Fortitudo

Fortitudo is a browser-only C/C++ compiler explorer for JupyterLab, JupyterLite, and a standalone Lumino application. All three hosts use the same workbench and compiler worker. No kernel or remote compiler is needed.

Edit a function, choose a language, target, and optimization level, then select Compile or press Ctrl/Cmd+Enter. Inspect assembly and compiler messages; select a diagnostic to jump to its source location. Compilation runs in a worker, so editing remains available. Cancel terminates that worker; the next compile loads a fresh one.

The defaults are C++23, WebAssembly, and O2. C23 and O0–O3 are available. The packaged LLVM runtime reports WebAssembly, x86-64, and AArch64 backends. Native targets produce assembly with Clang built-in headers only; the packaged C/C++ system headers are for WebAssembly.

Source, options, and docked pane layout are saved by the host. Jupyter also keeps a browser copy scoped to the current workspace, protecting recent edits while its workspace writes are deferred. Reopening restores editing state without compiling. Output is explicitly marked out of date when source or options change. Invalid saved state opens a usable default session with feedback.

Running locally

The prebuilt extension requires JupyterLab 4.6 or later. Once the release is published, install it with:

python -m pip install fortitudo

Restart JupyterLab and select Open Fortitudo in the launcher or command palette. The wheel includes the compiler; installing it does not require Node or a compiler build. The npm package also includes the runtime and exports the shared workbench for applications that supply their own Lumino host.

See CONTRIBUTING.md for the complete Pixi setup and build sequence. The compiler is a separate heavyweight build; frontend builds require its generated assets and verify their hashes.

For an already built checkout:

pixi run --as-is jupyter lab
pixi run --as-is jlpm serve
pixi run --as-is jlpm serve:standalone

In JupyterLab or JupyterLite, select Open Fortitudo in the launcher or command palette. The Lite testbed is served on port 8080; the standalone preview prints its local address. These commands run in separate terminals.

Built standalone and Lite directories can be served below a URL prefix. Keep each site's compiler assets at their generated relative location. Serve .js as JavaScript, .wasm as application/wasm, and .data as application/octet-stream. Compilation works offline after initialization; offline page reload is a separate feature.

Architecture

• Pure model, request construction, and diagnostic parsing.
• One instance-owned store outside React.
• Lumino commands orchestrate semantic changes and compiler effects.
• Functional React views, with explicit store and CodeMirror bridges.
• A shared Lumino workbench owns layout, workers, and view lifecycles.
• Thin Jupyter and standalone adapters supply shell and persistence.

The shared package entry exports these contracts. Only src/jupyter/ imports JupyterLab packages; the plugin retains fortitudo:plugin.

Runtime and limits

The runtime is reproduced from WasmBolt with LLVM 23.1.0 and Emscripten 4.0.9. runtime/README.md records its origin, pins, licenses, and build details. Generated compiler/manifest.json records asset sizes and SHA-256 hashes. Browser test attachments record timings and Wasm memory observations.

The current compiler is large and reserves 256 MiB of initial Wasm memory, with memory growth enabled and a 32 MiB stack. Browser memory limits still apply. Cancellation releases the worker; a later compile must initialize another. Initialization and runtime failures offer a retry path. Ordinary compiler errors retain diagnostics and raw output.

This implementation produces assembly. IR/AST views, MLIR tools, graphs, execution, a terminal, sharing, arbitrary flags, automatic compilation, and automatic timeouts are deferred. The optional upstream MLIR driver remains packaged to preserve the runtime build, but is not downloaded eagerly.

License

Fortitudo is BSD-3-Clause licensed. The compiler incorporates WasmBolt and other separately licensed software. Required notices are included in runtime/licenses/ and copied into each distribution.