The rule right now is that a function is called at comptime implicitly only when you’re already in a comptime context, otherwise it will need to be prefixed with comptime. There has been discussion about eventually have the compiler “try” to resolve function calls at comptime without explicitly asking for it, but none of that exists today.
So a function call in the top scope of a file is always comptime, while a function call inside main is always at runtime unless prefixed with comptime.
Not sure how that interacts with LLVM optimizations though, so you might get a non-comptime compile time resolution by LLVM when the function is simple enough.