ExecutionEngine::finalizeModule()

[Yaron Keren]

Hi,

Couple of ideas how to fix the FIXMEs.

finalizeModule should be called loadAndFinalizeModule
and
finalizeObject should be called loadAndFinalizeModules
to convey their functions.

About the data structures, while the combined list solves nicely the
looping find case it introduces a linear time find is time linear with
possibly a high constant:
http://llvm.org/docs/ProgrammersManual.html#list says "std::list
is an extremely inefficient class that is rarely useful. It performs a heap
allocation for every element inserted into it, thus having an extremely
high constant factor, particularly for small data types."

LLVM has many excellent datatypes available.

The SmallPtrSet
http://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallptrset-h
would be a great fit for MCJIT needs: Iteration is still supported and
other operations are constant time.
This will make a big difference when there are 1000s of modules.

The price we'd have to pay for using SmallPtrSet is that we'll have three
different SmallPtrSets so any operation on all three sets will require
three loops, set by set.

How bad is it ? there are two cases where a loop on all owned modules is
required:

1) In MCJIT.CPP, we need several times to find a module from the three sets.
This can be done by a find function of the OwningModuleContainer which will
find in the three sets and using it.

2) in ~OwningModuleContainer
We will need to destruct the three sets in three loops.

Not too bad in exchange for runtime performance that will scale much better
to the number of modules.

