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Browser Embedding

Unsurprisingly, one of WebAssembly's primary purposes is to run on the Web, embedded in Web browsers (though this is not its only purpose).

This means integrating with the Web ecosystem, leveraging Web APIs, supporting the Web's security model, preserving the Web's portability, and designing in room for evolutionary development. Many of these goals are clearly reflected in WebAssembly's high-level goals.

Implementation Details

We've identified interesting implementation approaches which help convince us that the design, especially that of the MVP, are sensible:

  • WebAssembly's security model should depend on CORS and subresource integrity to enable distribution, especially through content distribution networks and to implement dynamic linking.
  • A module can be loaded in the same way as an ES6 module (import statements, Reflect API, Worker constructor, etc) and the result is reflected to JS as an ES6 module object.
    • Exports are the ES6 module object exports.
    • An import first passes the module name to the module loader pipeline and resulting ES6 module (which could be implemented in JS or WebAssembly) is queried for the export name.
    • There is no special case for when one WebAssembly module imports another: they have separate heaps and pointers cannot be passed between the two. Module imports encapsulate the importer and importee. Dynamic linking should be used to share heaps and pointers across modules.
    • To synchronously call into JavaScript from C++, the C++ code would declare and call an undefined extern function and the target JavaScript function would be given the (mangled) name of the extern and put inside the imported ES6 module.
  • Once threads are supported, a WebAssembly module would initially be distributed between workers via postMessage().
    • This also has the effect of explicitly sharing code so that engines don't perform N fetches and compile N copies.
    • May later standardize a more direct way to create a thread from WebAssembly.
  • Once SIMD is supported, a Web implementation of WebAssembly would:
    • Be statically typed analogous to SIMD.js-in-asm.js;
    • Reuse specification of operation semantics (with TC39);
    • Reuse backend implementation (same IR nodes).