openJDK之sun.misc.Unsafe類CAS底層實現

注：這篇文章參考了http://www.javashuo.com/article/p-faklllqw-bs.html，然後本身結合hotspot源碼看的

1.sun.misc.Unsafe中CAS方法

在sun.misc.Unsafe中CAS方法以下：

1. compareAndSwapObject(java.lang.Object arg0, long arg1, java.lang.Object arg2, java.lang.Object arg3);
2. compareAndSwapInt(java.lang.Object arg0, long arg1, int arg2, int arg3);
3. compareAndSwapLong(java.lang.Object arg0, long arg1, long arg2, long arg3);

來看下openJDK8的hotspot中，unsafe是如何實現的，連接http://hg.openjdk.java.net/jdk8u/jdk8u60/hotspot/file/37240c1019fd/src/share/vm/prims/unsafe.cpp

 

                                                                    圖1

    List-1

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);//更新後的值
  oop e = JNIHandles::resolve(e_h);//指望值
  oop p = JNIHandles::resolve(obj);//更新的對象
  HeapWord* addr = (HeapWord *)index_oop_from_field_offset_long(p, offset);//獲取偏移地址，能夠理解爲獲取要更新的屬性的內存地址
  oop res = oopDesc::atomic_compare_exchange_oop(x, addr, e, true);//調用方法執行CAS操做
  jboolean success  = (res == e);//atomic_compare_exchange_oop的返回值是否等於指望值，若是等於，則success爲true
  if (success)
    update_barrier_set((void*)addr, x);//更新memory barrier
  return success;
UNSAFE_END

 

    atomic_exchange_oop聲明在http://hg.openjdk.java.net/jdk8u/jdk8u60/hotspot/file/37240c1019fd/src/share/vm/oops/oop.hpp中，以下

    List-2

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

    實如今http://hg.openjdk.java.net/jdk8u/jdk8u60/hotspot/file/37240c1019fd/src/share/vm/oops/oop.inline.hpp中，以下List-3所示：

    List-3

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
  }
}

    如上的List-3所示，核心的CAS調用Atomic::cmpxchg(val, (narrowOop*)dest, cmp)和Atomic::cmpxchg_ptr(exchange_value, (oop*)dest, compare_value)。

    cmpxchg的實現是在http://hg.openjdk.java.net/jdk8u/jdk8u60/hotspot/file/37240c1019fd/src/share/vm/runtime/atomic.cpp中，以下List-4所示

    List-4

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;//對象當前值
  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;
  //這裏有個for循環，資料上說「 比較當前值與指望值，若是相同則更新，不一樣則直接返回」？
  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];
}

    cmpxchg_ptr的實現由不一樣的系統而實現不一樣，以64位linux爲例，是在http://hg.openjdk.java.net/jdk8u/jdk8u60/hotspot/file/37240c1019fd/src/os_cpu/linux_x86/vm/atomic_linux_x86.inline.hpp中