JIT

The JIT compiles hot functions from LIR to native code using Cranelift.

Architecture

LIR → FunctionTranslator → Cranelift IR → Native code → JitCode

Key types

JitCompiler — manages the Cranelift JITModule, declares

runtime helper symbols, tracks compilation stats

FunctionTranslator — walks LIR basic blocks and instructions,

emitting Cranelift IR

JitCode — wraps the native function pointer; keeps the module

alive for the code's lifetime

RuntimeHelpers — extern symbols the JIT calls back into the

VM (allocation, GC barriers, signal checks)

Function selection

Functions become JIT candidates based on a hotness threshold (default 10, controlled by --jit=N). The VM increments a counter on each call; when it crosses the threshold, the function is compiled.

Rejection tracking

Not all functions can be JIT-compiled. The JIT rejects functions that:

Use features not yet implemented in the translator
Fail Cranelift verification

Rejected functions are marked so the VM doesn't retry them.

Yield-through-call

For functions that call other functions which might yield, the JIT collects yield-site metadata during LIR emission. This enables proper save/restore sequences so a yielded fiber can resume into JIT code.

CLI flags

--jit=0       Disable JIT entirely
--jit=N       Compile after N-1 calls (default: --jit=11, threshold 10)
--jit=1       Compile on first call
--stats       Print compilation stats on exit

Files

src/jit/compiler.rs    JitCompiler, module management
src/jit/translate.rs   FunctionTranslator, LIR → Cranelift IR
src/jit/code.rs        JitCode wrapper
src/jit/vtable.rs      Runtime helper dispatch table
src/jit/dispatch.rs    JIT dispatch integration with VM

JIT