WASM

03/12/2026

• What is WebAssembly (WASM)
• A binary instruction format that runs near-native-speed code in web browsers
• Not a programming language — a compilation target for C, C++, Rust, Go, etc.
• Runs in a sandboxed VM alongside JavaScript — W3C standard, all modern browsers
• Also runs outside browsers — Cloudflare Workers, Node.js, Deno


• Why WASM
• Performance: near-native speed for compute-heavy tasks
• Code reuse: bring existing C/C++ libraries to the web without rewriting in JS
• Portability: compile once, run anywhere that supports WASM


• How it works
• C/C++ source → compiled to .wasm binary via Emscripten
• Emscripten also generates a JS glue file (.js) that handles loading and binding
• Browser fetches and instantiates the .wasm, exported functions become callable from JS
• JS and WASM share a linear memory (ArrayBuffer) — data is copied into/out of this memory


• Emscripten & Embind
• Emscripten: the most widely used C/C++ → WASM compiler toolchain
• Compile with emcc (C/C++) or em++ (forces C++ mode), produces .js (glue) + .wasm (binary)
• Embind: Emscripten's binding system that maps C++ functions, structs, and types to JavaScript
• After compilation, JS can call C++ functions as if they were native JS functions


• Memory management
• Embind objects live on the WASM heap, not managed by JS garbage collector
• Must call .delete() on Embind class instances — forgetting causes memory leaks
• WASM heap grows dynamically but never shrinks


• Practical example: Instant Meshes as WASM
• Instant Meshes — C++ tool for field-aligned quad-dominant remeshing, compiled to WASM (~485 KB)
• Why WASM over rewriting in TS: research-grade algorithm, complex numerics, hard to validate a from-scratch port
• Key decisions:
  • Single-threaded: TBB replaced with a shim — avoids SharedArrayBuffer / pthreads
  • No file I/O: stubbed out since WASM has no filesystem
  • Pinned versions: Emscripten, source commit, Eigen all pinned for reproducible builds


Instant Meshes quad remeshing via WASM




• Limitations
• No DOM access — must call back to JS for browser APIs
• No threads by default — requires SharedArrayBuffer + COOP/COEP headers
• No filesystem — file I/O must be stubbed or use Emscripten's virtual FS
• Debugging — Chrome DevTools supports WASM debugging via DWARF debug info, but limited compared to JS

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