[llvm-dev] ORC C Interface & JITEventListeners

[Lang Hames via llvm-dev]

Hi Andres,

I started out using MCJIT but it looks like that's slowly on the way
> out.  My current concern is that neither the ORC MCJIT replacement, nor
> the ORC C bindings appear to provide integration into JIT event
> listeners.  Which I find a bit surprising - without gdb and/or profiler
> integration, how is one supposed to actually use a JIT successfully?


ORC is a set of components to build a JIT, rather than a black-box JIT like
LLVM. The aim is to give clients more visibility into the workings of the
JIT by hooking into compositions points (e.g. using a TransformLayer
between other JIT layers) or by writing custom components, rather than
being limited to the fixed callbacks provided by JITEventListener. Exactly
how this will fit into the C API has not been explored yet.

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.

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 also a bit confused about RTDyldObjectLinkingLayer's notify
> integration.  NotifyLoadedFtor is a class template parameter, but
> NotifyFinalizedFtor is just a std::function defaulting to empty?  Is
> that just for historical raisins, or is there a deeper reason?


Historical raisins. I'm working on a substantial ORC refactor that should
be landing soon and this will make NotifyLoaded a std::function.

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.

- Lang.


On Wed, Mar 8, 2017 at 11:47 AM, Andres Freund <andres at anarazel.de> wrote:

> Hi,
>
> I am working on using LLVM to compile parts of longrunning PostgreSQL
> queries into native code for faster code execution.  As postgres is,
> nearly, entirely written in C and has long-lived (5 years) supported
> branches (making the higher API stability important), I'm currently
> using the C API.
>
> I started out using MCJIT but it looks like that's slowly on the way
> out.  My current concern is that neither the ORC MCJIT replacement, nor
> the ORC C bindings appear to provide integration into JIT event
> listeners.  Which I find a bit surprising - without gdb and/or profiler
> integration, how is one supposed to actually use a JIT successfully?
>
> 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.
>
> In addition, it doesn't currently look there's C API to force the mcjit
> replacement being used?
>
> 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...
>
>
> I'm also a bit confused about RTDyldObjectLinkingLayer's notify
> integration.  NotifyLoadedFtor is a class template parameter, but
> NotifyFinalizedFtor is just a std::function defaulting to empty?  Is
> that just for historical raisins, or is there a deeper reason?
>
>
> 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.
>
> Greetings,
>
> Andres Freund
>
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