RE: Is const really that much better than value?

[Vernon Hamberg <vhamberg@xxxxxxxxxxx>]

To expand a little, I hope, on Charles' reply - data types like float 
and integer do better with VALUE because they are shorter than the 
length of a pointer, which is always 16. I think!

Seems to me that, except for these numeric types, CONST is the thing 
to use. It's a win-win - you lose nothing if the system has to 
convert for length - it'll do it either way - and you gain with 
matching types and length - character stuff, mostly, since it will 
use the pointer.

But then it is early here and my brain is barely awake. Off to work!

Vern

At 06:27 AM 10/30/2007, you wrote:

> Larry,
>
> There's been some discussion about this in the past.  In particular 
> look for posts from Barbara Morris
> on the subject.
>
> Here's one:
> http://archive.midrange.com/rpg400-l/200705/msg00155.html
>
> "Neither CONST nor VALUE is always better than the other. But if you 
> _were_ restricted to one or the
> other, CONST would be a better choice.
>
> For a few data types (pointer, float, integer), VALUE is a bit more 
> efficient. For other data types,
> CONST is usually more efficient; the larger the parameter, the 
> better CONST is. For large parameters,
> CONST is more efficient even if you have to copy the parameter to a 
> temporary within the procedure to
> have something you can modify; this is especially true of VARYING parameters."
>
> Whenever possible, if I'm passing large strings, or to be more 
> precise, if my procedure accepts large
> strings, I make them CONST VARYING.
>
>
> HTH,
> Charles
>
>
> > -----Original Message-----
> > From: rpg400-l-bounces@xxxxxxxxxxxx
> > [mailto:rpg400-l-bounces@xxxxxxxxxxxx] On Behalf Of Larry Ducie
> > Sent: Tuesday, October 30, 2007 8:17 AM
> > To: rpg400-l@xxxxxxxxxxxx
> > Subject: Is const really that much better than value?
> >
> > Hi Chaps,
> >
> > Quick question - when passing a variable value to a
> > subprocedure, and the passed variable does not have the same
> > length/precision as that defined on the prototype, what is
> > the real difference between passing it by constant reference
> > or by value? Why choose one over the other? For passing long
> > strings by constant reference and options(*varsize) is not
> > used then doesn't the OS simply copy the variable to a
> > temporary field anyway if the variable length does not match
> > the prototype definition? Isn't this the same as passing by
> > value in all real respects?
> >
> > I mean, when parameters are passed to a subprocedure the OS
> > passes an array of pointers together with operational
> > descriptors doesn't it? for passing by value the pointers
> > will point to temporary variables containing copies of the
> > original data. These are cast/padded/truncated accordingly.
> > When passing by constant reference doesn't the OS also do
> > this if the original variable definition does not exactly
> > match that on the prototype? This is why we can pass zoned
> > fields even though the prototype defines a packed parm, etc.
> >
> > I guess I'm basicaly asking - in this scenario is there any
> > real performance reason for passing a long string by constant
> > reference over passing it by value? Do we only get a
> > performance gain when we have an exact match and a temporary
> > field is not necessary when passing by constant reference? (I
> > must stress thet I'm talking about scenarios when
> > options(*varsize) and/or opdesc are NOT used).
> >
> > I'm very interested to know the mechanics behind this so I
> > can make the right design choices. If I have it all wrong
> > then I'd rather know now. :-)
> >
> > Cheers
> >
> > Larry Ducie
> >
> > --
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