WebAssembly

Backend

  • Emscripten emits WASM using the upstream LLVM Wasm backend
    • LLD is a linker from the LLVM project that is a drop-in replacement for system linkers and runs much faster than them. It also provides features that are useful for toolchain developers.
      • The WebAssembly version of lld takes WebAssembly binaries as inputs and produces a WebAssembly binary as its output. For the most part it tries to mimic the behaviour of traditional ELF linkers and specifically the ELF lld port. Where possible the command line flags and the semantics should be the same.
  • The LLVM Wasm backend avoids traps by adding more code around each possible trap (basically clamping the value if it would trap). This can increase code size and decrease speed, if you don’t need that extra code. The proper solution for this is to use newer Wasm instructions that do not trap, by calling emcc or clang with -mnontrapping-fptoint. That code may not run in older VMs, though.

Compiler output

When using emcc to build to WebAssembly you will see a .wasm file containing that code, as well as the usual .js file that is the main target of compilation.

  • Note that the .wasm file is not standalone - it’s not easy to manually run it without that .js code, as it depends on getting the proper imports that integrate with JS. For example, it receives imports for syscalls so that it can do things like print to the console. There is work in progress towards ways to create standalone .wasm files, see the WebAssembly Standalone page.
    • WebAssembly currently depends on a JavaScript runtime for features like longjmp, C++ exceptions, checking the date or time, printing to the console, using an API like WebGL, etc., and Emscripten will emit a JavaScript runtime to support those things.
      • if you don’t need those things - if you have mostly pure computational code - or if you want to write your own JavaScript runtime, you can tell Emscripten to emit a “standalone” wasm file, one which doesn’t depend on an Emscripten JS runtime. That lets you write your own runtime if you want, and you may be able to run the wasm in a a non-JS environment such as Wasmer, WAVM, or wasmtime.
  • You may also see additional files generated, like a .data file if you are preloading files into the virtual filesystem.