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    SubjectRe: [RFC] gcc feature request: Moving blocks into sections
    On Mon, Aug 12, 2013 at 07:56:10AM -0700, H. Peter Anvin wrote:
    > On 08/12/2013 02:17 AM, Peter Zijlstra wrote:
    > >
    > > I've been wanting to 'abuse' static_key/asm-goto to sort-of JIT
    > > if-forest functions like perf_prepare_sample() and perf_output_sample().
    > >
    > > They are of the form:
    > >
    > > void func(obj, args..)
    > > {
    > > unsigned long f = ...;
    > >
    > > if (f & F1)
    > > do_f1();
    > >
    > > if (f & F2)
    > > do_f2();
    > >
    > > ...
    > >
    > > if (f & FN)
    > > do_fn();
    > > }
    > >
    >
    > Am I reading this right that f can be a combination of any of these?

    Correct.

    > > Where f is constant for the entire lifetime of the particular object.
    > >
    > > So I was thinking of having these functions use static_key/asm-goto;
    > > then write the proper static key values unsafe so as to avoid all
    > > trickery (as these functions would never actually be used) and copy the
    > > end result into object private memory. The object will then use indirect
    > > calls into these functions.
    >
    > I'm really not following what you are proposing here, especially not
    > "copy the end result into object private memory."
    >
    > With asm goto you end up with at minimum a jump or NOP for each of these
    > function entries, whereas an actual JIT can elide that as well.
    >
    > On the majority of architectures, including x86, you cannot simply copy
    > a piece of code elsewhere and have it still work.

    I thought we used -fPIC which would allow just that.

    > You end up doing a
    > bunch of the work that a JIT would do anyway, and would end up with
    > considerably higher complexity and worse results than a true JIT.

    Well, less complexity but worse result, yes. We'd only poke the specific
    static_branch sites with either NOPs or the (relative) jump target for
    each of these branches. Then copy the result.

    > You
    > also say "the object will then use indirect calls into these
    > functions"... you mean the JIT or pseudo-JIT generated functions, or the
    > calls inside them?

    The calls to these pseudo-JIT generated functions.

    > > I suppose the question is, do people strenuously object to creativity
    > > like that and or is there something GCC can do to make this
    > > easier/better still?
    >
    > I think it would be much easier to just write a minimal JIT for this,
    > even though it is per architecture. However, I would really like to
    > understand what the value is.

    Removing a lot of the conditionals from the sample path. Depending on
    the configuration these can be quite expensive.


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