Recommend wasm-snip for Rust wasm binaries

[jamesmcm]

Hi,

One thing I noticed working through the tutorial is the moment you do anything that can panic e.g. FRAMEBUFFER.as_mut().unwrap() the binary size explodes from ~800 bytes to 27 kilobytes as Rust includes all the panicking and formatting code.

You can get a reasonable binary size by using wasm-snip to remove that code:

In Cargo.toml

[profile.release]
opt-level = "z"
lto = true
panic = 'abort'
debug = true

$ cargo build --release
$ wasm-snip --snip-rust-fmt-code --snip-rust-panicking-code target/wasm32-unknown-unknown/cart.wasm
$ w4 run target/wasm32-unknown-unknown/release/cart2.wasm

And it seems to work as expected (note this probably means having to avoid use std::fmt::* stuff though, but it's probably best practice to largely avoid the standard library anyway).

It might be worth adding language speciifc notes to the docs for stuff like this,

[ayuusweetfish]

I'd like to share my current approach for even smaller binary sizes: run the output through wasm-snip, followed by wasm-opt (from binaryen) followed by wasm-strip (from WABT) and another pass through wasm-opt (not sure how, but sometimes it further prunes off a few bytes).

Flags:

• wasm-snip --snip-rust-panicking-code (formatting support is kept)
• wasm-opt -Oz --zero-filled-memory --strip-producers --dce (enabled DCE in addition to the C template's Makefile)
• wasm-strip (no flags)

(Upd. Found that others are using exactly the same flags, with further experimentation here: christopher-kleine/wasm-4-tutorial-games#17)

Meanwhile, it is worth mentioning that string literals introduced by panicking subroutines still cannot be properly pruned by these tools. (source, archive; the problem persists as of today.)

Also, see the same blog post for suggestions for avoiding panics. Perhaps also consider going for unsafe code in a safe function when it comes to raw memory access.

[maxcurzi]

A note on the very first comment, the bash commands didn't work for me for two reasons 1) The "cart.wasm" was on the release subfolder 2) wasm-snip as is outputted the result in the command line so I needed to redirect it in another cart file

This is the set of commands that worked for me:

cargo build --release
wasm-snip --snip-rust-fmt-code --snip-rust-panicking-code target/wasm32-unknown-unknown/release/cart.wasm > target/wasm32-unknown-unknown/release/cart_snipped.wasm
w4 run target/wasm32-unknown-unknown/release/cart_snipped.wasm

and optionally (for further space reduction)

wasm-opt target/wasm32-unknown-unknown/release/cart_snipped.wasm -Oz --zero-filled-memory --strip-producers --dce --output target/wasm32-unknown-unknown/release/cart_snipped_optimized.wasm
w4 run target/wasm32-unknown-unknown/release/cart_snipped_optimized.wasm