lkml.org 
[lkml]   [2009]   [Feb]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2] ftrace: On PowerPC we don't need frame pointers for CALLER_ADDRs
On Tue, Feb 03, 2009 at 05:06:45PM +0100, Ingo Molnar wrote:
>
> * Anton Vorontsov <avorontsov@ru.mvista.com> 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: cbouatmailru@gmail.com
irc://irc.freenode.net/bd2


\
 
 \ /
  Last update: 2009-02-03 17:23    [W:2.148 / U:21.472 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site