alt Hacker NewsMostly because the behavior is implementation defined. So long as the behavior meets the contract, the compiler/interpreter is free to do whatever it wants.
Python could certainly optimize repeated code paths to make them more efficient. I don’t know that the standard implementation actually does, but it could. Spending extra time optimizing repeated code paths is a reasonable choice for an interpreted or JIT compiled language.
I would not expect C to change from invocation to invocation mostly because C is supposed to be boring and predictable. That’s kind of its thing. But again, it could. There’s nothing in the C spec I’m aware of that says the C compiler has to ensure that each invocation of a piece of code will execute the same machine instructions.
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Ok but how does that change the behaviour of the program?
>So long as the behavior meets the contract, the compiler/interpreter is free to do whatever it wants.
Yes, and that's how it's supposed to be. Any description that determines the totality of a problem space is an implementation itself.
Imagine the following requirements:
f(0) = 0, f(2) = 4
Both f(x) = x^2 and f(x) = 2x are correct ways to implement said requirements. But if you start relying on f(1) = 2, you might get in trouble with a coworker that relies on f(1) = 1. This is undefined behavior and should be avoided.
>There’s nothing in the C spec I’m aware of that says the C compiler has to ensure that each invocation of a piece of code will execute the same machine instructions.
It can't, because C can be written for any system you want. If I ask the compiler to compile x *= 2, it might use the mul primitive or it might use shl, both are ok.