PWR005: Disable default OpenMP scoping

Issue

Disable default OpenMP scoping to force having to declare the scoping of all variables.

Relevance

When the scope for a variable is not specified in an OpenMP parallel directive, a default scope is assigned to it. Even when set explicitly, using a default scope is considered a bad practice since it can lead to  wrong data scopes inadvertently being applied to variables. Thus, it is recommended to explicitly set the scope for each variable.

Actions

Add default(none) to disable default OpenMP scoping.

Code example

In the following code, a variable t is used in each iteration of the loop to hold a value that is then assigned to the array result. Since no data scoping is declared for those variables the default will be used. This makes the variable t shared which is incorrect since it introduces a race condition.

void foo() {
  int t;
  int result[10];

  #pragma omp parallel for
  for (int i = 0; i < 10; i++) {
    t = i + 1;
    result[i] = t;
  }
}

The following code disables the default scoping which will make the compiler raise an error due to unspecified scopes.

void foo() {
  int t;
  int result[10];

  #pragma omp parallel for default(none)
  for (int i = 0; i < 10; i++) {
    t = i + 1;
    result[i] = t;
  }
}

To fix the code the scope of each variable must be specified. The variable t must be made private to prevent the race condition.

void foo() {
  int t;
  int result[10];

  #pragma omp parallel for default(none) shared(result) private(t)
  for (int i = 0; i < 10; i++) {
    t = i + 1;
    result[i] = t;
  }
}

Related resources

• PWR005 examples at GitHub
• PWR004: Declare variable OpenMP scoping
• OpenMP 4.5 Complete Specifications, November 2015 [last checked May 2019]
• clang-tidy – openmp-use-default-none, [last checked July 2019]

References

• Privatization
• Race condition