((always_inline)) does not work when function is implemented in different file

Question

I have a file funcs.h where I have the declaration of function:

inline void some_func( void ) __attribute__((always_inline));

Then I have a file funcs.c where I have the implementation of the function:

inline void some_func( void ) { … }

Then in my main file main.c I #include the funcs.h and try to use some_func() somewhere in the code. However when I compile my program and look at the binary file, the function appears to be compiled as a regular separate function, and called just like any other regular function, instead of being embedded as inline.

Why is that happening, and is there a way to force actual inlining into this? (Except the option of just using #define macros instead of functions, of course.)

How is the compiler supposed to inline a definition that is found after compilation? — ghostofstandardspast
– ghostofstandardspast, Commented Jun 18, 2014 at 15:34
You will have to turn on link-time optimization (-flto switch). — The Paramagnetic Croissant
– The Paramagnetic Croissant, Commented Jun 18, 2014 at 15:36
@user3477950, That's a good point I guess. I remember Microsoft saying something about the optimizations the linker can do. — ghostofstandardspast
– ghostofstandardspast, Commented Jun 18, 2014 at 15:37
@user3477950 simple testing shows that even a+b function isn't inlined with gcc 4.8 and -flto. Moreover, using this attribute produces compile error, since compiler knows for sure that it can't inline, while programmer insists that it should. — keltar
– keltar, Commented Jun 18, 2014 at 15:50

Carl Norum · Accepted Answer · 2014-06-18 15:34:27Z

4

Put the implementations is the header. If they're not available in the translation unit in which you intend to do the inlining, you'll be out of luck. The linker (well, a traditional linker) can't do anything about that for you.

answered Jun 18, 2014 at 15:34

Carl Norum

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4 Comments

The Paramagnetic Croissant Over a year ago

Most modern toolchains support link-time optimization, relying on that would probably be a better idea that dumping huge pieces of definitions in headers.

Deduplicator Over a year ago

@user3477950: Yes, though relying on compiler optimization makes the force_inline-attribute superfluous.

The Paramagnetic Croissant Over a year ago

@Deduplicator …which tells me that one should probably leave low-level optimizations to the compiler, especially nowadays.

keltar Over a year ago

In header file, in case of C and not C++, it should be static inline. inline itself does not flag function as non-exportable (but in C++ it does).

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