GAP Message (Diagnostic ID 30531)

Message

Store the value of the upper-bound expression of the loop at line %d into a temporary local variable, and use this variable as the new upper-bound expression of the loop. To do this, insert a statement of the form "temp = upper-bound of loop" right before the loop, where "temp" is the newly created local variable. Choose a variable name that is unique, then replace the loop's original upper-bound expression with "temp".

Advice

Use a local-variable for the loop upper-bound if the upper-bound does not change during the execution of the loop. This enables the compiler to recognize the loop as a proper counted do loop, which enables various loop optimizations including vectorization and parallelization.

This message appears when the compiler cannot output the exact upper-bound variable to be replaced.

Example

Consider the following:

class FooClass  { 
public:
    const int getValue() { return m_numTimeSteps;}
        void Foo2(double*  vec); 
private:
        int m_numTimeSteps; 
}; 
void FooClass::Foo2(double*  vec) 
{
        // this will not vectorize
        for (int k=0; k < m_numTimeSteps; k++)
                vec[k] = 0.0;
 
        // this will not vectorize
        for (int k=0; k < getValue(); k++)
                vec[k] = 0.0;
 
        // this will vectorize
        int ub1 = m_numTimeSteps;
        for (int k=0; k < ub1; k++)
                vec[k] = 0.0;
 
        // this will vectorize
        int ub2 = getValue();
        for (int k=0; k < ub2; k++)
                vec[k] = 0.0; 
}

Verify

Confirm that the value of the upper-bound expression does not change throughout the entire execution of the loop.