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

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Message-ID: <AANLkTikq32EDVfP8SjNJSih2Z5jNafHGRPCDdc+w1GR6@mail.gmail.com>
Date:	Wed, 1 Dec 2010 20:18:24 +0800
From:	Hui Zhu <teawater@...il.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Américo Wang <xiyou.wangcong@...il.com>,
	linux-kernel@...r.kernel.org, gdb@...rceware.org,
	hellogcc@...elists.org, linux-kbuild@...r.kernel.org
Subject: Re: [PATCH] Built kernel without -O2 option

On Mon, Nov 29, 2010 at 17:44, Christian Borntraeger
<borntraeger@...ibm.com> wrote:
> Am 29.11.2010 09:16, schrieb Américo Wang:
>> 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.
>
> In essence -O2 just tells gcc to activate a list of optimizations
> gcc  -Q -O2 --help=optimizers
> tells you what.
>
> So what about making this patch much smaller by explicitely using the optimizations
> that are absolutely necessary?
> e.g:
> -finline-small-functions -finline-functions-called-once
> (what else do we need?)
>
> We might even  be able to collapse this with the optimize for size option,
> by providing a Kconfig entry that allows to choose between
>
> -O0 -finline-small-functions -finline-functions-called-once
> -O1 -finline-small-functions -finline-functions-called-once
> -Os
> -O2
> -O3
>
> Christian
>

Hi Christian.

If we can build without any optimization options just set some file to O2.
I think keep it it will better than add some optimization options to
all files, right?

But make for this file that I add O2 to them, I can change them to to
some others options like "-O0 -finline-small-functions
-finline-functions-called-once".

Thanks,
Hui
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