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    SubjectRe: [PATCH] Built kernel without -O2 option
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    On Mon, Nov 29, 2010 at 9:52 AM, Américo Wang <xiyou.wangcong@gmail.com> wrote:
    > 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@gmail.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().

    Can you please explain to me why some functions work only when they are inlined?
    Is it because of asm() black magic?

    > 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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    --
    Thanks,
    //richard
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