linux-kernel - Re: -Os versus -O2

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Date:	Mon, 25 Jun 2007 03:33:02 +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: Re: -Os versus -O2

On Sun, Jun 24, 2007 at 05:58:46PM -0700, Arjan van de Ven wrote:
> 
> > 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.
> 
> I think you are missing the point. You tell the kernel to
> OPTIMIZE_FOR_SIZE. *over performance*. Sure. Performance shouldn't be
> EXTREMELY pathetic, but it's not; and if it were, it's a problem with
> the gcc version you have (and if you are a distro, you can surely fix
> that)

My point is commit c45b4f1f1e149c023762ac4be166ead1818cefef

CC_OPTIMIZE_FOR_SIZE is currently known as an experimental feature to 
improve the _performance_.

> > The interesting questions are:
> > Does -Os still sometimes generate faster code with gcc 4.2?
> > If yes, why?
> 
> on a system level, size can help performance because you have more
> memory available for other things.

For a given gcc version, there's a finite number of differences between 
-Os and -O2. 

The interesting question is for which differences with gcc 4.2 we want 
the -Os version in the kernel for best performance. This should then be 
controllable through gcc options.

> It also reduces download size and 
> gives you more space on the live CD....

That's a different point.

If you don't care about performance but care about size then -Os is 
the best choice.

> if you want to make things bigger again, please do this OUTSIDE the
> "optimize for size" option. Because that TELLS you to go for size.

Agreed, but CONFIG_CC_OPTIMIZE_FOR_SIZE should again be under 
CONFIG_EMBEDDED.

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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