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From: Hui Zhu <teawater@gmail.com>
Date: Wed, 1 Dec 2010 21:10:51 +0800
Message-ID: <AANLkTinVH0X_wmHgQSeYT-JMVWAjO3CHvy=jh1817swj@mail.gmail.com>
Subject: Re: [PATCH] Built kernel without -O2 option
To: Arnaud Lacombe <lacombar@gmail.com>
Cc: linux-kernel@vger.kernel.org, gdb@sourceware.org, hellogcc@freelists.org
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On Tue, Nov 30, 2010 at 05:07, Arnaud Lacombe <lacombar@gmail.com> wrote:
> Hi,
>
> On Sun, Nov 28, 2010 at 10:56 PM, Hui Zhu <teawater@gmail.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
>> $3 = <value optimized out>
>> And the code execution order is not right.
>>
>> This is becuase the Kernel is bult with gcc -O2. ?Gcc will not
>> generate enough debug message with file with -O2.
>> So GDB cannot work very well with Linux kernel.
>>
>> So I make a patch that add a option in "Kernel hacking" called "Close
>> GCC optimization". ?It will make kernel be built without -O2.
>>
> no, it does not, see below ..
>
>> I built and use it in i386 and x86_64. ?I will try to make it OK in other arch.
>>
>> And I will put new patch in here and
>> http://code.google.com/p/kgtp/downloads/list
>>
>> Thanks,
>> Hui
>>
>> Signed-off-by: Hui Zhu <teawater@gmail.com>
>> ---
>> [...]
>> +ifdef CONFIG_CC_CLOSE_OPTIMIZATION
>> +CFLAGS_process_$(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...
>
> ?- Arnaud
>

You are really good at English.  But not mathematics.

Add -O2 to build 30 files is better than add -O2 to all files(who know
how much files in Linux Kernel?).

Thanks,
Hui
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