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Subject: Re: [PATCH v2] ftrace: On PowerPC we don't need frame pointers for
 CALLER_ADDRs
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: avorontsov@ru.mvista.com
Cc: Steven Rostedt <srostedt@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       Paul Mackerras <paulus@samba.org>, linuxppc-dev@ozlabs.org,
       linux-kernel@vger.kernel.org
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Date: Wed, 04 Feb 2009 11:34:03 +1100
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On Tue, 2009-02-03 at 21:59 +0300, Anton Vorontsov wrote:
> On Tue, Feb 03, 2009 at 11:32:18AM -0500, Steven Rostedt wrote:
> > 
> > On Tue, 2009-02-03 at 19:19 +0300, Anton Vorontsov wrote:
> > > On Tue, Feb 03, 2009 at 05:06:45PM +0100, Ingo Molnar wrote:
> > 
> > > [1] Btw, why exactly do we need the -fno-omit-frame-pointer for
> > > "FUNCTION_TRCER" tracer? Why just -pg isn't sufficient?..
> > > 
> > 
> > The problem is this that is in the toplevel Makefile:
> > 
> > 
> > ifdef CONFIG_FRAME_POINTER
> > KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls
> > else
> > KBUILD_CFLAGS	+= -fomit-frame-pointer
> > endif
> > 

And don't forget the gcc bug that miscompiles function epilogues on ppc
with -fno-omit-frame-pointer (iirc), which we need to work around using
-mno-sched-epilog.

Currently, we set that only if CONFIG_FUNCTION_TRACER is set, ie, we
assume that we only set -fno-omit-frame-pointer when
CONFIG_FUNCTION_TRACER is set.

Ben.