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

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Date:	Wed, 1 Dec 2010 21:42:18 +0800
From:	Hui Zhu <teawater@...il.com>
To:	Petr Hluzín <petr.hluzin@...il.com>
Cc:	Arnaud Lacombe <lacombar@...il.com>, linux-kernel@...r.kernel.org,
	gdb@...rceware.org, hellogcc@...elists.org
Subject: Re: [PATCH] Built kernel without -O2 option

On Wed, Dec 1, 2010 at 06:40, Petr Hluzín <petr.hluzin@...il.com> wrote:
> On 29 November 2010 22:07, Arnaud Lacombe <lacombar@...il.com> wrote:
>> Hi,
>>
>> On Sun, Nov 28, 2010 at 10:56 PM, Hui Zhu <teawater@...il.com> 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
>>> $3 = <value optimized out>
>>> And the code execution order is not right.
>>>
>>> This is becuase the Kernel is bult with gcc -O2.  Gcc will not
>>> generate enough debug message with file with -O2.
>>> So GDB cannot work very well with Linux kernel.
>>>
>>> So I make a patch that add a option in "Kernel hacking" called "Close
>>> GCC optimization".  It will make kernel be built without -O2.
>>>
>> no, it does not, see below ..
>>
>>> I built and use it in i386 and x86_64.  I will try to make it OK in other arch.
>>>
>>> And I will put new patch in here and
>>> http://code.google.com/p/kgtp/downloads/list
>>>
>>> Thanks,
>>> Hui
>>>
>>> Signed-off-by: Hui Zhu <teawater@...il.com>
>>> ---
>>> [...]
>>> +ifdef CONFIG_CC_CLOSE_OPTIMIZATION
>>> +CFLAGS_process_$(BITS).o       += -O2
>>> +CFLAGS_entry_$(BITS).o         += -O2
>>> +CFLAGS_traps.o                 += -O2
>>> +CFLAGS_i387.o                  += -O2
>>> +CFLAGS_xsave.o                 += -O2
>>> +CFLAGS_hpet.o                  += -O2
>>> +endif
>>>
>>> [...]
>>>
>>> --- a/lib/Kconfig.debug
>>> +++ b/lib/Kconfig.debug
>>> @@ -136,6 +136,14 @@ config DEBUG_SECTION_MISMATCH
>>>          - Enable verbose reporting from modpost to help solving
>>>            the section mismatches reported.
>>>
>>> +config CC_CLOSE_OPTIMIZATION
>>> +       bool "Close GCC optimization"
>>> +       default n
>>> +       help
>>> +         Enabling this option will let gcc build kernel without "-O2".
>>> +
>>> +         If unsure, say N.
>>> +
>> You are not consistent with yourself, you add an option saying "do not
>> build the kernel with -O2" and yet, you add "-O2" flags for unknown
>> reason all over the tree...
>
> The patch removes -O2 compilation flag from all files (see the
> fragment below) and adds the flag to the few files that actually need
> the flag. Unfortunately the patch does not explain that and the noise
> of adding the flag obscures the main thing.
>
>>> +++ b/Makefile
>>> @@ -540,8 +540,10 @@ all: vmlinux
>>>  ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
>>>  KBUILD_CFLAGS  += -Os
>>>  else
>>> +ifndef CONFIG_CC_CLOSE_OPTIMIZATION
>>>  KBUILD_CFLAGS  += -O2
>>>  endif
>>> +endif
>
> Hui, The name of the config option is close to meaningless. I suggest:
>
> - config CONFIG_CC_CLOSE_OPTIMIZATION
> -       bool "Close GCC optimization"
> + config CONFIG_CC_NO_OPTIMIZATION
> +       bool "Compile with almost no optimization"
>        default n
>        help
> -          Enabling this option will let gcc build kernel without "-O2".
> +          Enabling this option will let gcc build kernel with no
> optimization (where possible).
> +           This makes debugging friendlier.
>
>
> --
> Petr Hluzin
>

Thanks Petr.

I will fix them add try add some comments.

Thanks,
Hui
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