[llvm-dev] ORC JIT - different behaviour of ExecutionSession.lookup?

Tue Sep 29 00:03:50 PDT 2020

Hey Lang,

Thank you for your help and your patience – also for your answers in the “ORC JIT - Can modules independently managed with one LLJIT instance? + problems with ExecutionSession.lookup” mail. Both problems have the same origin so I keep writing about it here, to avoid duplication.
My big problem is still handling cross references between modules with “our” name scheme. Since our old loader loads object files, we resolved those references with object files and since they were already compiled, we knew all addresses right away. With the LLJIT as I finally understand, I will only get the addresses when I have resolved every references, which makes the code way safer.
However, another thing of our system is, that each object file was loaded from a different process, so sometimes not all symbols for ModuleA were present because ModuleB was not loaded/requested yet. That was okay, so we kept resolving the undefined references of ModuleA until ModuleB was loaded and everything was fine.

If I get it right… This would change now to having a single LLJIT representing the entire system. Each process would get it’s own DyLib for there module. However, I would need to check on IR-Level now which symbols would be undefined – correct? Because if I wait until “DefinitionGenerator::tryToGenerate” is called and have to wait for a module that might never be loaded, then I’m stuck there forever.
1.) If I find a symbol that is undefined – and it has our name scheme, then I would jump to for example ModuleB, which is also not jitted yet and would do a “replaceAllUsesWith” on the Symbol of ModuleA to ModuleB – right?
                - Would that mean, when I add ModuleA to DyLibA – is ModuleB then part of DyLibA as well?
1.1.) Alternatively I could rename the symbol
2.) If the ModuleB is already jitted, then I can take the address to  do the “replaceAllUsesWith” right?

When I resolved all those references, then I can add the IR Module to my DyLib and compile it. However is it a good idea to use “replaceAllUsesWith” with addresses? Seems like the DefinitionGenerator would be jobless…

Writing this… I actually wondered about something else but this feels like a terrible approach… But now I’m curious xD
So… if I load an IR-Module, could I use the static LLVM compiler to compile it to an object file and then use the source code of the LLD to load and resolve the symbols the same way/kind we did in the past?
That sounds more like the “addObjectFile” function of the LLJIT… And then I guess I have to write a LinkLayer or something? That is where my knowledge ends…
Disclaimer: I don’t like that approach but it would be interesting to know (also cause some people here would be happy with it .w.”)

Thank you so far!

Kind greetings
Björn

From: Lang Hames <lhames at gmail.com>
Sent: 29 September 2020 01:47
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 - different behaviour of ExecutionSession.lookup?

Hi Bjoern,

Even though the "tryToGenerate" function of my DefinitionGenerator returned a "llvm::orc::SymbolsNotFound" for the "?_Plansch_test@@3HA", I got an address for "?

That's because you're issuing the lookup with RequiredState == SymbolState::Resolved. This means that your query will return as soon as "?Sampler@@YAXXZ" is assigned an address. In the JIT linker(s) addresses are assigned before external references are looked up. So after your lookup returns the linker attempts to find "?_Plansch_test@@3HA", fails, and so moves "?Sampler@@YAXXZ" to the error state.

You almost always want to issue your lookups with RequiredState == SymbolState::Ready. This ensures that the query will not return until / unless the requested symbols (and all their dependencies) are successfully linked into the target process and ready to execute.

Question 1.)
Is there any way to reset the error state of "?Sampler@@YAXXZ" at this point?

No. However, the removable code feature will allow you to remove failed materialization units once it lands in the mainline.

- After my first call I used the "define" function of the JITDylib to define ?_Plansch_test@@3HA and then I tried calling the lookup function again and again, however I only got the error: "Failed to materialize symbols" even though "?_Plansch_test@@3HA" was defined now...
- Changing the order of the "define" and the "lookup" call works of course, but I'm interested in the case where I don't know the address yet.

The JIT doesn't re-try linking. Once a symbol has failed to link it remains in the error state. In theory, once removable code is added you could choose to remove and then re-add "?Sampler@@YAXXZ" after "?_Plansch_test@@3HA" is defined. The real solution though is just to make sure that "?_Plansch_test@@3HA" is defined (either directly or via a generator) before you look up "?Sampler@@YAXXZ".

Out of curiosity I repeated the previous scenario - but added "?_Plansch_test@@3HA" to the "lookupSet" which changed things drasticly. When executing "lookup" I now get the "llvm::orc::SymbolsNotFound" error from my DefinitionGenerator...

Yes. Because "?_Plansch_test@@3HA" is not defined. You should see a SymbolsNotFound error sent to your error reporter in the first scenario too, followed by a failure-to-materialize error for "?Sampler@@YAXXZ".

... and "?Sampler@@YAXXZ" is stuck as a pending query in the MaterializingInfos entries.

