[llvm-dev] ORC C Interface & JITEventListeners

Andres Freund via llvm-dev llvm-dev at lists.llvm.org
Mon Apr 3 15:59:37 PDT 2017


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.


> > 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. :)


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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