1 /*
   2  * Copyright (c) 1999, 2017, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "c1/c1_Canonicalizer.hpp"
  27 #include "c1/c1_IR.hpp"
  28 #include "c1/c1_ValueMap.hpp"
  29 #include "c1/c1_ValueSet.inline.hpp"
  30 #include "c1/c1_ValueStack.hpp"
  31 
  32 #ifndef PRODUCT
  33 
  34   int ValueMap::_number_of_finds = 0;
  35   int ValueMap::_number_of_hits = 0;
  36   int ValueMap::_number_of_kills = 0;
  37 
  38   #define TRACE_VALUE_NUMBERING(code) if (PrintValueNumbering) { code; }
  39 
  40 #else
  41 
  42   #define TRACE_VALUE_NUMBERING(code)
  43 
  44 #endif
  45 
  46 
  47 ValueMap::ValueMap()
  48   : _nesting(0)
  49   , _entries(ValueMapInitialSize, ValueMapInitialSize, NULL)
  50   , _killed_values()
  51   , _entry_count(0)
  52 {
  53   NOT_PRODUCT(reset_statistics());
  54 }
  55 
  56 
  57 ValueMap::ValueMap(ValueMap* old)
  58   : _nesting(old->_nesting + 1)
  59   , _entries(old->_entries.length(), old->_entries.length(), NULL)
  60   , _killed_values()
  61   , _entry_count(old->_entry_count)
  62 {
  63   for (int i = size() - 1; i >= 0; i--) {
  64     _entries.at_put(i, old->entry_at(i));
  65   }
  66   _killed_values.set_from(&old->_killed_values);
  67 }
  68 
  69 
  70 void ValueMap::increase_table_size() {
  71   int old_size = size();
  72   int new_size = old_size * 2 + 1;
  73 
  74   ValueMapEntryList worklist(8);
  75   ValueMapEntryArray new_entries(new_size, new_size, NULL);
  76   int new_entry_count = 0;
  77 
  78   TRACE_VALUE_NUMBERING(tty->print_cr("increasing table size from %d to %d", old_size, new_size));
  79 
  80   for (int i = old_size - 1; i >= 0; i--) {
  81     ValueMapEntry* entry;
  82     for (entry = entry_at(i); entry != NULL; entry = entry->next()) {
  83       if (!is_killed(entry->value())) {
  84         worklist.push(entry);
  85       }
  86     }
  87 
  88     while (!worklist.is_empty()) {
  89       entry = worklist.pop();
  90       int new_index = entry_index(entry->hash(), new_size);
  91 
  92       if (entry->nesting() != nesting() && new_entries.at(new_index) != entry->next()) {
  93         // changing entries with a lower nesting than the current nesting of the table
  94         // is not allowed because then the same entry is contained in multiple value maps.
  95         // clone entry when next-pointer must be changed
  96         entry = new ValueMapEntry(entry->hash(), entry->value(), entry->nesting(), NULL);
  97       }
  98       entry->set_next(new_entries.at(new_index));
  99       new_entries.at_put(new_index, entry);
 100       new_entry_count++;
 101     }
 102   }
 103 
 104   _entries = new_entries;
 105   _entry_count = new_entry_count;
 106 }
 107 
 108 
 109 Value ValueMap::find_insert(Value x) {
 110   const intx hash = x->hash();
 111   if (hash != 0) {
 112     // 0 hash means: exclude from value numbering
 113     NOT_PRODUCT(_number_of_finds++);
 114 
 115     for (ValueMapEntry* entry = entry_at(entry_index(hash, size())); entry != NULL; entry = entry->next()) {
 116       if (entry->hash() == hash) {
 117         Value f = entry->value();
 118 
 119         if (!is_killed(f) && f->is_equal(x)) {
 120           NOT_PRODUCT(_number_of_hits++);
 121           TRACE_VALUE_NUMBERING(tty->print_cr("Value Numbering: %s %c%d equal to %c%d  (size %d, entries %d, nesting-diff %d)", x->name(), x->type()->tchar(), x->id(), f->type()->tchar(), f->id(), size(), entry_count(), nesting() - entry->nesting()));
 122 
 123           if (entry->nesting() != nesting() && f->as_Constant() == NULL) {
 124             // non-constant values of of another block must be pinned,
 125             // otherwise it is possible that they are not evaluated
 126             f->pin(Instruction::PinGlobalValueNumbering);
 127           }
 128           assert(x->type()->tag() == f->type()->tag(), "should have same type");
 129 
 130           return f;
 131 
 132         }
 133       }
 134     }
 135 
 136     // x not found, so insert it
 137     if (entry_count() >= size_threshold()) {
