Building 5-Microsecond JIT Compilers with AI and Copy-and-Patch Stencils
Implementing custom JIT compilers historically required complex assembly skills, forcing engines to rely on heavy frameworks like LLVM. By leveraging AI assistance and copy-and-patch assembly stencils, developers can build lightweight ARM64 JIT compilers that execute in 5 microseconds.
Impact: Medium
Why it matters
You can replace slow AST interpreters or heavy LLVM compilation pipelines in custom engines with sub-5-microsecond copy-and-patch JIT execution.
TL;DR
- 01AI code tools simplify writing ARM64 assembly stencils for low-level performance optimization.
- 02Copy-and-patch JIT reduces compilation latency to ~5μs by patching runtime values directly into pre-compiled instructions.
- 03JIT compiling custom parsers and database queries yields 10x-20x speedups over AST interpreters.
Key facts
- Compilation Time
- ~5μs per query
- Performance Gain
- 10x-20x vs interpreter
- Target Architecture
- ARM64 (macOS)
- Core Technique
- Copy-and-patch assembly stencils
The Copy-and-Patch JIT Architecture
Historically, writing production-grade JIT compilers required manual assembly composition or heavy runtime infrastructures such as LLVM. Copy-and-patch compilation breaks execution into fixed assembly stencils. AI code generators now simplify authoring and validating these low-level stencils for target architectures like ARM64.
Implementing Assembly Stencils on ARM64
In a database engine like pgrust, every AST operator translates directly into a sequence of machine instructions. For instance, character comparisons rely on loading register bytes using ldrb w9, [x0] and evaluating branching instructions (b.ne). Memory layout conventions reserve registers like x0 for the input buffer pointer and x1 for the backtracking stack.
Ultra-Low Latency Performance
Benchmarking this copy-and-patch regex engine yields a 10x to 20x speedup over AST-based interpreters. Because the compilation overhead drops to approximately 5μs, applications can JIT-compile every incoming query dynamically rather than restricting JIT execution to long-running batch operations.
Try it in 2 minutes
// ARM64 stencil pattern for single character matching in JIT
// x0: input pointer, w9: current byte register
// 4: ldrb w9, [x0] ; load current byte
// c: b.ne fallback_label ; jump if mismatch
// 10: add x0, x0, #1 ; advance byte pointerrust
✓ When to use
- Building high-throughput custom database query engines or serialization parsers.
- Needing native code execution speed without paying high LLVM compilation latencies.
✕ When NOT to use
- Standard high-level web services where standard language runtimes provide sufficient throughput.
- Restricted deployment environments that block dynamic executable memory allocations (mprotect / W^X).
What to do today
- Evaluate copy-and-patch stencil techniques for internal DSL parsers or string matching bottlenecks.
- Use Claude Code or Cursor to inspect and generate target architecture ARM64 assembly snippets.
Sources