JDK14/Java14源码在线阅读

/*
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 * 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.
 *
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#ifndef SHARE_GC_SHARED_PTRQUEUE_HPP
#define SHARE_GC_SHARED_PTRQUEUE_HPP

#include "memory/padded.hpp"
#include "utilities/align.hpp"
#include "utilities/debug.hpp"
#include "utilities/globalDefinitions.hpp"
#include "utilities/lockFreeStack.hpp"
#include "utilities/sizes.hpp"

class Mutex;
class Monitor;

// There are various techniques that require threads to be able to log
// addresses.  For example, a generational write barrier might log
// the addresses of modified old-generation objects.  This type supports
// this operation.

class BufferNode;
class PtrQueueSet;
class PtrQueue {
  friend class VMStructs;

  NONCOPYABLE(PtrQueue);

  // The ptr queue set to which this queue belongs.
  PtrQueueSet* const _qset;

  // Whether updates should be logged.
  bool _active;

  // The (byte) index at which an object was last enqueued.  Starts at
  // capacity_in_bytes (indicating an empty buffer) and goes towards zero.
  // Value is always pointer-size aligned.
  size_t _index;

  // Size of the current buffer, in bytes.
  // Value is always pointer-size aligned.
  size_t _capacity_in_bytes;

  static const size_t _element_size = sizeof(void*);

  // Get the capacity, in bytes.  The capacity must have been set.
  size_t capacity_in_bytes() const {
    assert(_capacity_in_bytes > 0, "capacity not set");
    return _capacity_in_bytes;
  }

  static size_t byte_index_to_index(size_t ind) {
    assert(is_aligned(ind, _element_size), "precondition");
    return ind / _element_size;
  }

  static size_t index_to_byte_index(size_t ind) {
    return ind * _element_size;
  }

protected:
  // The buffer.
  void** _buf;

  size_t index() const {
    return byte_index_to_index(_index);
  }

  void set_index(size_t new_index) {
    size_t byte_index = index_to_byte_index(new_index);
    assert(byte_index <= capacity_in_bytes(), "precondition");
    _index = byte_index;
  }

  size_t capacity() const {
    return byte_index_to_index(capacity_in_bytes());
  }

  PtrQueueSet* qset() const { return _qset; }

  // Process queue entries and release resources.
  void flush_impl();

  // Process (some of) the buffer and leave it in place for further use,
  // or enqueue the buffer and allocate a new one.
  virtual void handle_completed_buffer() = 0;

  void allocate_buffer();

  // Enqueue the current buffer in the qset and allocate a new buffer.
  void enqueue_completed_buffer();

  // Initialize this queue to contain a null buffer, and be part of the
  // given PtrQueueSet.
  PtrQueue(PtrQueueSet* qset, bool active = false);

  // Requires queue flushed.
  ~PtrQueue();

public:

  // Forcibly set empty.
  void reset() {
    if (_buf != NULL) {
      _index = capacity_in_bytes();
    }
  }

  void enqueue(volatile void* ptr) {
    enqueue((void*)(ptr));
  }

  // Enqueues the given "obj".
  void enqueue(void* ptr) {
    if (!_active) return;
    else enqueue_known_active(ptr);
  }

  void handle_zero_index();

  void enqueue_known_active(void* ptr);

  // Return the size of the in-use region.
  size_t size() const {
    size_t result = 0;
    if (_buf != NULL) {
      assert(_index <= capacity_in_bytes(), "Invariant");
      result = byte_index_to_index(capacity_in_bytes() - _index);
    }
    return result;
  }

  bool is_empty() const {
    return _buf == NULL || capacity_in_bytes() == _index;
  }

  // Set the "active" property of the queue to "b".  An enqueue to an
  // inactive thread is a no-op.  Setting a queue to inactive resets its
  // log to the empty state.
  void set_active(bool b) {
    _active = b;
    if (!b && _buf != NULL) {
      reset();
    } else if (b && _buf != NULL) {
      assert(index() == capacity(),
             "invariant: queues are empty when activated.");
    }
  }

  bool is_active() const { return _active; }

  // To support compiler.

protected:
  template<typename Derived>
  static ByteSize byte_offset_of_index() {
    return byte_offset_of(Derived, _index);
  }

  static ByteSize byte_width_of_index() { return in_ByteSize(sizeof(size_t)); }

  template<typename Derived>
  static ByteSize byte_offset_of_buf() {
    return byte_offset_of(Derived, _buf);
  }

  static ByteSize byte_width_of_buf() { return in_ByteSize(_element_size); }

  template<typename Derived>
  static ByteSize byte_offset_of_active() {
    return byte_offset_of(Derived, _active);
  }

  static ByteSize byte_width_of_active() { return in_ByteSize(sizeof(bool)); }

};

class BufferNode {
  size_t _index;
  BufferNode* volatile _next;
  void* _buffer[1];             // Pseudo flexible array member.

