1 /* Code sinking for trees
2    Copyright (C) 2001-2018 Free Software Foundation, Inc.
3    Contributed by Daniel Berlin <dan@dberlin.org>
4 
5 This file is part of GCC.
6 
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.
11 
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.
16 
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/>.  */
20 
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"
38 
39 /* TODO:
40    1. Sinking store only using scalar promotion (IE without moving the RHS):
41 
42    *q = p;
43    p = p + 1;
44    if (something)
45      *q = <not p>;
46    else
47      y = *q;
48 
49 
50    should become
51    sinktemp = p;
52    p = p + 1;
53    if (something)
54      *q = <not p>;
55    else
56    {
57      *q = sinktemp;
58      y = *q
59    }
60    Store copy propagation will take care of the store elimination above.
61 
62 
63    2. Sinking using Partial Dead Code Elimination.  */
64 
65 
66 static struct
67 {
68   /* The number of statements sunk down the flowgraph by code sinking.  */
69   int sunk;
70 
71 } sink_stats;
72 
73 
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.  */
77 
78 static basic_block
find_bb_for_arg(gphi * phi,tree def)79 find_bb_for_arg (gphi *phi, tree def)
80 {
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)
86       {
87 	if (foundone)
88 	  return NULL;
89 	foundone = true;
90 	result = gimple_phi_arg_edge (phi, i)->src;
91       }
92   return result;
93 }
94 
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.  */
102 
103 static bool
all_immediate_uses_same_place(def_operand_p def_p)104 all_immediate_uses_same_place (def_operand_p def_p)
105 {
106   tree var = DEF_FROM_PTR (def_p);
107   imm_use_iterator imm_iter;
108   use_operand_p use_p;
109 
110   gimple *firstuse = NULL;
111   FOR_EACH_IMM_USE_FAST (use_p, imm_iter, var)
112     {
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;
120     }
121 
122   return true;
123 }
124 
125 /* Find the nearest common dominator of all of the immediate uses in IMM.  */
126 
127 static basic_block
nearest_common_dominator_of_uses(def_operand_p def_p,bool * debug_stmts)128 nearest_common_dominator_of_uses (def_operand_p def_p, bool *debug_stmts)
129 {
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;
137 
138   FOR_EACH_IMM_USE_FAST (use_p, imm_iter, var)
139     {
140       gimple *usestmt = USE_STMT (use_p);
141       basic_block useblock;
142 
143       if (gphi *phi = dyn_cast <gphi *> (usestmt))
144 	{
145 	  int idx = PHI_ARG_INDEX_FROM_USE (use_p);
146 
147 	  useblock = gimple_phi_arg_edge (phi, idx)->src;
148 	}
149       else if (is_gimple_debug (usestmt))
150 	{
151 	  *debug_stmts = true;
152 	  continue;
153 	}
154       else
155 	{
156 	  useblock = gimple_bb (usestmt);
157 	}
158 
159       /* Short circuit. Nothing dominates the entry block.  */
160       if (useblock == ENTRY_BLOCK_PTR_FOR_FN (cfun))
161 	return NULL;
162 
163       bitmap_set_bit (blocks, useblock->index);
164     }
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;
170 }
171 
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.
175 
176    We want the most control dependent block in the shallowest loop nest.
177 
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.
182 
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.  */
187 
188 static basic_block
select_best_block(basic_block early_bb,basic_block late_bb,gimple * stmt)189 select_best_block (basic_block early_bb,
190 		   basic_block late_bb,
191 		   gimple *stmt)
192 {
193   basic_block best_bb = late_bb;
194   basic_block temp_bb = late_bb;
195   int threshold;
196 
197   while (temp_bb != early_bb)
198     {
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;
203 
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);
207     }
208 
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;
214 
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))
220     {
221       threshold += 7;
222       if (threshold > 100)
223 	threshold = 100;
224     }
225 
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       /* If result of comparsion is unknown, preffer EARLY_BB.
