linux-kernel - Re: [tip:x86/asm] x86/defconfig: Turn on CONFIG_CC_OPTIMIZE_FOR

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Date:	Sun, 27 Jan 2013 13:49:42 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Jan Beulich <jbeulich@...e.com>, ling.ml@...pay.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/asm] x86/defconfig: Turn on
 CONFIG_CC_OPTIMIZE_FOR_SIZE= y in the 64-bit defconfig


* Steven Rostedt <rostedt@...dmis.org> wrote:

> On Sat, 2013-01-26 at 11:43 -0800, Linus Torvalds wrote:
> 
> > The problem, of course, is that most -O2 code generation is done
> > assuming hot loops that don't show much if any I$ issues. And the -Os
> > thing is done *purely* for size, not taking any performance into
> > account at all. There's no balanced middle ground, which is what _we_
> > would want.
> 
> Gcc needs to implement a -Olinus

What we really want is a sane default for 'library code' 
optimization:

 - cache-cold optimizations for run-through-once non-looping 
   code (-Os)

 - good loop optimizations for anything that arguably loops (-O2)

 - plus common-sense fixes to -Os like not throwing away 
   explicit branch hints we go to great pains to insert.

Possibly some time this decade.

Thanks,

	Ingo
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