This document discusses using Emscripten to compile C++ code to JavaScript. It shows how Emscripten adds an asm.js backend to LLVM, allowing any LLVM-supported language like C++ to target JavaScript. Benchmarks show Emscripten-compiled code runs faster than native JavaScript for tasks like summing arrays and matrix multiplication by using optimized libraries and compiler optimizations not available to JavaScript. The document advocates using Emscripten to compile C++ to JavaScript when performance is important, as it allows using the best tools and libraries for the job.

1. From C++ to JavaScript with Emscripten
Tom Nickson

2. asm.js
• Subset of JavaScript - so portable
• Predictable high performance
• Assembly code for the web

3. Emscripten
• Adds an asm.js backend to LLVM
• Any LLVM supported language can now target JS
• C++ supported by Clang

4. DEAR GOD WHY

5. ML = numerics
• Optimisation
• Linear algebra
• Other depres^H^H important things

6. LOOK AT THIS CODE
for (auto i = 0; i < 10000000; ++i) {
acc = acc + v[i] * v[i];
}
cout << acc << endl;
for (i=0; i<10000000; ++i) {
acc = acc + x[i] * x[i]
}
console.log(acc);

7. Benchmark: Sum numbers
• JS: 1 second
• Emscripten JS: 0.5 seconds
• C++ default: 0.1seconds

8. Matrix Multiplication
• Naïve algorithm has complexity O(n3)
• Order of memory access has major effect on speed
• Why write code when you don’t have to

9. ALSO THIS CODE
for (var r = 0; r < n; r++) {
for (var c = 0; c < m; c++) {
for (var i = 0; i < m; i++) {
v3[c*m + r] += v1[c*m + i] * v2[i*n + r];
}
}
}
for (auto r = 0; r < n; ++r) {
for (auto c = 0; c < m; ++c) {
for (auto i = 0; i < m; ++i) {
v3[r*m + c] += v1[r*m + i] * v2[i*n + c];
}
}
}

10. THAT CODE SUCKED
matrix<double> m3 = m1 * m2;
var matrix3 = matrix1.multiply(matrix2);

11. Benchmark: Matrix Multiplication
• Naïve JS: 25 Seconds
• Naïve Emscripten: 10.5 Seconds
• Sylvester JS: 46 Seconds
• dlib Emscripten: 7 Seconds

12. WHY SHOULD YOU CARE

13. Right tool for the job
• Better optimisers
• More and faster libraries
• Better debugging

14. complete
tom@osculate.it
@lordoftheengine