linux-kernel - Re: [PATCH][GIT PULL][v2.6.32] tracing/x86: Add check to detect GCC messing with mcount prologue

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Date:	Wed, 25 Nov 2009 16:53:27 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Jakub Jelinek <jakub@...hat.com>, Andrew Haley <aph@...hat.com>,
	"H.J. Lu" <hjl.tools@...il.com>, rostedt@...dmis.org,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	feng.tang@...el.com, Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	David Daney <ddaney@...iumnetworks.com>,
	Richard Guenther <richard.guenther@...il.com>,
	gcc <gcc@....gnu.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH][GIT PULL][v2.6.32] tracing/x86: Add check to detect GCC
 messing with mcount prologue

On Wed, 25 Nov 2009, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > On Tue, 24 Nov 2009, Jakub Jelinek wrote:
> > 
> > > On Tue, Nov 24, 2009 at 03:55:49PM +0100, Thomas Gleixner wrote:
> > > > > you should compile your code with -maccumulate-outgoing-args, and there's
> > > > > no need to use -mtune=generic.  Is that right?
> > > > 
> > > > Seems to work. What other side effects has that ?
> > > 
> > > Faster code, significant increase in code size though.  Note that on many
> > > architectures it is the only supported model.
> > 
> > Just checked on the affected -marchs. The increase in code size is 
> > about 3% which is not that bad and definitely acceptable for the 
> > tracing case. Will zap the -mtune=generic patch and use 
> > -maccumulate-outgoing-args instead.
> 
> hm, 3% sounds quite large :( dyn-ftrace is enabled in distro configs, so 
> 3% is a big deal IMO.

Distro-configs have -mtune=generic anyway. So it's not changing
anything for them.

I'm talking about the -march flags which result in that weird code
(pentium-mmx ....).

Thanks,

	tglx
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