linux-kernel - Re: BUG: GCC-4.4.x changes the function frame on some functions

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Message-ID: <alpine.LFD.2.00.0911200122190.24119@localhost.localdomain>
Date:	Fri, 20 Nov 2009 01:36:05 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jeff Law <law@...hat.com>
cc:	rostedt@...dmis.org, "H. Peter Anvin" <hpa@...or.com>,
	David Daney <ddaney@...iumnetworks.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Haley <aph@...hat.com>,
	Richard Guenther <richard.guenther@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	feng.tang@...el.com, Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, jakub@...hat.com,
	gcc@....gnu.org
Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions

On Thu, 19 Nov 2009, Jeff Law wrote:
> On 11/19/09 15:43, Steven Rostedt wrote:
> > On Thu, 2009-11-19 at 14:25 -0700, Jeff Law wrote:
> > 
> >    
> > > Having said all that, I don't expect to personally be looking at the
> > > problem, given the list of other codegen issues that need to be looked
> > > at (reload in particular), profiling/stack interactions would be around
> > > 87 millionth on my list.
> > >      
> > Is there someone else that can look at it?
> > 
> >    
> Unsure at the moment...  Like everyone else, GCC developers are busy and this
> probably isn't going to be a high priority item for anyone.
> 
> 
> > Or at the very least, could you point us to where that code is, and one
> > of us tracing folks could take a crack at switching hats to be a
> > compiler writer (with the obvious prerequisite of drinking a lot of beer
> > first, or is there a better drug to cope with the pain of writing gcc?).
> >    
> It _might_ be as easy as defining PROFILE_BEFORE_PROLOGUE in
> gcc-<someversion>gcc/config/i386/linux.h & rebuilding GCC.
> 
> Based on comments elsewhere, the sun386i support may have used
> PROFILE_BEFORE_PROLOGUE in the past and thus the x86 backend may not need
> further adjustment.  That is obviously the ideal case.
> 
> If that appears to work for your needs, I'll volunteer to test it more
> thoroughly and assuming those tests look good shepherd it into the source
> tree.

We definitely want to see that ASAP.

While testing various kernel configs we found out that the problem
comes and goes. Finally I started to compare the gcc command line
options and after some fiddling it turned out that the following
minimal deltas change the code generator behaviour:

Bad:  -march=pentium-mmx                -Wa,-mtune=generic32
Good: -march=i686        -mtune=generic -Wa,-mtune=generic32
Good: -march=pentium-mmx -mtune-generic -Wa,-mtune=generic32

I'm not supposed to understand the logic behind that, right ?

Thanks,

	tglx
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