linux-kernel - Re: [PATCH 1/3] powerpc, Makefile: Make it possible to safely select CONFIG_FRAME

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Date:	Sat, 02 May 2009 22:04:09 -0400
From:	Steven Rostedt <srostedt@...hat.com>
To:	Segher Boessenkool <segher@...nel.crashing.org>
Cc:	Anton Vorontsov <avorontsov@...mvista.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org, Paul Mackerras <paulus@...ba.org>,
	Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH 1/3] powerpc, Makefile: Make it possible to safely
	select CONFIG_FRAME_POINTER

On Sat, 2009-05-02 at 21:48 +0200, Segher Boessenkool wrote:
> > This patch introduces HAVE_NORMAL_FRAME_POINTER Kconfig symbol. When
> > defined, the top level Makefile won't add -fno-omit-frame-pointer
> > cflag (the flag is useless for PowerPC kernels, and also makes gcc
> > generate wrong code).
> 
> > +++ b/arch/powerpc/Kconfig
> 
> > +	select HAVE_NORMAL_FRAME_POINTER
> 
> > +config HAVE_NORMAL_FRAME_POINTER
> > +	bool
> > +	help
> > +	  Architectures should select this symbol if their ABI implies
> > +	  having a frame pointer.
> 
> I am totally confused what you call a frame pointer here.
> None of the relevant PowerPC ABIs have a frame pointer
> separate from the stack pointer; the compiler can create
> one, of course.  A better config symbol name and help text
> would help understand this patch :-)

Yeah, I agree. This needs a better description. I only know what's going
on because I was there for the start of the discussion.

But just to be sure, this is what I think is happening.

When we add "-pg" to gcc, it automatically causes frame pointers to be
used.

But with PPC, it always has frame pointers and there's no problem.

But with Linux, when you add CONFIG_FRAME_POINTER, it automatically
adds:  -fno-omit-frame-pointer. Thus the config will add
"-fomit-frame-pointer" when CONFIG_FRAME_POINTER is not set, or it will
add "-fno-omit-frame-pointer" when it is set.

The problem with PPC is that "-fno-omit-frame-pointer" is buggy and
causes gcc to produce bad code.

Perhaps a better name would be:

HAVE_FRAME_POINTER_AS_DEFAULT

??

Or am I totally wrong in my analysis?

-- Steve

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