User's Guide

Different Results in Optimized Programs

Here are some reasons why an optimized program might produce different results from those of an unoptimized one:

• Optimized code can fail if a program contains code that is not valid. For example, failure can occur if the program passes an actual argument that also appears in a common block in the called procedure, or if two or more dummy arguments are associated with the same actual argument.

• If a program that worked without optimization fails when compiled with it, check the cross-reference listing and the execution flow of the program for variables that are used before they are initialized. Compile with the option -qinitauto=ff to try to produce the incorrect results consistently. (Programs with uninitialized variables may appear to work properly when compiled without optimization because of the default assumptions the compiler makes, but may fail when compiled with optimization.)

• Use with caution debugging techniques that rely on examining values in storage. The compiler might have deleted or moved a common expression evaluation. It might have assigned some variables to registers, so that they do not appear in storage at all.