1 /*
   2  * Copyright (c) 1998, 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 "code/codeBlob.hpp"
  27 #include "code/codeCache.hpp"
  28 #include "code/nmethod.hpp"
  29 #include "code/scopeDesc.hpp"
  30 #include "compiler/oopMap.hpp"
  31 #include "gc_interface/collectedHeap.hpp"
  32 #include "memory/allocation.inline.hpp"
  33 #include "memory/resourceArea.hpp"
  34 #include "runtime/frame.inline.hpp"
  35 #include "runtime/signature.hpp"
  36 #ifdef COMPILER1
  37 #include "c1/c1_Defs.hpp"
  38 #endif
  39 
  40 // OopMapStream
  41 
  42 OopMapStream::OopMapStream(OopMap* oop_map) {
  43   if(oop_map->omv_data() == NULL) {
  44     _stream = new CompressedReadStream(oop_map->write_stream()->buffer());
  45   } else {
  46     _stream = new CompressedReadStream(oop_map->omv_data());
  47   }
  48   _mask = OopMapValue::type_mask_in_place;
  49   _size = oop_map->omv_count();
  50   _position = 0;
  51   _valid_omv = false;
  52 }
  53 
  54 
  55 OopMapStream::OopMapStream(OopMap* oop_map, int oop_types_mask) {
  56   if(oop_map->omv_data() == NULL) {
  57     _stream = new CompressedReadStream(oop_map->write_stream()->buffer());
  58   } else {
  59     _stream = new CompressedReadStream(oop_map->omv_data());
  60   }
  61   _mask = oop_types_mask;
  62   _size = oop_map->omv_count();
  63   _position = 0;
  64   _valid_omv = false;
  65 }
  66 
  67 
  68 void OopMapStream::find_next() {
  69   while(_position++ < _size) {
  70     _omv.read_from(_stream);
  71     if(((int)_omv.type() & _mask) > 0) {
  72       _valid_omv = true;
  73       return;
  74     }
  75   }
  76   _valid_omv = false;
  77 }
  78 
  79 
  80 // OopMap
  81 
  82 // frame_size units are stack-slots (4 bytes) NOT intptr_t; we can name odd
  83 // slots to hold 4-byte values like ints and floats in the LP64 build.
  84 OopMap::OopMap(int frame_size, int arg_count) {
  85   // OopMaps are usually quite so small, so pick a small initial size
  86   set_write_stream(new CompressedWriteStream(32));
  87   set_omv_data(NULL);
  88   set_omv_count(0);
  89 
  90 #ifdef ASSERT
  91   _locs_length = VMRegImpl::stack2reg(0)->value() + frame_size + arg_count;
  92   _locs_used   = NEW_RESOURCE_ARRAY(OopMapValue::oop_types, _locs_length);
  93   for(int i = 0; i < _locs_length; i++) _locs_used[i] = OopMapValue::unused_value;
  94 #endif
  95 }
  96 
  97 
  98 OopMap::OopMap(OopMap::DeepCopyToken, OopMap* source) {
  99   // This constructor does a deep copy
 100   // of the source OopMap.
 101   set_write_stream(new CompressedWriteStream(source->omv_count() * 2));
 102   set_omv_data(NULL);
 103   set_omv_count(0);
 104   set_offset(source->offset());
 105 
 106 #ifdef ASSERT
 107   _locs_length = source->_locs_length;
 108   _locs_used = NEW_RESOURCE_ARRAY(OopMapValue::oop_types, _locs_length);
 109   for(int i = 0; i < _locs_length; i++) _locs_used[i] = OopMapValue::unused_value;
 110 #endif
 111 
 112   // We need to copy the entries too.
