UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapInt(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jint e, jint x))
  UnsafeWrapper("Unsafe_CompareAndSwapInt");
  oop p = JNIHandles::resolve(obj);
  jint* addr = (jint *) index_oop_from_field_offset_long(p, offset);
  return (jint)(Atomic::cmpxchg(x, addr, e)) == e;
UNSAFE_END

return (jint)(Atomic::cmpxchg(x, addr, e)) == e;

分析,有三个值,内存值V,旧的预期值A,新的修改值B;

如果A==V,那么把B赋值给V,返回V,如果A！=V，直接返回V;

进入

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;
  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;
  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]; }

inline jlong Atomic::cmpxchg (jlong exchange_value, volatile jlong* dest, jlong compare_value) {

　　　　　　　　　　　　　　　　　　　　// 

return _Atomic_cmpxchg_long(exchange_value, dest, compare_value, os::is_MP());
}

　　　　　　　　　　　　　　　　　　　　　　　　// 

inline jlong _Atomic_cmpxchg_long(jlong exchange_value, volatile jlong* dest, jlong compare_value, int mp) {

#ifdef AMD64
    __asm__ __volatile__ (LOCK_IF_MP(%4) "cmpxchgq %1,(%3)"
                        : "=a" (exchange_value)
                        : "r" (exchange_value), "a" (compare_value), "r" (dest), "r" (mp)
                        : "cc", "memory");
    return exchange_value;
#else
    return _Atomic_cmpxchg_long_gcc(exchange_value, dest, compare_value, os::is_MP());

    #if 0
    // The code below does not work presumably because of the bug in gcc
    // The error message says:
    //   can't find a register in class BREG while reloading asm
    // However I want to save this code and later replace _Atomic_cmpxchg_long_gcc
    // with such inline asm code:

    volatile jlong_accessor evl, cvl, rv;
    evl.long_value = exchange_value;
    cvl.long_value = compare_value;
    int mp = os::is_MP();

    __asm__ volatile ("cmp $0, %%esi\n\t"
       "je 1f \n\t"
       "lock\n\t"
       "1: cmpxchg8b (%%edi)\n\t"
       : "=a"(cvl.words[0]),   "=d"(cvl.words[1])
       : "a"(cvl.words[0]), "d"(cvl.words[1]),
         "b"(evl.words[0]), "c"(evl.words[1]),
         "D"(dest), "S"(mp)
       :  "cc", "memory");
    return cvl.long_value;
    #endif // if 0
#endif // AMD64
  }