can someone explain "inline" once and for all?

can someone explain "inline" once and for all?

[Robert P. J. Day]

  apologies if this is an inappropriately trivial question but this
has been bugging me for a while.  what is the deal with "inline"?

  first, there appear to be three possible ways of specifying an
inline routine in the kernel source:

  $ grep -r "static inline " .
  $ grep -r "static __inline__ " .
  $ grep -r "static __inline " .

i vaguely recall that this has something to do with a distinction
between C99 inline and gcc inline and trying to avoid a clash between
the two, but i'm not going to put any money on that.  but the
confusion probably explains why so many people insist on creating new
macros to represent inline:

  $ grep -r "#define.*inline" .

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).

rdau

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

[Pekka Enberg]

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.

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

[Robert P. J. Day]

On Fri, 19 Jan 2007, 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.

  oh, *that* part i knew.  what i don't understand is the difference
between "inline", "__inline" and "__inline__".  you can see in
include/linux/compiler-gcc4.h:

#ifdef CONFIG_FORCED_INLINING
# undef inline
# undef __inline__
# undef __inline
# define inline                 inline          __attribute__((always_inline))
# define __inline__             __inline__      __attribute__((always_inline))
# define __inline               __inline        __attribute__((always_inline))
#endif

  so that header file certainly suggests that there's some sort of
difference.  after which it gets even more confusing as various macros
seem to mix and match:

  drivers/cdrom/sbpcd.c:#define INLINE inline
  arch/arm/nwfpe/ARM-gcc.h:#define INLINE extern __inline__
  arch/cris/arch-v10/kernel/fasttimer.c:#define __INLINE__ inline
  arch/alpha/mm/fault.c:#define __EXTERN_INLINE inline
  ... etc etc ...

  i mean, how many different kinds of inline *are* there?

rday

p.s.  apparently, some of the alpha people are less than thrilled with
the situation:

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.  */

#include <linux/compiler.h>
#undef inline
#undef __inline__
#undef __inline
#undef __always_inline
#define __always_inline         inline __attribute__((always_inline))

#endif /* __KERNEL__ */

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

[Adrian Bunk]

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.

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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

[Robert P. J. Day]

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@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.

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

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

[Robert P. J. Day]

On Fri, 19 Jan 2007, Adrian Bunk wrote:

> 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.

yes, that option was implemented in a half-assed sort of way.  if you
look at compiler-gcc4.h, at first glance the preprocessing looks like
it's doing the right thing for that config option:

==================================
#include <linux/compiler-gcc.h>

#ifdef CONFIG_FORCED_INLINING
# undef inline
# undef __inline__
# undef __inline
# define inline                 inline          __attribute__((always_inline))
# define __inline__             __inline__      __attribute__((always_inline))
# define __inline               __inline        __attribute__((always_inline))
#endif
==================================

  but it's too late for checking that kernel config option, since
compiler-gcc.h has already been included, which includes:

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

so, as you say, "__always_inline and inline are currently equivalent".
which is sort of confusing and might come as a nasty surprise to some
developers who weren't expecting that.

rday

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

[Andreas Schwab]

"Robert P. J. Day" <rpjday@mindspring.com> writes:

>   first, there appear to be three possible ways of specifying an
> inline routine in the kernel source:
>
>   $ grep -r "static inline " .
>   $ grep -r "static __inline__ " .
>   $ grep -r "static __inline " .
>
> i vaguely recall that this has something to do with a distinction
> between C99 inline and gcc inline

No, it doesn't (there is no C99 compatible inline in gcc before 4.3).  It
has to do with the fact that inline is not a keyword in C89, so you need
to use a different spelling when you want to stay compatible with strict
C89.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

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

[Robert P. J. Day]

On Fri, 19 Jan 2007, Andreas Schwab wrote:

> "Robert P. J. Day" <rpjday@mindspring.com> writes:
>
> >   first, there appear to be three possible ways of specifying an
> > inline routine in the kernel source:
> >
> >   $ grep -r "static inline " .
> >   $ grep -r "static __inline__ " .
> >   $ grep -r "static __inline " .
> >
> > i vaguely recall that this has something to do with a distinction
> > between C99 inline and gcc inline
>
> No, it doesn't (there is no C99 compatible inline in gcc before
> 4.3).  It has to do with the fact that inline is not a keyword in
> C89, so you need to use a different spelling when you want to stay
> compatible with strict C89.

ok, so based on that and a bit more surfing, i see that either
"__inline" or "__inline__" are acceptable variants in gcc, and there
is no distinction between them, is that right?

but in terms of strict C89 compatibility, it would seem to be a bit
late for that given:

  $ grep -r "static inline " .

no?

rday

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

[Andreas Schwab]

"Robert P. J. Day" <rpjday@mindspring.com> writes:

> but in terms of strict C89 compatibility, it would seem to be a bit
> late for that given:
>
>   $ grep -r "static inline " .
>
> no?

