[cpp] functions get wrapped in Dynamic

[nadako]

One of the intermediate issues that lead to the discovery of HaxeFoundation/hxcpp#575 is the following:

Source:

class C {
    static var _f:cpp.Function<Int->Void,cpp.abi.StdCall>;

    static function f(n) {
        _f.call(n + 1);
    }
}

Generated C++

void C_obj::f(int n){
            	HX_STACKFRAME(&_hx_pos_3831bbadf076dfac_5_f)
HXLINE(   5)		 ::Dynamic _hx_tmp = ::C_obj::_f;
HXDLIN(   5)		_hx_tmp((n + (int)1));
            	}

The function is wrapped in hxcpp's Dynamic which I believe quite costly performance-wise because of all the wrapping and even array allocation for arguments, so this is something we might care about.

The cause is probably that the cpp.Function ends up being typed as a simple function in the end, as we can see in the AST. I'm not sure why though.

AST:

static function f[Function:n : Int -> Void]
	[Arg:Int] [Local n(176):Int]
	[Block:Void]
		[Var _hx_tmp(194):Int -> Void]
			[Cast:cpp.FunctionData<Int -> Void, cpp.abi.StdCall>]
				[Cast:cpp.FunctionData<Int -> Void, cpp.abi.StdCall>]
					[Field:cpp.Function<Int -> Void, cpp.abi.StdCall>]
						[TypeExpr C:Class<C>]
						[FStatic:cpp.Function<Int -> Void, cpp.abi.StdCall>]
							C
							_f
		[Call:Void]
			[Local _hx_tmp(194):Int -> Void]
			[Binop:Int]
				[Local n(176):Int]
				+
				[Const:Int] 1

[nadako]

Hmm, right after submitting this issue, I've found the // Legacy Api comment on the cpp.Function.call property. And indeed if we remove the .call from the original example and call _f directly, we'll get much better output:

void C_obj::f(int n){
            	HX_STACKFRAME(&_hx_pos_3831bbadf076dfac_5_f)
HXLINE(   5)		 ::Dynamic _hx_tmp = hx::ClassOf< ::C >();
HXDLIN(   5)		::C_obj::_f((n + (int)1));
            	}

This is something @Gama11 and/or @jgranick might be interested in.

I'll leave this issue open for now because chances are @hughsando can do something to make .call generate the same code somehow :)

[jgranick]

@nadako We may be able to use conditional compilation on this part of the code. At what version can we call the function directly, without the .call method? Does it require 3.4.0, does it work on 3.2.1...?