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Re: [RFC][ PATCH -tip v2 3/7] Kbuild: disable gcc crossjumping
- From: Sam Ravnborg <sam at ravnborg dot org>
- To: Steven Rostedt <rostedt at goodmis dot org>
- Cc: Masami Hiramatsu <mhiramat at redhat dot com>, Ingo Molnar <mingo at elte dot hu>, Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, lkml <linux-kernel at vger dot kernel dot org>, systemtap <systemtap at sources dot redhat dot com>, DLE <dle-develop at lists dot sourceforge dot net>, Jim Keniston <jkenisto at us dot ibm dot com>, Srikar Dronamraju <srikar at linux dot vnet dot ibm dot com>, Christoph Hellwig <hch at infradead dot org>, Frederic Weisbecker <fweisbec at gmail dot com>, "H. Peter Anvin" <hpa at zytor dot com>, Anders Kaseorg <andersk at ksplice dot com>, Tim Abbott <tabbott at ksplice dot com>
- Date: Fri, 17 Jul 2009 22:28:25 +0200
- Subject: Re: [RFC][ PATCH -tip v2 3/7] Kbuild: disable gcc crossjumping
- References: <20090622212255.5384.53732.stgit@localhost.localdomain> <20090622212313.5384.84166.stgit@localhost.localdomain> <alpine.DEB.2.00.0906271611280.12592@gandalf.stny.rr.com>
On Sat, Jun 27, 2009 at 04:13:04PM -0400, Steven Rostedt wrote:
>
> A change like this requires an ACK from Sam Ravnborg.
>
> -- Steve
Overdue review...
I do not know the gcc option so I cannot comment on theeffect of it.
>
> >
> > diff --git a/Makefile b/Makefile
> > index 2903e13..f73b139 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -524,6 +524,10 @@ else
> > KBUILD_CFLAGS += -O2
> > endif
> >
> > +ifdef CONFIG_DISABLE_CROSSJUMP
> > +KBUILD_CFLAGS += -fno-crossjumping
> > +endif
> > +
> > include $(srctree)/arch/$(SRCARCH)/Makefile
Why do we add this option _before_ we include the arch specific Makefile?
If we do not want the arch specific Makefile to undo this then move it lower.
Also please add a comment what the effect of the disabling is.
There is no reason I should fire up info gcc to find out
what -fno-crossjumping is about.
> >
> > ifneq ($(CONFIG_FRAME_WARN),0)
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 8da7467..f88e6b8 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -673,6 +673,13 @@ config FRAME_POINTER
> > larger and slower, but it gives very useful debugging information
> > in case of kernel bugs. (precise oopses/stacktraces/warnings)
> >
> > +config DISABLE_CROSSJUMP
> > + bool "Disable cross-function jump optimization"
> > + help
> > + This build option disables cross-function jump optimization
> > + (crossjumping) of gcc. Disabling crossjumping might increase
> > + kernel binary size a little.
So the above comment tells me that is I disable crossjumping I will
increase the kernel image a little.
That is by no means a good explanation.
Please explain the potential advantage of disabling crossjumping.
Sam