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Building a JIT: Per-function Lazy Compilation

This tutorial is under active development. It is incomplete and details may change frequently. Nonetheless we invite you to try it out as it stands, and we welcome any feedback.

Chapter 3 Introduction

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.

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:

#include "llvm/ExecutionEngine/SectionMemoryManager.h"
#include "llvm/ExecutionEngine/Orc/CompileOnDemandLayer.h"
#include "llvm/ExecutionEngine/Orc/CompileUtils.h"

class KaleidoscopeJIT {
  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>)>

  IRTransformLayer<decltype(CompileLayer), OptimizeFunction> OptimizeLayer;
  CompileOnDemandLayer<decltype(OptimizeLayer)> CODLayer;

  typedef decltype(CODLayer)::ModuleSetHandleT ModuleHandle;

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.

    : TM(EngineBuilder().selectTarget()), DL(TM->createDataLayout()),
      CompileLayer(ObjectLayer, SimpleCompiler(*TM)),
                    [this](std::unique_ptr<Module> M) {
                      return optimizeModule(std::move(M));
          orc::createLocalCompileCallbackManager(TM->getTargetTriple(), 0)),
               [this](Function &F) { return std::set<Function*>({&F}); },
                 TM->getTargetTriple())) {

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),
// ...

// ...
return CODLayer.findSymbol(MangledNameStream.str(), true);
// ...

// ...
// ...

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

Here is the complete code listing for our running example with a CompileOnDemand layer added to enable lazy function-at-a-time compilation. To build this example, use:

# Compile
clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core orc native` -O3 -o toy
# Run

Here is the code:

Next: Extreme Laziness -- Using Compile Callbacks to JIT directly from ASTs