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

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Message-ID: <4CF37603.5080301@de.ibm.com>
Date:	Mon, 29 Nov 2010 10:44:35 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
CC:	Hui Zhu <teawater@...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

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
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