 138       increase_table_size();
 139     }
 140     int idx = entry_index(hash, size());
 141     _entries.at_put(idx, new ValueMapEntry(hash, x, nesting(), entry_at(idx)));
 142     _entry_count++;
 143 
 144     TRACE_VALUE_NUMBERING(tty->print_cr("Value Numbering: insert %s %c%d  (size %d, entries %d, nesting %d)", x->name(), x->type()->tchar(), x->id(), size(), entry_count(), nesting()));
 145   }
 146 
 147   return x;
 148 }
 149 
 150 
 151 #define GENERIC_KILL_VALUE(must_kill_implementation)                                     \
 152   NOT_PRODUCT(_number_of_kills++);                                                       \
 153                                                                                          \
 154   for (int i = size() - 1; i >= 0; i--) {                                                \
 155     ValueMapEntry* prev_entry = NULL;                                                    \
 156     for (ValueMapEntry* entry = entry_at(i); entry != NULL; entry = entry->next()) {     \
 157       Value value = entry->value();                                                      \
 158                                                                                          \
 159       must_kill_implementation(must_kill, entry, value)                                  \
 160                                                                                          \
 161       if (must_kill) {                                                                   \
 162         kill_value(value);                                                               \
 163                                                                                          \
 164         if (prev_entry == NULL) {                                                        \
 165           _entries.at_put(i, entry->next());                                             \
 166           _entry_count--;                                                                \
 167         } else if (prev_entry->nesting() == nesting()) {                                 \
 168           prev_entry->set_next(entry->next());                                           \
 169           _entry_count--;                                                                \
 170         } else {                                                                         \
 171           prev_entry = entry;                                                            \
 172         }                                                                                \
 173                                                                                          \
 174         TRACE_VALUE_NUMBERING(tty->print_cr("Value Numbering: killed %s %c%d  (size %d, entries %d, nesting-diff %d)", value->name(), value->type()->tchar(), value->id(), size(), entry_count(), nesting() - entry->nesting()));   \
 175       } else {                                                                           \
 176         prev_entry = entry;                                                              \
 177       }                                                                                  \
 178     }                                                                                    \
 179   }                                                                                      \
 180 
 181 #define MUST_KILL_MEMORY(must_kill, entry, value)                                        \
 182   bool must_kill = value->as_LoadField() != NULL || value->as_LoadIndexed() != NULL;
 183 
 184 #define MUST_KILL_ARRAY(must_kill, entry, value)                                         \
 185   bool must_kill = value->as_LoadIndexed() != NULL                                       \
 186                    && value->type()->tag() == type->tag();
 187 
 188 #define MUST_KILL_FIELD(must_kill, entry, value)                                         \
 189   /* ciField's are not unique; must compare their contents */                            \
