Baseline JIT compiler: AArch64 assembler infrastructure #138

Summary#

Build the low-level infrastructure for JIT compilation: an AArch64 machine code assembler, executable memory allocation, and the interface between interpreted and JIT-compiled code.

Background#

The JS engine uses register-based bytecode designed for JIT from day one. This issue implements the assembler layer that translates high-level JIT IR or bytecode directly into AArch64 machine code. This is unsafe code (allowed in the js crate per CLAUDE.md).

Acceptance Criteria#

• Executable memory allocator: mmap with PROT_READ | PROT_WRITE | PROT_EXEC (or W^X with mprotect flip)
• JitBuffer struct: manages allocated code pages, tracks used/free space
• Assembler struct with methods to emit AArch64 instructions:
  • Arithmetic: add, sub, mul, sdiv (register and immediate forms)
  • Logic: and, orr, eor, lsl, lsr, asr
  • Memory: ldr, str (register offset, immediate offset, pre/post-index)
  • Branch: b, b.cond, bl, blr, br, ret, cbz, cbnz
  • Comparison: cmp, tst
  • Move: mov, movz, movk (for loading 64-bit immediates)
  • FP: fmov, fadd, fsub, fmul, fdiv, fcmp, scvtf, fcvtzs (for f64 arithmetic)
• Instruction encoding tests: verify each emitted instruction matches expected byte patterns
• CodePtr type wrapping a function pointer for calling into JIT code
• Entry/exit stubs: save/restore callee-saved registers, set up frame pointer, bridge between VM and JIT code
• AArch64 calling convention support (registers x0-x7 for args, x8 for indirect result, x19-x28 callee-saved)

Implementation Notes#

• All JIT code lives in crates/js/src/jit/ (new module)
• Use libc::mmap / libc::mprotect / libc::munmap for memory management (these are syscalls, not external crates)
• W^X policy: write code with PROT_WRITE, then flip to PROT_READ|PROT_EXEC before execution
• AArch64 instructions are fixed 32-bit width — encoding is straightforward
• The assembler should support label-based forward references with backpatching (similar to bytecode jump patching)
• unsafe is allowed here per CLAUDE.md (JIT compiler)

Dependencies#

None — can be developed in parallel with shape/IC work.

Phase#

Phase 15: Performance