help understanding lock instruction in OrderAccess::fence()

[恶灵骑士]

hotspot/src/os_cpu/linux_x86/vm/orderAccess_linux_x86.inline.hpp
inline void OrderAccess::fence() {
  if (os::is_MP()) {
    // always use locked addl since mfence is sometimes expensive
#ifdef AMD64
    __asm__ volatile ("lock; addl $0,0(%%rsp)" : : : "cc", "memory");
#else
    __asm__ volatile ("lock; addl $0,0(%%esp)" : : : "cc", "memory");
#endif
  }
}


my classmates think that code “addl $0,0(%%esp)” having some specific effect，
because esp points to the top of stack .
it that true ？
or the code “addl $0,0(%%esp)”  just equals no op, 
needing one operation after lock at least,  otherwise  lock instruction will produce an error .


Thank you !


Arron