Emscripten Docs, Materials, Learnings

Emscripten lacks developer resources. Since we often reverse engineer Emscripten code, let's centralize our efforts, document our learnings, and use this issue as a hub for everything we learn about Emscripten.

• Compilation flags
  • There are two layers of flags: Upstream Clang flags (including some related to WebAssembly!) and Emscripten-specific flags
• Memory
  • Emscripten-built binaries are 32 bit
  • The 64 bit memory proposal is in the works
  • Meanwhile, would the -sMEMORY64=2 flag help with anything? The docs says: "The “architecture” to compile for. 0 means the default wasm32, 1 is the full end-to-end wasm64 mode, and 2 is wasm64 for clang/lld but lowered to wasm32 in Binaryen (such that it can run on wasm32 engines, while internally using i64 pointers). Assumes WASM_BIGINT."
• WASI
  • Emscripten is unlikely to support WASI. Any WASI support in Chrome will come through a third-party JavaScript integration library.
  • https://github.com/WordPress/wordpress-playground/issues/290
• Async
  • Asyncify PHP builds are unstable
  • JSPI – learnings from migrating Asyncify -> JSPI
• Filesystem
  • The JavaScript filesystem API works, but it may not get significantly more features
  • The Emscripten team heavily invests in a C-level filesystem implementation called WASMFS. It requires pthreads or JSPI.
  • adamziel/emscripten-wasmfs-demo repository has a few examples of WASMFS
  • OPFS Explorer enables browsing OPFS in chrome devtools
  • I don't know any prior art on synchronizing concurrent writes from multiple workers to OPFS

cc @brandonpayton @bgrgicak @mho22 @psrpinto @akirk

Start by reviewing the existing Emscripten references in settings.js, the asyncify documentation, the settings reference, and the linked WASMFS examples. Organize the documented learnings about compilation flags, memory, WASI, async behavior, and filesystems into a maintainable developer-resource hub, with each topic supported by the cited references.