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

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Message-ID: <AANLkTinVH0X_wmHgQSeYT-JMVWAjO3CHvy=jh1817swj@mail.gmail.com>
Date:	Wed, 1 Dec 2010 21:10:51 +0800
From:	Hui Zhu <teawater@...il.com>
To:	Arnaud Lacombe <lacombar@...il.com>
Cc:	linux-kernel@...r.kernel.org, gdb@...rceware.org,
	hellogcc@...elists.org
Subject: Re: [PATCH] Built kernel without -O2 option

On Tue, Nov 30, 2010 at 05:07, 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...
>
>  - 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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