[llvm-dev] ORC C Interface & JITEventListeners

[Lang Hames via llvm-dev]

Hi Andres,

Unless JITed objects are actually written out as proper shared objects I
> don't see how profilers would understand this?


Yes - The objects would need to be written out. The key point is to try to
keep the semantics and formats as close as possible to the statically
compiled equivalent.


> There might also be functions getting mapped to points where another
> function might have
> been mapped to previously.


This will depend on the nature of the specific JIT being constructed. For
JITs that do re-compilation of functions (rather than just deferring the
initial compilation until first call) we will need a mechanism to
communicate what has happened to the tools.


>    I suspect this'll continue to need something like the JITEventListener
> thing.


When I say that we don't want a JITEventListener I mean that we don't want
the JITEventListener interface itself (or any fixed set of callbacks).
There's no dispute that we need general visibility into the JIT though.
Decomposing the JIT into components is meant to improve that.

 I'm quite concerned about the API stability around all of this. Postgres
> releases yearly, and supports 5 years of release branches (so there's 5
> release branches most of the time).  Having to constantly adapt to
> changing LLVM APIs in each of those release branches and the dev branch,
> is going to make using LLVM pretty painful.  If working on the C API is
> the best way to address that concern, I'm willing to do some of that.


That's a fair concern. LLVM policy is that we don't guarantee stability
in either the C or C++ APIs. However we don't make change for changes sake
(especially not in the C APIs), so once we have a decent ORC C API I'd
expect the churn to be infrequent and hopefully minimally disruptive.

Getting *to* a decent ORC C API is another question. Nobody has invested
much effort in that yet. Once someone does I'd expect plenty of churn until
a decent, tested design emerges.

Cheers,
Lang.


On Mon, Apr 3, 2017 at 3:59 PM, Andres Freund <andres at anarazel.de> wrote:

> Hi,
>
>
> On 2017-04-03 15:48:38 -0700, Lang Hames wrote:
> > Regarding debugging and profiling specifically: the long-term goal is to
> > integrate the JIT with the dynamic loader so that JIT'd functions appear
> to
> > the system the same way they would have if they had been dlopen'd
> > libraries. This should allow existing profilers and debuggers to be
> > used with JIT'd code.
>
> I doubt that that'll be sufficient for profilers at least - frequently
> profiling results will be analyzed when the program has been shut down.
> Unless JITed objects are actually written out as proper shared objects I
> don't see how profilers would understand this? There might also be
> functions getting mapped to points where another function might have
> been mapped to previously.   I suspect this'll continue to need
> something like the JITEventListener thing.
>
>
> > > It'd not be too bad if I had to use a small bit of, optional, code to
> > > register a JIT event listener, but otherwise use the C API (that's what
> > > I currently do for perf support in MCJIT), but it doesn't look like
> > > that's an option with the ORC C bindings (RTDyldObjectLinkingLayer's
> > > integration is a class template parameter defaulting to
> > > DoNothingOnNotifyLoaded), and it's not used by OrcCBindingsStack.
>
> > The C-bindings haven't received much interest yet, so they're only
> > being improved slowly. Adding some callbacks should be easy though.
>
> Cool.
>
>
> > > Is there interest in addressing these issues, or is the position more
> > > generally that the C bindings aren't going to be useful enough?  I'm
> > > willing to work on that, but only if there's actual interest in
> > > integrating things...
> >
> >
> > There's interest, and I'd love to have some help with it. :)
>
> Ok.
>
>
> I'm quite concerned about the API stability around all of this. Postgres
> releases yearly, and supports 5 years of release branches (so there's 5
> release branches most of the time).  Having to constantly adapt to
> changing LLVM APIs in each of those release branches and the dev branch,
> is going to make using LLVM pretty painful.  If working on the C API is
> the best way to address that concern, I'm willing to do some of that.
>
>
> > >To be able to use existing JITEventListeners - it'd surely be a shame to
> > > have to rewrite them anew - in custom stacks it also appears that
> > > there's no easy way to call JITEventListener->NotifyFreeingObject() -
> > > the to-be-freed objects aren't readily available in
> > > RTDyldObjectLinkingLayer.
>
> > I think it would be easy enough to hook up the existing event listener
> > interface to RTDyldObjectLayer, it's just that nobody has done it yet.
>
> I'll take a stab then.
>
> - Andres
>
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