【不简单的Atomic工具】getAndAddInt in getAndIncrement

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上一篇Atmoic文章中提到，JDK8在AtomicInteger的getAndIncrement方法实现上发生了变化：

JDK7
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public final int getAndIncrement() {
  for (;;) {
    int current = get();
    int next = current + 1;
    if (compareAndSet(current, next))
      return current;
  }
}
JDK8
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public final int getAndIncrement() {
    return unsafe.getAndAddInt(this, valueOffset, 1);
}

getAndAddInt

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public final int getAndAddInt(Object paramObject, long paramLong, int paramInt) {
    int i;
    do
      i = getIntVolatile(paramObject, paramLong);
    while (!(compareAndSwapInt(paramObject, paramLong, i, i + paramInt)));
    return i;
}

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jint
sun::misc::Unsafe::getIntVolatile (jobject obj, jlong offset)
{
  volatile jint *addr = (jint *) ((char *) obj + offset);
  jint result = *addr;
  read_barrier ();
  return result;
}

汇编之后

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0x0000000002c207f5: lock cmpxchg %r8d,0xc(%rdx)
0x0000000002c207fb: sete   %r11b
0x0000000002c207ff: movzbl %r11b,%r11d
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0x0000000002cf49c7: mov    %rbp,0x10(%rsp)
0x0000000002cf49cc: mov    $0x1,%eax
0x0000000002cf49d1: lock xadd %eax,0xc(%rdx)

原本的lock cmpxchg被lock xadd所替换，也就是在1.8之后，在硬件支持lock XADD这样的操作情况下，都会用fetch-and-add来替换CAS操作。

参考：