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Previous writings about Mozilla work

I am currently a compiler engineer at Mozilla corporation, the company making the Firefox browser among else. Our JavaScript virtual machine, Spidermonkey, is split in several tiers, including an highly optimizing Just-In-Time (JIT) compiler able to compile JavaScript to assembly at runtime. My previous work has involved efficiently compiling Float32 arithmetic to hardware instructions and implement a new SIMD API for the Web.

About Float32 optimizations

The full blog post is there. It has been written in November 2013.

The main idea is that if you have float32 inputs to an operation; and you cast them to doubles; and you apply an arithmetic operation to these inputs; and you cast the result back to a float32, then you’d have the same result as if you did the entire computation with float32 values and operations.

So we’ve introduced an operation in JavaScript that converts a Number to its closest float32 IEEE754 representation: Math.fround. Said differently, the above equivalence says that:

function f(x, y) {
    return x + y;

function g(x, y) {
    var xf = Math.fround(x);
    var yf = Math.fround(y);
    return Math.fround(xf + yf);

// For all x, y that can be represented exactly as float32:
assert(f(x, y) === g(x, y));

Yes, ===. The same === you’ve been told not to use for floating-point Numbers. But here, we have bitwise equality, so we can use strict equality*.

Processors have special instructions for carrying out float32 arithmetic, which have higher throughput than the equivalent double ones. With this result in mind, we could add a pass that would spot opportunities where the computations are equivalent (thanks to Math.fround hints) and emit float32 instructions instead of double instructions. This sped up a some numerical applications and games engines by a few points.

* a careful reader would object that this is wrong for x = y = NaN, which I’ve put away for the sake of simplicity.

About SIMD.js

The full blog post is there. It has been written in March 2015.

Nowadays, processors have instructions sets that allow them to execute several simple arithmetic operations at once. For instance, let’s say you have two arrays of integers and you want to add each element to the corresponding one in the other array. If both arrays have size N, this means you’ll have to carry out N scalar additions. But processors can actually group these into bundles of several additions, with SIMD; for the case of 32-bits wide integers, on most modern processors, you need at most Math.ceil(N / 4) instructions. The blog post details what SIMD.js is and what bottlenecks we hit during implementation.


This was a small reminder about previously written blog posts. If you’re into JavaScript, compilers or low-level optimization, I can only recommend you to go read the Mozilla’s JavaScript blog.