[Bug, Reduction] Dependency violation in kernels containing reductions nested within conditionals

[kaushikcfd]

Consider the kernel:

knl = lp.make_kernel(
    "{[i, j, k]: 0<=i,j,k<10}",
    """
    y[i] = 0 if sum(j, j) else sum(k, k)
    """)
print(lp.generate_code_v2(knl).device_code())

which generates:

void loopy_kernel(double* x, double* y)
{
  int acc_j;
  double acc_k;
  int tmp[10];

  for (int i = 0; i <= 9; ++i)
  {
    acc_k = 0.0;
    tmp[i] = i;
    acc_j = 0;
    if (!acc_j)
      for (int k = 0; k <= 9; ++k)
        acc_k = acc_k + x[10 * i + k];
    for (int j = 0; j <= 9; ++j)
      acc_j = acc_j + tmp[j];
    y[i] = (acc_j ? 0.0 : acc_k);
  }
}

Notice how acc_j is read before it is written. The reason being realize_reduction doesn't enforce dependencies between the statements during the transformation.

[inducer]

Thanks for doing the detective work to find why this happens!

[kaushikcfd]

To see that this actually results in wrong results:

(ceesd) kgk2@bock:~/temp$ cat file.py 
import loopy as lp
import pyopencl as cl

ctx = cl.create_some_context()
cq = cl.CommandQueue(ctx)

knl = lp.make_kernel(
    "{[i, j, k]: 0<=i,j,k<10}",
    """
    y[i] = 1729 if (sum(j, j) == 0) else sum(k, k)
    """, seq_dependencies=True)

evt, (out,) = knl(cq)
print(out)

(ceesd) kgk2@bock:~/temp$ python file.py 
[0 0 0 0 0 0 0 0 0 0]