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

[Lang Hames via llvm-dev]

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> 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
> 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, Junichi Tajika, Ergin
> Cansiz.
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