[llvm-dev] [ORC JIT] -Resolving cross references in a multi-process scenario

Thu Mar 19 02:44:14 PDT 2020

Hey Lang,

> Thanks for your patience.
I'm happy you consider me patient x3

> Out of interest -- Is it necessary for your JIT itself to be split across two or more processes?
In general I agree with  your approach, but sadly - the current software design does not allow this yet. So I try understanding how to tackle both solutions - one process managing all the modules vs multiple processes manging just there modules. I would prefer just having one process doing it, because it makes way more sense.

However... About your solution. This looks really promising – Thank you! - but sadly, I'm not experienced enough with ORC yet to understand how to use it.

I create a llvm::orc::LLJIT and then add my llvm::Module to it.
Can I use the getExecutionSession function of LLJIT to get access to this special lookup function after I added the module?

Also I have some difficulties with providing the parameters .w. For example... I don't have a "JITDylibSearchList" nor "SymbolNameSet" (I probably have it hidden somewhere). I do know those from when I do the symbol resolving. I didn't created a JITDylib for it, instead I attached a generator function to the MainJITDylib – because I have to lookup symbols from the shared memory and that list changes.

So I'm not sure how to use it yet - sorry for the noobie question >o<

Kind greetings
Björn

From: Lang Hames <lhames at gmail.com>
Sent: 18 March 2020 17:26
To: Gaier, Bjoern <Bjoern.Gaier at horiba.com>
Cc: LLVM Developers Mailing List <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [ORC JIT] -Resolving cross references in a multi-process scenario

Hi Bjoern,

Thanks for your patience. The good news is that there is a neater way to do this: ExecutionSession's lookup methods take a orc::SymbolState parameter which describes the state that symbols must reach before a query can return (See https://github.com/llvm/llvm-project/blob/d1a7bfca74365c6523e647fcedd84f8fa1972dd3/llvm/include/llvm/ExecutionEngine/Orc/Core.h#L1273).In your case you want to use orc::SymbolState::Resolved, which will cause the query to return as soon as the searched-for symbols have been assigned an address.

So if your process is generating a set of symbols that may need to be transmitted to other sessions then you would write something like:

auto Syms = ES.lookup(SearchOrder, SymbolsToTransmit, LookupKind::Static, SymbolState::Resolved);
if (!Syms)
  reportError(Syms.takeError());
sendSymbolsToRemote(*Syms);

Out of interest -- Is it necessary for your JIT itself to be split across two or more processes? I had always anticipated having a single ExecutionSession attached to the target process and then running just the compilers on other processes. That gives you a star-like IPC network with N + 1 connections: One between the execution session and each of the N compilers, and one between the execution session and the target processs. In your model it sounds like the risk is that you may end up with N(N + 1) IPC links with every execution session having to communicate with every other and also with the target process.

-- Lang.

On Tue, Mar 17, 2020 at 12:34 AM Gaier, Bjoern <Bjoern.Gaier at horiba.com<mailto:Bjoern.Gaier at horiba.com>> wrote:
Hello LLVM-Mailing-List and Lang,

I played around with this problem a bit and found a way of doing this – however I’m not sure about the consequences and if this is really a good idea.

I went trough all of the global symbols and functions being defined in the llvm::Module and set for each of them a new section name (based on there name). With this approach the memorymanager could tell me already the addresses of my symbols, which I could then share with the other process. However, this approach needed to load the module two times, first to get the future address and in a second run to use the addresses to resolve the symbols.

The only disadvantage I noticed so far was that the overall size of the emitted code was bigger then having the regular sections. Are there any other risks I might have overseen?

Kind greetings
Björn

From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> On Behalf Of Gaier, Bjoern via llvm-dev
Sent: 06 March 2020 14:24
To: LLVM Developers Mailing List <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; Lang Hames <lhames at gmail.com<mailto:lhames at gmail.com>>
Subject: [llvm-dev] [ORC JIT] -Resolving cross references in a multi-process scenario

Hello LLVM-Mailing-List and Lang,

I have a very weird (or strict?) scenario and was curious if the ORC  JIT can help me with it. Soo here it comes:

I have currently a windows process ("Runtime") which does nothing but creating a shared memory with read, write and execute rights - and also writing some function addresses like from printf to it.
Then I have two or more processes which are using the ORC JIT to load modules to this shared memory, in "perspective" of the process I mentioned above ("Runtime"). The sections are remapped from the other processes to be correct for "Runtime", also they resolve the undefined references for "Runtime" - like for printf and so on.
This works quite well so far! Wuhuhu x3

However, there is one issue about it. Given that those two (or more processes) are now loading modules that reference each other. Like Module A is using a function of Module B - but Module B also uses a function of Module A. How could I resolve those modules when they are loaded from different processes? Normally I would use llvm-link to link those modules but in my current scenario this is sadly not possible.

Can the ORC JIT help me with that? Can I solve this problem differently? Like replacing all the functions of Module A that rely on Module B with function pointers? Is something like that possible?

I know this situation is pretty uncommon but sadly I ran into that issue.

Kind greetings
Björn

Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816, USt.ID-Nr. DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert Plank, Markus Bode, Heiko Lampert, Takashi Nagano, Takeshi Fukushima. Junichi Tajika
Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816, USt.ID-Nr. DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert Plank, Markus Bode, Heiko Lampert, Takashi Nagano, Takeshi Fukushima. Junichi Tajika
Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816, USt.ID-Nr. DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert Plank, Markus Bode, Heiko Lampert, Takashi Nagano, Takeshi Fukushima. Junichi Tajika
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