VMS Help MACRO, /MIGRATION, Compiler Directives, .GLOBAL LABEL, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The .GLOBAL_LABEL directive declares a global label within a
    routine that is not a routine entry point. Unless declared with
    .GLOBAL_LABEL, global labels in code (specified with "::") are
    assumed to be entry point labels, which require declaration. If
    they are not declared, they are flagged as errors.

    The compiler also allows the address of a global label to be
    stored (for instance, by means of  PUSHAL instruction). (The
    compiler flags as an error any attempt to store a label that has
    not been declared as a global label or an entry point.)

    By using the .GLOBAL_LABEL directive, the user is acknowledging
    that the stored code address will not be the target of a CALL
    or JSB instruction. Global labels must appear inside routine
    boundaries.

    Labels declared with the .GLOBAL_LABEL directive can be used as
    the newpc argument in calls to the $UNWIND (Unwind Call Stack)
    system service because it allows the address of the label to be
    stored.

    However, there is no provision in the compiler to automatically
    adjust the stack pointer at such labels to remove arguments
    passed on the stack or compensate for stack alignment. If
    the call stack is unwound back to an alternate PC in the
    calling routine, the stack may still contain arguments and
    alignment bytes, and any stack-based references that expect this
    adjustment to the caller's original stack depth (which happened
    automatically on VAX) will be incorrect.

    Code that contains labels declared with this directive that are
    to be used as alternate PC targets for $UNWIND must be examined
    carefully to ensure correct behavior, with particular emphasis on
    any references based on the stack pointer.