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

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Date:	Mon, 29 Nov 2010 16:55:29 +0800
From:	Steven <mqyoung@...il.com>
To:	Hui Zhu <teawater@...il.com>
Cc:	hellogcc@...elists.org,
	Américo Wang <xiyou.wangcong@...il.com>,
	linux-kernel@...r.kernel.org, gdb@...rceware.org
Subject: Re: [hellogcc] Re: [PATCH] Built kernel without -O2 option

I replace -O2 with -O0 in Makefile of kernel source code root directory?
I just do it and my gdb works well with this method.

You mean it is not enough?

Steven

On Mon, 2010-11-29 at 16:47 +0800, Hui Zhu wrote:
> In this way, you will got a lot of error.
> 
> Hui
> 
> On Mon, Nov 29, 2010 at 16:41, 乔崇 <qiaochong@...ngson.cn> wrote:
> > does not need change kernel,just write a shell mycc as bellow:
> >
> > #!/bin/sh
> > gcc "$@" -O0
> >
> > then
> >
> > make CC=./mycc vmlinux
> >
> > Hui Zhu said
> >
> > On Mon, Nov 29, 2010 at 16:16, Américo Wang <xiyou.wangcong@...il.com>
> > wrote:
> >
> >
> > On Mon, Nov 29, 2010 at 11:56:15AM +0800, Hui Zhu wrote:
> >
> >
> > Hi,
> >
> > 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.
> >
> > This is becuase the Kernel is bult with gcc -O2.  Gcc will not
> > generate enough debug message with file with -O2.
> > So GDB cannot work very well with Linux kernel.
> >
> > So I make a patch that add a option in "Kernel hacking" called "Close
> > GCC optimization".  It will make kernel be built without -O2.
> >
> > I built and use it in i386 and x86_64.  I will try to make it OK in other
> > arch.
> >
> >
> >
> > The problem is that some functions _have to_ be inlined and gcc without -O2
> > doesn't inline them. Have check all the cases? I doubt.
> >
> >
> > If they really need O2, I set  them to O2.
> > Actually, this is the main work, find out the file that need the O2.  :)
> >
> > For example:
> > ifdef CONFIG_CC_CLOSE_OPTIMIZATION
> > CFLAGS_fpu.o                           += -O2
> > CFLAGS_aesni-intel_glue.o              += -O2
> > CFLAGS_ghash-clmulni-intel_glue.o      += -O2
> > endif
> >
> > And I will try to find more of these type files.
> >
> >
> >
> > Also, what is size of vmlinux before applying your patch and after that?
> > Does it increase too much?
> >
> >
> >
> > Before the patch:
> > ls -alh vmlinuz-2.6.37-rc3+
> > -rw-r--r-- 1 root root 4.1M 2010-11-25 12:02 vmlinuz-2.6.37-rc3+
> > ls -alh b26no/vmlinux
> > -rwxr-xr-x 1 teawater teawater 135M 2010-11-25 13:31 b26no/vmlinux
> >
> > After the patch:
> > ls -alh vmlinuz-2.6.37-rc3debug+
> > -rw-r--r-- 1 root root 4.6M 2010-11-25 14:02 vmlinuz-2.6.37-rc3debug+
> > ls -alh b26/vmlinux
> > -rwxr-xr-x 1 teawater teawater 140M 2010-11-25 11:14 b26/vmlinux
> >
> > Thanks,
> > Hui
> >
> >
> >
> >
> > --
> >
> > 乔崇 qiaochong@...ngson.cn
> >
> > 2010年 11月 29日 星期一 16:35:38 CST

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