Method-Based JIT Compilation in JavaScript

This challenge explores a simplified form of Just-In-Time (JIT) compilation in JavaScript, focusing on optimizing frequently called methods. The goal is to create a system that identifies and compiles methods based on their call frequency, improving performance by converting them into a more efficient, compiled representation. This is a simplified model, but it demonstrates core JIT concepts.

Problem Description

You are tasked with building a basic method-based JIT compiler in JavaScript. The system should monitor the execution of JavaScript functions and, when a function is called frequently enough, compile it into a faster, equivalent representation. The compiled representation will be a simple string of JavaScript code that performs the same operations as the original function, but without the overhead of the function call itself. The system should then use this compiled code instead of the original function for subsequent calls.

What needs to be achieved:

Method Tracking: Track the number of times each method is called.
Compilation Threshold: Define a threshold (a number of calls) above which a method should be compiled.
Code Generation: When a method exceeds the threshold, generate a string of JavaScript code that replicates the method's functionality. This generated code should be a simple, direct implementation of the original function's body. Assume the function body is a single return statement with a simple expression (no loops, conditionals, or complex operations).
Method Replacement: Replace the original function with the generated code.
Execution: Execute the compiled code instead of the original function for subsequent calls.

Key Requirements:

The system should be able to handle functions with a single return statement containing a simple expression (e.g., return a + b;, return x * 2;, return c;).
The generated code should be functionally equivalent to the original function.
The system should not modify the original function definition. It should replace the function with the compiled code.
The system should be able to handle multiple functions.

Expected Behavior:

Initially, all functions should be executed as normal JavaScript functions.
As functions are called, the call count should be tracked.
When a function's call count exceeds the compilation threshold, the function should be compiled.
After compilation, subsequent calls to the function should execute the compiled code.
The system should not introduce any errors or unexpected behavior.

Edge Cases to Consider:

Functions that are never called.
Functions that are called only a few times (below the threshold).
Functions with complex expressions (although the problem statement limits this, consider how the system might handle it).
Functions that are redefined after compilation. (This is outside the scope of this simplified challenge, but good to consider).

Examples

Example 1:

Input:
const add = (a, b) => a + b;
const jit = new JITCompiler(10); // Threshold of 10 calls

jit.compile(add);

add(1, 2); // Call 1
add(3, 4); // Call 2
add(5, 6); // Call 3
// ... (8 more calls)
add(7, 8); // Call 10 - Compilation should trigger

add(9, 10); // Call 11 - Should execute compiled code

Output: add(9, 10) should return 19 and execute significantly faster than the original function. The internal representation of add should be replaced with a string like "return a + b;".

Explanation: The add function is called 10 times, exceeding the threshold. The JIT compiler generates the code "return a + b;" and replaces the original add function with this compiled code. Subsequent calls to add execute this compiled code directly.

Example 2:

Input:
const multiply = (x) => x * 2;
const jit = new JITCompiler(5);

jit.compile(multiply);

multiply(2); // Call 1
multiply(3); // Call 2
multiply(4); // Call 3
// ... (1 more call)
multiply(5); // Call 5 - Compilation should trigger

multiply(6); // Call 6 - Should execute compiled code

Output: multiply(6) should return 12 and execute faster. The internal representation of multiply should be replaced with a string like "return x * 2;".

Explanation: The multiply function is called 5 times, reaching the threshold. The JIT compiler generates the code "return x * 2;" and replaces the original function.

Example 3: (Edge Case)

Input:
const neverCalled = () => 1 + 1;
const jit = new JITCompiler(10);

jit.compile(neverCalled);

neverCalled(); // Call 1

Output: neverCalled() should return 2. The function is compiled after the first call, even though it's below the threshold. This demonstrates that the compilation happens on the first call, regardless of the threshold.

Explanation: The neverCalled function is called once, triggering compilation. The JIT compiler generates the code "return 1 + 1;" and replaces the original function.

Constraints

Threshold: The compilation threshold should be configurable.
Function Body: The function body must be a single return statement with a simple expression (no loops, conditionals, or complex operations). Assume the expression only involves basic arithmetic operators (+, -, *, /) and variables passed as arguments.
Performance: While a full performance benchmark isn't required, the compiled code should demonstrably execute faster than the original function for subsequent calls.
Input: The input will be a JavaScript function and a JITCompiler object with a defined threshold.

Notes

This is a simplified JIT compiler. Real-world JIT compilers are significantly more complex.
Focus on the core concepts of method tracking, code generation, and replacement.
Consider using a simple data structure (e.g., a JavaScript object) to store the call counts and compiled code for each method.
The code generation should be straightforward. You don't need to handle all possible JavaScript expressions.
Error handling is not required for invalid function bodies. Assume the input functions are always valid according to the constraints.
The JITCompiler class should have a compile method that takes a function as input.
The JITCompiler class should internally track the call counts and compiled code.
The compile method should replace the original function with the compiled code.
The compile method should return the compiled function.