Yaron



2013/10/19 Kaylor, Andrew <andrew.kaylor at intel.com>

>  Hi Yaron,****
>
> ** **
>
> Here’s a first attempt at implementing what I described below.  It’s still
> got one significant place that I know needs revision (marked by a FIXME
> comment), but it seems to be functional.****
>
> ** **
>
> Would you mind checking it out and telling me what you think?****
>
> ** **
>
> Thanks,****
>
> Andy****
>
> ** **
>
> *From:* Yaron Keren [mailto:yaron.keren at gmail.com]
> *Sent:* Friday, October 18, 2013 3:04 PM
>
> *To:* Kaylor, Andrew
> *Cc:* llvm-commits at cs.uiuc.edu
> *Subject:* Re: ExecutionEngine::finalizeModule()****
>
> ** **
>
> This data structure sounds good .****
>
> ** **
>
> I actually didn't consider lazy compilation with regard about module
> states because I knew that "MCJIT does not have lazy compilation".  So the
> loaded state purpose is having modules in "standby" should they be
> required. OK. ****
>
> ** **
>
> If you put one function into one module exactly (call this a
> function-module) you almost have the lazy compilation capability. Maybe
> it's possible to split a module into function-modules automatically in a
> addModuleFunctions() function.****
>
> ** **
>
> Some questions -****
>
> ** **
>
> 1) Is the purpose of having loaded and finalized states to enable an
> external action on the module between loading and finalizing?****
>
> ** **
>
> 2) Is it OK to finalize a set of modules, add more modules and finalize
> again?****
>
> ** **
>
> Yaron****
>
> ** **
>
> ** **
>
> 2013/10/19 Kaylor, Andrew <andrew.kaylor at intel.com>****
>
>  I’ve actually got an idea for this and am working on a patch today.
> I’ll send it to you for input when I have something working.****
>
>  ****
>
> Basically my idea is to replace the existing Modules vector that MCJIT
> inherits from ExecutionEngine with a smarter container class that will
> internally maintain separate lists/sets for added, loaded and finalized
> modules.   This class would provide iterator access that MCJIT could use to
> iterate through the entire combined set of modules (I think I can make that
> work reasonably) but also iterators to go through just the modules in a
> particular state.  This would let us get rid of the clumsy ModuleState
> thing completely and provide faster access to the subsets we need.****
>
>  ****
>
> The “loaded” state is needed because when relocations are processed
> (during the finalize handling) they are applied to all modules that have
> been loaded and the finalize operation on the memory manager (which sets
> page protection, invalidates code caches, etc.) is applied to the sections
> of all loaded modules.  It would require a significant amount of extra
> infrastructure to make this behave otherwise and I couldn’t think of a
> compelling reason why it would be needed.****
>
>  ****
>
> But maybe there’s some confusion here.  The emitted state is entirely
> useless because it is always immediately followed by object loading.  The
> state I’m currently calling “loaded” is probably what you’re referring to
> as “compiled”.  We definitely do not want to immediately compile modules
> when addModule is called because that would eliminate the only facsimile of
> lazy compilation MCJIT currently has.  The idea is that you can add a
> module to provide function definitions but if nothing in that module is
> referenced by a module in which a function is executed (note the wording
> here because it’s not exactly the ideal case) then the module never gets
> compiled.****
>
>  ****
>
> Eventually, we’ll want to add another level of lazy compilation in which
> we generate stubs to resolve external function during linking/finalization
> and only compile their dependent modules if the stub is called.  But that’s
> a job for another day.****
>
>  ****
>
> -Andy****
>
>  ****
>
> *From:* Yaron Keren [mailto:yaron.keren at gmail.com]
> *Sent:* Friday, October 18, 2013 1:41 PM****
>
>
> *To:* Kaylor, Andrew
> *Cc:* llvm-commits at cs.uiuc.edu
> *Subject:* Re: ExecutionEngine::finalizeModule()****
>
>  ****
>
> Ok, it's simpler than I thought since only three states are relevant.****
>
>  ****
>
> But first - do we need really all three states? what good is loaded state?
> is just means being on the list, and any operation that is possible on a
> loaded module should be possible before adding the module to MCJIT, right?
> ****
>
>  ****
>
> If we merge loading and emitted by having the addModule immediately emit
> the loaded module we'll need only two states:  "Compiled" and "Linked" (or
> "Finalized").****
>
>  ****
>
> addModule emits module and adds the module to an "Compiled" vector of
> modules.****
>
> Finalize loops over "Compiled" vector, each module is finalized, added to
> "Linked" list and then the "Compiled" list is cleared.****
>
>  ****
>
> If we still need three lists then just the same, three vectors would be
> required with add inserting the loaded vector, emit moving modules from
> loaded to compiled and finalize moving them from compiled to linked.****
>
>  ****
>
> Yaron****
>
>  ****
>
>  ****
>
>  ****
>
>  ****
>
> 2013/10/18 Kaylor, Andrew <andrew.kaylor at intel.com>****
>
>  Your suggestion to introduce an optimization within MCJIT to avoid
> inefficiency in finalizeLoadedModules is the correct direction.  I
> definitely prefer to improve the MCJIT implementation rather than
> increasing the surface of the interface and requiring clients to optimize
> their own use of MCJIT.****
>
>  ****
>
> We’ve talked about removing runFunction from the MCJIT interface.  It
> really doesn’t belong there and was just implemented as a convenience to
> ease the transition of legacy code to MCJIT.  As such, I think of it in the
> same way I think of finalizeObject.  That is, I’m hoping it will go away.
> It might make sense to provide an independent class or perhaps a static
> function that provides the signature decoding that’s contained in
> MCJIT::runFunction, which would ease the pain of deprecating it.  That
> method isn’t entirely complete as it stands, BTW.****
>
>  ****
>
> The Module state handling in MCJIT definitely isn’t mature.  The only
> states that really have any significance are “Added” (which needs a better
> name), “Loaded” and “Finalized”.  The current state tracking is just
> something I put in there to get the multiple module support up and
> running.  I would be happy to see it replaced with something better.****
>
>  ****
>
> -Andy****
>
>  ****
>
>  ****
>
> *From:* Yaron Keren [mailto:yaron.keren at gmail.com]
> *Sent:* Thursday, October 17, 2013 4:02 PM
> *To:* Kaylor, Andrew
> *Cc:* llvm-commits at cs.uiuc.edu
> *Subject:* Re: ExecutionEngine::finalizeModule()****
>
>  ****
>
> The changes you are making make sense. Return valid ready-to-run function
> address rather than depend upon the user calling finalize is right.****
>
>  ****
>
> The issue is with scalability of the current  implementation. Let's take
> your Kaliedoscope example (which was very helpful) as the use-case but
> every REPL will do the same.****
>
>  ****
>
> After adding  a new module it and only it needs to be finalized to run the
> new code. But finalizeLoadedModules loops over all modules to see which one
> should be finalized. It's not efficient. ****
>
>  ****
>
> I initially thought using finalizeModule would be the solution, calling it
> after creating a module, saving the loop.****
>
>  ****
>
> However this goes deeper and finalizeModule isn't the solution.  As you
> say getFunctionAddress calls finalizeLoadedModules() if a symbol found.
> finalizeLoadedModules always loops on all modules so it's wasteful. In an
> extreme case, we may know that all modules are already finalized but can't
> prevent the needless loop on module.****
>
>  ****
>
> The solution would be other data structures. ****
>
> Simplified case, instead of looping on all modules and checking module
> states  MCJIT could have a vector of un-finalized modules and a vector of
> finalized modules. New modules enter the un-finalized vector
> and finalizeLoadedModules() loops over the un-finalized vector, finalize
> every module, appends them to finalized vector and clears the unfinalized
> vector. ****
>
>  ****
>
> This is simplified since there are more ModuleStates so maybe other data
> structures should be used. I have not looked in depth the ModuleStates.
>
> Regarding getFunctionAddress, runFunction() does not use it but calls the
> older getPointerToFunction, so either runFunction() should be changed to
> the newer API or we still need to manually finalize modules.
>
> Yaron****
>
>  ****
>
> 2013/10/18 Kaylor, Andrew <andrew.kaylor at intel.com>****
>
>  Hi Yaron,****
>
>  ****
>
> What do you see as the use case for adding this?  I was thinking about
> removing finalizeObject() from the interface -- it’s mostly there to
> support code that was written when it was necessary.  It was probably a
> misstep to introduce finalizeObject in the EE interface in the first
> place.  It’s a case of leaking an implementation detail into the interface.
> ****
>
>  ****
>
> With the current trunk implementation of MCJIT, calling getFunctionAddress
> will implicitly finalize the module that contains the function (as well as
> any other modules that have been loaded).  The idea is that the address you
> get back is immediately useful (which was not true of the older
> getPointerToFunction method).****
>
>  ****
>
> Also note that within MCJIT calling finalizeModule has the effect of
> finalizing all loaded modules.  In fact, I plan to update the
> implementation of finalizeModule to call finalizeLoadedModules soon.****
>
>  ****
>
> -Andy****
>
>  ****
>
>  ****
>
> *From:* Yaron Keren [mailto:yaron.keren at gmail.com]
> *Sent:* Thursday, October 17, 2013 2:32 PM
> *To:* llvm-commits at cs.uiuc.edu; Kaylor, Andrew
> *Subject:* ExecutionEngine::finalizeModule()****
>
>  ****
>
> Hi,****
>
>  ****
>
> Make finalizeModule() accessible for MCJIT created through ExecutionEngine.
> ****
>
>  ****
>
> Maybe it would make sense to expose finalizeLoadedModules() as well.****
>
>  ****
>
> Yaron****
>
>  ****
>
>  ****
>
>   ****
>
>   ****
>
>  ** **
>
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