linux-kernel - Re: [PATCH] Built kernel without -O2 option

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Date:	Mon, 29 Nov 2010 11:12:16 +0100
From:	Mark Wielaard <mjw@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Hui Zhu <teawater@...il.com>, linux-kernel@...r.kernel.org,
	gdb@...rceware.org, hellogcc@...elists.org
Subject: Re: [PATCH] Built kernel without -O2 option

On Mon, 2010-11-29 at 10:59 +0100, Andi Kleen wrote:
> Hui Zhu <teawater@...il.com> writes:
> 
> > Now, there are a lot of ways to debug the Linux kernel with GDB, like
> > qemu, kgtp or kgdb and so on.
> > But the developer more like add a printk. It have a lot of reason, a big one is:
> > (gdb) p ret
> > $3 = <value optimized out>
> > And the code execution order is not right.
> 
> Really the right place to fix a lot of this would be gcc (and perhaps
> gdb). I suspect with some careful work the debugging experience
> for -O2 could be improved a lot.
> I also believe the latest gccs already have improvements in this area.

The VTA branch was merged for GCC 4.5
http://gcc.gnu.org/wiki/Var_Tracking_Assignments
Which helps a lot with generating dwarf for previously "optimized out"
values. There were also a couple of papers on generating even better
debuginfo at the recent GCC Summit: http://gcc.gnu.org/wiki/summit2010

Jakub Jelínek, Improving debug info for optimized away parameters
http://gcc.gnu.org/wiki/summit2010?action=AttachFile&do=get&target=jelinek.pdf
Alexandre Oliva, Consistent Views at Recommended Breakpoints
http://gcc.gnu.org/wiki/summit2010?action=AttachFile&do=get&target=oliva.pdf

Cheers,

Mark

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