Before 2017

• Write from scratch
• Use the kernel wrapper approach

2017: release of xeus

• A modern C++ implementation of the Jupyter Kernel Protocol
• Not a kernel, but a tool to make kernel

After 2017: xeus-based kernels

• xeus-python
• xeus-sql, xeus-sqlite
• xeus-cling, based on Cling, a C++ interpreter developped at the CERN

2022: split of xeus

• xeus: implementation of the Jupyter Kernel protocol with no middleware
• xeus-zmq: ZMQ-based middleware for xeus
• xeus-lite: emscripten-based middleware for xeus

Limitations of Cling

• Based on an outdated version of Clang
• Limited capabilities and compatibility with modern C++ features
• Hard to maintain on all the platforms

xeus-cpp, based on Clang

• Recent versions of Clang incorporated REPL in their codebase
• Extensible architecture based on plugins
• Supports modern versions of C++, and many features (CUDA, python)

Clang-Repl on Native Platforms

• Uses LLVM’s ORC JIT for runtime compilation
• Compiles code & executes directly from memory

Clang-Repl in the Browser

• WebAssembly enforces a strict sandbox model
• Can’t write or modify executable memory at runtime
• Memory is separated into code and data (Harvard architecture)

LLVM 17 introduced a WASM-specific IncrementalExecutor that avoids LLVM's ORC JIT

The new WasmIncrementalExecutor handles the wasm execution as follows:

• Each REPL input is parsed into a Partial Translation Unit (PTU)
• PTU is lowered to LLVM IR, then compiled to a WASM object file
• The object is linked with wasm-ld into a standalone binary (incr_module_x.wasm)
• The side module is dynamically loaded with emscripten’s dlopen, extending the main module

These modules:

• Share the same memory as the main wasm module
• Resolve symbols from earlier cells (cross-cell linking)
• Mimic dynamic linking, even though WASM doesn’t support shared libraries traditionally

🚀 This model effectively turned Clang-Repl into a live REPL for WebAssembly, enabling dynamic incremental C++ in the browser!

// custom_module.cpp
#include <iostream>

extern "C" {
  void my_custom_function() {
    std::cout << "Hello from my_custom_function!" << std::endl;
  }
}
    

emcc custom_module.cpp \
    -O2 \
    -s SIDE_MODULE=1 \
    -s WASM=1 \
    -fPIC \
    -o custom_module.so
    

• Last Value Printing
• Debugger Support
• CUDA Support
• Python Interoperatability
• Multiple Interpreters

Compiler Research Group

Google Summer of Code

Scan for slides