GAP Message (Diagnostic ID 30521)

Intel® C++ Compiler Classic Developer Guide and Reference

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ID 767249

Date 12/16/2022

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Public

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GAP Message (Diagnostic ID 30521)

Message

Assign a value to the variable(s) "%s" at the beginning of the body of the loop in line %d. This will allow the loop to be parallelized.

Advice

Check to see if you can unconditionally initialize the scalar variables at the beginning of the specified loop. If so, do the code change for such initialization (standard), or list the variables in the private clause of a parallel pragma (advanced). This allows the parallelizer to privatize those variables for each iteration and to parallelize the loop.

Example

Consider the following:

#define N 100000 
double A[N], B[N];
 
void foo(int cond1, int cond2){
  int i, t=7;
  for (i=0; i<N; i++){
    if (cond1) {
      t = i+1;
    }
    if (cond2) {
      t = i-1;
    }
    A[i] = t;
  } 
}

In this case, the compiler does not parallelize the loop because it cannot privatize the variable t without further information. If you know that cond1 or cond2 is true, or both cond1 and cond2 are true, then you can assist the parallelizer by ensuring that any iteration that uses t also writes to t before its use in the same iteration. One of the ways to do this is to assign a value to t at the top of each iteration.

If you determine it is safe to do so, you can modify the program code as follows:

#define N 100000 
double A[N], B[N];
  
void foo(int cond1, int cond2){
  int i, t=7;
  for (i=0; i<N; i++){
    t=0;
    if (cond1) {
      t = i+1;
    }
    if (cond2) {
      t = i-1;
    }
    A[i] = t;
  } 
}

Verify

Confirm that in the original program, any variables read in any iteration of the loop have been defined earlier in the same iteration.

Parent topic: Guided Auto Parallelism Messages

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Intel® C++ Compiler Classic Developer Guide and Reference

GAP Message (Diagnostic ID 30521)

Message

Advice

Example

Verify

See Also