[llvm-dev] New ORC v2 LLJIT - global variables

Tue Aug 13 14:49:01 PDT 2019

Hi Chris,

Thanks for clarifying that. That makes sense.
>

So I apologize. It was not an error I got on the AddLazyIRModule. The issue
> is actually after I call AddLazyIRModule(). Performing a lookup on that
> global variable causes a hang. I've stepped through the debugger, and it
> seems to hang inside Core.cpp, ExecutionSession::lookup().

Are you (a) trying to JIT concurrently and (b) loading all modules on the
same ThreadSafeContext? Notionally you should be able to do both, but in my
experience bugs in this area are the most frequent source of deadlocks in
lookup.

Are you able to get a backtrace for each thread in the debugger while the
lookup is deadlocked? If you're using LLDB you can use "thread list" to
list the threads, "thread select N" to switch to each, and "bt" to get the
backtrace. I am not sure what the equivalent commands in GDB are. Either
way, the lookup thread is the least interesting one: I expect to see some
other thread waiting on a context lock, and the thing we *really* want to
know is who owns the lock and how did they end up blocking while holding it.

Are you able to share any of your code / test cases?

Cheers,
Lang.

Prior to r367686, ORCv2 took out a lock on the context when a module was
emitted and did not relinquish it until that module had been fully emitted.
The problem with that is that if the module being emitted has dependencies
on other modules living on the same

On Tue, Aug 13, 2019 at 2:09 PM chris boese <chris107565 at gmail.com> wrote:

> Hi Lang,
>
> Thanks for clarifying that. That makes sense.
>
> So I apologize. It was not an error I got on the AddLazyIRModule. The
> issue is actually after I call AddLazyIRModule(). Performing a lookup on
> that global variable causes a hang. I've stepped through the debugger, and
> it seems to hang inside Core.cpp, ExecutionSession::lookup().
>
> Thanks,
> Chris
>
> On Mon, Aug 12, 2019 at 2:33 PM Lang Hames <lhames at gmail.com> wrote:
>
>> Hi Chris,
>>
>>
>>> Thanks so much for the quick help. I was able to find them after
>>> changing the linkage. Does this new orc JIT follow different standards than
>>> the MCJitReplacement? We used to set a lot of our functions/globals to
>>> internal linkage before doing a lookup.
>>
>>
>> They are different: ORCv2 follows the static linker's lookup rules,
>> whereas MCJITReplacement mimicked MCJIT's rules which were never clearly
>> spelled out, but in practice allowed you to search for internal symbols.
>>
>> The advantage of the new scheme is that if you are compiling/running
>> something on the command line. E.g.:
>>
>> $ llc -filetype=obj -o extra.o extra.ll
>> $ llc -filetype=obj -o prog.o prog.ll
>> $ clang -o prog prog.o extra.o
>> $ ./prog
>>
>> Then you can add the same modules to the JIT and expect the resulting
>> JIT'd code to behave the same as the statically compiled code. E.g.
>> (omitting some error handling for clarity):
>>
>> J->addIRModule(loadModule("extra.ll"));
>> J->addIRModule(loadModule("main.ll"));
>> auto Main = (int(*)())J->lookup("main").getAddress();
>> Main();
>>
>> If you are generating code specifically for the JIT it is often easiest
>> to just use external linkage and default visibility for all symbols.
>> However, for people who want to hook up an existing front-end, or to use
>> more advanced features in their JIT (duplicate symbol names with internal
>> linkage, symbols with weak linkage, hidden visibility, etc.) this scheme
>> has the nice property that "What does linker-feature X do in the JIT?" is
>> more-or-less equivalent to "What does linker-feature X do in a regular
>> build". The answer to the latter question is generally more widely known,
>> and is much easier to test from the command line.
>>
>> When adding a new module with a single global variable in it, I get an
>>> error during the addLazyIRModule.
>>
>>
>> Are you hitting an assertion, or is addLazyIRModule returning a
>> non-success value? If the latter, what is the error message?
>> (You can find this by calling 'logAllUnhandledErrors(std::move(Err),
>> errs(), "")', or, in tool code, using the ExitOnError utility:
>> http://llvm.org/docs/ProgrammersManual.html#using-exitonerror-to-simplify-tool-code
>> ).
>>
>> Right now we are treating the module as one big single module, and when
>>> we go to lookup a symbol, I believe we are jitting the entire thing. Once
>>> we make the switch to LLLazyJIT, we want to start splitting up our modules.
>>
>>
>> If you are using MCJITReplacement you are definitely JITing the whole
>> module up-front, and in that case lazy compilation may help reduce your
>> compile time. I'd love to hear how it works for you!
>>
>> Cheers,
>> Lang.
>>
>> On Mon, Aug 12, 2019 at 10:26 AM David Blaikie <dblaikie at gmail.com>
>> wrote:
>>
>>> +Lang Hames <lhames at gmail.com>, Admiral of the Orcish Fleet (Lang's the
>>> primary developer/creator of the ORC JIT & might have some answers to your
>>> question)
>>>
>>> On Thu, Aug 8, 2019 at 4:15 PM chris boese via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> We are trying to switch to the new orc v2 lljit engine at my work with
>>>> hopes of parallel jitting. We are switching from the ExecutionEngine w/
>>>> OrcMCJitReplacement. We are having a hard time with global variables. We
>>>> have found a way to create/emit new globals during jitting by using the old
>>>> ExecutionEngine::getOrEmitGlobalVariable. Is there an easier way to do this
>>>> with the new jit engine? We were hoping to create a new ThreadSafeModule,
>>>> add a GlobalVariable, and then add the module to the JIT engine through
>>>> addLazyIRModule(), but this doesn't seem to work.
>>>>
>>>> 2nd question; Is the lookup function supposed to work with global
>>>> variables by name? Looking through the CompileOnDemandLayer it seems to
>>>> change the names of globals to "__orc_lcl.<name><id>". Does this affect the
>>>> lookup methods? I noticed that all of the globals we are looking for are
>>>> tagged with Internal linkage and end up getting emitted through
>>>> extractSubModule().
>>>>
>>>> Any help is appreciated.
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
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