/*
 * Copyright (c) 2020, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */

#include "precompiled.hpp"

#include "gc/shared/objectMarker.hpp"
#include "memory/iterator.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/biasedLocking.hpp"

// This implementation uses the existing mark bits in an object for
// marking. Objects that are marked must later have their headers restored.
// As most objects are unlocked and don't have their identity hash computed
// we don't have to save their headers. Instead we save the headers that
// are "interesting". Later when the headers are restored this implementation
// restores all headers to their initial value and then restores the few
// objects that had interesting headers.
//
// Future work: This implementation currently uses growable arrays to save
// the oop and header of interesting objects. As an optimization we could
// use the same technique as the GC and make use of the unused area
// between top() and end().
//

// An ObjectClosure used to restore the mark bits of an object
class RestoreMarksClosure : public ObjectClosure {
 public:
  void do_object(oop o) {
    if (o != NULL) {
      markWord mark = o->mark();
      if (mark.is_marked()) {
        o->init_mark();
      }
    }
  }
};

DefaultObjectMarker::DefaultObjectMarker() :
  _saved_oop_stack(NULL),
  _saved_mark_stack(NULL) {
  set_needs_reset(true);
}

// initialize ObjectMarker - prepares for object marking
bool DefaultObjectMarker::init() {
  assert(Thread::current()->is_VM_thread(), "must be VMThread");

  // prepare heap for iteration
  Universe::heap()->ensure_parsability(false);  // no need to retire TLABs

  // create stacks for interesting headers
  _saved_mark_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<markWord>(4000, true);
  _saved_oop_stack = new (ResourceObj::C_HEAP, mtInternal) GrowableArray<oop>(4000, true);

  if (UseBiasedLocking) {
    BiasedLocking::preserve_marks();
  }

  return true;
}

// Object marking is done so restore object headers
void DefaultObjectMarker::done() {
  // iterate over all objects and restore the mark bits to
  // their initial value
  RestoreMarksClosure blk;
  if (needs_reset()) {
    Universe::heap()->object_iterate(&blk);
  } else {
    // We don't need to reset mark bits on this call, but reset the
    // flag to the default for the next call.
    set_needs_reset(true);
  }

  // now restore the interesting headers
  for (int i = 0; i < _saved_oop_stack->length(); i++) {
    oop o = _saved_oop_stack->at(i);
    markWord mark = _saved_mark_stack->at(i);
    o->set_mark(mark);
  }

  if (UseBiasedLocking) {
    BiasedLocking::restore_marks();
  }

  // free the stacks
  delete _saved_oop_stack;
  delete _saved_mark_stack;

  delete this;
}

// mark an object
bool DefaultObjectMarker::mark(oop o) {
  assert(Universe::heap()->is_in(o), "sanity check");

  // object's mark word
  markWord mark = o->mark();
  if (mark.is_marked()) {
    return false;
  }

  if (o->mark_must_be_preserved(mark)) {
    _saved_mark_stack->push(mark);
    _saved_oop_stack->push(o);
  }

  // mark the object
  o->set_mark(markWord::prototype().set_marked());
  return true;
}

// return true if object is visited
bool DefaultObjectMarker::visited(oop o) {
  return o->mark().is_marked();
}