RFC: powerpc libgomp locking

[Richard Henderson]

On 11/17/2011 01:56 AM, Alan Modra wrote:
> This is part of work in progress getting locking back into shape on
> powerpc.  (If we ever were in shape, which is doubtful.)  Using the
> ia64 version means we have a needless sync at the start of
> gomp_mutex_lock (courtesy of __sync_val_compare_and_swap) and a
> needless isync at the end of gomp_mutex_unlock (slightly odd use
> of __sync_lock_test_and_set to release a lock).  It would be nice to
> get rid of the __sync_val_compare_and_swap in config/linux/mutex.c
> too.

We should be able to use this new file as a replacement for all
targets, not just PPC.

And, yes, updating all of the __sync uses in libgomp would be a
good thing.

> +  __atomic_compare_exchange_4 (mutex, &oldval, 1, false,
> +			       MEMMODEL_ACQUIRE, MEMMODEL_RELAXED);
> +  if (__builtin_expect (oldval != 0, 0))
> +    gomp_mutex_lock_slow (mutex, oldval);

__atomic_compare_exchange_4 returns a boolean success:

  if (__builtin_expect (!__atomic_compare_exchange_4
		        (mutex, &oldval, 1, false,
			 MEMMODEL_ACQUIRE, MEMMODEL_RELAXED), 0))

In theory that should avoid an extra comparison on some targets.


r~