Add a Basic JIT to Your VM

Template JIT Basics

Interpretation pays dispatch tax every opcode. A template JIT copies pre-built machine-code snippets into an executable buffer, patches immediates, and jumps to native code. LuaJIT and V8 sit farther along this spectrum; template JIT is the entry point.

x86-64 Emission

Each bytecode maps to a native template:

• `mov rax, imm64`: `48 B8` plus 8 immediate bytes
• `add rax, rbx`: `48 01 D8`
• `ret`: `C3`

Executable pages come from `mmap` with `PROT_READ | PROT_WRITE | PROT_EXEC`, or `mprotect` after writing. Modern OS hardening (W^X on iOS, SELinux policies) restricts this, which is why browser JITs need special entitlements.

A switch-based interpreter might spend 5-10 cycles per opcode on fetch and dispatch. JIT'd straight-line code runs at near-native speed for hot sequences.

For example, `PUSH 10; PUSH 20; ADD` becomes a few `mov` instructions and one `add` without a central dispatch loop.

Design Notes

Flush the instruction cache after writing machine code on architectures that require it. Template JIT is not peak performance, but it teaches the exact steps production engines hide inside megabytes of optimization logic.

For This Exercise

This exercise asks you to emit x86-64 machine code for a short bytecode program using `mmap`. You will implement byte emitters and demonstrate calling the generated function from C.