[Omp] static variables question

[Scott Paine]

Certainly Tim is right-- even in a single-threaded envrionment, where in
principle the order of recursive function calls is knowable in advance,
things would be conceptually very difficult to keep track of if the
recursively-called function were not reentrant.

Regarding recursion in OpenMP, I've done the following kind of thing
using nested parallelism.  In the example below, which is from a
recursive fft, the number (nbranch) of recursive calls to fft_dif_rec()
doubles at each recursion level.  New threads are generated unless
nbranch exceeds the maximum number of OMP threads.  The idea is to pass
a parameter to each recursive call which lets that call know how deep in
the recursion it is.  Then, an if clause on a parallel sections
directive can be used to control whether subsequent recursion is done
serially or in parallel.

Scott Paine
Smithsonian Astrophysical Observatory
spaine at cfa.harvard.edu

========================================
/*
 * External interface calls recursive routine with an initial branch
count of 1
 */
void fft_dif(double *z, int n)
{
fft_dif_rec(z, n, 1);
return;
} /* fft_dif() */

static void fft_dif_rec(double *z, int n, int nbranch)
{
. . . code omitted here . . .
	nh = n >> 1;
	nbranch <<= 1;
	#ifdef _OPENMP
	#pragma omp parallel sections if (nbranch <= omp_get_max_threads())
	#endif
	{
		#ifdef _OPENMP
		#pragma omp section
		#endif
		fft_dif_rec(z, nh, nbranch);
		#ifdef _OPENMP
		#pragma omp section
		#endif
		fft_dif_rec(z + n, nh, nbranch);
	}
	return;
} /* fft_dif_rec() */


On Sun, 2007-01-07 at 16:29 -0800, Tim Prince wrote:
> Klaas.Vantournhout at UGent.be wrote:
> > 
> > I have a question regarding static variables which are declared in 
> > routines.
> > 
> > 
> > What I am thinking about are recursive functions which use static 
> > variables to obtain a meaningful result.  
> Any variable which has a value belonging to the current scope of a 
> recursive function must be automatic, regardless of openmp.  I'm having 
> difficulty imagining how a recursive function would be employed in 
> openmp, other than within task sharing, with each thread having its own 
> data (presumably thread private).
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