Re: [Tinycc-devel] RE :Re: inline functions

[Rob]

'inline' alone should not generate code, I'm pretty certain. It's an
inline-only definition. If it can't be inlined, you get a linker error.

For example:

inline int f()
{
        return 3;
}

int main()
{
        return f();
}

A standalone function named 'f' will never appear in the generated code.
If the compiler inlines 'f' into main, the program will link. If the
compiler doesn't or can't inline 'f', you'll get a linker error.

So we get a linker error with 'gcc -std=c99 -O0', but the program links
fine with 'gcc -std=c99 -O1', as f isn't referenced (since it's
inlined by an optimiser).

Also, I checked - gcc defaults to -std=gnu89, which means gnu89 inline
semantics, so when testing, make sure you give it '-std=c99'.

Clang exhibits the same behaviour w.r.t. inline, so I'm pretty sure I've
interpreted the standard correctly. Note that clang defaults to C99.


Thanks,
Rob


On Wed, 11 Dec 2013, Pierre wrote:
> So inline alone should export nothing if not refferenced (with code or forward 
> declared), and at most, create a static function if cannot be inlined ?
>
> And an 'extern inline ' simply ignore 'inline' ?
>
>
> Le 11/12/2013 22:33, Rob a écrit :
>       On Wed, 11 Dec 2013, Thomas Preud'homme wrote:
>             Le mercredi 11 décembre 2013, 09:28:07 Christian JULLIEN a écrit :
>             I knew about the fact that it is a hint and I knew even when 
> inlined, the
>             function still needs to be output in case its address is used. 
> However I
>             forgot about the other details. I stand corrected, thanks.
>
>             I remembered extern and inline has a special meaning as well but I 
> forgot so I
>             checked online and I think the documentation of gcc [0] explains 
> pretty well
>             how inline behave. I'm a bit surprised though because I thought the 
> special
>             meaning of extern inline was not in the standard but something gcc 
> specific.
>             Again, I stand corrected.
>
>             [0] http://gcc.gnu.org/onlinedocs/gcc/Inline.html
>
>                   If you want to avoid problems when you define an inlined 
> function in a .h is
>                   to declare this function static. This way, if compiler is not 
> able to
>                   inline function, the two or more translation units using it 
> will have their
>                   own static copy that will make linker happy.
>
>
>             Or extern inline which has a slightly different meaning. With 
> static inline you
>             need to provide two implementation of the function, one with static 
> inline in
>             the header, and one normal implementation somewhere else in case 
> there is a
>             call that cannot be inlined or if the address of the function is 
> used. With
>             extern inline you make it clear that you want the function inlined 
> no matter
>             what. At least that is my understanding after a quick read but 
> again, I might
>             have read too quickly.
>
>
>
>       There are very subtle differences and with gcc I'm pretty sure you have
>       to specify -std=c99 otherwise it defaults to gnu89 which has different,
>       GNU inline semantics.
>
>       'inline', unfortunately is not just a hint to the compiler. There are a
>       few checks compilers have to perform too.
>
>
>       So we have three forms of inline:
>
>       extern inline void func() { ... } // 1
>       static inline void func() { ... } // 2
>              inline void func() { ... } // 3
>
>
>       The first form, 'extern inline', will always cause a
>       standalone/non-inlined function to be emitted, just as if 'inline'
>       wasn't present. The inline modifier here simply acts as a hint that this
>       function may be inlined.
>
>       The second form, 'static inline', will cause function code to be
>       emitted, but the compiler may omit this if it knows the function is
>       never used (since it's static, it's not called from another translation
>       unit). Again, 'inline' acts only as a hint.
>
>       The final form, 'inline' without a storage class is where we have
>       special handling. In this case, standalone function code is never
>       emitted, this version is _only_ to be used for inline substitution.
>
>
>       For the final form to take affect, none of the declarations of `func'
>       must mention extern, for example:
>
>
>       inline f();
>       inline f();
>       extern inline f(); // this causes `f' to be in form 2
>       inline f() { ... }
>
>
>
>       At least, that's what I've gathered from the standard and various
>       searches.
>
>
>       Thanks, Rob
>
>
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