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Date: Mon, 29 Nov 2010 16:07:20 -0500
Message-ID: <AANLkTinpX0p8hv_RJojASbjDNYQ60bODo3kLBobRDtmf@mail.gmail.com>
Subject: Re: [PATCH] Built kernel without -O2 option
From: Arnaud Lacombe <lacombar@gmail.com>
To: Hui Zhu <teawater@gmail.com>
Cc: linux-kernel@vger.kernel.org, gdb@sourceware.org, hellogcc@freelists.org
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Hi,

On Sun, Nov 28, 2010 at 10:56 PM, Hui Zhu <teawater@gmail.com> 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.
>
no, it does not, see below ..

> I built and use it in i386 and x86_64. ?I will try to make it OK in other arch.
>
> And I will put new patch in here and
> http://code.google.com/p/kgtp/downloads/list
>
> Thanks,
> Hui
>
> Signed-off-by: Hui Zhu <teawater@gmail.com>
> ---
> [...]
> +ifdef CONFIG_CC_CLOSE_OPTIMIZATION
> +CFLAGS_process_$(BITS).o ? ? ? += -O2
> +CFLAGS_entry_$(BITS).o ? ? ? ? += -O2
> +CFLAGS_traps.o ? ? ? ? ? ? ? ? += -O2
> +CFLAGS_i387.o ? ? ? ? ? ? ? ? ?+= -O2
> +CFLAGS_xsave.o ? ? ? ? ? ? ? ? += -O2
> +CFLAGS_hpet.o ? ? ? ? ? ? ? ? ?+= -O2
> +endif
>
> [...]
>
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -136,6 +136,14 @@ config DEBUG_SECTION_MISMATCH
> ? ? ? ? ?- Enable verbose reporting from modpost to help solving
> ? ? ? ? ? ?the section mismatches reported.
>
> +config CC_CLOSE_OPTIMIZATION
> + ? ? ? bool "Close GCC optimization"
> + ? ? ? default n
> + ? ? ? help
> + ? ? ? ? Enabling this option will let gcc build kernel without "-O2".
> +
> + ? ? ? ? If unsure, say N.
> +
You are not consistent with yourself, you add an option saying "do not
build the kernel with -O2" and yet, you add "-O2" flags for unknown
reason all over the tree...

 - Arnaud
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