openjdk源码分析之AtomicLong

关键代码:

    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset; // 底层value变量的偏移地址
    private volatile long value; //底层变量


static {
        try {
            valueOffset = unsafe.objectFieldOffset
                (AtomicLong.class.getDeclaredField("value"));
        } catch (Exception ex) { throw new Error(ex); }
    }

    public final boolean compareAndSet(long expect, long update) {
        return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
    }

然后看unsafe.cpp中compareAndSwapLong的定义:

JNINativeMethod

{CC "compareAndSetLong", CC "(" OBJ "J""J""J"")Z", FN_PTR(Unsafe_CompareAndSetLong)}

UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSetLong(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jlong e, jlong x)) {
  oop p = JNIHandles::resolve(obj);
  if (p == NULL) {
    volatile jlong* addr = (volatile jlong*)index_oop_from_field_offset_long(p, offset);
    return RawAccess<>::atomic_cmpxchg(x, addr, e) == e;
  } else {
    assert_field_offset_sane(p, offset);
    return HeapAccess<>::atomic_cmpxchg_at(x, p, (ptrdiff_t)offset, e) == e;
  }
} UNSAFE_END

template <DecoratorSet decorators>
template <DecoratorSet ds, typename T>
inline typename EnableIf<
  HasDecorator<ds, MO_SEQ_CST>::value, T>::type
RawAccessBarrier<decorators>::atomic_cmpxchg_internal(T new_value, void* addr, T compare_value) {
  return Atomic::cmpxchg(new_value,
                         reinterpret_cast<volatile T*>(addr),
                         compare_value,
                         memory_order_conservative);
}

template<typename T, typename D, typename U>
inline D Atomic::cmpxchg(T exchange_value,
                         D volatile* dest,
                         U compare_value,
                         atomic_memory_order order) {
  return CmpxchgImpl<T, D, U>()(exchange_value, dest, compare_value, order);
}

// Handle cmpxchg for integral and enum types.
//
// All the involved types must be identical.
template<typename T>
struct Atomic::CmpxchgImpl<
  T, T, T,
  typename EnableIf<IsIntegral<T>::value || IsRegisteredEnum<T>::value>::type>
{
  T operator()(T exchange_value, T volatile* dest, T compare_value,
               atomic_memory_order order) const {
    // Forward to the platform handler for the size of T.
    return PlatformCmpxchg<sizeof(T)>()(exchange_value,
                                        dest,
                                        compare_value,
                                        order);
  }
};

然后转到具体平台相关的实现类，比如mac下的src/hotspot/os_cpu/bsd_x86/atomic_bsd_x86.hpp

template<>
template<typename T>
inline T Atomic::PlatformCmpxchg<8>::operator()(T exchange_value,
                                                T volatile* dest,
                                                T compare_value,
                                                atomic_memory_order /* order */) const {
  STATIC_ASSERT(8 == sizeof(T));
  __asm__ __volatile__ (  "lock cmpxchgq %1,(%3)"
                        : "=a" (exchange_value)
                        : "r" (exchange_value), "a" (compare_value), "r" (dest)
                        : "cc", "memory");
  return exchange_value;
}

可见mac下采用的是cmpxchgq汇编指令实现：

__asm__ 内联汇编

lock 锁消息总线保证互斥地使用这个内存地址

AT&T汇编格式： 指令 源操作数 目的操作数

cmpxchg : 用RAX中的值与目的操作数的值进行比较，如果相等，则设置ZF标志并把源操作数加载到目的操作数中，否则清除ZF标志并把目的操作数加载到RAX中。

分析：lock cmpxchgq %1,(%3) 其中%1指exchange_value,%3指dest。
首先把compare_value加载到RAX,然后比较RAX与目的操作数(%3即dest)的值，如果相等则把%1即exchange_value的值设置到dest,返回RAX中的值即compare_value。否则把%3即dest中的值加载的RAX寄存器，然后返回RAX中的值。即如果相等返回的是compare_value,否则返回的是dest原来的值。最后那返回的值与比较的值进行比较，相等则返回true表示设置成功。
return HeapAccess<>::atomic_cmpxchg_at(x, p, (ptrdiff_t)offset, e) == e;