Since the old compiler had only very limited support for the BYVALUE attribute, you are not likely to see this message when compiling old code:
IBM2628I W BYVALUE parameters should ideally be no larger than 32 bytes.
However as you start to use the BYVALUE attribute more, you may see this message, and in that case you should heed it. You should reserve the use of the BYVALUE attribute for small scalars and ideally for variables that could be passed in a register. Typically these would be declared as
You should never use the BYVALUE attribute with strings or aggreates that are larger than 4096 bytes in size.