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1=============================================2Building a JIT: Per-function Lazy Compilation3=============================================4 5.. contents::6 :local:7 8**This tutorial is under active development. It is incomplete and details may9change frequently.** Nonetheless we invite you to try it out as it stands, and10we welcome any feedback.11 12Chapter 3 Introduction13======================14 15**Warning: This text is currently out of date due to ORC API updates.**16 17**The example code has been updated and can be used. The text will be updated18once the API churn dies down.**19 20Welcome to Chapter 3 of the "Building an ORC-based JIT in LLVM" tutorial. This21chapter discusses lazy JITing and shows you how to enable it by adding an ORC22CompileOnDemand layer the JIT from `Chapter 2 <BuildingAJIT2.html>`_.23 24Lazy Compilation25================26 27When we add a module to the KaleidoscopeJIT class from Chapter 2 it is28immediately optimized, compiled and linked for us by the IRTransformLayer,29IRCompileLayer and RTDyldObjectLinkingLayer respectively. This scheme, where all the30work to make a Module executable is done up front, is simple to understand and31its performance characteristics are easy to reason about. However, it will lead32to very high startup times if the amount of code to be compiled is large, and33may also do a lot of unnecessary compilation if only a few compiled functions34are ever called at runtime. A truly "just-in-time" compiler should allow us to35defer the compilation of any given function until the moment that function is36first called, improving launch times and eliminating redundant work. In fact,37the ORC APIs provide us with a layer to lazily compile LLVM IR:38*CompileOnDemandLayer*.39 40The CompileOnDemandLayer class conforms to the layer interface described in41Chapter 2, but its addModule method behaves quite differently from the layers42we have seen so far: rather than doing any work up front, it just scans the43Modules being added and arranges for each function in them to be compiled the44first time it is called. To do this, the CompileOnDemandLayer creates two small45utilities for each function that it scans: a *stub* and a *compile46callback*. The stub is a pair of a function pointer (which will be pointed at47the function's implementation once the function has been compiled) and an48indirect jump through the pointer. By fixing the address of the indirect jump49for the lifetime of the program we can give the function a permanent "effective50address", one that can be safely used for indirection and function pointer51comparison even if the function's implementation is never compiled, or if it is52compiled more than once (due to, for example, recompiling the function at a53higher optimization level) and changes address. The second utility, the compile54callback, represents a re-entry point from the program into the compiler that55will trigger compilation and then execution of a function. By initializing the56function's stub to point at the function's compile callback, we enable lazy57compilation: The first attempted call to the function will follow the function58pointer and trigger the compile callback instead. The compile callback will59compile the function, update the function pointer for the stub, then execute60the function. On all subsequent calls to the function, the function pointer61will point at the already-compiled function, so there is no further overhead62from the compiler. We will look at this process in more detail in the next63chapter of this tutorial, but for now we'll trust the CompileOnDemandLayer to64set all the stubs and callbacks up for us. All we need to do is to add the65CompileOnDemandLayer to the top of our stack and we'll get the benefits of66lazy compilation. We just need a few changes to the source:67 68.. code-block:: c++69 70 ...71 #include "llvm/ExecutionEngine/SectionMemoryManager.h"72 #include "llvm/ExecutionEngine/Orc/CompileOnDemandLayer.h"73 #include "llvm/ExecutionEngine/Orc/CompileUtils.h"74 ...75 76 ...77 class KaleidoscopeJIT {78 private:79 std::unique_ptr<TargetMachine> TM;80 const DataLayout DL;81 RTDyldObjectLinkingLayer ObjectLayer;82 IRCompileLayer<decltype(ObjectLayer), SimpleCompiler> CompileLayer;83 84 using OptimizeFunction =85 std::function<std::shared_ptr<Module>(std::shared_ptr<Module>)>;86 87 