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    SubjectRe: [PATCH 2/2] jump label: disable due to compiler bug
    * Steven Rostedt (rostedt@goodmis.org) wrote:
    > On Fri, 2010-10-29 at 14:22 +0200, Ingo Molnar wrote:
    > > * Steven Rostedt <rostedt@goodmis.org> wrote:
    > >
    > > > On Fri, 2010-10-29 at 08:34 +0200, Ingo Molnar wrote:
    >
    > > > Do you mean to have it dependent on !EMBEDDED, or perhaps !CC_OPTIMIZE_FOR_SIZE ?
    > >
    > > CC_OPTIMIZE_FOR_SIZE dependency probably not. Might even leave out !EMBEDDED, to
    > > make it generally configurable.
    > >
    > > Also, we should make it default off - we need to see whether there are any compiler
    > > bugs lurking.
    >
    > Ah, so we should then add a:
    >
    > config JUMP_LABEL
    > prompt "Optimize trace point call sites"
    > depends on HAVE_ARCH_JUMP_LABEL
    > help
    > If it is detected that the compiler has support for
    > "asm goto", the kernel will compile trace points locations
    > with just a nop instruction. When trace points are enabled
    > the nop will be converted to a jump to the trace function.
    > This technique lowers overhead and stress on the branch
    > prediction of the processor.
    >
    > On i386, the options added to make this work may increase
    > the size of the kernel slightly.
    >
    >
    > Then in the code have:
    >
    > -#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_HAVE_ARCH_JUMP_LABEL)
    > +#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_HAVE_ARCH_JUMP_LABEL) \

    Technically, you could remove the dependency on
    "defined(CONFIG_HAVE_ARCH_JUMP_LABEL)", because it is now dealt with at
    the Kconfig level (CONFIG_JUMP_LABEL depends on it).

    > + && defined(JUMP_LABEL)

    this would be defined(CONFIG_JUMP_LABEL)

    The rest looks fine,

    Thanks,

    Mathieu

    > # include <asm/jump_label.h>
    > # define HAVE_JUMP_LABEL
    > #endif
    >
    >
    > If you agree, I can write up a patch.
    >
    > -- Steve
    >
    > >
    > > > I can see people making a big stink about adding back a gcc option that makes the
    > > > kernel bigger.
    > >
    > > Depends on exactly how much .text we are talking about here. Plus it would be
    > > default-off.
    > >
    > > Thanks,
    > >
    > > Ingo
    >
    >

    --
    Mathieu Desnoyers
    Operating System Efficiency R&D Consultant
    EfficiOS Inc.
    http://www.efficios.com


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