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    SubjectRe: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out
    On Thu, Oct 06, 2011 at 11:29:25AM -0700, H. Peter Anvin wrote:
    > On 10/06/2011 11:26 AM, Steven Rostedt wrote:
    > > On Thu, 2011-10-06 at 14:10 -0400, Jason Baron wrote:
    > >
    > >>> Looks like jmp2 is about 5% faster than jmp5 on Sandybridge with this
    > >>> benchmark.
    > >>>
    > >>> But insignificant difference on Nehalem.
    > >>>
    > >>> J
    > >>
    > >> It would be cool if we could make the total width 2-bytes, when
    > >> possible. It might be possible by making the initial 'JUMP_LABEL_INITIAL_NOP'
    > >> as a 'jmp' to the 'l_yes' label. And then patching that with a no-op at boot
    > >> time or link time - letting the compiler pick the width. In that way we could
    > >> get the optimal width...
    > >
    > > Why not just do it?
    > >
    > > jump_label is encapsulated in arch_static_branch() which on x86 looks
    > > like:
    > >
    > > static __always_inline bool arch_static_branch(struct jump_label_key *key)
    > > {
    > > asm goto("1:"
    > > JUMP_LABEL_INITIAL_NOP
    > > ".pushsection __jump_table, \"aw\" \n\t"
    > > _ASM_ALIGN "\n\t"
    > > _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
    > > ".popsection \n\t"
    > > : : "i" (key) : : l_yes);
    > > return false;
    > > l_yes:
    > > return true;
    > > }
    > >
    > >
    > > That jmp to l_yes should easily be a two byte jump.

    remember the compiler is moving the l_yes out of line, so its not
    necessarily always a two byte jump. Also, I plan to look at a possible
    'cold' label for the 'l_yes' branch, so that it can moved to a separate
    'cold' section, but we might only want that for some cases...

    > >
    > > If not I'm sure it would be easy to catch it before modifying the code.
    > > And then complain real loudly about it.
    > >
    >
    > The important thing is that it requires the build-time elimination of
    > jumps. It's just work.
    >
    > -hpa
    >

    Right, its certainly doable, but I'm not sure its so simple, since we'll
    need a pass to eliminate the jumps - which can be keyed off the
    '__jump_table' section.

    thanks,

    -Jason


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