linux-kernel - Re: [PATCH] include: kernel.h: rewrite min3, max3 and clamp using min and max

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Message-Id: <20140616163525.46933b64320e220a6d806bb4@linux-foundation.org>
Date:	Mon, 16 Jun 2014 16:35:25 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Michal Nazarewicz <mina86@...a86.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Hagen Paul Pfeifer <hagen@...u.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] include: kernel.h: rewrite min3, max3 and clamp using
 min and max

On Mon, 16 Jun 2014 16:25:15 -0700 (PDT) David Rientjes <rientjes@...gle.com> wrote:

> On Mon, 16 Jun 2014, Andrew Morton wrote:
> 
> > > It appears that gcc is better at optimising a double call to min
> > > and max rather than open coded min3 and max3.  This can be observed
> > > here:
> > > 
> > > ...
> > >
> > > Furthermore, after ___make allmodconfig && make bzImage modules___ this is the
> > > comparison of image and modules sizes:
> > > 
> > >     # Without this patch applied
> > >     $ ls -l arch/x86/boot/bzImage **/*.ko |awk '{size += $5} END {print size}'
> > >     350715800
> > > 
> > >     # With this patch applied
> > >     $ ls -l arch/x86/boot/bzImage **/*.ko |awk '{size += $5} END {print size}'
> > >     349856528
> > 
> > We saved nearly a megabyte by optimising min3(), max3() and clamp()? 
> > 
> > I'm counting a grand total of 182 callsites for those macros.  So the
> > saving is 4700 bytes per invokation?  I don't believe it...
> > 
> 
> I was checking just the instances of min3() in mm/ and gcc ends up 
> inlining transfer_objects() in mm/slab.c as a result of this change and 
> increases its text size:
> 
>    text	   data	    bss	    dec	    hex	filename
>   28369	  21559	      4	  49932	   c30c	slab.o.before
>   28399	  21559	      4	  49962	   c32a	slab.o.after

Maybe that's a good thing in disguise: gcc said "hey this thing is now
small enough to inline it".

> It also seems to use one additional temp variable of type typeof(x) on the 
> stack, so I do think the old version was superior.
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