 113   for (OopMapStream oms(source); !oms.is_done(); oms.next()) {
 114     OopMapValue omv = oms.current();
 115     omv.write_on(write_stream());
 116     increment_count();
 117   }
 118 }
 119 
 120 
 121 OopMap* OopMap::deep_copy() {
 122   return new OopMap(_deep_copy_token, this);
 123 }
 124 
 125 
 126 void OopMap::copy_to(address addr) {
 127   memcpy(addr,this,sizeof(OopMap));
 128   memcpy(addr + sizeof(OopMap),write_stream()->buffer(),write_stream()->position());
 129   OopMap* new_oop = (OopMap*)addr;
 130   new_oop->set_omv_data_size(write_stream()->position());
 131   new_oop->set_omv_data((unsigned char *)(addr + sizeof(OopMap)));
 132   new_oop->set_write_stream(NULL);
 133 }
 134 
 135 
 136 int OopMap::heap_size() const {
 137   int size = sizeof(OopMap);
 138   int align = sizeof(void *) - 1;
 139   if(write_stream() != NULL) {
 140     size += write_stream()->position();
 141   } else {
 142     size += omv_data_size();
 143   }
 144   // Align to a reasonable ending point
 145   size = ((size+align) & ~align);
 146   return size;
 147 }
 148 
 149 // frame_size units are stack-slots (4 bytes) NOT intptr_t; we can name odd
 150 // slots to hold 4-byte values like ints and floats in the LP64 build.
 151 void OopMap::set_xxx(VMReg reg, OopMapValue::oop_types x, VMReg optional) {
 152 
 153   assert(reg->value() < _locs_length, "too big reg value for stack size");
 154   assert( _locs_used[reg->value()] == OopMapValue::unused_value, "cannot insert twice" );
 155   debug_only( _locs_used[reg->value()] = x; )
 156 
 157   OopMapValue o(reg, x);
 158 
 159   if(x == OopMapValue::callee_saved_value) {
 160     // This can never be a stack location, so we don't need to transform it.
 161     assert(optional->is_reg(), "Trying to callee save a stack location");
 162     o.set_content_reg(optional);
 163   } else if(x == OopMapValue::derived_oop_value) {
 164     o.set_content_reg(optional);
 165   }
 166 
 167   o.write_on(write_stream());
 168   increment_count();
 169 }
 170 
 171 
 172 void OopMap::set_oop(VMReg reg) {
 173   set_xxx(reg, OopMapValue::oop_value, VMRegImpl::Bad());
 174 }
 175 
 176 
 177 void OopMap::set_value(VMReg reg) {
 178   // At this time, we only need value entries in our OopMap when ZapDeadCompiledLocals is active.
 179   if (ZapDeadCompiledLocals)
 180     set_xxx(reg, OopMapValue::value_value, VMRegImpl::Bad());
 181 }
 182 
 183 
 184 void OopMap::set_narrowoop(VMReg reg) {
 185   set_xxx(reg, OopMapValue::narrowoop_value, VMRegImpl::Bad());
 186 }
 187 
 188 
 189 void OopMap::set_callee_saved(VMReg reg, VMReg caller_machine_register ) {
 190   set_xxx(reg, OopMapValue::callee_saved_value, caller_machine_register);
 191 }
 192 
 193 
 194 void OopMap::set_derived_oop(VMReg reg, VMReg derived_from_local_register ) {
 195   if( reg == derived_from_local_register ) {
 196     // Actually an oop, derived shares storage with base,
 197     set_oop(reg);
 198   } else {
 199     set_xxx(reg, OopMapValue::derived_oop_value, derived_from_local_register);
