Features, caveats and tricks
----------------------------

 _Optlink
 --------

    The _Optlink implementation differs from VACPP implementation in the
    following ways:

    o _Optlink is not enabled unless you enable optimization (-O).

    o The floating-point parameters are always passed on stack, never in
      FPU registers.

    Use _Optlink for VACPP compatibility only. If you want performance,
    use the __attribute__((regparm=x)), where x is the number of parameters
    to be passed in registers (_Optlink uses three). The 'regparm' calling
    convention is much more optimal than _Optlink: it does not allocate
    stack for parameters passed in registers (and does not store registers
    in that location right at the function entry), does not have the strange
    limit of 12 arguments ('eyecatcher bitfields') considered for conformance
    and other benefits.