jit-compilation

JIT Compilation

Purpose

Guide agents through just-in-time compilation: LLVM ORC JIT v2 (ExecutionSession, IRLayer, ObjectLayer), LLJIT for simpler use cases, Cranelift JIT, inline caches for dynamic dispatch, trampolines for lazy compilation, security considerations (W^X, code signing), and Rust dynasm for x86 codegen.

When to Use

• Building an interpreter with a JIT hot-path
• Implementing lazy function compilation on first call
• Embedding dynamic code generation in a REPL or game engine
• Prototyping codegen without writing a full linker
• Creating inline caches for polymorphic call sites
• Generating x86 machine code from Rust with dynasm

Workflow

1. JIT architecture overview

Source/AST/Bytecode
    → IR (LLVM IR or Cranelift CLIF)
    → Object file (in memory)
    → Runtime linker (RTDyldObjectLinkingLayer)
    → Executable code in R+X memory
    → Function pointer call

2. LLJIT — simplest LLVM JIT

#include "llvm/ExecutionEngine/Orc/LLJIT.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/IRBuilder.h"

using namespace llvm;
using namespace llvm::orc;

int main() {
    auto JIT = cantFail(LLJITBuilder().create());

    LLVMContext Context;
    auto M = std::make_unique<Module>("jit", Context);
    IRBuilder<> Builder(Context);

    // int add(int a, int b) { return a + b; }
    Function *AddFn = Function::Create(
        FunctionType::get(Builder.getInt32Ty(),
                          {Builder.getInt32Ty(), Builder.getInt32Ty()}, false),
        Function::ExternalLinkage, "add", M.get());

    BasicBlock *BB = BasicBlock::Create(Context, "entry", AddFn);
    Builder.SetInsertPoint(BB);
    auto Args = AddFn->arg_begin();
    Value *Sum = Builder.CreateAdd(Args, Args + 1);
    Builder.CreateRet(Sum);

    cantFail(JIT->addIRModule(ThreadSafeModule(std::move(M), std::make_unique<LLVMContext>())));

    auto AddSym = JIT->lookup("add");
    auto *AddPtr = (int (*)(int, int))AddSym->getValue();
    int result = AddPtr(3, 4);  // 7
    return 0;
}