Intel® C++ Compiler Classic Developer Guide and Reference

ID 767249
Date 3/31/2023
Public

A newer version of this document is available. Customers should click here to go to the newest version.

Document Table of Contents

GAP Message (Diagnostic ID 30519)

Message

Insert a "%s parallel" statement right before the loop at line %d to parallelize the loop.

Advice

Add "#pragma parallel" before the specified loop. This pragma enables the parallelization of the loop at the specified line by ignoring assumed cross-iteration data dependencies.

Example

Consider the following:

void foo(float *a, int m, int n) {
  int i;
  for (i=0; i<n; i++) {
    a[i] = a[i+m]+1;
  }
  return; 
}

In this case, the compiler is unable to parallelize the loop without further information about m. For example, if m is negative, then each iteration will be dependent on the previous iteration. However, if m is known to be greater than n, you can parallelize the loop.

If you determine it is safe to do so, you can add the pragma as follows:

void foo(float *a, int m, int n) {
  int i; 
#pragma parallel
  for (i=0; i<n; i++) {
    a[i] = a[i+m]+1;
  }
  return; 
}

Verify

Confirm that any arrays in the loop do not have cross-iteration dependencies. A cross-iteration dependency exists if a memory location is modified in an iteration of the loop and accessed by a read or a write statement in another iteration of the loop.