230 	 Thus use !(...>=..) rather than (...<...)  */
231       && !(best_bb->count.apply_scale (100, 1)
232 	   >= early_bb->count.apply_scale (threshold, 1)))
233     return best_bb;
234 
235   /* No better block found, so return EARLY_BB, which happens to be the
236      statement's original block.  */
237   return early_bb;
238 }
239 
240 /* Given a statement (STMT) and the basic block it is currently in (FROMBB),
241    determine the location to sink the statement to, if any.
242    Returns true if there is such location; in that case, TOGSI points to the
243    statement before that STMT should be moved.  */
244 
245 static bool
statement_sink_location(gimple * stmt,basic_block frombb,gimple_stmt_iterator * togsi,bool * zero_uses_p)246 statement_sink_location (gimple *stmt, basic_block frombb,
247 			 gimple_stmt_iterator *togsi, bool *zero_uses_p)
248 {
249   gimple *use;
250   use_operand_p one_use = NULL_USE_OPERAND_P;
251   basic_block sinkbb;
252   use_operand_p use_p;
253   def_operand_p def_p;
254   ssa_op_iter iter;
255   imm_use_iterator imm_iter;
256 
257   *zero_uses_p = false;
258 
259   /* We only can sink assignments and non-looping const/pure calls.  */
260   int cf;
261   if (!is_gimple_assign (stmt)
262       && (!is_gimple_call (stmt)
263 	  || !((cf = gimple_call_flags (stmt)) & (ECF_CONST|ECF_PURE))
264 	  || (cf & ECF_LOOPING_CONST_OR_PURE)))
265     return false;
266 
267   /* We only can sink stmts with a single definition.  */
268   def_p = single_ssa_def_operand (stmt, SSA_OP_ALL_DEFS);
269   if (def_p == NULL_DEF_OPERAND_P)
270     return false;
271 
272   /* There are a few classes of things we can't or don't move, some because we
273      don't have code to handle it, some because it's not profitable and some
274      because it's not legal.
275 
276      We can't sink things that may be global stores, at least not without
277      calculating a lot more information, because we may cause it to no longer
278      be seen by an external routine that needs it depending on where it gets
279      moved to.
280 
281      We can't sink statements that end basic blocks without splitting the
282      incoming edge for the sink location to place it there.
283 
284      We can't sink statements that have volatile operands.
285 
286      We don't want to sink dead code, so anything with 0 immediate uses is not
287      sunk.
288 
289      Don't sink BLKmode assignments if current function has any local explicit
290      register variables, as BLKmode assignments may involve memcpy or memset
291      calls or, on some targets, inline expansion thereof that sometimes need
292      to use specific hard registers.
293 
294   */
295   if (stmt_ends_bb_p (stmt)
296       || gimple_has_side_effects (stmt)
297       || (cfun->has_local_explicit_reg_vars
298 	  && TYPE_MODE (TREE_TYPE (gimple_get_lhs (stmt))) == BLKmode))
299     return false;
300 
301   /* Return if there are no immediate uses of this stmt.  */
302   if (has_zero_uses (DEF_FROM_PTR (def_p)))
303     {
304       *zero_uses_p = true;
305       return false;
306     }
307 
308   if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (DEF_FROM_PTR (def_p)))
309     return false;
310 
311   FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_ALL_USES)
312     {
313       tree use = USE_FROM_PTR (use_p);
314       if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (use))
315 	return false;
316     }
317 
318   use = NULL;
319 
320   /* If stmt is a store the one and only use needs to be the VOP
321      merging PHI node.  */
322   if (virtual_operand_p (DEF_FROM_PTR (def_p)))
323     {
324       FOR_EACH_IMM_USE_FAST (use_p, imm_iter, DEF_FROM_PTR (def_p))
325 	{
326 	  gimple *use_stmt = USE_STMT (use_p);
327 
328 	  /* A killing definition is not a use.  */