 200   }
 201 }
 202 
 203 // OopMapSet
 204 
 205 OopMapSet::OopMapSet() {
 206   set_om_size(MinOopMapAllocation);
 207   set_om_count(0);
 208   OopMap** temp = NEW_RESOURCE_ARRAY(OopMap*, om_size());
 209   set_om_data(temp);
 210 }
 211 
 212 
 213 void OopMapSet::grow_om_data() {
 214   int new_size = om_size() * 2;
 215   OopMap** new_data = NEW_RESOURCE_ARRAY(OopMap*, new_size);
 216   memcpy(new_data,om_data(),om_size() * sizeof(OopMap*));
 217   set_om_size(new_size);
 218   set_om_data(new_data);
 219 }
 220 
 221 
 222 void OopMapSet::copy_to(address addr) {
 223   address temp = addr;
 224   int align = sizeof(void *) - 1;
 225   // Copy this
 226   memcpy(addr,this,sizeof(OopMapSet));
 227   temp += sizeof(OopMapSet);
 228   temp = (address)((intptr_t)(temp + align) & ~align);
 229   // Do the needed fixups to the new OopMapSet
 230   OopMapSet* new_set = (OopMapSet*)addr;
 231   new_set->set_om_data((OopMap**)temp);
 232   // Allow enough space for the OopMap pointers
 233   temp += (om_count() * sizeof(OopMap*));
 234 
 235   for(int i=0; i < om_count(); i++) {
 236     OopMap* map = at(i);
 237     map->copy_to((address)temp);
 238     new_set->set(i,(OopMap*)temp);
 239     temp += map->heap_size();
 240   }
 241   // This "locks" the OopMapSet
 242   new_set->set_om_size(-1);
 243 }
 244 
 245 
 246 void OopMapSet::add_gc_map(int pc_offset, OopMap *map ) {
 247   assert(om_size() != -1,"Cannot grow a fixed OopMapSet");
 248 
 249   if(om_count() >= om_size()) {
 250     grow_om_data();
 251   }
 252   map->set_offset(pc_offset);
 253 
 254 #ifdef ASSERT
 255   if(om_count() > 0) {
 256     OopMap* last = at(om_count()-1);
 257     if (last->offset() == map->offset() ) {
 258       fatal("OopMap inserted twice");
 259     }
 260     if(last->offset() > map->offset()) {
 261       tty->print_cr( "WARNING, maps not sorted: pc[%d]=%d, pc[%d]=%d",
 262                       om_count(),last->offset(),om_count()+1,map->offset());
 263     }
 264   }
 265 #endif // ASSERT
 266 
 267   set(om_count(),map);
 268   increment_count();
 269 }
 270 
 271 
 272 int OopMapSet::heap_size() const {
 273   // The space we use
 274   int size = sizeof(OopMap);
 275   int align = sizeof(void *) - 1;
 276   size = ((size+align) & ~align);
 277   size += om_count() * sizeof(OopMap*);
 278 
 279   // Now add in the space needed for the indivdiual OopMaps
 280   for(int i=0; i < om_count(); i++) {
 281     size += at(i)->heap_size();
 282   }
 283   // We don't need to align this, it will be naturally pointer aligned
 284   return size;
 285 }
 286 
 287 
 288 OopMap* OopMapSet::singular_oop_map() {
 289   guarantee(om_count() == 1, "Make sure we only have a single gc point");
 290   return at(0);
 291 }
 292 
 293 
 294 OopMap* OopMapSet::find_map_at_offset(int pc_offset) const {
 295   int i, len = om_count();
 296   assert( len > 0, "must have pointer maps" );
 297 
 298   // Scan through oopmaps. Stop when current offset is either equal or greater
 299   // than the one we are looking for.