 190   LoadField* lf = value->as_LoadField();                                                 \
 191   bool must_kill = lf != NULL                                                            \
 192                    && lf->field()->holder() == field->holder()                           \
 193                    && (all_offsets || lf->field()->offset() == field->offset());
 194 
 195 
 196 void ValueMap::kill_memory() {
 197   GENERIC_KILL_VALUE(MUST_KILL_MEMORY);
 198 }
 199 
 200 void ValueMap::kill_array(ValueType* type) {
 201   GENERIC_KILL_VALUE(MUST_KILL_ARRAY);
 202 }
 203 
 204 void ValueMap::kill_field(ciField* field, bool all_offsets) {
 205   GENERIC_KILL_VALUE(MUST_KILL_FIELD);
 206 }
 207 
 208 void ValueMap::kill_map(ValueMap* map) {
 209   assert(is_global_value_numbering(), "only for global value numbering");
 210   _killed_values.set_union(&map->_killed_values);
 211 }
 212 
 213 void ValueMap::kill_all() {
 214   assert(is_local_value_numbering(), "only for local value numbering");
 215   for (int i = size() - 1; i >= 0; i--) {
 216     _entries.at_put(i, NULL);
 217   }
 218   _entry_count = 0;
 219 }
 220 
 221 
 222 #ifndef PRODUCT
 223 
 224 void ValueMap::print() {
 225   tty->print_cr("(size %d, entries %d, nesting %d)", size(), entry_count(), nesting());
 226 
 227   int entries = 0;
 228   for (int i = 0; i < size(); i++) {
 229     if (entry_at(i) != NULL) {
 230       tty->print("  %2d: ", i);
 231       for (ValueMapEntry* entry = entry_at(i); entry != NULL; entry = entry->next()) {
 232         Value value = entry->value();
 233         tty->print("%s %c%d (%s%d) -> ", value->name(), value->type()->tchar(), value->id(), is_killed(value) ? "x" : "", entry->nesting());
 234         entries++;
 235       }
 236       tty->print_cr("NULL");
 237     }
 238   }
 239 
 240   _killed_values.print();
 241   assert(entry_count() == entries, "entry_count incorrect");
 242 }
 243 
 244 void ValueMap::reset_statistics() {
 245   _number_of_finds = 0;
 246   _number_of_hits = 0;
 247   _number_of_kills = 0;
 248 }
 249 
 250 void ValueMap::print_statistics() {
 251   float hit_rate = 0;
 252   if (_number_of_finds != 0) {
 253     hit_rate = (float)_number_of_hits / _number_of_finds;
 254   }
 255 
 256   tty->print_cr("finds:%3d  hits:%3d   kills:%3d  hit rate: %1.4f", _number_of_finds, _number_of_hits, _number_of_kills, hit_rate);
 257 }
 258 
 259 #endif
 260 
 261 
 262 
 263 class ShortLoopOptimizer : public ValueNumberingVisitor {
 264  private:
 265   GlobalValueNumbering* _gvn;
 266   BlockList             _loop_blocks;
 267   bool                  _too_complicated_loop;
 268   bool                  _has_field_store[T_ARRAY + 1];
 269   bool                  _has_indexed_store[T_ARRAY + 1];
 270 
 271   // simplified access to methods of GlobalValueNumbering
 272   ValueMap* current_map()                        { return _gvn->current_map(); }
 273   ValueMap* value_map_of(BlockBegin* block)      { return _gvn->value_map_of(block); }
 274 
 275   // implementation for abstract methods of ValueNumberingVisitor
 276   void      kill_memory()                                 { _too_complicated_loop = true; }
 277   void      kill_field(ciField* field, bool all_offsets)  {
 278     current_map()->kill_field(field, all_offsets);
 279     assert(field->type()->basic_type() >= 0 && field->type()->basic_type() <= T_ARRAY, "Invalid type");
 280     _has_field_store[field->type()->basic_type()] = true;
 281   }
 282   void      kill_array(ValueType* type)                   {
 283     current_map()->kill_array(type);
 284     BasicType basic_type = as_BasicType(type); assert(basic_type >= 0 && basic_type <= T_ARRAY, "Invalid type");
 285     _has_indexed_store[basic_type] = true;
 286   }
 287 
 288  public:
 289   ShortLoopOptimizer(GlobalValueNumbering* gvn)
 290     : _gvn(gvn)
 291     , _loop_blocks(ValueMapMaxLoopSize)
 292     , _too_complicated_loop(false)
 293   {
 294     for (int i=0; i<= T_ARRAY; i++){
 295       _has_field_store[i] = false;
 296       _has_indexed_store[i] = false;
