[llvm-dev] VModuleKey K not valid here

Tue Aug 13 15:04:25 PDT 2019

Hi Dibyendu,

Yes indeed that is what happened. So to prevent that I need to check
> if VModuleKey was ever initialized ... how do I do that? I would have
> to have to add another flag to track the initialized state.
> The reason for it not being initialized is that sometimes I cannot
> generate JIT code because some bytecode is not yet supported.

I'm not sure I follow. In ORCv1 VModuleKeys are just integers:
initialization is up to you, and the standard way to get a unique key is to
always initialize keys by calling ExecutionSession::allocateVModuleKey().
The intent is that you should use a unique key for each module that you
add, and keep a copy of the key if it is associated with a module if you
want to be able to remove that module later. E.g.

// Module that I will never want to remove:
CODLayer.addModule(ES.createVModuleKey(), std::move(M)); // unique
throwaway key.

// Module that I want to be able to remove later:
auto TemporaryModuleKey = ES.createVModuleKey();
CODLayer.addModule(TemporaryModuleKey, std::move(M));
// Do stuff.
CODLayer.removeModule(TemporaryModuleKey);

The main use cases of removable modules that I'm aware of are: (1)
temporary expressions in REPLs, and (2) lower-optimized modules in
re-optimizing JITs. In the former case you can usually name the key
individually (as above). In the latter case you usually want to stash it in
a map or context object somewhere and remove it after you've replaced the
low-optimization-level code with a higher optimization level.

> > There should not be any check other than the assertion. Assertions
> aren't for recoverable errors or logging, they're only for verifying that
> code is being used according to contract. In this case the contract is that
> removeModule is only called with valid keys. If the assertion is triggering
> then either the key you're using is invalid, or my implementation of
> CompileOnDemandLayer (or the assert itself) is invalid. We just need to fix
> the offending code.
> >
> Sure - in that case there ought be some value of VModuleKey that is
> invalid. Example NULL for pointers. Otherwise users have to track
> separately whether it is initialized or not.
>

In ORCv1 the solution is to always use ES.createVModuleKey(). Users should
only have to keep track keys that they want to be able to remove.

> Having said that just as free(NULL) doesn't do anything, and
> fclose(NULL) doesn't either, it seems better to not continue to try to
> access 'I' if it doesn't exist. Or the assertion should trigger even
> in release builds.

I don't have strong opinions about this idiom when applied to free or
fclose, but I definitely wouldn't want it to apply here. What is a
programmer who calls "removeModule(<invalid key>)" trying to do? Whatever
it is, it's definitely a mistake. The contract is: Only call removeModule
with a valid key that is associated with a module that you want to remove.
Tracking that is definitely up to the client, but they'd have to do that
anyway, otherwise how would you know what needed removing?

Ideally it should be possible to delete modules after the code is compiled.
> And also delete compiled functions if they are no longer needed.
> In my case, the language is Lua based, functions are garbage
> collected. So any associated stuff should be deleted when the function
> is collected. Sometimes each function is in its own module, but
> sometimes several functions are in one module. Either way when the
> last function in a module is deleted, I invoke removeModule().

Yep. That makes perfect sense. You just want to create a unique key for
each module and track it in your garbage collection context somewhere.

-- Lang.

On Tue, Aug 13, 2019 at 2:33 PM Dibyendu Majumdar <mobile at majumdar.org.uk>
wrote:

> Hi Lang,
>
> On Tue, 13 Aug 2019 at 22:15, Lang Hames <lhames at gmail.com> wrote:
> >
> >> 1) Can 0 ever be a valid VModuleKey? How can one reliably detect an
> invalid VModuleKey?
> >
> >
> > I believe 0 was a valid VModuleKey in ORCv1. The assertion is checking
> the the VModuleKey is present in the LogicalDylibs map. That means that you
> have to have used that key in an addModule call, e.g.:
> >
> > CODLayer.addModule(K, std::move(M));
> >
> > and not subsequently removed the key with a call to removeModule.
>
> Yes indeed that is what happened. So to prevent that I need to check
> if VModuleKey was ever initialized ... how do I do that? I would have
> to have to add another flag to track the initialized state.
> The reason for it not being initialized is that sometimes I cannot
> generate JIT code because some bytecode is not yet supported.
>
> >
> > 2) Secondly it seems to me that  following the assertion there should be
> a check so that the code dosn't continue? It is causing segmentation fault
> in release builds.
> >
> > There should not be any check other than the assertion. Assertions
> aren't for recoverable errors or logging, they're only for verifying that
> code is being used according to contract. In this case the contract is that
> removeModule is only called with valid keys. If the assertion is triggering
> then either the key you're using is invalid, or my implementation of
> CompileOnDemandLayer (or the assert itself) is invalid. We just need to fix
> the offending code.
> >
>
> Sure - in that case there ought be some value of VModuleKey that is
> invalid. Example NULL for pointers. Otherwise users have to track
> separately whether it is initialized or not.
> Having said that just as free(NULL) doesn't do anything, and
> fclose(NULL) doesn't either, it seems better to not continue to try to
> access 'I' if it doesn't exist. Or the assertion should trigger even
> in release builds.
>
> > Out of interest: What are your plans for removeModule? It's currently
> unimplemented in ORCv2. Understanding people's use cases will help with
> design and prioritization.
> >
>
> Ideally it should be possible to delete modules after the code is compiled.
> And also delete compiled functions if they are no longer needed.
> In my case, the language is Lua based, functions are garbage
> collected. So any associated stuff should be deleted when the function
> is collected. Sometimes each function is in its own module, but
> sometimes several functions are in one module. Either way when the
> last function in a module is deleted, I invoke removeModule().
>
> I think in ORCv2 stuff never gets deleted?
>
> Regards
> Dibyendu
>
> > On Tue, Aug 13, 2019 at 1:50 PM Dibyendu Majumdar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> Hi,
> >>
> >> I am getting following assertion failure when attempting to remove a
> module.
> >>
> >> /llvm/ExecutionEngine/Orc/CompileOnDemandLayer.h:311: llvm::Error
> >> llvm::orc::LegacyCompileOnDemandLayer<BaseLayerT, CompileCallbackMgrT,
> >> IndirectStubsMgrT>::removeModule(llvm::orc::VModuleKey) [with
> >> BaseLayerT =
> llvm::orc::LegacyIRTransformLayer<llvm::orc::LegacyIRCompileLayer<llvm::orc::LegacyRTDyldObjectLinkingLayer,
> >> llvm::orc::SimpleCompiler>,
> >>
> std::function<std::unique_ptr<llvm::Module>(std::unique_ptr<llvm::Module>)>
> >> >; CompileCallbackMgrT = llvm::orc::JITCompileCallbackManager;
> >> IndirectStubsMgrT = llvm::orc::IndirectStubsManager;
> >> llvm::orc::VModuleKey = long unsigned int]: Assertion `I !=
> >> LogicalDylibs.end() && "VModuleKey K not valid here"' failed.
> >>
> >> 1) Can 0 ever be a valid VModuleKey? How can one reliably detect an
> >> invalid VModuleKey?
> >> 2) Secondly it seems to me that  following the assertion there should
> >> be a check so that the code dosn't continue? It is causing
> >> segmentation fault in release builds.
> >>
> >> Regards
> >> Dibyendu
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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