Re: [PATCH] Built kernel without -O2 option

Re: [PATCH] Built kernel without -O2 option

[Christian Borntraeger]

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

Re: [PATCH] Built kernel without -O2 option

[Segher Boessenkool]

> In essence -O2 just tells gcc to activate a list of optimizations
> gcc  -Q -O2 --help=optimizers
> tells you what.

Not quite; see http://gcc.gnu.org/wiki/FAQ#optimization-options .


Segher

Re: [PATCH] Built kernel without -O2 option

[Christian Borntraeger]

Am 29.11.2010 12:12, schrieb Segher Boessenkool:
>> In essence -O2 just tells gcc to activate a list of optimizations
>> gcc  -Q -O2 --help=optimizers
>> tells you what.
> 
> Not quite; see http://gcc.gnu.org/wiki/FAQ#optimization-options .
> 

Erm...right. Thank you.

Re: [PATCH] Built kernel without -O2 option

[Nicholas Mc Guire]

On Mon, 29 Nov 2010, Christian Borntraeger wrote:

> Am 29.11.2010 12:12, schrieb Segher Boessenkool:
> >> In essence -O2 just tells gcc to activate a list of optimizations
> >> gcc  -Q -O2 --help=optimizers

I think for completness you would need to pass

 gcc -Wextra -Q -O2 --help=optimizers

or you could miss some options

> >> tells you what.
> > 
> > Not quite; see http://gcc.gnu.org/wiki/FAQ#optimization-options .
> > 
> 
> Erm...right. Thank you.

well I guess you always could explicidly turn off all optioins with 
-fnoWHATEVER you want to disable and thus remove all unwanted flags 
- admitedly a bit painfull though.

hofrat

Re: [PATCH] Built kernel without -O2 option

[Hui Zhu]

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