Optimize for Size

When serving a WASM binary, the smaller it is, the faster the browser can download it. Faster downloads means faster page load times and less data bandwidth use, and that means happier users.

This page gives some suggestions for how to make your WASM files smaller.

Do not prematurely optimize! You probably don't need small WASM files during development, and many of these techniques can get in the way of your workflow! They may come at the cost of longer compile times or less debuggability.

Depending on the nature of your application, your mileage may vary, and performing measurements of binary size and execution speed is recommended.

Twiggy is a code size profiler for WASM binaries, which you can use to make measurements.

For additional information and more techniques, refer to the Code Size chapter in the Rust WASM book.

Compiling for size instead of speed

You can change the optimization profile of the compiler, to tell it to prioritize small output size, rather than performance.

(although in some rare cases, optimizing for size can actually improve speed)

In Cargo.toml, add one of the following:

opt-level = 's'
opt-level = 'z'

These are two different profiles for size optimization. Usually, z produces smaller files than s, but sometimes it can be the opposite. Measure to confirm which one works better for you.

In Cargo.toml, add the following:

lto = "thin"

LTO tells the compiler to optimize all code together, considering all crates as if they were one. It may be able to inline and prune functions much more aggressively.

This typically results in smaller size and better performance, but do measure to confirm. Sometimes, the size can actually be larger.

The downside here is that compilation will take much longer. Do this only for release builds you publish for other users.

Use the wasm-opt tool

The binaryen toolkit is a set of extra tools for working with WASM. One of them is wasm-opt. It goes much further than what the compiler can do, and can be used to further optimize for either speed or size:

# Optimize for size (s profile).
wasm-opt -Os -o output.wasm input.wasm

# Optimize for size (z profile).
wasm-opt -Oz -o output.wasm input.wasm

# Optimize aggressively for speed.
wasm-opt -O3 -o output.wasm input.wasm

# Optimize aggressively for both size and speed.
wasm-opt -O -ol 100 -s 100 -o output.wasm input.wasm

Do you know of more WASM size-optimization techniques? Post about them in the GitHub Issue Tracker so that they can be added to this page!