monitoring variable evaluation time

[via Digitalmars-d-learn]

On Monday, 20 July 2015 at 08:53:52 UTC, Clayton wrote:
> What could be the best-tool for monitoring the evaluation time 
> of a variable . What I usually do is run the command :-
>
> - dmd -J. program.d
>
> Then I inspect the program.o file using vi for presence of 
> compile-time constants and enums. I am wondering if this is the 
> right way to do this. Am a bit skeptical about this as am 
> working on pushing everything that the compiler can do to the 
> compiler for faster run-time.

I'm not sure that's what you want to know, but...

The evaluation time of expressions is completely deterministic. 
Global (module level) or static variables, default values for 
struct/class members, template arguments, as well as enums, are 
evaluated at compile-time and thus their construction has no 
runtime costs.

Everything else can, and usually does, happen at runtime (modulo 
anything I might have overlooked in the list above). But even 
then, compilers are free to evaluate things during optimization, 
as long as it's possible and makes no observable difference in 
behaviour. For example, with optimization enabled, both GDC and 
LDC can even evaluate some complex recursive function calls to 
their result. But these optimizations are optional. If you want a 
guarantee, you need to use one of the above contexts.

You can use a simple helper template for that (untested):

template forceCTFE(Args...) if(Args.length == 1) {
     alias forceCTFE = Args[0];
}

void main() {
     auto myvar = forceCTFE!(foo(23));
}