linux-kernel - Re: [RFC][ PATCH -tip v2 3/7] Kbuild: disable gcc crossjumping

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Message-ID: <20090717202825.GA6872@merkur.ravnborg.org>
Date:	Fri, 17 Jul 2009 22:28:25 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Jim Keniston <jkenisto@...ibm.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Anders Kaseorg <andersk@...lice.com>,
	Tim Abbott <tabbott@...lice.com>
Subject: Re: [RFC][ PATCH -tip v2 3/7] Kbuild: disable gcc crossjumping

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
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