
53 if (something)
54 *q = <not p>;
55 else
57 *q = sinktemp;
58 y = *q
60 Store copy propagation will take care of the store elimination above.
63 2. Sinking using Partial Dead Code Elimination. */
66 static struct
68 /* The number of statements sunk down the flowgraph by code sinking. */
69 int sunk;
71 } sink_stats;
74 /* Given a PHI, and one of its arguments (DEF), find the edge for
75 that argument and return it. If the argument occurs twice in the PHI node,
76 we return NULL. */
78 static basic_block
79 find_bb_for_arg (gphi *phi, tree def)
81 size_t i;
82 bool foundone = false;
83 basic_block result = NULL;
84 for (i = 0; i < gimple_phi_num_args (phi); i++)
85 if (PHI_ARG_DEF (phi, i) == def)
87 if (foundone)
88 return NULL;
89 foundone = true;
90 result = gimple_phi_arg_edge (phi, i)->src;
92 return result;
95 /* When the first immediate use is in a statement, then return true if all
96 immediate uses in IMM are in the same statement.
97 We could also do the case where the first immediate use is in a phi node,
98 and all the other uses are in phis in the same basic block, but this
99 requires some expensive checking later (you have to make sure no def/vdef
100 in the statement occurs for multiple edges in the various phi nodes it's
101 used in, so that you only have one place you can sink it to. */
103 static bool
104 all_immediate_uses_same_place (def_operand_p def_p)
106 tree var = DEF_FROM_PTR (def_p);
107 imm_use_iterator imm_iter;
108 use_operand_p use_p;
110 gimple *firstuse = NULL;
111 FOR_EACH_IMM_USE_FAST (use_p, imm_iter, var)
113 if (is_gimple_debug (USE_STMT (use_p)))
114 continue;
115 if (firstuse == NULL)
116 firstuse = USE_STMT (use_p);
117 else
118 if (firstuse != USE_STMT (use_p))
119 return false;
122 return true;
125 /* Find the nearest common dominator of all of the immediate uses in IMM. */
127 static basic_block
128 nearest_common_dominator_of_uses (def_operand_p def_p, bool *debug_stmts)
130 tree var = DEF_FROM_PTR (def_p);
131 auto_bitmap blocks;
132 basic_block commondom;
133 unsigned int j;
134 bitmap_iterator bi;
135 imm_use_iterator imm_iter;
136 use_operand_p use_p;
138 FOR_EACH_IMM_USE_FAST (use_p, imm_iter, var)
140 gimple *usestmt = USE_STMT (use_p);
141 basic_block useblock;
143 if (gphi *phi = dyn_cast <gphi *> (usestmt))
145 int idx = PHI_ARG_INDEX_FROM_USE (use_p);
147 useblock = gimple_phi_arg_edge (phi, idx)->src;
149 else if (is_gimple_debug (usestmt))
151 *debug_stmts = true;
152 continue;
154 else
156 useblock = gimple_bb (usestmt);
159 /* Short circuit. Nothing dominates the entry block. */
160 if (useblock == ENTRY_BLOCK_PTR_FOR_FN (cfun))
161 return NULL;
163 bitmap_set_bit (blocks, useblock->index);
165 commondom = BASIC_BLOCK_FOR_FN (cfun, bitmap_first_set_bit (blocks));
166 EXECUTE_IF_SET_IN_BITMAP (blocks, 0, j, bi)
167 commondom = nearest_common_dominator (CDI_DOMINATORS, commondom,
168 BASIC_BLOCK_FOR_FN (cfun, j));
169 return commondom;
172 /* Given EARLY_BB and LATE_BB, two blocks in a path through the dominator
173 tree, return the best basic block between them (inclusive) to place
174 statements.
176 We want the most control dependent block in the shallowest loop nest.
178 If the resulting block is in a shallower loop nest, then use it. Else
179 only use the resulting block if it has significantly lower execution
180 frequency than EARLY_BB to avoid gratutious statement movement. We
181 consider statements with VOPS more desirable to move.
