linux-kernel - Re: [PATCH v3 2/9] rbtree: add __rb_change

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Date:	Wed, 26 Sep 2012 15:43:45 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Daniel Santos <daniel.santos@...ox.com>
Cc:	Daniel Santos <danielfsantos@....net>,
	Michel Lespinasse <walken@...gle.com>, riel@...hat.com,
	peterz@...radead.org, aarcange@...hat.com, dwmw2@...radead.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH v3 2/9] rbtree: add __rb_change_child() helper function

On Wed, 26 Sep 2012 17:34:11 -0500
Daniel Santos <danielfsantos@....net> wrote:

> Sorry to resurrect the dead here, but I'm playing catch-up and this
> looks important.
> 
> On 08/20/2012 05:17 PM, Andrew Morton wrote:
> > I'm inclined to agree with Peter here - "inline" is now a vague,
> > pathetic and useless thing.  The problem is that the reader just
> > doesn't *know* whether or not the writer really wanted it to be
> > inlined.
> >
> > If we have carefully made a decision to inline a function, we should
> > (now) use __always_inline.
> Are we all aware here that __always_inline (a.k.a.
> "__attribute__((always_inline))") just means "inline even when not
> optimizing"?  This appears to be a very common misunderstanding (unless
> the gcc docs are wrong, see
> http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html#index-g_t_0040code_007bflatten_007d-function-attribute-2512).
> 
> If you want to *force* gcc to inline a function (when inlining is
> enabled), you can currently only do it from the calling function by
> adding the |flatten attribute to it, which I have proposed adding here:
> https://lkml.org/lkml/2012/9/25/643.
> 
> Thus, all of the __always_inline markings we have in the kernel only
> affect unoptimized builds (and maybe -O1?).  If we need this feature
> (and I think it would be darned handy!) we'll have to work on gcc to get it.

When I replace the four __always_inline's in fs/namei.c with "inline",
namei.o's .text shrinks 2kbytes (gcc-4.4.4), so __always_inline does
allear to be doing what we think it does?


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