329 	  if ((gimple_has_lhs (use_stmt)
330 	       && operand_equal_p (gimple_get_lhs (stmt),
331 				   gimple_get_lhs (use_stmt), 0))
332 	      || stmt_kills_ref_p (use_stmt, gimple_get_lhs (stmt)))
333 	    {
334 	      /* If use_stmt is or might be a nop assignment then USE_STMT
335 	         acts as a use as well as definition.  */
336 	      if (stmt != use_stmt
337 		  && ref_maybe_used_by_stmt_p (use_stmt,
338 					       gimple_get_lhs (stmt)))
339 		return false;
340 	      continue;
341 	    }
342 
343 	  if (gimple_code (use_stmt) != GIMPLE_PHI)
344 	    return false;
345 
346 	  if (use
347 	      && use != use_stmt)
348 	    return false;
349 
350 	  use = use_stmt;
351 	}
352       if (!use)
353 	return false;
354     }
355   /* If all the immediate uses are not in the same place, find the nearest
356      common dominator of all the immediate uses.  For PHI nodes, we have to
357      find the nearest common dominator of all of the predecessor blocks, since
358      that is where insertion would have to take place.  */
359   else if (gimple_vuse (stmt)
360 	   || !all_immediate_uses_same_place (def_p))
361     {
362       bool debug_stmts = false;
363       basic_block commondom = nearest_common_dominator_of_uses (def_p,
364 								&debug_stmts);
365 
366       if (commondom == frombb)
367 	return false;
368 
369       /* If this is a load then do not sink past any stores.
370 	 ???  This is overly simple but cheap.  We basically look
371 	 for an existing load with the same VUSE in the path to one
372 	 of the sink candidate blocks and we adjust commondom to the
373 	 nearest to commondom.  */
374       if (gimple_vuse (stmt))
375 	{
376 	  /* Do not sink loads from hard registers.  */
377 	  if (gimple_assign_single_p (stmt)
378 	      && TREE_CODE (gimple_assign_rhs1 (stmt)) == VAR_DECL
379 	      && DECL_HARD_REGISTER (gimple_assign_rhs1 (stmt)))
380 	    return false;
381 
382 	  imm_use_iterator imm_iter;
383 	  use_operand_p use_p;
384 	  basic_block found = NULL;
385 	  FOR_EACH_IMM_USE_FAST (use_p, imm_iter, gimple_vuse (stmt))
386 	    {
387 	      gimple *use_stmt = USE_STMT (use_p);
388 	      basic_block bb = gimple_bb (use_stmt);
389 	      /* For PHI nodes the block we know sth about
390 		 is the incoming block with the use.  */
391 	      if (gimple_code (use_stmt) == GIMPLE_PHI)
392 		bb = EDGE_PRED (bb, PHI_ARG_INDEX_FROM_USE (use_p))->src;
393 	      /* Any dominator of commondom would be ok with
394 	         adjusting commondom to that block.  */
395 	      bb = nearest_common_dominator (CDI_DOMINATORS, bb, commondom);
396 	      if (!found)
397 		found = bb;
398 	      else if (dominated_by_p (CDI_DOMINATORS, bb, found))
399 		found = bb;
400 	      /* If we can't improve, stop.  */
401 	      if (found == commondom)
402 		break;
403 	    }
404 	  commondom = found;
405 	  if (commondom == frombb)
406 	    return false;
407 	}
408 
409       /* Our common dominator has to be dominated by frombb in order to be a
410 	 trivially safe place to put this statement, since it has multiple
411 	 uses.  */
412       if (!dominated_by_p (CDI_DOMINATORS, commondom, frombb))
413 	return false;
414 
415       commondom = select_best_block (frombb, commondom, stmt);
416 
417       if (commondom == frombb)
418 	return false;
419 
420       *togsi = gsi_after_labels (commondom);
421 
422       return true;
423     }
424   else
425     {
426       FOR_EACH_IMM_USE_FAST (one_use, imm_iter, DEF_FROM_PTR (def_p))
427 	{
428 	  if (is_gimple_debug (USE_STMT (one_use)))
429 	    continue;
430 	  break;
431 	}
432       use = USE_STMT (one_use);
433 
434       if (gimple_code (use) != GIMPLE_PHI)
435 	{
436 	  sinkbb = gimple_bb (use);
437 	  sinkbb = select_best_block (frombb, gimple_bb (use), stmt);
438 
439 	  if (sinkbb == frombb)
