linux-kernel - Re: [PATCH v2] ftrace: On PowerPC we don't need frame pointers for CALLER

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Date:	Tue, 3 Feb 2009 21:59:33 +0300
From:	Anton Vorontsov <avorontsov@...mvista.com>
To:	Steven Rostedt <srostedt@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ftrace: On PowerPC we don't need frame pointers for
	CALLER_ADDRs

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
> 
> 
> -pg is incompatible with -fomit-frame-pointer

Ah...

$ gcc -pg -fomit-frame-pointer -S c.c
gcc: -pg and -fomit-frame-pointer are incompatible

It's hard-coded in gcc, in the code that don't know about
architecture details.

But on PowerPC -O1 implies -fomit-frame-pointer, that is

gcc -pg -O1 -fno-omit-frame-pointer
and
gcc -pg -O1

produce different outputs. Thus -pg -O should be the same
as "-pg -O -fomit-framepointer".

-- 
Anton Vorontsov
email: cbouatmailru@...il.com
irc://irc.freenode.net/bd2
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