Larry Ducie wrote:
Also, as the return variable is aIways passed by value I would like to know
whether it is also subject to the low-level extra copy described by Barbara.
I know this was described as a low-level parameter passing function by the
OS, but I'd be interested to know if this applies to return variables too.
I'd also like to know if the passing of the return variable is subject to
the same casting/truncating/padding rules as parameters?
I just want to start by quoting Donald Knuth:
"We should forget about small efficiencies, say about 97% of the time:
premature optimization is the root of all evil."
See
http://en.wikipedia.org/wiki/Optimization_(computer_science)
It's worth reading the short section on "When to optimize" before
reading any further here.
For VALUE vs CONST, choosing CONST over VALUE rarely impacts the design,
so it seems reasonable to use CONST instead of VALUE as a general rule.
The same doesn't apply to return values, so it's important to bear
Knuth's statement in mind when reading the rest of what I say here.
So ... yes, the issue with passing parameters by value does apply to
return values too. If necessary, the compiler sets up temporary of the
returned type, and then the OS works with the entire size of the
returned value.
Unfortunately, there isn't such a good alternative as CONST vs VALUE.
One alternative is to define your string-handling procedures to have the
"returned value" passed by reference, and use the same LIKE type to
define all such return values, and use the same LIKE type to define all
string temporaries. Then it's not TOO bad to code like this:
D temp1 s like(string_t)
center (temp1 : title : 80);
result = temp1;
instead of the much nicer but possibly poorer performing
result = center (title : 80);
You could even provide two procedures each, where the one that returns a
value calls through to the other one. Callers can then choose between
convenience and performance, using the less convenient version when the
performance of the other version is _has been determined to be a problem_.
Choose
result = center (val : len);
or
center_R (temp : val : len);
result = center;
D center b
D center pi like(string_t)
D val like(string_t) const
D len 10i 0 value
D result s like(string_t)
/free
center_R (result : val : len);
return result;
/end-free
D center e
midrange.com