 300   for( i = 0; i < len; i++) {
 301     if( at(i)->offset() >= pc_offset )
 302       break;
 303   }
 304 
 305   assert( i < len, "oopmap not found" );
 306 
 307   OopMap* m = at(i);
 308   assert( m->offset() == pc_offset, "oopmap not found" );
 309   return m;
 310 }
 311 
 312 class DoNothingClosure: public OopClosure {
 313  public:
 314   void do_oop(oop* p)       {}
 315   void do_oop(narrowOop* p) {}
 316 };
 317 static DoNothingClosure do_nothing;
 318 
 319 static void add_derived_oop(oop* base, oop* derived) {
 320 #ifndef TIERED
 321   COMPILER1_PRESENT(ShouldNotReachHere();)
 322 #endif // TIERED
 323 #ifdef COMPILER2
 324   DerivedPointerTable::add(derived, base);
 325 #endif // COMPILER2
 326 }
 327 
 328 
 329 #ifndef PRODUCT
 330 static void trace_codeblob_maps(const frame *fr, const RegisterMap *reg_map) {
 331   // Print oopmap and regmap
 332   tty->print_cr("------ ");
 333   CodeBlob* cb = fr->cb();
 334   OopMapSet* maps = cb->oop_maps();
 335   OopMap* map = cb->oop_map_for_return_address(fr->pc());
 336   map->print();
 337   if( cb->is_nmethod() ) {
 338     nmethod* nm = (nmethod*)cb;
 339     // native wrappers have no scope data, it is implied
 340     if (nm->is_native_method()) {
 341       tty->print("bci: 0 (native)");
 342     } else {
 343       ScopeDesc* scope  = nm->scope_desc_at(fr->pc());
 344       tty->print("bci: %d ",scope->bci());
 345     }
 346   }
 347   tty->cr();
 348   fr->print_on(tty);
 349   tty->print("     ");
 350   cb->print_value_on(tty);  tty->cr();
 351   reg_map->print();
 352   tty->print_cr("------ ");
 353 
 354 }
 355 #endif // PRODUCT
 356 
 357 void OopMapSet::oops_do(const frame *fr, const RegisterMap* reg_map, OopClosure* f) {
 358   // add derived oops to a table
 359   all_do(fr, reg_map, f, add_derived_oop, &do_nothing);
 360 }
 361 
 362 
 363 void OopMapSet::all_do(const frame *fr, const RegisterMap *reg_map,
 364                        OopClosure* oop_fn, void derived_oop_fn(oop*, oop*),
 365                        OopClosure* value_fn) {
 366   CodeBlob* cb = fr->cb();
 367   assert(cb != NULL, "no codeblob");
 368 
 369   NOT_PRODUCT(if (TraceCodeBlobStacks) trace_codeblob_maps(fr, reg_map);)
 370 
 371   OopMapSet* maps = cb->oop_maps();
 372   OopMap* map = cb->oop_map_for_return_address(fr->pc());
 373   assert(map != NULL, "no ptr map found");
 374 
 375   // handle derived pointers first (otherwise base pointer may be
 376   // changed before derived pointer offset has been collected)
 377   OopMapValue omv;
 378   {
 379     OopMapStream oms(map,OopMapValue::derived_oop_value);
 380     if (!oms.is_done()) {
 381 #ifndef TIERED
 382       COMPILER1_PRESENT(ShouldNotReachHere();)
 383 #endif // !TIERED
 384       // Protect the operation on the derived pointers.  This
 385       // protects the addition of derived pointers to the shared
 386       // derived pointer table in DerivedPointerTable::add().
 387       MutexLockerEx x(DerivedPointerTableGC_lock, Mutex::_no_safepoint_check_flag);
 388       do {
 389         omv = oms.current();
 390         oop* loc = fr->oopmapreg_to_location(omv.reg(),reg_map);
 391         if ( loc != NULL ) {
 392           oop *derived_loc = loc;
 393           oop *base_loc    = fr->oopmapreg_to_location(omv.content_reg(), reg_map);
 394           // Ignore NULL oops and decoded NULL narrow oops which
 395           // equal to Universe::narrow_oop_base when a narrow oop
 396           // implicit null check is used in compiled code.
 397           // The narrow_oop_base could be NULL or be the address
 398           // of the page below heap depending on compressed oops mode.