 297     }
 298   }
 299 
 300   bool has_field_store(BasicType type) {
 301     assert(type >= 0 && type <= T_ARRAY, "Invalid type");
 302     return _has_field_store[type];
 303   }
 304 
 305   bool has_indexed_store(BasicType type) {
 306     assert(type >= 0 && type <= T_ARRAY, "Invalid type");
 307     return _has_indexed_store[type];
 308   }
 309 
 310   bool process(BlockBegin* loop_header);
 311 };
 312 
 313 class LoopInvariantCodeMotion : public StackObj  {
 314  private:
 315   GlobalValueNumbering* _gvn;
 316   ShortLoopOptimizer*   _short_loop_optimizer;
 317   Instruction*          _insertion_point;
 318   ValueStack *          _state;
 319   bool                  _insert_is_pred;
 320 
 321   void set_invariant(Value v) const    { _gvn->set_processed(v); }
 322   bool is_invariant(Value v) const     { return _gvn->is_processed(v); }
 323 
 324   void process_block(BlockBegin* block);
 325 
 326  public:
 327   LoopInvariantCodeMotion(ShortLoopOptimizer *slo, GlobalValueNumbering* gvn, BlockBegin* loop_header, BlockList* loop_blocks);
 328 };
 329 
 330 LoopInvariantCodeMotion::LoopInvariantCodeMotion(ShortLoopOptimizer *slo, GlobalValueNumbering* gvn, BlockBegin* loop_header, BlockList* loop_blocks)
 331   : _gvn(gvn), _short_loop_optimizer(slo) {
 332 
 333   TRACE_VALUE_NUMBERING(tty->print_cr("using loop invariant code motion loop_header = %d", loop_header->block_id()));
 334   TRACE_VALUE_NUMBERING(tty->print_cr("** loop invariant code motion for short loop B%d", loop_header->block_id()));
 335 
 336   BlockBegin* insertion_block = loop_header->dominator();
 337   if (insertion_block->number_of_preds() == 0) {
 338     return;  // only the entry block does not have a predecessor
 339   }
 340 
 341   assert(insertion_block->end()->as_Base() == NULL, "cannot insert into entry block");
 342   _insertion_point = insertion_block->end()->prev();
 343   _insert_is_pred = loop_header->is_predecessor(insertion_block);
 344 
 345   BlockEnd *block_end = insertion_block->end();
 346   _state = block_end->state_before();
 347 
 348   if (!_state) {
 349     // If, TableSwitch and LookupSwitch always have state_before when
 350     // loop invariant code motion happens..
 351     assert(block_end->as_Goto(), "Block has to be goto");
 352     _state = block_end->state();
 353   }
 354 
 355   // the loop_blocks are filled by going backward from the loop header, so this processing order is best
 356   assert(loop_blocks->at(0) == loop_header, "loop header must be first loop block");
 357   process_block(loop_header);
 358   for (int i = loop_blocks->length() - 1; i >= 1; i--) {
 359     process_block(loop_blocks->at(i));
 360   }
 361 }
 362 
 363 void LoopInvariantCodeMotion::process_block(BlockBegin* block) {
 364   TRACE_VALUE_NUMBERING(tty->print_cr("processing block B%d", block->block_id()));
 365 
 366   Instruction* prev = block;
 367   Instruction* cur = block->next();
 368 
 369   while (cur != NULL) {
 370 
 371     // determine if cur instruction is loop invariant
 372     // only selected instruction types are processed here
 373     bool cur_invariant = false;
 374 
 375     if (cur->as_Constant() != NULL) {
 376       cur_invariant = !cur->can_trap();
 377     } else if (cur->as_ArithmeticOp() != NULL || cur->as_LogicOp() != NULL || cur->as_ShiftOp() != NULL) {
 378       assert(cur->as_Op2() != NULL, "must be Op2");
 379       Op2* op2 = (Op2*)cur;
 380       cur_invariant = !op2->can_trap() && is_invariant(op2->x()) && is_invariant(op2->y());
 381     } else if (cur->as_LoadField() != NULL) {
 382       LoadField* lf = (LoadField*)cur;
 383       // deoptimizes on NullPointerException
 384       cur_invariant = !lf->needs_patching() && !lf->field()->is_volatile() && !_short_loop_optimizer->has_field_store(lf->field()->type()->basic_type()) && is_invariant(lf->obj()) && _insert_is_pred;
 385     } else if (cur->as_ArrayLength() != NULL) {
 386       ArrayLength *length = cur->as_ArrayLength();
 387       cur_invariant = is_invariant(length->array());
