Messages in this thread | | | Date | Fri, 19 Jan 2007 15:13:55 +0100 | From | Adrian Bunk <> | Subject | Re: can someone explain "inline" once and for all? |
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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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