 399           if (base_loc != NULL && *base_loc != (oop)NULL && !Universe::is_narrow_oop_base(*base_loc)) {
 400             derived_oop_fn(base_loc, derived_loc);
 401           }
 402         }
 403         oms.next();
 404       }  while (!oms.is_done());
 405     }
 406   }
 407 
 408   // We want coop, value and oop oop_types
 409   int mask = OopMapValue::oop_value | OopMapValue::value_value | OopMapValue::narrowoop_value;
 410   {
 411     for (OopMapStream oms(map,mask); !oms.is_done(); oms.next()) {
 412       omv = oms.current();
 413       oop* loc = fr->oopmapreg_to_location(omv.reg(),reg_map);
 414       if ( loc != NULL ) {
 415         if ( omv.type() == OopMapValue::oop_value ) {
 416           oop val = *loc;
 417           if (val == (oop)NULL || Universe::is_narrow_oop_base(val)) {
 418             // Ignore NULL oops and decoded NULL narrow oops which
 419             // equal to Universe::narrow_oop_base when a narrow oop
 420             // implicit null check is used in compiled code.
 421             // The narrow_oop_base could be NULL or be the address
 422             // of the page below heap depending on compressed oops mode.
 423             continue;
 424           }
 425 #ifdef ASSERT
 426           if ((((uintptr_t)loc & (sizeof(*loc)-1)) != 0) ||
 427              !Universe::heap()->is_in_or_null(*loc)) {
 428             tty->print_cr("# Found non oop pointer.  Dumping state at failure");
 429             // try to dump out some helpful debugging information
 430             trace_codeblob_maps(fr, reg_map);
 431             omv.print();
 432             tty->print_cr("register r");
 433             omv.reg()->print();
 434             tty->print_cr("loc = %p *loc = %p\n", loc, (address)*loc);
 435             // do the real assert.
 436             assert(Universe::heap()->is_in_or_null(*loc), "found non oop pointer");
 437           }
 438 #endif // ASSERT
 439           oop_fn->do_oop(loc);
 440         } else if ( omv.type() == OopMapValue::value_value ) {
 441           assert((*loc) == (oop)NULL || !Universe::is_narrow_oop_base(*loc),
 442                  "found invalid value pointer");
 443           value_fn->do_oop(loc);
 444         } else if ( omv.type() == OopMapValue::narrowoop_value ) {
 445           narrowOop *nl = (narrowOop*)loc;
 446 #ifndef VM_LITTLE_ENDIAN
 447           if (!omv.reg()->is_stack()) {
 448             // compressed oops in registers only take up 4 bytes of an
 449             // 8 byte register but they are in the wrong part of the
 450             // word so adjust loc to point at the right place.
 451             nl = (narrowOop*)((address)nl + 4);
 452           }
 453 #endif
 454           oop_fn->do_oop(nl);
 455         }
 456       }
 457     }
 458   }
 459 }
 460 
 461 
 462 // Update callee-saved register info for the following frame
 463 void OopMapSet::update_register_map(const frame *fr, RegisterMap *reg_map) {
 464   ResourceMark rm;
 465   CodeBlob* cb = fr->cb();
 466   assert(cb != NULL, "no codeblob");
 467 
 468   // Any reg might be saved by a safepoint handler (see generate_handler_blob).
