c - force inline function in other translation unit

Question

This part of the gcc manual is pretty obscure and i can't understand the usage of the forceinline attribute after repeated attempts.

I'm defining an object and certain functions to manipulate that object. Few of those functions can use atomic instructions and i want the compiler to inline those functions. However i do not want to write those functions in the header file and declare them with "static inline" like in the linux kernel.

Is there a way to force gcc to inline functions from another translation unit ?

Answer 1

you can use the always_inline attribute, for example:

void foo () __attribute__((always_inline));

From the docs

always_inline Generally, functions are not inlined unless optimization is specified. For functions declared inline, this attribute inlines the function even if no optimization level was specified.

Note1: There's no need to use inline if you use the always_inline attribute

Note2: If the function could not be inlined you will get a warning, if for example the definition is not available when compiling, however, at a higher optimization gcc can still inline it into the caller, there's a specific switch for that too:

-funit-at-a-time

From the docs:

Optimization levels -O2 and above, in particular, enable unit-at-a-time mode, which allows the compiler to consider information gained from later functions in the file when compiling a function. Compiling multiple files at once to a single output file in unit-at-a-time mode allows the compiler to use information gained from all of the files when compiling each of them.

Note3: It is not necessary to have an explicit prototype so you can use the attribute on the function defintion:

__attribute__((always_inline)) void foo() {
   //some code
}

Also see this discussion, it answers some of your questions.