The kernel does not use strict C89, it uses GNUC89.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

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

[Robert P. J. Day]

On Fri, 19 Jan 2007, Andreas Schwab wrote:

> "Robert P. J. Day" <rpjday@mindspring.com> writes:
>
> > but in terms of strict C89 compatibility, it would seem to be a bit
> > late for that given:
> >
> >   $ grep -r "static inline " .
> >
> > no?
>
> The kernel does not use strict C89, it uses GNUC89.

  in that case, why did you just waste my time and all that bandwidth
by writing in your previous post:

"... so you need to use a different spelling when you want to stay
compatible with strict C89."

  please do me a favour and don't post another reply until you go back
and actually read my original question.

rday

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

[Alexandre Oliva]

On Jan 19, 2007, "Robert P. J. Day" <rpjday@mindspring.com> wrote:

>   first, there appear to be three possible ways of specifying an
> inline routine in the kernel source:

inline, __inline and __inline__ are equivalent as far as GCC is
concerned, as you've already figured out.

> i vaguely recall that this has something to do with a distinction
> between C99 inline and gcc inline

I suspect you're thinking of a different issue.

In C99, static inline means the same as in GNU89, non-static
non-extern inline means 'use this definition, that does not define
objects with static storage nor references identifiers with internal
linkage, for inlining or for a local definition, but make calls to it
fast and don't generate any out-of-line definition', and extern inline
means 'compile this code into a global out-of-line function, but also
inline it wherever it makes sense'.

In GNU89, static inline means 'compile this code into a local
out-of-line function if needed, but also inline it wherever it makes
sense', non-static non-extern inline means 'compile this code into a
global out-of-line function, but also inline it wherever it makes
sense', and extern inline means 'use this definition for inlining, but
don't generate any out-of-line definition; because either I have a
non-inline definition in this or in another translation unit, or I
want undefined-symbol errors at link time if inlining fails.'

So you see that the meaning of extern inline and non-extern inline are
also reversed comparing GNU89 with C99.  That's quite unfortunate,
and GNU libc went to some trouble to encapsulate the intended inline
meaning into preprocessor macros even in user headers, such that the
intended meaning is obtained regardless of the compiler version.

Fortunately, static inline is probably the most useful and thus common
case anyway.  Other constructs will work as in GNU89 up to GCC 4.3,
even with -std=c99, but the meaning of inline in C99 and GNU99 is
intended to be fixed to the C99 semantics in GCC 4.4, according to
http://gcc.gnu.org/ml/gcc/2006-11/msg00006.html

That's still a long way ahead (the 4.3 development cycle has just
started), but it wouldn't hurt to start fixing incompatibilities
sooner rather than later, and coming up with a clean and uniform set
of inline macros that express intended meaning for the kernel to use.

-- 
Alexandre Oliva         http://www.lsd.ic.unicamp.br/~oliva/
FSF Latin America Board Member         http://www.fsfla.org/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}

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

[Adrian Bunk]

On Fri, Jan 19, 2007 at 03:15:03PM -0200, Alexandre Oliva wrote:
>...
> That's still a long way ahead (the 4.3 development cycle has just
> started), but it wouldn't hurt to start fixing incompatibilities
> sooner rather than later, and coming up with a clean and uniform set
> of inline macros that express intended meaning for the kernel to use.

I had already removed most of the "extern inline"s in the kernel since 
they give warnings with -Wmissing-prototypes (which I'd like to enable 
long-term in the kernel since it helps discovering a class of nasty 
runtime errors).

As far as I can see, all we need is "static inline" with the semantics
"force inlining" for functions in header files and perhaps a handful of 
functions in C files (if any).

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed