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

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Message-ID: <4A60E972.1060301@redhat.com>
Date:	Fri, 17 Jul 2009 17:13:22 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>, 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

Sam Ravnborg wrote:
> 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.

Thanks for comment. however, as far as I can see, I couldn't find
in the difference between binary with -fno-crossjump and without it.
So I decided to drop this patch...

> 
>>> 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.

Sure, I'll write a comment on, next time.

> 
> 
>>>  
>>>  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.

Agreed.

Thank you,
-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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