Re: NOOPT D-Spec keyword

[Hans Boldt <boldt@xxxxxxxxxx>]

David & Eileen Keck wrote:
> I was doing a little light reading in the RPG IV reference and came across
> the NOOPT d-spec keyword, which befuddles me.  It says we should always
> specify NOOPT when prototyping for an OPM RPG program ... that problems with
> data currency "could" occur if it's not used.  It also says that "some" data
> may not be "current" during exception processing unless NOOPT is specified.
> This keyword is defined in the manual without respect to any optimization on
> any CRT* commands.  This all seems very cloudy ... can anyone clear this up
> for me ?  Thanks - Dave K.

The RPG IV Reference is "light" reading?

The key words regarding this particular keyword (and optimization in
general) are "may" and "might".  First, I wouldn't worry about it
much unless you're compiling your modules with optimization levels
higher that *NONE.  Second, I wouldn't worry much about using NOOPT
unless some variable used in the *PSSR or an INFSR does not have the
value you know it should have.  Thirdly, if your application does a
lot of I/O, I wouldn't worry much at all about using optimization
levels higher than *NONE.  Analyzing and optimizing your I/O logic
will always pay off better than bumping up the optimization level in
the compiler.

Why might some variable not have a "current" value?  There are a lot
of possible optimizations that an optimizing back-end can do.  One
example is that if a value intended to be assigned to some variable
is in a register, the back-end can refer to the value in the
register instead of the value in the variable.  The back-end can in
some cases defer the assignment of the value to the variable to a
point where the back-end knows the value has to be actually
contained by the variable.  But if an exception occurs in the
mean-time, the value in the register would be lost.  But considering
how pessimistic the RPG front-end is regarding optimization, I'm not
even sure if this situation would ever crop up in practice.  But in
case you do run into trouble with an unexpectedly aggressive
optimization, you do have the option to tell the optimizer to lay
off a particular variable.

Cheers!  Hans

BTW, for light reading, try a Terry Pratchett novel instead.