 388     } else if (cur->as_LoadIndexed() != NULL) {
 389       LoadIndexed *li = (LoadIndexed *)cur->as_LoadIndexed();
 390       cur_invariant = !_short_loop_optimizer->has_indexed_store(as_BasicType(cur->type())) && is_invariant(li->array()) && is_invariant(li->index()) && _insert_is_pred;
 391     }
 392 
 393     if (cur_invariant) {
 394       // perform value numbering and mark instruction as loop-invariant
 395       _gvn->substitute(cur);
 396 
 397       if (cur->as_Constant() == NULL) {
 398         // ensure that code for non-constant instructions is always generated
 399         cur->pin();
 400       }
 401 
 402       // remove cur instruction from loop block and append it to block before loop
 403       Instruction* next = cur->next();
 404       Instruction* in = _insertion_point->next();
 405       _insertion_point = _insertion_point->set_next(cur);
 406       cur->set_next(in);
 407 
 408       //  Deoptimize on exception
 409       cur->set_flag(Instruction::DeoptimizeOnException, true);
 410 
 411       //  Clear exception handlers
 412       cur->set_exception_handlers(NULL);
 413 
 414       TRACE_VALUE_NUMBERING(tty->print_cr("Instruction %c%d is loop invariant", cur->type()->tchar(), cur->id()));
 415 
 416       if (cur->state_before() != NULL) {
 417         cur->set_state_before(_state->copy());
 418       }
 419       if (cur->exception_state() != NULL) {
 420         cur->set_exception_state(_state->copy());
 421       }
 422 
 423       cur = prev->set_next(next);
 424 
 425     } else {
 426       prev = cur;
 427       cur = cur->next();
 428     }
 429   }
 430 }
 431 
 432 bool ShortLoopOptimizer::process(BlockBegin* loop_header) {
 433   TRACE_VALUE_NUMBERING(tty->print_cr("** loop header block"));
 434 
 435   _too_complicated_loop = false;
 436   _loop_blocks.clear();
 437   _loop_blocks.append(loop_header);
 438 
 439   for (int i = 0; i < _loop_blocks.length(); i++) {
 440     BlockBegin* block = _loop_blocks.at(i);
 441     TRACE_VALUE_NUMBERING(tty->print_cr("processing loop block B%d", block->block_id()));
 442 
 443     if (block->is_set(BlockBegin::exception_entry_flag)) {
 444       // this would be too complicated
 445       return false;
 446     }
 447 
 448     // add predecessors to worklist
 449     for (int j = block->number_of_preds() - 1; j >= 0; j--) {
 450       BlockBegin* pred = block->pred_at(j);
 451 
 452       if (pred->is_set(BlockBegin::osr_entry_flag)) {
 453         return false;
 454       }
 455 
 456       ValueMap* pred_map = value_map_of(pred);
 457       if (pred_map != NULL) {
 458         current_map()->kill_map(pred_map);
 459       } else if (!_loop_blocks.contains(pred)) {
 460         if (_loop_blocks.length() >= ValueMapMaxLoopSize) {
 461           return false;
 462         }
 463         _loop_blocks.append(pred);
 464       }
 465     }
 466 
 467     // use the instruction visitor for killing values
 468     for (Value instr = block->next(); instr != NULL; instr = instr->next()) {
 469       instr->visit(this);
 470       if (_too_complicated_loop) {
 471         return false;
 472       }
 473     }
 474   }
 475 
 476   bool optimistic = this->_gvn->compilation()->is_optimistic();
 477 
 478   if (UseLoopInvariantCodeMotion && optimistic) {
 479     LoopInvariantCodeMotion code_motion(this, _gvn, loop_header, &_loop_blocks);
 480   }
 481 
 482   TRACE_VALUE_NUMBERING(tty->print_cr("** loop successfully optimized"));
 483   return true;
 484 }
 485 
 486 
 487 GlobalValueNumbering::GlobalValueNumbering(IR* ir)
 488   : _current_map(NULL)
 489   , _value_maps(ir->linear_scan_order()->length(), ir->linear_scan_order()->length(), NULL)
 490   , _compilation(ir->compilation())
 491 {
 492   TRACE_VALUE_NUMBERING(tty->print_cr("****** start of global value numbering"));
 493 
 494   ShortLoopOptimizer short_loop_optimizer(this);
 495 
 496   BlockList* blocks = ir->linear_scan_order();
 497   int num_blocks = blocks->length();
 498 
 499   BlockBegin* start_block = blocks->at(0);
