5.13.5 Generating Unique Location Labels

Macros often contain the definitions of location labels that must not duplicate when expanding a macro multiple times. To define a number of location labels in the local scope of a macro, you can define local symbols for them using lines of code like these:

lu0     def     u##__UNIQUE__
lu1     def     u##__UNIQUE__
lu2     def     u##__UNIQUE__

The above directives define the symbols lu0, lu1, and lu2 with values that have the form ui, where i is an increasing integer number. When expanding the macro the first time, the symbols lu0, lu1, and lu2 take the values u1, u2, u3 (if there are no definitions of other symbols using the predefined symbol ‘__UNIQUE__’). When expanding the macro the second time, the symbols lu0, lu1, and lu2 take the values u4, u5, u6, and so on. Thus, the values of the symbols lu0, lu1, and lu2 are always unique, and you can use them for location labels within macros expanded multiple times. The following example of a macro illustrates this:

emit_ch macro

lu0     def     u##__UNIQUE__
lu1     def     u##__UNIQUE__
lu2     def     u##__UNIQUE__

        choice
        case    0.33, lu0
        case    0.33, lu1
        end     choice

        emit    "A"
        jmp     lu2

lu0:    emit    "B"
        jmp     lu2

lu1:    emit    "C"

lu2:
        end     macro

This macro emits the character ‘A’, ‘B’, or ‘C’ by the corresponding user instruction emit. You can safely expand the macro an arbitrary number of times—location labels are unique in every expansion. For example, to emit a sequence of two characters, expand the macro twice:

        ...
        emit_ch
        emit_ch
        ...