ecmascript/jit

Manages JIT (Just-In-Time) compilation: decides when to compile hot methods, schedules compilation tasks on background threads, and installs compiled code into the runtime. Primary Language: C++

Design

The JIT module is the runtime-side coordinator that connects hotness detection to the compiler:

  Interpreter executes bytecode
       │  increments hotness counter
       ▼
  CompileDecision
    ├── Checks hotness threshold
    ├── Checks function kind support
    ├── Checks VM state (GC, etc.)
    └── Decides: compile baseline or optimized?
       │
       ▼
  Jit::Compile()
       │  creates JitTask
       ▼
  JitTask (posted to JitThread)
    ├── Sets up JitCompilationEnv
    ├── Calls into compiler pipeline (PassManager)
    └── Installs compiled code on completion
       │
       ▼
  JitResources
    ├── Manages compilation memory
    └── Tracks concurrent compilation state

Key design decisions:

• Async compilation — JIT compilation runs on JitThread to avoid blocking the main JS thread.
• Tiered compilation — CompileDecision can trigger either baseline (fast, unoptimized) or optimized (slow, PGO-guided) compilation based on method characteristics.
• Profiling integration — JitProfiler collects per-bytecode type feedback during interpretation, feeding the optimizing compiler.
• Diagnostics — JitDfx provides logging and timing information for debugging JIT behavior.

Key Files

File	Role
jit.cpp/.h	JIT lifecycle management and Compile() entry point
compile_decision.cpp/.h	Hotness-based compilation trigger heuristics
jit_task.cpp/.h	Compilation task lifecycle (create → compile → install)
jit_thread.cpp/.h	Background compilation thread management
jit_resources.cpp/.h	Compilation memory and resource tracking
jit_profiler.cpp/.h	Bytecode-level type feedback collection
jit_dfx.cpp/.h	JIT diagnostics and timing

How to Modify

Tune JIT trigger heuristics:

1. Modify CompileDecision in compile_decision.cpp
2. Key factors: hotness threshold (--compiler-jit-hotness-threshold), function kind filtering, VM state checks
3. Test with varying workloads to avoid over/under-compilation

Building

Standalone repo (from repo root ../../):

python ark.py x64.debug

Full OHOS tree:

./build.sh --product-name <product-name> --build-target ark_js_host_linux_tools_packages

Tests

Standalone repo (from repo root ../../):

python ark.py x64.debug unittest   # all unit tests (includes JIT tests)

Full OHOS tree:

./build.sh --product-name <product-name> --build-target ecmascript/jit/tests:host_unittest

• tests/jit_test.cpp — JIT compilation flow tests
• tests/jit_time_scope_test.cpp — Timing/profiler tests

Add new tests in tests/BUILD.gn under group("host_unittest").

Boundaries

• Compile-trigger policy changes must be synchronized across compile_decision.cpp, jit_task.cpp, and jit_thread.cpp.
• Backend compilation logic belongs in ecmascript/compiler/ — this module only schedules and coordinates.
• Keep timing/profiler assertions in tests deterministic (avoid flaky timing-only checks).
• Deoptimization handling is in ecmascript/deoptimizer/, not here.