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

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Message-ID: <20101129085250.GF5218@cr0.nay.redhat.com>
Date:	Mon, 29 Nov 2010 16:52:50 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Hui Zhu <teawater@...il.com>
Cc:	Américo Wang <xiyou.wangcong@...il.com>,
	linux-kernel@...r.kernel.org, gdb@...rceware.org,
	hellogcc@...elists.org
Subject: Re: [PATCH] Built kernel without -O2 option

On Mon, Nov 29, 2010 at 04:24:42PM +0800, Hui Zhu wrote:
>On Mon, Nov 29, 2010 at 16:16, Américo Wang <xiyou.wangcong@...il.com> wrote:
>> 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.
>
>If they really need O2, I set  them to O2.
>Actually, this is the main work, find out the file that need the O2.  :)
>
>For example:
>ifdef CONFIG_CC_CLOSE_OPTIMIZATION
>CFLAGS_fpu.o                           += -O2
>CFLAGS_aesni-intel_glue.o              += -O2
>CFLAGS_ghash-clmulni-intel_glue.o      += -O2
>endif

No, I didn't mean this, I meant some function that have to be inlined
are those who only work when being inlined, e.g. current_text_addr().

I think it is not alone. :)

Also, since many inline functions sit in hot-path, are there any
performance regressions with your patch applied?

BTW, you need to Cc kbuild for makefile changes like this.
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