 469   assert( reg_map->_update_for_id == NULL || fr->is_older(reg_map->_update_for_id),
 470          "already updated this map; do not 'update' it twice!" );
 471   debug_only(reg_map->_update_for_id = fr->id());
 472 
 473   // Check if caller must update oop argument
 474   assert((reg_map->include_argument_oops() ||
 475           !cb->caller_must_gc_arguments(reg_map->thread())),
 476          "include_argument_oops should already be set");
 477 
 478   // Scan through oopmap and find location of all callee-saved registers
 479   // (we do not do update in place, since info could be overwritten)
 480 
 481   address pc = fr->pc();
 482   OopMap* map  = cb->oop_map_for_return_address(pc);
 483   assert(map != NULL, "no ptr map found");
 484   DEBUG_ONLY(int nof_callee = 0;)
 485 
 486   for (OopMapStream oms(map, OopMapValue::callee_saved_value); !oms.is_done(); oms.next()) {
 487     OopMapValue omv = oms.current();
 488     VMReg reg = omv.content_reg();
 489     oop* loc = fr->oopmapreg_to_location(omv.reg(), reg_map);
 490     reg_map->set_location(reg, (address) loc);
 491     DEBUG_ONLY(nof_callee++;)
 492   }
 493 
 494   // Check that runtime stubs save all callee-saved registers
 495 #ifdef COMPILER2
 496   assert(cb->is_compiled_by_c1() || !cb->is_runtime_stub() ||
 497          (nof_callee >= SAVED_ON_ENTRY_REG_COUNT || nof_callee >= C_SAVED_ON_ENTRY_REG_COUNT),
 498          "must save all");
 499 #endif // COMPILER2
 500 }
 501 
 502 //=============================================================================
 503 // Non-Product code
 504 
 505 #ifndef PRODUCT
 506 
 507 bool OopMap::has_derived_pointer() const {
 508 #ifndef TIERED
 509   COMPILER1_PRESENT(return false);
 510 #endif // !TIERED
 511 #ifdef COMPILER2
 512   OopMapStream oms((OopMap*)this,OopMapValue::derived_oop_value);
 513   return oms.is_done();
 514 #else
 515   return false;
 516 #endif // COMPILER2
 517 }
 518 
 519 #endif //PRODUCT
 520 
 521 // Printing code is present in product build for -XX:+PrintAssembly.
 522 
 523 static
 524 void print_register_type(OopMapValue::oop_types x, VMReg optional,
 525                          outputStream* st) {
 526   switch( x ) {
 527   case OopMapValue::oop_value:
 528     st->print("Oop");
 529     break;
 530   case OopMapValue::value_value:
 531     st->print("Value");
 532     break;
 533   case OopMapValue::narrowoop_value:
 534     st->print("NarrowOop");
 535     break;
 536   case OopMapValue::callee_saved_value:
 537     st->print("Callers_");
 538     optional->print_on(st);
 539     break;
 540   case OopMapValue::derived_oop_value:
 541     st->print("Derived_oop_");
 542     optional->print_on(st);
 543     break;
 544   default:
 545     ShouldNotReachHere();
 546   }
 547 }
 548 
 549 
 550 void OopMapValue::print_on(outputStream* st) const {
 551   reg()->print_on(st);
 552   st->print("=");
 553   print_register_type(type(),content_reg(),st);
 554   st->print(" ");
 555 }
 556 
 557 
 558 void OopMap::print_on(outputStream* st) const {
 559   OopMapValue omv;
 560   st->print("OopMap{");
 561   for(OopMapStream oms((OopMap*)this); !oms.is_done(); oms.next()) {
 562     omv = oms.current();
 563     omv.print_on(st);
 564   }
 565   st->print("off=%d}", (int) offset());
 566 }
 567 
 568 
 569 void OopMapSet::print_on(outputStream* st) const {
 570   int i, len = om_count();
 571 
 572   st->print_cr("OopMapSet contains %d OopMaps\n",len);
 573 
 574   for( i = 0; i < len; i++) {
 575     OopMap* m = at(i);
 576     st->print_cr("#%d ",i);
 577     m->print_on(st);
 578     st->cr();
 579   }
 580 }
 581 
 582 
 583 
 584 //------------------------------DerivedPointerTable---------------------------
 585 
 586 #ifdef COMPILER2
 587 
 588 class DerivedPointerEntry : public CHeapObj<mtCompiler> {
 589  private:
 590   oop*     _location; // Location of derived pointer (also pointing to the base)
 591   intptr_t _offset;   // Offset from base pointer
 592  public:
 593   DerivedPointerEntry(oop* location, intptr_t offset) { _location = location; _offset = offset; }
 594   oop* location()    { return _location; }
 595   intptr_t  offset() { return _offset; }
 596 };
 597 
 598 
 599 GrowableArray<DerivedPointerEntry*>* DerivedPointerTable::_list = NULL;
 600 bool DerivedPointerTable::_active = false;
 601 
 602 
 603 void DerivedPointerTable::clear() {
 604   // The first time, we create the list.  Otherwise it should be
 605   // empty.  If not, then we have probably forgotton to call
 606   // update_pointers after last GC/Scavenge.