440 	    return false;
441 
442 	  if (sinkbb == gimple_bb (use))
443 	    *togsi = gsi_for_stmt (use);
444 	  else
445 	    *togsi = gsi_after_labels (sinkbb);
446 
447 	  return true;
448 	}
449     }
450 
451   sinkbb = find_bb_for_arg (as_a <gphi *> (use), DEF_FROM_PTR (def_p));
452 
453   /* This can happen if there are multiple uses in a PHI.  */
454   if (!sinkbb)
455     return false;
456 
457   sinkbb = select_best_block (frombb, sinkbb, stmt);
458   if (!sinkbb || sinkbb == frombb)
459     return false;
460 
461   /* If the latch block is empty, don't make it non-empty by sinking
462      something into it.  */
463   if (sinkbb == frombb->loop_father->latch
464       && empty_block_p (sinkbb))
465     return false;
466 
467   *togsi = gsi_after_labels (sinkbb);
468 
469   return true;
470 }
471 
472 /* Perform code sinking on BB */
473 
474 static void
sink_code_in_bb(basic_block bb)475 sink_code_in_bb (basic_block bb)
476 {
477   basic_block son;
478   gimple_stmt_iterator gsi;
479   edge_iterator ei;
480   edge e;
481   bool last = true;
482 
483   /* If this block doesn't dominate anything, there can't be any place to sink
484      the statements to.  */
485   if (first_dom_son (CDI_DOMINATORS, bb) == NULL)
486     goto earlyout;
487 
488   /* We can't move things across abnormal edges, so don't try.  */
489   FOR_EACH_EDGE (e, ei, bb->succs)
490     if (e->flags & EDGE_ABNORMAL)
491       goto earlyout;
492 
493   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
494     {
495       gimple *stmt = gsi_stmt (gsi);
496       gimple_stmt_iterator togsi;
497       bool zero_uses_p;
498 
499       if (!statement_sink_location (stmt, bb, &togsi, &zero_uses_p))
500 	{
501 	  gimple_stmt_iterator saved = gsi;
502 	  if (!gsi_end_p (gsi))
503 	    gsi_prev (&gsi);
504 	  /* If we face a dead stmt remove it as it possibly blocks
505 	     sinking of uses.  */
506 	  if (zero_uses_p
507 	      && ! gimple_vdef (stmt))
508 	    {
509 	      gsi_remove (&saved, true);
510 	      release_defs (stmt);
511 	    }
512 	  else
513 	    last = false;
514 	  continue;
515 	}
516       if (dump_file)
517 	{
518 	  fprintf (dump_file, "Sinking ");
519 	  print_gimple_stmt (dump_file, stmt, 0, TDF_VOPS);
520 	  fprintf (dump_file, " from bb %d to bb %d\n",
521 		   bb->index, (gsi_bb (togsi))->index);
522 	}
523 
524       /* Update virtual operands of statements in the path we
525          do not sink to.  */
526       if (gimple_vdef (stmt))
527 	{
528 	  imm_use_iterator iter;
529 	  use_operand_p use_p;
530 	  gimple *vuse_stmt;
531 
532 	  FOR_EACH_IMM_USE_STMT (vuse_stmt, iter, gimple_vdef (stmt))
533 	    if (gimple_code (vuse_stmt) != GIMPLE_PHI)
534 	      FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
535 		SET_USE (use_p, gimple_vuse (stmt));
536 	}
537 
538       /* If this is the end of the basic block, we need to insert at the end
539          of the basic block.  */
540       if (gsi_end_p (togsi))
541 	gsi_move_to_bb_end (&gsi, gsi_bb (togsi));
542       else
543 	gsi_move_before (&gsi, &togsi);
544 
545       sink_stats.sunk++;
546 
547       /* If we've just removed the last statement of the BB, the
548 	 gsi_end_p() test below would fail, but gsi_prev() would have
549 	 succeeded, and we want it to succeed.  So we keep track of
550 	 whether we're at the last statement and pick up the new last
551 	 statement.  */
552       if (last)
553 	{
554 	  gsi = gsi_last_bb (bb);
555 	  continue;
556 	}
557 
558       last = false;
559       if (!gsi_end_p (gsi))
560 	gsi_prev (&gsi);
561 
562     }
563  earlyout:
564   for (son = first_dom_son (CDI_POST_DOMINATORS, bb);
565        son;
566        son = next_dom_son (CDI_POST_DOMINATORS, son))
567     {
568       sink_code_in_bb (son);
569     }
570 }
571 
572 /* Perform code sinking.
