I know you're getting bombarded with a dozen answers but I don't think anyone's also mentioned speculative optimization/deoptimization. Jit can and often does assume things that can't be proven like a specific call site can be inlined through a virtual call because in practice it's always the same hard type. It can back out the optimization later if that proves to be untrue. Or if a function is always called with the same variable because it was read from a config: it can assume that it won't change.