183 This pass would obviously benefit from PDO as it utilizes block
184 frequencies. It would also benefit from recomputing frequencies
185 if profile data is not available since frequencies often get out
186 of sync with reality. */
188 static basic_block
189 select_best_block (basic_block early_bb,
190 basic_block late_bb,
191 gimple *stmt)
193 basic_block best_bb = late_bb;
194 basic_block temp_bb = late_bb;
195 int threshold;
197 while (temp_bb != early_bb)
199 /* If we've moved into a lower loop nest, then that becomes
200 our best block. */
201 if (bb_loop_depth (temp_bb) < bb_loop_depth (best_bb))
202 best_bb = temp_bb;
204 /* Walk up the dominator tree, hopefully we'll find a shallower
205 loop nest. */
206 temp_bb = get_immediate_dominator (CDI_DOMINATORS, temp_bb);
209 /* If we found a shallower loop nest, then we always consider that
210 a win. This will always give us the most control dependent block
211 within that loop nest. */
212 if (bb_loop_depth (best_bb) < bb_loop_depth (early_bb))
213 return best_bb;
215 /* Get the sinking threshold. If the statement to be moved has memory
216 operands, then increase the threshold by 7% as those are even more
217 profitable to avoid, clamping at 100%. */
218 threshold = PARAM_VALUE (PARAM_SINK_FREQUENCY_THRESHOLD);
219 if (gimple_vuse (stmt) || gimple_vdef (stmt))
221 threshold += 7;
222 if (threshold > 100)
223 threshold = 100;
226 /* If BEST_BB is at the same nesting level, then require it to have
227 significantly lower execution frequency to avoid gratutious movement. */
228 if (bb_loop_depth (best_bb) == bb_loop_depth (early_bb)
229 && best_bb->frequency < (early_bb->frequency * threshold / 100.0))
230 return best_bb;
232 /* No better block found, so return EARLY_BB, which happens to be the
233 statement's original block. */
234 return early_bb;
237 /* Given a statement (STMT) and the basic block it is currently in (FROMBB),
238 determine the location to sink the statement to, if any.
239 Returns true if there is such location; in that case, TOGSI points to the
240 statement before that STMT should be moved. */
242 static bool
243 statement_sink_location (gimple *stmt, basic_block frombb,
244 gimple_stmt_iterator *togsi, bool *zero_uses_p)
246 gimple *use;
247 use_operand_p one_use = NULL_USE_OPERAND_P;
248 basic_block sinkbb;
249 use_operand_p use_p;
250 def_operand_p def_p;
251 ssa_op_iter iter;
252 imm_use_iterator imm_iter;
254 *zero_uses_p = false;
256 /* We only can sink assignments and non-looping const/pure calls. */
257 int cf;
258 if (!is_gimple_assign (stmt)
259 && (!is_gimple_call (stmt)
260 || !((cf = gimple_call_flags (stmt)) & (ECF_CONST|ECF_PURE))
261 || (cf & ECF_LOOPING_CONST_OR_PURE)))
262 return false;
264 /* We only can sink stmts with a single definition. */
265 def_p = single_ssa_def_operand (stmt, SSA_OP_ALL_DEFS);
266 if (def_p == NULL_DEF_OPERAND_P)
267 return false;
269 /* There are a few classes of things we can't or don't move, some because we
270 don't have code to handle it, some because it's not profitable and some
271 because it's not legal.
273 We can't sink things that may be global stores, at least not without
274 calculating a lot more information, because we may cause it to no longer
275 be seen by an external routine that needs it depending on where it gets
276 moved to.
278 We can't sink statements that end basic blocks without splitting the
279 incoming edge for the sink location to place it there.
281 We can't sink statements that have volatile operands.
283 We don't want to sink dead code, so anything with 0 immediate uses is not
284 sunk.
286 Don't sink BLKmode assignments if current function has any local explicit
287 register variables, as BLKmode assignments may involve memcpy or memset
288 calls or, on some targets, inline expansion thereof that sometimes need
289 to use specific hard registers.
292 if (stmt_ends_bb_p (stmt)
293 || gimple_has_side_effects (stmt)
294 || (cfun->has_local_explicit_reg_vars
295 && TYPE_MODE (TREE_TYPE (gimple_get_lhs (stmt))) == BLKmode))
296 return false;
298 /* Return if there are no immediate uses of this stmt. */
299 if (has_zero_uses (DEF_FROM_PTR (def_p)))
301 *zero_uses_p = true;
302 return false;
305 if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (DEF_FROM_PTR (def_p)))
306 return false;
308 FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_ALL_USES)
310 tree use = USE_FROM_PTR (use_p);
311 if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (use))
312 return false;
315 use = NULL;
317 /* If stmt is a store the one and only use needs to be the VOP
318 merging PHI node. */
319 if (virtual_operand_p (DEF_FROM_PTR (def_p)))
321 FOR_EACH_IMM_USE_FAST (use_p, imm_iter, DEF_FROM_PTR (def_p))
323 gimple *use_stmt = USE_STMT (use_p);
325 /* A killing definition is not a use. */
326 if ((gimple_has_lhs (use_stmt)
327 && operand_equal_p (gimple_get_lhs (stmt),
328 gimple_get_lhs (use_stmt), 0))
329 || stmt_kills_ref_p (use_stmt, gimple_get_lhs (stmt)))
331 /* If use_stmt is or might be a nop assignment then USE_STMT
332 acts as a use as well as definition. */
333 if (stmt != use_stmt
334 && ref_maybe_used_by_stmt_p (use_stmt,
335 gimple_get_lhs (stmt)))
336 return false;
337 continue;
340 if (gimple_code (use_stmt) != GIMPLE_PHI)
341 return false;
343 if (use
344 && use != use_stmt)
345 return false;
347 use = use_stmt;
349 if (!use)
350 return false;
352 /* If all the immediate uses are not in the same place, find the nearest
353 common dominator of all the immediate uses. For PHI nodes, we have to
354 find the nearest common dominator of all of the predecessor blocks, since
355 that is where insertion would have to take place. */
356 else if (gimple_vuse (stmt)
357 || !all_immediate_uses_same_place (def_p))
359 bool debug_stmts = false;
360 basic_block commondom = nearest_common_dominator_of_uses (def_p,
361 &debug_stmts);
363 if (commondom == frombb)
364 return false;
366 /* If this is a load then do not sink past any stores.
