Test for array literal arguments?

[Peter Alexander]

On Tuesday, 5 June 2012 at 23:48:48 UTC, bearophile wrote:
> So we are back to an idea Walter has tried and refused few 
> years ago, of compile-time known arguments with "static". The 
> small difference here is (I think) that both iPow templates are 
> allowed to exist in the code at the same time, and the iPow 
> overload with "static" is preferred by the argument is known at 
> compile-time:

I was also thinking about this idea today. I was writing a small 
math function (Van der Corput sequence generator) with the 
signature vdc(int n, int b) and noticed that the code could be 
faster when b == 2 (the most common case) because divides can be 
turned into shifts and mods turned to bitwise AND etc.

You could duplicate the function to take the static args as 
template args, but that's ugly.

I also came to the conclusion of using 'static' as a parameter 
"storage class" to specify that the parameter is known at 
compile-time.

One problem with this approach is that it only solves some cases 
and cannot work in general. It also has other implications:
- Code with 1 or more optimised versions will require extra 
maintenance/testing.
- Makes code more difficult to reason about (can be difficult to 
tell which is version is called).
- Adds more rules for overload resolution.

However, the biggest problem with this proposal (in my opinion) 
is that it is unnecessary. I care deeply about performance, but 
tiny optimisations like this are simply not important 99% of the 
time. When they are important, just write a specific optimised 
version and use that. Yes, you lose generality, but special needs 
call for special cases. Let's not complicate the language and 
bloat the codebase further for questionable gain.