Performance optimizations

[Bromeon]

There are some low-hanging fruits for making the library faster, this issue tracks such approaches.

Codegen

• Check rustfmt performance -- parallelize or allow skipping
• Cache intermediate results in Context, e.g. to_rust_type

Runtime

• Cache function pointers used by engine + builtin classes in a global method table
• Provide maximum type information to GDScript, so it can use ptrcalls whenever possible
• Benchmark if #[inline] has a true impact (otherwise don't clutter the whole codebase)
• Varargs (e.g. Object::call) could use impl AsVararg + passing as tuple, to allocate argument array on the stack

[lilizoey]

caching StringNames in function calls can have a significant improvement. running these functions 1 million times (in a std::hint::black_box(..) call):

fn generate_string_name() {
    let foo = StringName::from("foo");
}

fn cached_string_name() {
    static CACHED: OnceLock<StringName> = OnceLock::new();
    let foo = CACHED.get_or_init(|| StringName::from("foo"));
}

takes on average 0.15 seconds for the first one and 0.0003 seconds for the second one in release mode.

we do make calls like StringName::from("name") a lot in our bindings like for method calls and such.

this does require implementing Send and Sync for StringName, but i think that should be safe. StringName is immutable and i believe thread-safe.

[lilizoey]

Manually writing an implementation of StringName::length where i cache: __method_name only, __call_fn only, and both, then running each function 1 million times with black_box in release mode, i get:

	Uncached	__method_name	__call_fn	both
Total Average Time	0.1979221145	0.04312633755	0.170186928	0.01706689335
Percent	100.00%	21.79%	85.99%	8.62%
Percent Diff	0.00%	-78.21%	-14.01%	-91.38%

[WinstonHartnett]

Caching the method bind, builtin method, and utility function pointers with a static OnceLock produces the following speedups:

Function	Kind	Uncached	Cached	Percent
Node::get_child_count	Method Bind	308ns	8ns	2.60%
Node::is_inside_tree	Method Bind	212ns	5ns	2.36%
Node::is_node_ready	Method Bind	209ns	5ns	2.39%
minf	Utility	94ns	8ns	8.51%
floorf	Utility	86ns	5ns	5.81%
StringName::length	Builtin	86ns	7ns	8.14%

[Bromeon]

The cached function pointers have been implemented, and with them, StringNames also no longer need to be instantiated on each call.

For anyone interested in deeper insights, I wrote about it here:
https://godot-rust.github.io/dev/ffi-optimizations-benchmarking

[Bromeon]

I think we can close this, several things have been optimized and new ones probably deserve their own issue.

[Ughuuu]

Damn, impressive. I am ready to give these changes a try. Thanks everyone!