[caspar]

Well, sometimes your compiler will work out how to more efficiently compile a thing (e.g. vectorize a loop), and other times you'll rework some code to an equivalent formulation and suddenly it won't get vectorized because you've tripped some invisible flag that prevents an inlining operation that was crucial to enabling that vectorization, and now that hot path runs at a quarter of the speed it did before.

Technically it's (usually) deterministic for a given input, and you can follow various best practices to increase your odds of triggering the right optimizations.

But practically speaking "will I get the good & fast code for this formulation" is a crap shoot, and something like 99% (99.99%?) of programmers live with that. (you have guidelines and best practices you can follow, kinda like steering agents, but rarely get guarantees)

Maybe in the future the vast majority of programmers put up with a non-deterministic & variable amount of correctness in their programs, similar to how most of us put up with a (in practice) non-deterministic & variable amount of performance in our programs now?