Huh. That sounds like a bug: All references to the query should be removed from the state machine before it returns its result (in this case an error). I'll see if I can reproduce this locally and fix it up, but it doesn't affect the discussion here.

When I then add a definition for "?_Plansch_test@@3HA" and call "lookup" the second time, it will succeed and give me the addresses. Also I'm able to execute the code now. This is great! However...

When a lookup fails we try to restore the ExecutionSession state to what it was prior to the query. This is why the sequence "lookup -> symbols not found -> define -> lookup again" worked.

Question 2.)
Why did the first call to lookup not return the address of "?Sampler@@YAXXZ" like in the first scenario? I expected it would return an address for it.

A lookup must match against all symbols before anything is JIT'd. When it failed to match "?_Plansch_test@@3HA" we immediately bailed out with an error. There was no further attempt to compile "?Sampler@@YAXXZ".

Question 3.)
Can I somehow combine both behaviours? Getting the address for all the symbols (like in scenario 1) while still being able to provide definitions later (like in scenario 2)?

Sort of.

Definition generators allow you to provide a definition at the last minute (i.e. in response to a query). The best mental model though is: "All definitions that a generator can generate are part of the interface of the dylib". E.g. if you use a DynamicLibrarySearchGenerator to mirror symbols from a dynamic library containing "foo", "bar" and "baz" then you should think of your JITDylib as containing definitions for "foo", "bar" and "baz", even if the generator hasn't actually added them to the JITDylib yet. The reason is that it will add them in response to any query for them, so it's indistinguishable (except for timing and debug logging) from the case where they're already present.

If you need to be able to defer adding a "real" definition beyond the initial lookup then your only option (and this only applies to functions) is a lazy-reexport. This allows you to provide a definition for a function while deferring lookup until the first execution of the re-export at runtime. I wouldn't generally use this to break dependencies though: You want a definition of the real function body for "?_Plansch_test@@3HA" already added to your JIT because (in general) you never know when JIT'd code will need it.

My turn to ask a question: How is "?_Plansch_test@@3HA" created, and why not just add it up-front? :)

-- Lang.

On Mon, Sep 28, 2020 at 4:57 AM Gaier, Bjoern via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hey everyone,

I felt this question is different from my other question - hope this is okay.

So - I was playing around with the lookup function of the ExecutionSession and there are some things I don't understand.
I have a .BC file with a function "?Sampler@@YAXXZ" referencing a value "?_Plansch_test@@3HA" that is not defined in that module itself. I first planed on not providing an address for "?_Plansch_test@@3HA" but wanted to know the address of "?Sampler@@YAXXZ". So I issued something like that:

                auto &ES = this->jit->getExecutionSession();
                SymbolLookupSet lookupSet;

                lookupSet.add("?Sampler@@YAXXZ", llvm::orc::SymbolLookupFlags::WeaklyReferencedSymbol);
                ES.lookup({{&jit->getMainJITDylib(), llvm::orc::JITDylibLookupFlags::MatchAllSymbols}}, lookupSet, llvm::orc::LookupKind::Static, llvm::orc::SymbolState::Resolved);

Even though the "tryToGenerate" function of my DefinitionGenerator returned a "llvm::orc::SymbolsNotFound" for the "?_Plansch_test@@3HA", I got an address for "?Sampler@@YAXXZ". Dumping the "MainJITDylib" I saw, that the "?Sampler@@YAXXZ" was in an Error state. Which made sense - I guess.

Question 1.)
Is there any way to reset the error state of "?Sampler@@YAXXZ" at this point?
- After my first call I used the "define" function of the JITDylib to define ?_Plansch_test@@3HA and then I tried calling the lookup function again and again, however I only got the error: "Failed to materialize symbols" even though "?_Plansch_test@@3HA" was defined now...
- Changing the order of the "define" and the "lookup" call works of course, but I'm interested in the case where I don't know the address yet.

Out of curiosity I repeated the previous scenario - but added "?_Plansch_test@@3HA" to the "lookupSet" which changed things drasticly. When executing "lookup" I now get the "llvm::orc::SymbolsNotFound" error from my DefinitionGenerator and "?Sampler@@YAXXZ" is stuck as a pending query in the MaterializingInfos entries. When I then add a definition for "?_Plansch_test@@3HA" and call "lookup" the second time, it will succeed and give me the addresses. Also I'm able to execute the code now. This is great! However...

Question 2.)
Why did the first call to lookup not return the address of "?Sampler@@YAXXZ" like in the first scenario? I expected it would return an address for it.

Question 3.)
Can I somehow combine both behaviours? Getting the address for all the symbols (like in scenario 1) while still being able to provide definitions later (like in scenario 2)?

Thank you in advance and kind greetings,
Björn
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