4.22 Automatic function inlining and static functions

At -O2 and -O3 levels of optimization, or when --autoinline is specified, the compiler can automatically inline functions if it is practical and possible to do so, even if the functions are not declared as __inline or inline.

This works best for static functions, because if all use of a static function can be inlined, no out-of-line copy is required. Unless a function is explicitly declared as static (or __inline), the compiler has to retain the out-of-line version of it in the object file in case it is called from some other module.
It is best to mark all non-inline functions as static if they are not used outside the translation unit where they are defined (a translation unit being the preprocessed output of a source file together with all of the headers and source files included as a result of the #include directive). Typically, you do not want to place definitions of non-inline functions in header files.
If you fail to declare functions that are never called from outside a module as static, code can be adversely affected. In particular, you might have:
  • A larger code size, because out-of-line versions of functions are retained in the image.
    When a function is automatically inlined, both the in-line version and an out-of-line version of the function might end up in the final image, unless the function is declared as static. This might increase code size.
  • An unnecessarily complicated debug view, because there are both inline versions and out-of-line versions of functions to display.
    Retaining both inline and out-of-line copies of a function in code can sometimes be confusing when setting breakpoints or single-stepping in a debug view. The debugger has to display both in-line and out-of-line versions in its interleaved source view so that you can see what is happening when stepping through either the in-line or out-of-line version.
Because of these problems, declare non-inline functions as static when you are sure that they can never be called from another module.
Related concepts
4.20 Inline functions
4.21 Compiler decisions on function inlining
4.23 Inline functions and removal of unused out-of-line functions at link time
4.24 Automatic function inlining and multifile compilation
4.26 Compiler modes and inline functions
4.27 Inline functions in C++ and C90 mode
4.28 Inline functions in C99 mode
4.29 Inline functions and debugging
Related reference
4.25 Restriction on overriding compiler decisions about function inlining
7.11 --autoinline, --no_autoinline
7.120 -Onum
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