  BufferNode() : _index(0), _next(NULL) { }
  ~BufferNode() { }

  NONCOPYABLE(BufferNode);

  static size_t buffer_offset() {
    return offset_of(BufferNode, _buffer);
  }

  static BufferNode* volatile* next_ptr(BufferNode& bn) { return &bn._next; }

  // Allocate a new BufferNode with the "buffer" having size elements.
  static BufferNode* allocate(size_t size);

  // Free a BufferNode.
  static void deallocate(BufferNode* node);

public:
  typedef LockFreeStack<BufferNode, &next_ptr> Stack;

  BufferNode* next() const     { return _next;  }
  void set_next(BufferNode* n) { _next = n;     }
  size_t index() const         { return _index; }
  void set_index(size_t i)     { _index = i; }

  // Return the BufferNode containing the buffer, after setting its index.
  static BufferNode* make_node_from_buffer(void** buffer, size_t index) {
    BufferNode* node =
      reinterpret_cast<BufferNode*>(
        reinterpret_cast<char*>(buffer) - buffer_offset());
    node->set_index(index);
    return node;
  }

  // Return the buffer for node.
  static void** make_buffer_from_node(BufferNode *node) {
    // &_buffer[0] might lead to index out of bounds warnings.
    return reinterpret_cast<void**>(
      reinterpret_cast<char*>(node) + buffer_offset());
  }

  class Allocator;              // Free-list based allocator.
  class TestSupport;            // Unit test support.
};

// Allocation is based on a lock-free free list of nodes, linked through
// BufferNode::_next (see BufferNode::Stack).  To solve the ABA problem,
// popping a node from the free list is performed within a GlobalCounter
// critical section, and pushing nodes onto the free list is done after
// a GlobalCounter synchronization associated with the nodes to be pushed.
// This is documented behavior so that other parts of the node life-cycle
// can depend on and make use of it too.
class BufferNode::Allocator {
  friend class TestSupport;

  // Since we don't expect many instances, and measured >15% speedup
  // on stress gtest, padding seems like a good tradeoff here.
#define DECLARE_PADDED_MEMBER(Id, Type, Name) \
  Type Name; DEFINE_PAD_MINUS_SIZE(Id, DEFAULT_CACHE_LINE_SIZE, sizeof(Type))

  const size_t _buffer_size;
  char _name[DEFAULT_CACHE_LINE_SIZE - sizeof(size_t)]; // Use name as padding.
  DECLARE_PADDED_MEMBER(1, Stack, _pending_list);
  DECLARE_PADDED_MEMBER(2, Stack, _free_list);
  DECLARE_PADDED_MEMBER(3, volatile size_t, _pending_count);
  DECLARE_PADDED_MEMBER(4, volatile size_t, _free_count);
  DECLARE_PADDED_MEMBER(5, volatile bool, _transfer_lock);

#undef DECLARE_PADDED_MEMBER

  void delete_list(BufferNode* list);
  bool try_transfer_pending();

  NONCOPYABLE(Allocator);

public:
  Allocator(const char* name, size_t buffer_size);
  ~Allocator();

  const char* name() const { return _name; }
  size_t buffer_size() const { return _buffer_size; }
  size_t free_count() const;
  BufferNode* allocate();
  void release(BufferNode* node);

  // Deallocate some of the available buffers.  remove_goal is the target
  // number to remove.  Returns the number actually deallocated, which may
  // be less than the goal if there were fewer available.
  size_t reduce_free_list(size_t remove_goal);
};

// A PtrQueueSet represents resources common to a set of pointer queues.
// In particular, the individual queues allocate buffers from this shared
// set, and return completed buffers to the set.

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