The bitwise operations (prefix NOT, infix AND, infix OR, and infix EXCLUSIVE OR) are often evaluated by calls to library routines. These operations are, however, handled without a library call if either of the following conditions is true:
For certain assignments, expressions, and built-in function references, the compiler generates calls to library routines. If you avoid these calls, your code generally runs faster.
To help you determine when the compiler generates such calls, the compiler generates a message whenever a conversion is done using a library routine.
When your code refers to a member of a BASED structure using REFER, the compiler often has to generate one or more calls to a library routine to map the structure at run-time. These calls can be expensive, and so when the compiler makes these calls, it will issue a message so that you can locate these potential hot-spots in your code.
If you do have code that uses BASED structures with REFER and which the compiler flags with this message, you may be able to get better performance by passing the structure to a subroutine that declares a corresponding structure with * extents. This will cause the structure to be mapped once at the CALL statement, but there will no further remappings when it is accessed in the called subroutine.