Re: Explanation

----- Original Message -----
From: Albert York [mailto:alfromme@xxxxxxxxx]
Sent: Tuesday, June 19, 2012 04:25 PM
To: RPG programming on the IBM i / System i <rpg400-l@xxxxxxxxxxxx>
Subject: Re: Explanation

Parameter safety can always be assured by using a temporary field to
hold the parameter. Then, no matter what, the original data is
untouched.

Albert

On Tue, Jun 19, 2012 at 12:03 PM, Barbara Morris <bmorris@xxxxxxxxxx> wrote:

On 6/18/2012 2:03 PM, Jon Paris wrote:

Not really performance since a copy is being made both ways - VALUE or "real" CONST. But see below.

Actually it would make a big difference for performance, especially for
large varying length parameters, but for any "large" parameter.

When a parameter is passed by value, even if a temporary is not needed,
the entire parameter is copied to the parameter stack. When it's passed
by reference, only the address is copied to the parameter stack.

Say the parameter is 1000A varying, and the passed value is 'abc'.

For both CONST and VALUE, 5 bytes are set when the parameter is setup by
the compiler ('abc' plus the 2-byte length prefix).

For CONST, the 16 byte address is copied to the parameter stack. For
VALUE, the entire 1002 bytes is copied to the parameter stack.

So for that particular parameter, CONST requires 16+5=21 bytes to be
handled, and VALUE requires 5+1002=1007.

For a data structure of 1000 bytes where a compiler temp is not needed,
CONST requires 16 bytes to be handled, and VALUE requires 1000 bytes.

Personally, I agree that it would be nice to have a by-copy parameter
passing option, both for performance (vs VALUE) and guaranteed parameter
safety (vs CONST).

--
Barbara
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