573    This moves code down the flowgraph when we know it would be
574    profitable to do so, or it wouldn't increase the number of
575    executions of the statement.
576 
577    IE given
578 
579    a_1 = b + c;
580    if (<something>)
581    {
582    }
583    else
584    {
585      foo (&b, &c);
586      a_5 = b + c;
587    }
588    a_6 = PHI (a_5, a_1);
589    USE a_6.
590 
591    we'll transform this into:
592 
593    if (<something>)
594    {
595       a_1 = b + c;
596    }
597    else
598    {
599       foo (&b, &c);
600       a_5 = b + c;
601    }
602    a_6 = PHI (a_5, a_1);
603    USE a_6.
604 
605    Note that this reduces the number of computations of a = b + c to 1
606    when we take the else edge, instead of 2.
607 */
608 namespace {
609 
610 const pass_data pass_data_sink_code =
611 {
612   GIMPLE_PASS, /* type */
613   "sink", /* name */
614   OPTGROUP_NONE, /* optinfo_flags */
615   TV_TREE_SINK, /* tv_id */
616   /* PROP_no_crit_edges is ensured by running split_critical_edges in
617      pass_data_sink_code::execute ().  */
618   ( PROP_cfg | PROP_ssa ), /* properties_required */
619   0, /* properties_provided */
620   0, /* properties_destroyed */
621   0, /* todo_flags_start */
622   TODO_update_ssa, /* todo_flags_finish */
623 };
624 
625 class pass_sink_code : public gimple_opt_pass
626 {
627 public:
pass_sink_code(gcc::context * ctxt)628   pass_sink_code (gcc::context *ctxt)
629     : gimple_opt_pass (pass_data_sink_code, ctxt)
630   {}
631 
632   /* opt_pass methods: */
gate(function *)633   virtual bool gate (function *) { return flag_tree_sink != 0; }
634   virtual unsigned int execute (function *);
635 
636 }; // class pass_sink_code
637 
638 unsigned int
execute(function * fun)639 pass_sink_code::execute (function *fun)
640 {
641   loop_optimizer_init (LOOPS_NORMAL);
642   split_critical_edges ();
643   connect_infinite_loops_to_exit ();
644   memset (&sink_stats, 0, sizeof (sink_stats));
645   calculate_dominance_info (CDI_DOMINATORS);
646   calculate_dominance_info (CDI_POST_DOMINATORS);
647   sink_code_in_bb (EXIT_BLOCK_PTR_FOR_FN (fun));
648   statistics_counter_event (fun, "Sunk statements", sink_stats.sunk);
649   free_dominance_info (CDI_POST_DOMINATORS);
650   remove_fake_exit_edges ();
651   loop_optimizer_finalize ();
652 
653   return 0;
654 }
655 
656 } // anon namespace
657 
658 gimple_opt_pass *
make_pass_sink_code(gcc::context * ctxt)659 make_pass_sink_code (gcc::context *ctxt)
660 {
661   return new pass_sink_code (ctxt);
662 }
663