linux-kernel - Re: [rfc] the kernel workflow & trivial "global -> static" patches

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Message-ID: <20080506061825.GF1544@cs181133002.pp.htv.fi>
Date:	Tue, 6 May 2008 09:18:25 +0300
From:	Adrian Bunk <bunk@...nel.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sam Ravnborg <sam@...nborg.org>,
	Alexander Viro <viro@....linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [rfc] the kernel workflow & trivial "global -> static" patches

On Tue, May 06, 2008 at 02:21:31AM +0200, Andi Kleen wrote:
> 
> I don't think the code changes actually with current gcc for integer
> code if you change something from global to static (unless it causes
> gcc to inline the function, but then it might be well larger if you're
> unlucky)

It's a common case that a function has only one caller. It should always 
be an (at least tiny) space win to get them inlined.

> The only file size change you'll see will be from a smaller symbol
> table in the vmlinux ELF file, but that is not even loaded at run time
> or included into the bzImage (and the kallsyms table has statics too)
>...

I'm not attaching size change information to these patches since 
whatever change one sees anyway also depends on other factors like
the exact kernel configuration, so it's non-trivial to get numbers
that could be taken seriously.

There are many small aspects, e.g. both gcc with -Wmissing-prototypes 
and sparse give warnings, and the problem might either be needlessly 
global code or the fact that a function prototype is either not in a 
header or the header not #include'd by the file. Although I've only
2 or 3 times catched such bugs in the kernel that is a nasty to debug 
class of bugs and gcc can find such problems at compile time.

> I could see some advantage from static in future compiler versions 
> though from better optimization, but it's quite remote.
>...

The best case I've actually seen in practice was a variable I made 
static, and with CONFIG_DEBUG_FOOBAR=n gcc was now able to prove that 
the value never changed resulting in the variable plus quite a chunk
of code no longer emitted.

> -Andi

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

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