Fix PR51298, libgomp barrier failure

[Richard Henderson]

On 11/28/2011 06:02 AM, Alan Modra wrote:
> -  unsigned int ret = bar->generation & ~3;
> -  /* Do we need any barrier here or is __sync_add_and_fetch acting
> -     as the needed LoadLoad barrier already?  */
> -  ret += __sync_add_and_fetch (&bar->awaited, -1) == 0;
> +  unsigned int ret = __atomic_load_4 (&bar->generation, MEMMODEL_ACQUIRE) & ~3;
> +  ret += __atomic_add_fetch (&bar->awaited, -1, MEMMODEL_ACQ_REL) == 0;

Given that the read from bar->generation is ACQ, we don't need a duplicate 
barrier from the REL on the atomic add.  I believe both can be MEMMODEL_ACQUIRE
both in order to force the ordering of these two memops, as well as force these
to happen before anything subsequent.

The s/_4/_n/ change needs doing.

Otherwise ok.


r~