linux-kernel - Re: [Patch] Always include <linux/types.h>

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Date:	Wed, 5 Nov 2008 23:32:12 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Jörn Engel <joern@...fs.org>
Cc:	Sam Ravnborg <sam@...nborg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Patch] Always include <linux/types.h>

On Wed, Nov 05, 2008 at 09:02:17PM +0100, Jörn Engel wrote:
> On Wed, 5 November 2008 20:16:28 +0100, Jörn Engel wrote:
> > 
> > My hope was actually to speed up compilation.  If the average c file
> > includes 10 headers, on types.h will get included by most of them,
> > possibly multiple times.  Each run after the first still has to parse
> > the whole file, just to drop everything between  #ifndef _LINUX_TYPES_H
> > and #endif.
> > 
> > By passing types.h on the command line we can drop it from all headers
> > and only have to parse it once.  Just the intermediate step of parsing
> > types.h 11 times instead of 10 will slow things down.  By about .4% on
> > my not very beefy notebook.
> > 
> > Before:
> > real    4m33.241s
> > user    3m58.524s
> > sys     0m18.539s
> > 
> > After:
> > real    4m29.707s
> > user    3m59.674s
> > sys     0m18.182s
> 
> And after removing all explicit #include <linux/types.h> from headers:
> real    4m31.946s
> user    3m59.521s
> sys     0m18.752s
> 
> All this may be lost in the noise.  The build machine wasn't completely
> idle and the variation in system time is rather random.  So it neither
> helps nor hurts much.  Nor does it solve any real problem.
> 
> We might just as well drop it, I guess.

If you are working on improving compile times, it's better concentrate
on removing unneeded includes. If just removing "extern"s from prototypes
can reliably save several seconds, reducing headers can do wonders.
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