Understand and Implement a Basic JIT Compiler

Compile at Runtime

JIT compilers generate machine code in memory and execute it immediately. JVM HotSpot, JavaScriptCore, and LuaJIT profile hot loops, compile them natively, and jump from interpreter to generated code. JIT trades compile latency for peak performance.

Interpreter to Native Handoff

Profile bytecode or AST execution. When a function gets hot, lower IR to machine code into an executable buffer (mmap with PROT_EXEC). Patch call sites to jump to generated code.

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• Executable memory requires W^X awareness: write then mprotect
• Deoptimization returns to interpreter on guard failure
• OS must flush instruction cache after patching (ARM)
• Security: JIT surfaces are attack targets

Connecting to real compilers

Production compilers embed this step inside a longer pipeline. GCC flows through cpp, cc1, assembly, and ld; Clang uses the driver, Sema, LLVM IR passes, and a target backend. LLVM bitcode, JVM bytecode, and WASM are other familiar IRs at the same layer. The exercise isolates one pass so you can test it alone before chaining it to the next stage.

Why for this exercise

You will understand and implement a basic JIT compiler. This exercise requires allocating executable memory, emitting simple machine code, and transferring control from an interpreter stub to generated code.