[boost] lightweight mutex for MacOS

From: Martin Taylor (martin.taylor_at_[hidden])
Date: 2004-01-27 10:08:04

Hi

I would like to put forward an implementation of a lightweight mutex 
for MacOS.
I have tested it with shared_ptr_mt_test.cpp without problem.
It's fairly short, so I hope you don't mind me posting it directly here 
for review.
I should add also that I didn't really write this code (that's not a 
disclaimer - Howard Hinnant cooked it up after I asked the Codewarrior 
newsgroup for help!)

Anyway, here's the code:

#if __MACH__
        #include <sched.h>
#else
        #include <Multiprocessing.h>
#endif

namespace boost
{

namespace detail
{

class lightweight_mutex
{
private:

     volatile int a_;

     lightweight_mutex(lightweight_mutex const &);
     lightweight_mutex & operator=(lightweight_mutex const &);

public:

     lightweight_mutex(): a_(0)
     {
     }

     class scoped_lock;
     friend class scoped_lock;

     class scoped_lock
     {
     private:

         lightweight_mutex & m_;

         scoped_lock(scoped_lock const &);
         scoped_lock & operator=(scoped_lock const &);

                void yield_thread()
                {
#if __MACH__
                        sched_yield();
#else
                        MPYield();
#endif
                }
                
     public:

         explicit scoped_lock(lightweight_mutex & m);

         ~scoped_lock()
         {
             m_.a_ = 0;
         }
     };
};

inline lightweight_mutex::scoped_lock::scoped_lock(lightweight_mutex & 
m)
        : m_(m)
{
        register volatile int *p = &m_.a_;
        register int f;
        register int one = 1;

     asm
     {
     loop:
         lwarx  f, 0, p
         cmpwi  f, 0
         bne-   yield
         stwcx. one, 0, p
         beq+   done
     }

     yield:
             yield_thread();
             goto loop;
     done:
             ;
}

} // namespace detail

} // namespace boost

Martin