Re: [RFC] gcc feature request: Moving blocks into sections

Peter Zijlstra
2013-08-12T16:10:11Z

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.