This document discusses Emscripten, an LLVM-based project that compiles C and C++ code into highly optimized JavaScript in asm.js format, allowing the code to run on the web near-natively and without plugins. It provides an example of using Emscripten to compile and run a simple "hello world" C program in JavaScript. Additionally, it notes the large number of math operations, in the billions, required for a music transcription problem involving analyzing audio samples.

1. EMSCRIPTEN, ASM.JS
AND BILLIONS OF MATH OPS
Luka Zakrajšek
@bancek
Webcamp Ljubljana 2016
March 12, 2016

2. PROBLEM
music transcription - from audio samples to music notes
⬇

3. ASM.JS
an extraordinarily optimizable, low-level subset of JavaScript
"Assembler for Web"

4. EMSCRIPTEN
LLVM-based project that compiles C and C++ into highly-
optimizable JavaScript in asm.js format
It lets you run C and C++ on the web at near-native speed,
without plugins.

5. C CODE
#include<stdio.h>
intmain(){
printf("hello,world!n");
return0;
}

6. COMPILE
$emcchello_world.c
And run:
$nodea.out.js
hello,world!

7. C LIBRARY EXAMPLE
varsamplesData=Module._malloc(samples.length*8);
varoutJson=Module._malloc(4);
varoutJsonSize=Module._malloc(4);
Module.HEAPF64.set(samples,samplesData/8);
Module.ccall('samplesToTtm','number',['number','number','number','number',
[samplesData,samplesCount,sampleRate,layer,outJson,outJsonSize]);
Module._free(samplesData);
varjsonSize=Module.getValue(outJsonSize,'i32');
varjsonPtr=Module.getValue(outJson,'i32');
varjsonBytes=Module.HEAPU8.subarray(jsonPtr,jsonPtr+jsonSize);
varjson=newTextDecoder('utf-8').decode(jsonBytes);
JSON.parse(json);

8. SOME NUMBERS
180s × 41 000 samples per second = 7 380 000 samples
3 600 hops × 345 buckets × 4 410 samples × 2 =
10 954 440 000 multiplications

9. RESULTS

10. DEMO