Optimize JS compiler caching for standard builds (#27026) Linker system stubs, such as libpthread_stub.js, are dynamically appended to settings.JS_LIBRARIES for all standard C/C++ builds. Previously, the JS output cache check did not distinguish between these system stubs and user-defined JS libraries (passed via --js-library). As a result, standard builds always bypassed the compilation cache, triggering a Node.js compilation run on every invocation. Restrict the cache bypass condition to check only for user libraries (libraries located outside the Emscripten source tree). Standard compilations now successfully hit the JS output cache, decreasing baseline compilation wall clock time of a hello world C program by 52% (from 472 ms down to 282 ms). Add an integration test to validate JavaScript compilation caching (covering hits, misses, user library bypasses, and option-based cache entries).
Main project page: https://emscripten.org
GitHub CI status: 
Chromium builder status: emscripten-releases
Emscripten compiles C and C++ to WebAssembly using LLVM and Binaryen. Emscripten output can run on the Web, in Node.js, and in wasm runtimes.
Emscripten provides Web support for popular portable APIs such as OpenGL and SDL2, allowing complex graphical native applications to be ported, such as the Unity game engine and Google Earth. It can probably port your codebase, too!
While Emscripten mostly focuses on compiling C and C++ using Clang, it can be integrated with other LLVM-using compilers (for example, Rust has Emscripten integration, with the wasm32-unknown-emscripten target).
Emscripten is available under 2 licenses, the MIT license and the University of Illinois/NCSA Open Source License.
Both are permissive open source licenses, with little if any practical difference between them.
The reason for offering both is that (1) the MIT license is well-known and suitable for a compiler toolchain, while (2) LLVM‘s original license, the University of Illinois/NCSA Open Source License, was also offered to allow Emscripten’s code to be integrated upstream into LLVM. The second reason became less important after Emscripten switched to the LLVM wasm backend, at which point there isn't any code we expect to move back and forth between the projects; also, LLVM relicensed to Apache 2.0 + exceptions meanwhile. In practice you can just consider Emscripten as MIT licensed (which allows you to do pretty much anything you want with a compiler, including commercial and non-commercial use).
See LICENSE for the full content of the licenses.