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Re: Patch for PR libgomp/37938, IA64 specific.


On Thu, Nov 06, 2008 at 10:33:11AM -0800, Steve Ellcey wrote:
> On Thu, 2008-11-06 at 09:56 -0800, H.J. Lu wrote:
> 
> > Shouldn't we use __sync_lock_test_and_set  to initialize mutux?
> > 
> > 
> > H.J.
> > --- libgomp/config/linux/mutex.h.sync	2006-11-18 06:22:18.000000000 -0800
> > +++ libgomp/config/linux/mutex.h	2008-11-06 09:50:46.000000000 -0800
> > @@ -38,7 +38,7 @@ typedef int gomp_mutex_t;
> > 
> >  static inline void gomp_mutex_init (gomp_mutex_t *mutex)
> >  {
> > -  *mutex = 0;
> > +  __sync_lock_test_and_set (mutex, 0);
> >  }
> > 
> >  extern void gomp_mutex_lock_slow (gomp_mutex_t *mutex);
> 
> I was wondering about the initialization but this patch didn't seem to
> fix the test case in PR 37938 when I added it and then removed my
> patches.

Why would mutex initialization need it?  A mutex is always initialized
uncontended, so other threads either don't exist at that point, or are
prevented from trying to look it, usually by some other mutex, barrier etc.

I guess the problem is that the __sync_* builtins don't have explicit
acquire vs. release semantics variants.  We need to use
__sync_bool_compare_and_swap for locking the mutex, and I believe on ia64
__sync_bool_compare_and_swap has release, not acquire semantics, which is
desirable for mutex lock, and for unlocking the mutex we use
__sync_lock_test_and_set (which has acquire semantics?), but want release
semantics, right?  Putting __sync_synchronize into gomp_mutex_unlock
would mean slowing down other targets, for which already
__sync_lock_test_and_set (and __sync_bool_compare_and_swap) act as a full
barrier.  So I'm afraid ia64 needs to define config/linux/ia64/mutex.[ch]
which uses __asm instead of __sync builtins.

	Jakub


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