Question

I implemented a simple matrix vector multiplication for sparse matrices in CRS using an implicit openMP directive in the multiplication loop.

The complete code is in GitHub: https://github.com/torbjoernk/openMP-Examples/blob/icc_gcc_problem/matxvec_sparse/matxvec_sparse.cpp
Note: It’s ugly 😉

To control the private and shared memory I’m using restrict pointers. Compiling it with GCC 4.6.3 on 64bit Linux works fine (besides two warnings about %u and unsigned int in a printf command, but that’s not the point).

However, compiling it with ICC 12.1.0 on 64bit Linux failes with the error:

matxvec_sparse.cpp(79): error: "default_n_row" must be specified in a variable list at enclosing OpenMP parallel pragma
    #pragma omp parallel \
    ^

with the definition of the variable and pointer in question

int default_n_row = 4;
int *n_row = &default_n_row;

and the openMP directive defined as

#pragma omp parallel \
  default(none) \
  shared(n_row, aval, acolind, arowpt, vval, yval) \
  private(x, y)
{
  #pragma omp for \
    schedule(static)
  for ( x = 0; x < *n_row; x++ ) {
    yval[x] = 0;
    for ( y = arowpt[x]; y < arowpt[x+1]; y++ ) {
      yval[x] += aval[y] * vval[ acolind[y] ];
    }
  }
} /* end PARALLEL */

Compiled with g++:

c++ -fopenmp -O0 -g -std=c++0x -Wall -o matxvec_sparse matxvec_sparse.cpp

Compiled with icc:

icc -openmp -O0 -g -std=c++0x -Wall -restrict -o matxvec_sparse matxvec_sparse.cpp

---

• Is it an error in usage of GCC/ICC?
• Is this a design issue in my code causing undefined behaviour?
  If so, which line(s) is/are causing it?
• Is it just inconsistency between ICC and GCC?
  If so, what would be a good way to achieve compiler independence and compatibility?

Answer 1

Huh. Looking at the code, it’s clear what icpc thinks the problem is, but I’m not sure without going through the specification which compiler is doing the right thing here, g++ or icpc.

The issue isn’t the restrict keyword; if you take all those out and lose the -restrict option to icpc, the problem remains. The issue is that you’ve got in that parallel section default(none) shared(n_row...), but n_row is, at the start of the program, a pointer to default_n_row. And icpc is requiring that default_n_row also be shared (or, at least, something) in that omp parallel section.