linux-kernel - Re: Please be aware that __always_inline doesn't mean "always inline"!

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Message-Id: <20120926165044.46b8f7d6.akpm@linux-foundation.org>
Date:	Wed, 26 Sep 2012 16:50:44 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Daniel Santos <daniel.santos@...ox.com>
Cc:	Daniel Santos <danielfsantos@....net>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	torvalds@...ux-foundation.org
Subject: Re: Please be aware that __always_inline doesn't mean
 "always inline"!

On Wed, 26 Sep 2012 18:20:44 -0500
Daniel Santos <danielfsantos@....net> wrote:

> I've noticed that there's a lot of misperception about the meaning of
> the __always_inline, or more specifically,
> __attribute__((always_inline)), which does not actually cause the
> function to always be inlined.  Rather, it *allows* gcc to inline the
> function, even when compiling without optimizations.  Here is the
> description of the attribute from gcc's docs
> (http://gcc.gnu.org/onlinedocs/gcc-4.7.2/gcc/Function-Attributes.html)
> 
> always_inline
> Generally, functions are not inlined unless optimization is specified.
> For functions declared inline, this attribute inlines the function even
> if no optimization level was specified.
> 
> This would even appear to imply that such functions aren't even marked
> as "inline" (something I wasn't aware of until today).  The only
> mechanism I'm currently aware of to force gcc to inline a function is
> the flatten attribute (see https://lkml.org/lkml/2012/9/25/643) which
> works backwards, you declare it on the calling function, and it forces
> gcc to inline all functions (marked as inline) that it calls.

As I mentioned in the other thread, the __always_inline's in fs/namei.c
(at least) are doing exactly what we want them to do, so some more
investigation is needed here?

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