 607   assert (!_active, "should not be active");
 608   assert(_list == NULL || _list->length() == 0, "table not empty");
 609   if (_list == NULL) {
 610     _list = new (ResourceObj::C_HEAP, mtCompiler) GrowableArray<DerivedPointerEntry*>(10, true); // Allocated on C heap
 611   }
 612   _active = true;
 613 }
 614 
 615 
 616 // Returns value of location as an int
 617 intptr_t value_of_loc(oop *pointer) { return cast_from_oop<intptr_t>((*pointer)); }
 618 
 619 
 620 void DerivedPointerTable::add(oop *derived_loc, oop *base_loc) {
 621   assert(Universe::heap()->is_in_or_null(*base_loc), "not an oop");
 622   assert(derived_loc != base_loc, "Base and derived in same location");
 623   if (_active) {
 624     assert(*derived_loc != (oop)base_loc, "location already added");
 625     assert(_list != NULL, "list must exist");
 626     intptr_t offset = value_of_loc(derived_loc) - value_of_loc(base_loc);
 627     // This assert is invalid because derived pointers can be
 628     // arbitrarily far away from their base.
 629     // assert(offset >= -1000000, "wrong derived pointer info");
 630 
 631     if (TraceDerivedPointers) {
 632       tty->print_cr(
 633         "Add derived pointer@" INTPTR_FORMAT
 634         " - Derived: " INTPTR_FORMAT
 635         " Base: " INTPTR_FORMAT " (@" INTPTR_FORMAT ") (Offset: " INTX_FORMAT ")",
 636         p2i(derived_loc), p2i((address)*derived_loc), p2i((address)*base_loc), p2i(base_loc), offset
 637       );
 638     }
 639     // Set derived oop location to point to base.
 640     *derived_loc = (oop)base_loc;
 641     assert_lock_strong(DerivedPointerTableGC_lock);
 642     DerivedPointerEntry *entry = new DerivedPointerEntry(derived_loc, offset);
 643     _list->append(entry);
 644   }
 645 }
 646 
 647 
 648 void DerivedPointerTable::update_pointers() {
 649   assert(_list != NULL, "list must exist");
 650   for(int i = 0; i < _list->length(); i++) {
 651     DerivedPointerEntry* entry = _list->at(i);
 652     oop* derived_loc = entry->location();
 653     intptr_t offset  = entry->offset();
 654     // The derived oop was setup to point to location of base
 655     oop  base        = **(oop**)derived_loc;
 656     assert(Universe::heap()->is_in_or_null(base), "must be an oop");
 657 
 658     *derived_loc = (oop)(((address)base) + offset);
 659     assert(value_of_loc(derived_loc) - value_of_loc(&base) == offset, "sanity check");
 660 
 661     if (TraceDerivedPointers) {
 662       tty->print_cr("Updating derived pointer@" INTPTR_FORMAT
 663                     " - Derived: " INTPTR_FORMAT "  Base: " INTPTR_FORMAT " (Offset: " INTX_FORMAT ")",
 664           p2i(derived_loc), p2i((address)*derived_loc), p2i((address)base), offset);
 665     }
 666 
 667     // Delete entry
 668     delete entry;
 669     _list->at_put(i, NULL);
 670   }
 671   // Clear list, so it is ready for next traversal (this is an invariant)
 672   if (TraceDerivedPointers && !_list->is_empty()) {
 673     tty->print_cr("--------------------------");
 674   }
 675   _list->clear();
 676   _active = false;
 677 }
 678 
 679 #endif // COMPILER2