Reduction results in wrong results. Bug?


I’m currentliy working on this little piece of code:

#pragma acc parallel loop gang private(i,j) copy(force[0:DIM][0:N]) copyin(pos[0:DIM][0:N])
   for(i=0;i < num_particles ; ++i){
      double tmp_x = 0.0;
      double tmp_y = 0.0;
      double tmp_z = 0.0;
#pragma acc loop //reduction(+:tmp_x,tmp_z,tmp_z)
      for(j=0;j < num_particles; ++j){
         double dx = (pos[0][j] - pos[0][i]);
         double dy = (pos[1][j] - pos[1][i]);
         double dz = (pos[2][j] - pos[2][i]);
         double r = dx * dx + dy * dy + dz * dz;
         double tmp_f;
         if(r != 0.0){
            double s = 1.0 / r;
            s = s * s * s;
            tmp_f = 100.0 * s/r * (1.0 - 2.0 * s);
            tmp_f = 0.0;
         tmp_x += tmp_f * dx;
         tmp_y += tmp_f * dy;
         tmp_z += tmp_f * dz;
      force[0][i] += tmp_x;
      force[1][i] += tmp_y;
      force[2][i] += tmp_z;

If I compile this code as it is here, it is working perfectly (same results as sequential C code) and I receive the following compiler feedback:

    472, Accelerator kernel generated
        472, CC 2.0 : 47 registers; 16 shared, 96 constant, 0 local memory bytes
        476, #pragma acc loop gang /* blockIdx.x */
        481, #pragma acc loop vector(256) /* threadIdx.x */

However, I think that a reduction for tmp_x, tmp_y and tmp_z would be required. So if I uncomment the reduction I receive the exact same compiler feedback (i.e. nothing about added reductions) but the results are wrong.

What am I missing here?


Hi Paul,

In order the parallelize the inner loop, the compiler must be automatically generating the reductions, otherwise you’d be getting wrong answers without the reduction clause. I’m not sure why adding the reduction clause would then yield wrong answers. Seems like a compiler error.

Can you either post or send to PGI Customer Support ( a reproducing example?


thanks for the quick reply, I’ll file a bug-report shortly.

Do you know why I don’t see any compiler feedback which tells me that a reduction clause has been generated (in both cases)?


Hi Paul,

I just found the error:

#pragma acc loop reduction(+:tmp_x,tmp_z,tmp_z)

Changing one of the “tmp_z” to “tmp_y” fixes the problem.

I’ll need to talk with one of our compiler engineers about why we’re not printing a message for inner loop reductions.

  • Mat

ups :) thanks for catching that. Yes, a compiler feedback would be nice.

Maybe another question:
I’m curious if the private(i,j) clause is really necessary. In OpenMP it would be, at least private(j), is it true for OpenACC as well?
Because if I don’t use the private clause my programm runs 15% faster and still gives the same results.


I’m curious if the private(i,j) clause is really necessary

Scalars are privatized by default, so no, privatizing i and j is not necessary. As you found out, privatizing them can actually slow down your code. Privatizing a scalar variable will create an array of the variables, one for each thread, in global memory. If it’s not privatized, the variable is declared locally in the kernel and thus more likely to be stored in a register which is much faster to access.

  • Mat

Is that true for the PGI compiler or for OpenACC in general?

Is that true for the PGI compiler or for OpenACC in general?

I’m not sure. I believe it’s true for Cray, but don’t know for CAPS. The OpenACC 2.0 spec does clear this up a bit by adding a “default(none)” clause which will require the user to explicitly which variables are private. The exception being the loop index variables which are always private.

  • Mat

We have added compiler feedback when a reduction is performed.