367 ??? This is overly simple but cheap. We basically look
368 for an existing load with the same VUSE in the path to one
369 of the sink candidate blocks and we adjust commondom to the
370 nearest to commondom. */
371 if (gimple_vuse (stmt))
373 /* Do not sink loads from hard registers. */
374 if (gimple_assign_single_p (stmt)
375 && TREE_CODE (gimple_assign_rhs1 (stmt)) == VAR_DECL
376 && DECL_HARD_REGISTER (gimple_assign_rhs1 (stmt)))
377 return false;
379 imm_use_iterator imm_iter;
380 use_operand_p use_p;
381 basic_block found = NULL;
382 FOR_EACH_IMM_USE_FAST (use_p, imm_iter, gimple_vuse (stmt))
384 gimple *use_stmt = USE_STMT (use_p);
385 basic_block bb = gimple_bb (use_stmt);
386 /* For PHI nodes the block we know sth about
387 is the incoming block with the use. */
388 if (gimple_code (use_stmt) == GIMPLE_PHI)
389 bb = EDGE_PRED (bb, PHI_ARG_INDEX_FROM_USE (use_p))->src;
390 /* Any dominator of commondom would be ok with
391 adjusting commondom to that block. */
392 bb = nearest_common_dominator (CDI_DOMINATORS, bb, commondom);
393 if (!found)
394 found = bb;
395 else if (dominated_by_p (CDI_DOMINATORS, bb, found))
396 found = bb;
397 /* If we can't improve, stop. */
398 if (found == commondom)
399 break;
401 commondom = found;
402 if (commondom == frombb)
403 return false;
406 /* Our common dominator has to be dominated by frombb in order to be a
407 trivially safe place to put this statement, since it has multiple
408 uses. */
409 if (!dominated_by_p (CDI_DOMINATORS, commondom, frombb))
410 return false;
412 commondom = select_best_block (frombb, commondom, stmt);
414 if (commondom == frombb)
415 return false;
417 *togsi = gsi_after_labels (commondom);
419 return true;
421 else
423 FOR_EACH_IMM_USE_FAST (one_use, imm_iter, DEF_FROM_PTR (def_p))
425 if (is_gimple_debug (USE_STMT (one_use)))
426 continue;
427 break;
429 use = USE_STMT (one_use);
431 if (gimple_code (use) != GIMPLE_PHI)
433 sinkbb = gimple_bb (use);
434 sinkbb = select_best_block (frombb, gimple_bb (use), stmt);
436 if (sinkbb == frombb)
437 return false;
439 *togsi = gsi_for_stmt (use);
441 return true;
445 sinkbb = find_bb_for_arg (as_a <gphi *> (use), DEF_FROM_PTR (def_p));
447 /* This can happen if there are multiple uses in a PHI. */
448 if (!sinkbb)
449 return false;
451 sinkbb = select_best_block (frombb, sinkbb, stmt);
452 if (!sinkbb || sinkbb == frombb)
453 return false;
455 /* If the latch block is empty, don't make it non-empty by sinking
456 something into it. */
457 if (sinkbb == frombb->loop_father->latch
458 && empty_block_p (sinkbb))
459 return false;
461 *togsi = gsi_after_labels (sinkbb);
463 return true;
466 /* Perform code sinking on BB */
468 static void
469 sink_code_in_bb (basic_block bb)
471 basic_block son;
472 gimple_stmt_iterator gsi;
473 edge_iterator ei;
474 edge e;
475 bool last = true;
477 /* If this block doesn't dominate anything, there can't be any place to sink
478 the statements to. */
479 if (first_dom_son (CDI_DOMINATORS, bb) == NULL)
480 goto earlyout;
482 /* We can't move things across abnormal edges, so don't try. */
483 FOR_EACH_EDGE (e, ei, bb->succs)
484 if (e->flags & EDGE_ABNORMAL)
485 goto earlyout;
487 for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
489 gimple *stmt = gsi_stmt (gsi);
490 gimple_stmt_iterator togsi;
491 bool zero_uses_p;
493 if (!statement_sink_location (stmt, bb, &togsi, &zero_uses_p))
495 gimple_stmt_iterator saved = gsi;
496 if (!gsi_end_p (gsi))
497 gsi_prev (&gsi);
498 /* If we face a dead stmt remove it as it possibly blocks
499 sinking of uses. */
500 if (zero_uses_p