IRTransformLayer<decltype(CompileLayer), OptimizeFunction> OptimizeLayer;88 89 std::unique_ptr<JITCompileCallbackManager> CompileCallbackManager;90 CompileOnDemandLayer<decltype(OptimizeLayer)> CODLayer;91 92 public:93 using ModuleHandle = decltype(CODLayer)::ModuleHandleT;94 95First we need to include the CompileOnDemandLayer.h header, then add two new96members: a std::unique_ptr<JITCompileCallbackManager> and a CompileOnDemandLayer,97to our class. The CompileCallbackManager member is used by the CompileOnDemandLayer98to create the compile callback needed for each function.99 100.. code-block:: c++101 102 KaleidoscopeJIT()103 : TM(EngineBuilder().selectTarget()), DL(TM->createDataLayout()),104 ObjectLayer([]() { return std::make_shared<SectionMemoryManager>(); }),105 CompileLayer(ObjectLayer, SimpleCompiler(*TM)),106 OptimizeLayer(CompileLayer,107 [this](std::shared_ptr<Module> M) {108 return optimizeModule(std::move(M));109 }),110 CompileCallbackManager(111 orc::createLocalCompileCallbackManager(TM->getTargetTriple(), 0)),112 CODLayer(OptimizeLayer,113 [this](Function &F) { return std::set<Function*>({&F}); },114 *CompileCallbackManager,115 orc::createLocalIndirectStubsManagerBuilder(116 TM->getTargetTriple())) {117 llvm::sys::DynamicLibrary::LoadLibraryPermanently(nullptr);118 }119 120Next we have to update our constructor to initialize the new members. To create121an appropriate compile callback manager we use the122createLocalCompileCallbackManager function, which takes a TargetMachine and an123ExecutorAddr to call if it receives a request to compile an unknown124function. In our simple JIT this situation is unlikely to come up, so we'll125cheat and just pass '0' here. In a production quality JIT you could give the126address of a function that throws an exception in order to unwind the JIT'd127code's stack.128 129Now we can construct our CompileOnDemandLayer. Following the pattern from130previous layers we start by passing a reference to the next layer down in our131stack -- the OptimizeLayer. Next we need to supply a 'partitioning function':132when a not-yet-compiled function is called, the CompileOnDemandLayer will call133this function to ask us what we would like to compile. At a minimum we need to134compile the function being called (given by the argument to the partitioning135function), but we could also request that the CompileOnDemandLayer compile other136functions that are unconditionally called (or highly likely to be called) from137the function being called. For KaleidoscopeJIT we'll keep it simple and just138request compilation of the function that was called. Next we pass a reference to139our CompileCallbackManager. Finally, we need to supply an "indirect stubs140manager builder": a utility function that constructs IndirectStubManagers, which141are in turn used to build the stubs for the functions in each module. The142CompileOnDemandLayer will call the indirect stub manager builder once for each143call to addModule, and use the resulting indirect stubs manager to create144stubs for all functions in all modules in the set. If/when the module set is145removed from the JIT the indirect stubs manager will be deleted, freeing any146memory allocated to the stubs. We supply this function by using the147createLocalIndirectStubsManagerBuilder utility.148 149.. code-block:: c++150 151 // ...152 if (auto Sym = CODLayer.findSymbol(Name, false))153 // ...154 return cantFail(CODLayer.addModule(std::move(Ms),155 std::move(Resolver)));156 // ...157 158 // ...159 return CODLayer.findSymbol(MangledNameStream.str(), true);160 // ...161 162 // ...163 CODLayer.removeModule(H);164 // ...165 166Finally, we need to replace the references to OptimizeLayer in our addModule,167findSymbol, and removeModule methods. With that, we're up and running.168 169**To be done:**170 171** Chapter conclusion.**172 173Full Code Listing174=================175 176Here is the complete code listing for our running example with a CompileOnDemand177layer added to enable lazy function-at-a-time compilation. To build this example, use:178 179.. code-block:: bash180 181 # Compile182 clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core orcjit native` -O3 -o toy183 # Run184 ./toy185 186Here is the code:187 188.. literalinclude:: ../../examples/Kaleidoscope/BuildingAJIT/Chapter3/KaleidoscopeJIT.h189 :language: c++190 191`Next: Extreme Laziness -- Using Compile Callbacks to JIT directly from ASTs <BuildingAJIT4.html>`_192