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| -rw-r--r-- | llvm/docs/tutorial/BuildingAJIT3.rst | 133 |
1 files changed, 127 insertions, 6 deletions
diff --git a/llvm/docs/tutorial/BuildingAJIT3.rst b/llvm/docs/tutorial/BuildingAJIT3.rst index 35b0e24dc74..3e8d3fbef5a 100644 --- a/llvm/docs/tutorial/BuildingAJIT3.rst +++ b/llvm/docs/tutorial/BuildingAJIT3.rst @@ -16,15 +16,136 @@ Welcome to Chapter 3 of the "Building an ORC-based JIT in LLVM" tutorial. This chapter discusses lazy JITing and shows you how to enable it by adding an ORC CompileOnDemand layer the JIT from `Chapter 2 <BuildingAJIT2.html>`_. -**To be done:** +Lazy Compilation +================ + +When we add a module to the KaleidoscopeJIT class described in Chapter 2 it is +immediately optimized, compiled and linked for us by the IRTransformLayer, +IRCompileLayer and ObjectLinkingLayer respectively. This scheme, where all the +work to make a Module executable is done up front, is relatively simple to +understand its performance characteristics are easy to reason about. However, +it will lead to very high startup times if the amount of code to be compiled is +large, and may also do a lot of unnecessary compilation if only a few compiled +functions are ever called at runtime. A truly "just-in-time" compiler should +allow us to defer the compilation of any given function until the moment that +function is first called, improving launch times and eliminating redundant work. +In fact, the ORC APIs provide us with a layer to lazily compile LLVM IR: +*CompileOnDemandLayer*. + +The CompileOnDemandLayer conforms to the layer interface described in Chapter 2, +but the addModuleSet method behaves quite differently from the layers we have +seen so far: rather than doing any work up front, it just constructs a *stub* +for each function in the module and arranges for the stub to trigger compilation +of the actual function the first time it is called. Because stub functions are +very cheap to produce CompileOnDemand's addModuleSet method runs very quickly, +reducing the time required to launch the first function to be executed, and +saving us from doing any redundant compilation. By conforming to the layer +interface, CompileOnDemand can be easily added on top of our existing JIT class. +We just need a few changes: + +.. code-block:: c++ + + ... + #include "llvm/ExecutionEngine/SectionMemoryManager.h" + #include "llvm/ExecutionEngine/Orc/CompileOnDemandLayer.h" + #include "llvm/ExecutionEngine/Orc/CompileUtils.h" + ... + + ... + class KaleidoscopeJIT { + private: + std::unique_ptr<TargetMachine> TM; + const DataLayout DL; + std::unique_ptr<JITCompileCallbackManager> CompileCallbackManager; + ObjectLinkingLayer<> ObjectLayer; + IRCompileLayer<decltype(ObjectLayer)> CompileLayer; + + typedef std::function<std::unique_ptr<Module>(std::unique_ptr<Module>)> + OptimizeFunction; + + IRTransformLayer<decltype(CompileLayer), OptimizeFunction> OptimizeLayer; + CompileOnDemandLayer<decltype(OptimizeLayer)> CODLayer; + + public: + typedef decltype(CODLayer)::ModuleSetHandleT ModuleHandle; -**(1) Describe lazy function-at-a-time JITing and how it differs from the kind -of eager module-at-a-time JITing that we've been doing so far.** +First we need to include the CompileOnDemandLayer.h header, then add two new +members: a std::unique_ptr<CompileCallbackManager> and a CompileOnDemandLayer, +to our class. The CompileCallbackManager is a utility that enables us to +create re-entry points into the compiler for functions that we want to lazily +compile. In the next chapter we'll be looking at this class in detail, but for +now we'll be treating it as an opaque utility: We just need to pass a reference +to it into our new CompileOnDemandLayer, and the layer will do all the work of +setting up the callbacks using the callback manager we gave it. -**(2) Discuss CompileCallbackManagers and IndirectStubManagers.** -**(3) Describe CompileOnDemandLayer (automates these components and builds stubs -and lazy compilation callbacks for IR) and how to add it to the JIT.** + KaleidoscopeJIT() + : TM(EngineBuilder().selectTarget()), DL(TM->createDataLayout()), + CompileLayer(ObjectLayer, SimpleCompiler(*TM)), + OptimizeLayer(CompileLayer, + [this](std::unique_ptr<Module> M) { + return optimizeModule(std::move(M)); + }), + CompileCallbackManager( + orc::createLocalCompileCallbackManager(TM->getTargetTriple(), 0)), + CODLayer(OptimizeLayer, + [this](Function &F) { return std::set<Function*>({&F}); }, + *CompileCallbackManager, + orc::createLocalIndirectStubsManagerBuilder( + TM->getTargetTriple())) { + llvm::sys::DynamicLibrary::LoadLibraryPermanently(nullptr); + } + +Next we have to update our constructor to initialize the new members. To create +an appropriate compile callback manager we use the +createLocalCompileCallbackManager function, which takes a TargetMachine and a +TargetAddress to call if it receives a request to compile an unknown function. +In our simple JIT this situation is unlikely to come up, so we'll cheat and +just pass '0' here. In a production quality JIT you could give the address of a +function that throws an exception in order to unwind the JIT'd code stack. + +Now we can construct our CompileOnDemandLayer. Following the pattern from +previous layers we start by passing a reference to the next layer down in our +stack -- the OptimizeLayer. Next we need to supply a 'partitioning function': +when a not-yet-compiled function is called, the CompileOnDemandLayer will call +this function to ask us what we would like to compile. At a minimum we need to +compile the function being called (given by the argument to the partitioning +function), but we could also request that the CompileOnDemandLayer compile other +functions that are unconditionally called (or highly likely to be called) from +the function being called. For KaleidoscopeJIT we'll keep it simple and just +request compilation of the function that was called. Next we pass a reference to +our CompileCallbackManager. Finally, we need to supply an "indirect stubs +manager builder". This is a function that constructs IndirectStubManagers, which +are in turn used to build the stubs for each module. The CompileOnDemandLayer +will call the indirect stub manager builder once for each call to addModuleSet, +and use the resulting indirect stubs manager to create stubs for all functions +in all modules added. If/when the module set is removed from the JIT the +indirect stubs manager will be deleted, freeing any memory allocated to the +stubs. We supply this function by using the +createLocalIndirectStubsManagerBuilder utility. + + // ... + if (auto Sym = CODLayer.findSymbol(Name, false)) + // ... + return CODLayer.addModuleSet(std::move(Ms), + make_unique<SectionMemoryManager>(), + std::move(Resolver)); + // ... + + // ... + return CODLayer.findSymbol(MangledNameStream.str(), true); + // ... + + // ... + CODLayer.removeModuleSet(H); + // ... + +Finally, we need to replace the references to OptimizeLayer in our addModule, +findSymbol, and removeModule methods. With that, we're up and running. + +**To be done:** + +** Discuss CompileCallbackManagers and IndirectStubManagers in more detail.** Full Code Listing ================= |
