I'm looking for how to make a code optimizer.

Not a compiler, just an optimizer; ideally one that can be reused across different languages without much change. (I think that's impossible, but whatever)

I'm not sure if optimization / codegen is even taught anymore.

Generally, you'll hear not to prematurely optimize code, but a code optimizer can be used to improve performance while maintaining correctness.

Many compiled languages already have an optimizing compiler, ex: LLVM languages, Java, C#, Dart, etc.

But then there are languages that don't have any optimizations whatsoever, this is prevalent especially in interpreted languages.

I'm talking about a program that takes code like this:

type f64 = number;

const sqrt5: f64 = 5.0 ** 0.5;
const phi: f64 = (sqrt5 + 1.0) / 2.0;

export const fib = (n: f64): f64 => {
	const phi_pow: f64 = phi ** n;

	const fraction: f64 = -phi ** -n;

	return Math.floor((phi_pow + fraction) / sqrt5);
};

and outputs stuff more like:

type f64 = number;

export const fib = (n: f64): f64 => Math.floor(
	1.0 / (n = 1.618033988749895 ** n) + n / 2.23606797749979
);

Not minified, but optimized.
Yet, I tried Google's closure compiler, and it had generated worse code.

That is only a trivial example. There are an infinite number of cases that could be optimized better.

I was thinking about making a compiled language but with the differences between different brands of compilers being so drastic, I was thinking that I would be better off making my own. (also, most optimizing compilers are designed to output a specific target, and can't be used on say, Python)

Does anyone know where I would start with automated code optimization?

Furthermore, would want to help make the optimizer?

Does anyone think that I would be better off just contributing to an open-source project like LLVM?