The cost of doing compile time introspection

[Biotronic via Digitalmars-d]

On Wednesday, 10 May 2017 at 11:45:05 UTC, Moritz Maxeiner wrote:
[CTFE slow]

First, as you may know, Stefan Koch is working on an improved 
CTFE engine that will hopefully make things a lot better.

As for making the code faster right now, could this be done with 
mixin templates instead?

Something like:

import functions = llvm.functions.link;
import std.meta, std.traits;

template isCFunction(alias member)
{
     static if (isFunction!(member) &&
                (functionLinkage!(member) == "C" ||
                 functionLinkage!(member) == "Windows")) {
         enum isCFunction = true;
     } else {
         enum isCFunction = false;
     }
}

template CFunctions(alias scope_)
{
     alias GetSymbol(string member) = 
AliasSeq!(__traits(getMember, scope_, member));
     alias CFunctions = Filter!(isCFunction, staticMap!(GetSymbol, 
__traits(allMembers, scope_)));
}

mixin template declareStubsImpl(T...)
{
     static if (T.length == 0) {
     } else {
         mixin("extern (C) typeof(T[0])* "~__traits(identifier, 
T[0])~";");
         mixin declareStubsImpl!(T[1..$]);
     }
}

mixin template declareStubs(alias scope_)
{
     mixin declareStubsImpl!(CFunctions!scope_);
}

mixin declareStubs!functions;

(tests indicate this compiles but doesn't run, and I'm helping 
you on company time, so I can't fix everything right now :p)


> if the new static foreach works the way I suspect, then that 
> part could essentially be replaced (at module scope) with 
> something like
>
> ---
> static foreach (fn; CFunctions!functions) {
>     typeof(functions.fn)* __traits(identifier, fn);
> }

A few things here - functions.fn would not do what you want, and 
neither would __traits(identifier).
functions.fn would treat "fn" like a part of name, not a string 
value, so this will make the poor compiler barf.
__traits(identifier, fn) expects fn to be a symbol, while here 
it's a string. In fact, it's exactly the string you want __traits 
to return.
Lastly, you'll still need a mixin, whether it's for 
__traits(identifier, fn) or just fn - they're just strings. 
Something like this:

static foreach (fn; CFunctions!functions) {
     mixin("typeof(__traits(getMember, functions, fn))* "~fn~";");
}