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

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Message-ID: <AANLkTinnJjqmXmbpAYmV9Tsc0VJV=WDM+GY6Gi1cJFCB@mail.gmail.com>
Date:	Mon, 29 Nov 2010 16:24:42 +0800
From:	Hui Zhu <teawater@...il.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
Cc:	linux-kernel@...r.kernel.org, gdb@...rceware.org,
	hellogcc@...elists.org
Subject: Re: [PATCH] Built kernel without -O2 option

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
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