linux-kernel - -Os versus -O2

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Date:	Mon, 25 Jun 2007 02:41:06 +0200
From:	Adrian Bunk <bunk@...sta.de>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Benjamin LaHaise <bcrl@...ck.org>,
	Oleg Verych <olecom@...wer.upol.cz>, rae l <crquan@...il.com>,
	linux-kernel@...r.kernel.org
Subject: -Os versus -O2

On Sun, Jun 24, 2007 at 05:23:42PM -0700, Arjan van de Ven wrote:
> On Sun, 2007-06-24 at 20:12 -0400, Benjamin LaHaise wrote:
> > On Sun, Jun 24, 2007 at 05:09:16PM -0700, Arjan van de Ven wrote:
> > > if you care about the last cycle, don't specify -Os but -O2.
> > > simple as that... you get what you tell the compiler you want.
> > 
> > Certain distros are shipping kernels compiled with -Os.  And it's more 
> > than just a couple of cycles.
> 
> so those distros pick space over some cycles. Who are you to then
> override that choice ? ;-)
> 
> seriously, why are we even talking about overriding a choice the user
> (or distro vendor as user) made here?

There is a real issue in the fact that compiling with -Os is available
through a kconfig option and AFAIR used by some distributions.

I doubt distros enable CONFIG_CC_OPTIMIZE_FOR_SIZE due to size 
considerations, but due to speed considerations.

I wouldn't care if CONFIG_CC_OPTIMIZE_FOR_SIZE was hidden behind
CONFIG_EMBEDDED, but as long as it's available as a general purpose
option we have to consider it's performance.

The interesting questions are:
Does -Os still sometimes generate faster code with gcc 4.2?
If yes, why?

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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