g++ 4.2.x x86: code generation for __sync_lock_test_and_set() - builtin

[Daniel Lohmann <daniel dot lohmann at informatik dot uni-erlangen dot de>]

g++ 4.2.3

Hi,

I have the following code, which uses the new  
__sync_lock_test_and_set() builtin:

class Mutex {
  int locked;
public:
  Mutex() {
    locked = 0;
  }
  void lock();
  void unlock();
};

void Mutex::lock() {
  while( __sync_lock_test_and_set( &locked, 1) == 0 )
    ;
}
void Mutex::unlock() {
  __sync_lock_release( &locked );
}

After compiling with -03 -fomit-frame-pointer, the resulting code for  
the Mutex::lock() method looks as follows:

00000010 <Mutex::lock()>:
  10:	8b 54 24 04          	mov    0x4(%esp),%edx
  14:	b8 01 00 00 00       	mov    $0x1,%eax
  19:	87 02                	xchg   %eax,(%edx)
  1b:	85 c0                	test   %eax,%eax
  1d:	74 f5                	je     14 <Mutex::lock()+0x4>
  1f:	f3 c3                	repz ret


I am wondering about the repz prefix before the ret. A "do RET until Z- 
Flag is set" obviously does not make sense from the functional point  
of view. So I assume that it actually is a side effects of the repz  
prefix that is exploited here to guarantee "something" with respect to  
instruction reordering, fetching, caching, or ...?

So what exactly is this "something"?
And what exactly could happen under which circumstances if we don't  
use it?

 Google does not reveal much. If one googles for "repz ret" one gets  
a *load* of hits --  but just because of the fact that "ret" appears  
immediately after "repz" in the alphabetically sorted list of x86  
instructions :-)

Thanks!

Daniel