# data present clause with 2D array in a C function

In the case of Jacobi iteration, I’m trying to put the computation clauses into a function, and the code is

``````#include <math.h>
#include <string.h>
#include "timer.h"
#include <stdio.h>

#define NN 1024
#define NM 1024

float A[NN][NM];
float Anew[NN][NM];
const int n = NN;
const int m = NM;
const int iter_max = 1000;

const double tol = 1.0e-6;
double error     = 1.0;

void testfunction(float A[NN][NM], float Anew[NN][NM])
{
#pragma acc data present(A, Anew)
#pragma acc kernels
{
error = 0.0;

for( int j = 1; j < n-1; j++)
{
for( int i = 1; i < m-1; i++ )
{
Anew[j][i] = 0.25 * ( A[j][i+1] + A[j][i-1]
+ A[j-1][i] + A[j+1][i]);
error = fmax( error, fabs(Anew[j][i] - A[j][i]));
}
}

for( int j = 1; j < n-1; j++)
{
for( int i = 1; i < m-1; i++ )
{
A[j][i] = Anew[j][i];
}
}
}

}

int main(int argc, char** argv)
{

memset(A, 0, n * m * sizeof(float));
memset(Anew, 0, n * m * sizeof(float));

for (int j = 0; j < n; j++)
{
A[j][0]    = 1.0;
Anew[j][0] = 1.0;
}

printf("Jacobi relaxation Calculation: %d x %d mesh\n", n, m);

StartTimer();
int iter = 0;

#pragma acc data copy(A), create(Anew)
while ( error > tol && iter < iter_max )
{
testfunction(A, Anew);

if(iter % 100 == 0) printf("%5d, %0.6f\n", iter, error);

iter++;
}

double runtime = GetTimer();

printf(" total: %f s\n", runtime / 1000);
}
``````

But it seems to be some errors, and the compiler reported as

``````shawnl@shawnl-ubuntu:~/Desktop/OpenACC_Projects/test\$ pgc++ -acc -Minfo=accel -ta=nvidia test2Dfunction_2modi.cpp testfunction(float (*)[1024], float (*)[1024]):
22, Generating present(A[:][:],Anew[:][:])
25, Complex loop carried dependence of A->,Anew-> prevents parallelization
Accelerator scalar kernel generated
27, Complex loop carried dependence of A->,Anew-> prevents parallelization
35, Complex loop carried dependence of Anew->,A-> prevents parallelization
Accelerator scalar kernel generated
37, Complex loop carried dependence of Anew->,A-> prevents parallelization
main:
64, Generating copy(A[:][:])
Generating create(Anew[:][:])
``````

My questions are:

Does the present clause work for 2D/3D arrays in a function?
How to manage the data transformation in a function?

Or,
How to use the restrict keyword to 2D/3D arrays?
Should it be rewritten as a 1D form?

Many thanks for helping.

Hi cwu87488,

The compiler analysis can tell that the two arrays are independent in this case. Hence, the simplest solution is to add “loop independent” directives before each loop.

``````void testfunction(float A[NN][NM], float Anew[NN][NM])
{
#pragma acc data present(A, Anew)
#pragma acc kernels
{
error = 0.0;
#pragma acc loop independent
for( int j = 1; j < n-1; j++)
{
#pragma acc loop independent
for( int i = 1; i < m-1; i++ )
{
Anew[j][i] = 0.25 * ( A[j][i+1] + A[j][i-1]
+ A[j-1][i] + A[j+1][i]);
error = fmax( error, fabs(Anew[j][i] - A[j][i]));
}
}
#pragma acc loop independent
for( int j = 1; j < n-1; j++)
{
#pragma acc loop independent
for( int i = 1; i < m-1; i++ )
{
A[j][i] = Anew[j][i];
}
}
}

}
``````

Hope this helps,
Mat

It works.
Thank you Mat!