Andres Freund via llvm-dev
2017-Mar-08 19:47 UTC
[llvm-dev] ORC C Interface & JITEventListeners
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
Lang Hames via llvm-dev
2017-Apr-03 22:48 UTC
[llvm-dev] ORC C Interface & JITEventListeners
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 >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170403/038b6923/attachment.html>
Andres Freund via llvm-dev
2017-Apr-03 22:59 UTC
[llvm-dev] ORC C Interface & JITEventListeners
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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