A compiler front end for the C++ language

cxx-frontend is a work-in-progress compiler frontend for C++26 and C23

The compiler frontend is designed to be a powerful tool for developers, enabling them to parse, analyze, and modify C++ source code. This project aims to provide a robust foundation for building a complete C++ frontend, staying up-to-date with the latest language features and standards.

Playground showing the cxx compiler frontend in action at https://robertoraggi.github.io/cplusplus/

The API Reference is available at https://robertoraggi.github.io/cplusplus/docs/

Build the cxx compiler and the MLIR based backed

uv sync && source .venv/bin/activate # optonal, for the lit-based unit tests

cmake --preset default-mlir
cmake --build build
ctest --test-dir build --progress

The cxx tool will be available at ./build/src/frontend/cxx, and the default target is wasm32-wasip1.

./build/src/frontend/cxx --help
Usage: cxx [options] file...
Options:
  --help                       Display this information
  -D <macro>[=<val>]           Define a <macro> with <val> as its value. If just <macro> is given, <val> is taken to be 1
  -I <dir>                     Add <dir> to the end of the main include path
  -L <dir>                     Add <dir> to the end of the library path
  -U <macro>                   Undefine <macro>
  -std=<standard>              Assume that the input sources are for <standard>, one of 'c++14', 'c++17', 'c++20', 'c++23', 'c++26', or 'c23'
  --sysroot=<directory>        Use <directory> as the root directory for headers and libraries
...

Inspect and transform MLIR

After an MLIR-enabled build, use cxx and cxx-mlir-opt to inspect and transform MLIR, for example:

Convert the C++ code to MLIR and lift control flow to structured control flow.

echo 'int f(int n) { return n < 10 ? 123 : 321; }' |
    ./build/src/frontend/cxx -emit-ir -xc++ -std=c++26 - |
    ./build/src/mlir/opt/cxx-mlir-opt --cxx-lift-cf-to-scf

Compile to MLIR, lift control flow to structured control flow, convert back to control flow, and lower to LLVM IR:

echo 'int f(int n) { return n < 10 ? 123 : 321; }' |
    ./build/src/frontend/cxx -emit-ir -xc++ -std=c++26 - |
    ./build/src/mlir/opt/cxx-mlir-opt --cxx-lift-cf-to-scf |
    ./build/src/mlir/opt/cxx-mlir-opt --convert-scf-to-cf --convert-cxx-to-llvm |
    mlir-translate --mlir-to-llvmir

Covert to the ptr dialect and lower to LLVM IR

echo 'int read(int* p, int n) { return p[n]; }' |
    ./build/src/frontend/cxx -emit-ir -xc++ -std=c++26 - |
    ./build/src/mlir/opt/cxx-mlir-opt --cxx-lift-cf-to-scf --convert-cxx-to-ptr |
    ./build/src/mlir/opt/cxx-mlir-opt --convert-scf-to-cf --convert-cxx-to-llvm |
    mlir-translate --mlir-to-llvmir

Define a pipeline that outputs a mix of cxx, memref, ptr, and scf dialects:

./build/src/frontend/cxx -emit-ir \
    '-mlir-pass-pipeline=cxx.func(cxx-lift-cf-to-scf),convert-cxx-to-memref,convert-cxx-to-ptr' \
    source.cc -o source.mlir

Stage 2 bootstrap (on macOS)

cmake --workflow default-mlir
cmake --workflow stage1-macos
cmake --workflow stage2-macos

ls -la build-stage*/src/frontend/cxx

Build the NPM package and the Playground

The playground uses the Monaco Editor to demonstrate how to use the compiler frontend LSP implementation, and the MLIR based codegen pipeline.

https://robertoraggi.github.io/cplusplus/

# set EMSCRIPTEN_ROOT to the emscripten root, e.g. on macOS with emscripten installed via homebrew
export EMSCRIPTEN_ROOT=/opt/homebrew/opt/emscripten/libexec/

uv sync
source .venv/bin/activate

npm ci
npm run download-mlir
npm run build:cxx-frontend
npm run build:playground-sysroot
npm run playground

License

Copyright (c) 2026 Roberto Raggi [email protected]

Licensed under the MIT license.