This can be accomplished more simply and reliably by marking modulus as const. The compiler currently has to reason about the whole compilation unit to determine that modulus is not modified, which works. However, if future code modifies modulus (either on purpose or accidentally) or something changes that prevents the compiler from performing global reasoning, the optimization will be lost. By marking the actual intention, any modifications of modulus turn into compiler errors. Plus, if it becomes important to expose modulus to another compilation unit, now that's possible.
This is a common issue with C code (including lots of code I've written). It's really easy to forget to const something, which forces the compiler to do global reasoning or to generate worse code. I've gotten into the habit of making things const unless I know I plan on mutating them, but I wish there was tooling that encouraged it. (BTW, this is something Rust does well by making things constant by default and requiring "mut" if it's mutable.)
This is a common issue with C code (including lots of code I've written). It's really easy to forget to const something, which forces the compiler to do global reasoning or to generate worse code. I've gotten into the habit of making things const unless I know I plan on mutating them, but I wish there was tooling that encouraged it. (BTW, this is something Rust does well by making things constant by default and requiring "mut" if it's mutable.)