RE: Is const really that much better than value?

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
>
> --
> This is the RPG programming on the AS400 / iSeries (RPG400-L)
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