Re: can someone explain "inline" once and for all?

[Adrian Bunk <bunk@stusta.de>]

On Fri, Jan 19, 2007 at 03:01:44PM +0200, Pekka Enberg wrote:
> On 1/19/07, Robert P. J. Day <rpjday@mindspring.com> wrote:
> >is there a simple explanation for how to *properly* define inline
> >routines in the kernel?  and maybe this can be added to the
> >CodingStyle guide (he mused, wistfully).
> 
> AFAIK __always_inline is the only reliable way to force inlining where
> it matters for correctness (for example, when playing tricks with
> __builtin_return_address like we do in the slab).
> 
> Anything else is just a hint to the compiler that might be ignored if
> the optimizer thinks it knows better.

With the current implementation in the kernel (and considering that 
CONFIG_FORCED_INLINING was implemented in a way that it never had any 
effect), __always_inline and inline are currently equivalent.

__always_inline is mostly an annotation that really bad things might 
happen if the code doesn't get inlined.

But I'm not sure whether such a distinction is required at all - the 
rule of thumb should be that static functions in headers should be 
inline (otherwise, they belong into a C file), and functions in C files 
should never be marked inline. [1]

cu
Adrian

[1] For the latter there might be a handful of exceptions in the whole 
    kernel in real fastpath code, but usually gcc knows best when to 
    inline a function - and we have a global CONFIG_CC_OPTIMIZE_FOR_SIZE 
    knob for influencing the decision.

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