Developer Guide and Reference

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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.

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