Subject: Re: [PATCH v2] ftrace: On PowerPC we don't need frame pointers for CALLER_ADDRs

From: Anton Vorontsov <[email-masked]>
Date: Tue, 3 Feb 2009 19:19:55 +0300

On Tue, Feb 03, 2009 at 05:06:45PM +0100, Ingo Molnar wrote:
> 
> * Anton Vorontsov <[email-masked]> wrote:
> 
> > According to this discussion:
> > 
> > http://lkml.org/lkml/2008/7/25/338
> > http://lkml.org/lkml/2008/7/26/72
> > 
> > Frame pointers do nothing useful on PowerPC, so lib/Kconfig.debug
> > makes CONFIG_FRAME_POINTER unselectable on PPC targets. But ftrace.h
> > requires CONFIG_FRAME_POINTER for CALLER_ADDR macros. [...]
> 
> hm, why not add PPC to FRAME_POINTERS list of architectures, and select it 
> from the powerpc arch Kconfig? Does that cause complications somewhere?

-fno-omit-frame-pointers makes the code worse w/o any actual
benefit that we would use. Plus, there is a long standing bug in
gcc that makes -fno-omit-frame-pointer generate wrong code for PPC
targets:

http://lkml.org/lkml/2008/9/2/25

That is, the only tracer that needs[1] -fno-omit-frame-pointer is
"FUNCTION_TRCER", but we workaround the issue via -mno-sched-epilog,
quoting arch/powerpc/Makefile:

# Work around a gcc code-gen bug with -fno-omit-frame-pointer.
ifeq ($(CONFIG_FUNCTION_TRACER),y)
KBUILD_CFLAGS           += -mno-sched-epilog
endif

[1] Btw, why exactly do we need the -fno-omit-frame-pointer for
"FUNCTION_TRCER" tracer? Why just -pg isn't sufficient?..

-- 
Anton Vorontsov
email: [email-masked]
irc://irc.freenode.net/bd2