Rational Developer for System z
PL/I for Windows, Version 8.0, Programming Guide

FIXEDOVERFLOW

On some platforms, the FIXEDOVERFLOW condition is raised by the hardware and the compiler does not need to generate any extra code to detect it. With personal computers, however, the hardware does not raise this condition so the compiler must generate extra code. This extra code can negatively impact the performance of your program; and unless your program requires (or expects) this condition to be raised, specify PREFIX(NOFIXEDOVERFLOW) to improve performance.

Rational Developer for System z
PL/I for Windows, Version 8.0, Programming Guide