Re: Use atomics in libgomp mutex

[Alan Modra <amodra at gmail dot com>]

On Mon, Nov 28, 2011 at 09:27:20AM +1030, Alan Modra wrote:
> This is the mutex part.  Depends on
> http://gcc.gnu.org/ml/gcc-patches/2011-11/msg02454.html for MEMMODEL_*
> values.

Arrgh, I posted the wrong patch.  I know it needs work elsewhere in
libgomp to comply with the OMP spec, which says of flush:

14 A flush region without a list is implied at the following locations:
15 â During a barrier region.
16 â At entry to and exit from parallel, critical, and ordered regions.
[snip]

Yet COMP_critical_start simply calls gomp_mutex_lock, relying on the
lock acquisition to do the flush.  This won't necessarily happen with
MEMMODEL_ACQUIRE.

This bit in gomp_mutex_lock
> +  __atomic_compare_exchange_4 (mutex, &oldval, 1, false,
> +			       MEMMODEL_ACQUIRE, MEMMODEL_RELAXED);
should be
  __atomic_compare_exchange_4 (mutex, &oldval, 1, false,
			       MEMMODEL_ACQ_REL, MEMMODEL_ACQ_REL);

and once you do that it's hard to justify the patch for stage3 as
fixing a bug (unnecessary sync instructions emitted by __sync
builtins).

-- 
Alan Modra
Australia Development Lab, IBM