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Re: [PATCH] Built kernel without -O2 option
- From: Hui Zhu <teawater at gmail dot com>
- To: Christian Borntraeger <borntraeger at de dot ibm dot com>
- Cc: Américo Wang <xiyou dot wangcong at gmail dot com>, linux-kernel at vger dot kernel dot org, gdb at sourceware dot org, hellogcc at freelists dot org, linux-kbuild at vger dot kernel dot org
- Date: Wed, 1 Dec 2010 20:18:24 +0800
- Subject: Re: [PATCH] Built kernel without -O2 option
- References: <AANLkTimewZ4QhxzL2vOYYV85pegy8Fu6Rb4XUAPqGUXS@mail.gmail.com> <20101129081619.GD5218@cr0.nay.redhat.com> <4CF37603.5080301@de.ibm.com>
On Mon, Nov 29, 2010 at 17:44, Christian Borntraeger
<borntraeger@de.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