Static inline considered harmful

12 Dec 2020 - John Z. Li

The worst thing about the inline keyword in the C programming language is its name, because it has little to do with the compilation optimization technique called inclining, that is in-place code substitution of function calls. Modern compilers inline function calls as they see proper for performance reasons, with or without the inline keyword. What the keyword really means is as below:

• It loosens the One Definition Rule (ODR) of the language. ODR means, any entity, being it a variable or a function, can only be defined once. If a function is specified with inline, the restriction is relaxed for it. Now it can be defined in multiple translation unit, with the requirement that
• all definitions of the same inline function are identical. The language standard spent quite some words to specify what it means that two functions are considered identical. It is the compiler’s responsibility to ensure that this identical requirement is satisfied.
• Furthermore, if a function specified as inline is not inlined during compilation, it is just a normal function. If, otherwise, it gets inlined, for an external observer (user of compiled binary), he does not have to care about whether it is inlined or not. In other words, he can treat the function just as a normal function. This means, a symbol of the function is exported, the address of the function can be taken and passed around, or anything one can do with normal functions (remember that inline functions have external linkage).
• If an inline function is not inlined, there is no binary bloating. It costs as much as a normal function in terms of function call overhead.

If inline functions are used correctly, the programmer have all the above guarantees. However, if they are marked as static inline instead, we have problems:

• First, the static keyword nullifies the semantic meaning of the inline keyword. A function is marked with static inline are just static functions, which means, whether it gets inlined, each translation unit keeps a copy of the function leadingto binary bloat.
• More importantly, there are no guarantee that each copy of the function in different translation units is identical with each other. It is possible that some macro definitions or some function scope or file scope static variables have made the same inline function in different translation units behave differently. But the programmer may be left with the false assumption that they are identical. Imagine that you have to debug a program that is broken because the identical requirements are silently violated.

To sum up, do not mark your function as static inline. If you mean static, use static. If you mean inline, use it properly.