501 && ! gimple_vdef (stmt))
503 gsi_remove (&saved, true);
504 release_defs (stmt);
506 else
507 last = false;
508 continue;
510 if (dump_file)
512 fprintf (dump_file, "Sinking ");
513 print_gimple_stmt (dump_file, stmt, 0, TDF_VOPS);
514 fprintf (dump_file, " from bb %d to bb %d\n",
515 bb->index, (gsi_bb (togsi))->index);
518 /* Update virtual operands of statements in the path we
519 do not sink to. */
520 if (gimple_vdef (stmt))
522 imm_use_iterator iter;
523 use_operand_p use_p;
524 gimple *vuse_stmt;
526 FOR_EACH_IMM_USE_STMT (vuse_stmt, iter, gimple_vdef (stmt))
527 if (gimple_code (vuse_stmt) != GIMPLE_PHI)
528 FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
529 SET_USE (use_p, gimple_vuse (stmt));
532 /* If this is the end of the basic block, we need to insert at the end
533 of the basic block. */
534 if (gsi_end_p (togsi))
535 gsi_move_to_bb_end (&gsi, gsi_bb (togsi));
536 else
537 gsi_move_before (&gsi, &togsi);
539 sink_stats.sunk++;
541 /* If we've just removed the last statement of the BB, the
542 gsi_end_p() test below would fail, but gsi_prev() would have
543 succeeded, and we want it to succeed. So we keep track of
544 whether we're at the last statement and pick up the new last
545 statement. */
546 if (last)
548 gsi = gsi_last_bb (bb);
549 continue;
552 last = false;
553 if (!gsi_end_p (gsi))
554 gsi_prev (&gsi);
557 earlyout:
558 for (son = first_dom_son (CDI_POST_DOMINATORS, bb);
559 son;
560 son = next_dom_son (CDI_POST_DOMINATORS, son))
562 sink_code_in_bb (son);
566 /* Perform code sinking.
567 This moves code down the flowgraph when we know it would be
568 profitable to do so, or it wouldn't increase the number of
569 executions of the statement.
571 IE given
573 a_1 = b + c;
574 if (<something>)
577 else
579 foo (&b, &c);
580 a_5 = b + c;
582 a_6 = PHI (a_5, a_1);
583 USE a_6.
585 we'll transform this into:
587 if (<something>)
589 a_1 = b + c;
591 else
593 foo (&b, &c);
594 a_5 = b + c;
596 a_6 = PHI (a_5, a_1);
597 USE a_6.
599 Note that this reduces the number of computations of a = b + c to 1
600 when we take the else edge, instead of 2.
602 namespace {
604 const pass_data pass_data_sink_code =
606 GIMPLE_PASS, /* type */
607 "sink", /* name */
608 OPTGROUP_NONE, /* optinfo_flags */
609 TV_TREE_SINK, /* tv_id */
610 /* PROP_no_crit_edges is ensured by running split_critical_edges in
611 pass_data_sink_code::execute (). */
612 ( PROP_cfg | PROP_ssa ), /* properties_required */
613 0, /* properties_provided */
614 0, /* properties_destroyed */
615 0, /* todo_flags_start */
616 TODO_update_ssa, /* todo_flags_finish */
619 class pass_sink_code : public gimple_opt_pass
621 public:
622 pass_sink_code (gcc::context *ctxt)
623 : gimple_opt_pass (pass_data_sink_code, ctxt)
626 /* opt_pass methods: */
627 virtual bool gate (function *) { return flag_tree_sink != 0; }
628 virtual unsigned int execute (function *);
630 }; // class pass_sink_code
632 unsigned int
633 pass_sink_code::execute (function *fun)
635 loop_optimizer_init (LOOPS_NORMAL);
636 split_critical_edges ();
637 connect_infinite_loops_to_exit ();
638 memset (&sink_stats, 0, sizeof (sink_stats));
639 calculate_dominance_info (CDI_DOMINATORS);
640 calculate_dominance_info (CDI_POST_DOMINATORS);
641 sink_code_in_bb (EXIT_BLOCK_PTR_FOR_FN (fun));
642 statistics_counter_event (fun, "Sunk statements", sink_stats.sunk);
643 free_dominance_info (CDI_POST_DOMINATORS);
644 remove_fake_exit_edges ();
645 loop_optimizer_finalize ();
647 return 0;
650 } // anon namespace
652 gimple_opt_pass *
653 make_pass_sink_code (gcc::context *ctxt)
655 return new pass_sink_code (ctxt);