 500   assert(start_block == ir->start() && start_block->number_of_preds() == 0 && start_block->dominator() == NULL, "must be start block");
 501   assert(start_block->next()->as_Base() != NULL && start_block->next()->next() == NULL, "start block must not have instructions");
 502 
 503   // method parameters are not linked in instructions list, so process them separateley
 504   for_each_state_value(start_block->state(), value,
 505      assert(value->as_Local() != NULL, "only method parameters allowed");
 506      set_processed(value);
 507   );
 508 
 509   // initial, empty value map with nesting 0
 510   set_value_map_of(start_block, new ValueMap());
 511 
 512   for (int i = 1; i < num_blocks; i++) {
 513     BlockBegin* block = blocks->at(i);
 514     TRACE_VALUE_NUMBERING(tty->print_cr("**** processing block B%d", block->block_id()));
 515 
 516     int num_preds = block->number_of_preds();
 517     assert(num_preds > 0, "block must have predecessors");
 518 
 519     BlockBegin* dominator = block->dominator();
 520     assert(dominator != NULL, "dominator must exist");
 521     assert(value_map_of(dominator) != NULL, "value map of dominator must exist");
 522 
 523     // create new value map with increased nesting
 524     _current_map = new ValueMap(value_map_of(dominator));
 525 
 526     if (num_preds == 1 && !block->is_set(BlockBegin::exception_entry_flag)) {
 527       assert(dominator == block->pred_at(0), "dominator must be equal to predecessor");
 528       // nothing to do here
 529 
 530     } else if (block->is_set(BlockBegin::linear_scan_loop_header_flag)) {
 531       // block has incoming backward branches -> try to optimize short loops
 532       if (!short_loop_optimizer.process(block)) {
 533         // loop is too complicated, so kill all memory loads because there might be
 534         // stores to them in the loop
 535         current_map()->kill_memory();
 536       }
 537 
 538     } else {
 539       // only incoming forward branches that are already processed
 540       for (int j = 0; j < num_preds; j++) {
 541         BlockBegin* pred = block->pred_at(j);
 542         ValueMap* pred_map = value_map_of(pred);
 543 
 544         if (pred_map != NULL) {
 545           // propagate killed values of the predecessor to this block
 546           current_map()->kill_map(value_map_of(pred));
 547         } else {
 548           // kill all memory loads because predecessor not yet processed
 549           // (this can happen with non-natural loops and OSR-compiles)
 550           current_map()->kill_memory();
 551         }
 552       }
 553     }
 554 
 555     // phi functions are not linked in instructions list, so process them separateley
 556     for_each_phi_fun(block, phi,
 557       set_processed(phi);
 558     );
 559 
 560     TRACE_VALUE_NUMBERING(tty->print("value map before processing block: "); current_map()->print());
 561 
 562     // visit all instructions of this block
 563     for (Value instr = block->next(); instr != NULL; instr = instr->next()) {
 564       // check if instruction kills any values
 565       instr->visit(this);
 566       // perform actual value numbering
 567       substitute(instr);
 568     }
 569 
 570     // remember value map for successors
 571     set_value_map_of(block, current_map());
 572   }
 573 
 574   if (_has_substitutions) {
 575     SubstitutionResolver resolver(ir);
 576   }
 577 
 578   TRACE_VALUE_NUMBERING(tty->print("****** end of global value numbering. "); ValueMap::print_statistics());
 579 }
 580 
 581 void GlobalValueNumbering::substitute(Instruction* instr) {
 582   assert(!instr->has_subst(), "substitution already set");
 583   Value subst = current_map()->find_insert(instr);
 584   if (subst != instr) {
 585     assert(!subst->has_subst(), "can't have a substitution");
 586 
 587     TRACE_VALUE_NUMBERING(tty->print_cr("substitution for %d set to %d", instr->id(), subst->id()));
 588     instr->set_subst(subst);
 589     _has_substitutions = true;
 590   }
 591   set_processed(instr);
 592 }