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

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Message-ID: <Pine.LNX.4.64.0701190939470.25798@CPE00045a9c397f-CM001225dbafb6>
Date:	Fri, 19 Jan 2007 09:44:59 -0500 (EST)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Adrian Bunk <bunk@...sta.de>
cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: can someone explain "inline" once and for all?

On Fri, 19 Jan 2007, Adrian Bunk wrote:

> On Fri, Jan 19, 2007 at 03:01:44PM +0200, Pekka Enberg wrote:
> > On 1/19/07, Robert P. J. Day <rpjday@...dspring.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.

right, and that last part explains that snippet i previously posted
from include/asm-alpha/compiler.h

========================
#ifdef __KERNEL__
/* Some idiots over in <linux/compiler.h> thought inline should imply
   always_inline.  This breaks stuff.  We'll include this file whenever
   we run into such problems.  */
========================

  which is a result of this from include/linux/compiler.h:

========================

#define inline          inline          __attribute__((always_inline))
#define __inline__      __inline__      __attribute__((always_inline))
#define __inline        __inline        __attribute__((always_inline))

which certainly seems to suggest that *ever* explicitly stating
"always inline" is redundant, no?  maybe i'm missing something
critical here but this just seems wrong.

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

and that makes sense.  it has no effect, it's more for just
commenting.  but it's still kind of misleading.

rday
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