小编anL*_*856的帖子

我在阅读sun.misc.Unsafe.Java.

CAS 是像自旋一样的循环吗？

起初，我认为CAS只是一种低生活方式的原子操作。但是，当我尝试查找函数的源代码时compareAndSwapInt，我发现 cpp 代码如下：

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 …