openJDK之sun.misc.Unsafe类CAS底层实现

UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapObject(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jobject e_h, jobject x_h))
  UnsafeWrapper("Unsafe_CompareAndSwapObject");
  oop x = JNIHandles::resolve(x_h);//更新后的值
  oop e = JNIHandles::resolve(e_h);//期望值
  oop p = JNIHandles::resolve(obj);//更新的对象
  HeapWord* addr = (HeapWord *)index_oop_from_field_offset_long(p, offset);//获取偏移地址，可以理解为获取要更新的属性的内存地址
  oop res = oopDesc::atomic_compare_exchange_oop(x, addr, e, true);//调用方法执行CAS操作
  jboolean success  = (res == e);//atomic_compare_exchange_oop的返回值是否等于期望值，如果等于，则success为true
  if (success)
    update_barrier_set((void*)addr, x);//更新memory barrier
  return success;
UNSAFE_END

  static oop atomic_compare_exchange_oop(oop exchange_value,
                                         volatile HeapWord *dest,
                                         oop compare_value,
                                         bool prebarrier = false);

inline oop oopDesc::atomic_compare_exchange_oop(oop exchange_value,
                                                volatile HeapWord *dest,
                                                oop compare_value,
                                                bool prebarrier) {
  if (UseCompressedOops) {
    if (prebarrier) {
      update_barrier_set_pre((narrowOop*)dest, exchange_value);
    }
    // encode exchange and compare value from oop to T
    narrowOop val = encode_heap_oop(exchange_value);
    narrowOop cmp = encode_heap_oop(compare_value);

    narrowOop old = (narrowOop) Atomic::cmpxchg(val, (narrowOop*)dest, cmp);//核心
    // decode old from T to oop
    return decode_heap_oop(old);
  } else {
    if (prebarrier) {
      update_barrier_set_pre((oop*)dest, exchange_value);
    }
    return (oop)Atomic::cmpxchg_ptr(exchange_value, (oop*)dest, compare_value);//核心
  }
}

jbyte Atomic::cmpxchg(jbyte exchange_value, volatile jbyte* dest, jbyte compare_value) {
  assert(sizeof(jbyte) == 1, "assumption.");
  uintptr_t dest_addr = (uintptr_t)dest;
  uintptr_t offset = dest_addr % sizeof(jint);
  volatile jint* dest_int = (volatile jint*)(dest_addr - offset);
  jint cur = *dest_int;//对象当前值
  jbyte* cur_as_bytes = (jbyte*)(&cur);
  jint new_val = cur;
  jbyte* new_val_as_bytes = (jbyte*)(&new_val);
  new_val_as_bytes[offset] = exchange_value;
  //这里有个for循环，资料上说“ 比较当前值与期望值，如果相同则更新，不同则直接返回”？
  while (cur_as_bytes[offset] == compare_value) {
    jint res = cmpxchg(new_val, dest_int, cur);
    if (res == cur) break;
    cur = res;
    new_val = cur;
    new_val_as_bytes[offset] = exchange_value;
  }
  return cur_as_bytes[offset];
}