Re: openacc reference reductions

[Nathan Sidwell <nathan@acm.org>]

On 04/08/16 00:40, Jakub Jelinek wrote:

>> +/* OpenACC parallel reductions need a present_or_copy clause to ensure
>> +   that the original variable used in the reduction gets updated on
>> +   the host.  Scan the list of clauses for reduction so that any existing
>> +   data clause can be adjusted if necessary.  */
>> +  if (region_type == ORT_ACC_PARALLEL)
>> +    {
>> +      for (c = *list_p; c; c = OMP_CLAUSE_CHAIN (c))
>> +	{
>> +	  tree decl = NULL_TREE;
>> +
>> +	  if (OMP_CLAUSE_CODE (c) != OMP_CLAUSE_REDUCTION)
>> +	    continue;
>> +
>> +	  decl = OMP_CLAUSE_DECL (c);
>> +	  omp_add_variable (ctx, decl, GOVD_REDUCTION);
>> +	}
>> +    }
>> +
>
> And this looks also wrong, why?

And besides, won't it break
#pragma acc parallel firstprivate (x)  reduction(+:x)
{...}

which the openacc guys at GTC indicated they didn't want to disallow (without 
further thinking).

It seems to me the interaction of reduction and data copying at a parallel is 
still  not sufficiently well defined.

nathan