Compile a Sum Rust Function

RustWasm.md

Install rust using rustup

brew install rustup
rustup-init
# new shell for env (or source .cargo/env), then:
rustup target add wasm32-wasi

Compile Cowsay

git clone https://github.com/wapm-packages/cowsay
cd cowsay
cargo build --target wasm32-wasi --release

gives:

Updating crates.io index
Downloaded atty v0.2.14
...
Compiling cowsay v0.1.0 (/private/tmp/cowsay)
warning: use of deprecated function `rand::sample`: renamed to seq::sample_iter
...
warning: `cowsay` (bin "cowsay") generated 5 warnings (run `cargo fix --bin "cowsay"` to apply 2 suggestions)
  Finished release [optimized] target(s) in 9.62s

Cowsay is smaller? Hm.

du -sh target/wasm32-wasi/release/cowsay.wasm
712K	target/wasm32-wasi/release/cowsay.wasm

But still 712k vs 4k for the native binary, aka 178⨯ the size.

ls -lh /opt/homebrew/Cellar/cowsay/3.04_1/bin/cowsay
-rwxr-xr-x  1 helge  admin   4.1K Mar  8  2023 /opt/homebrew/Cellar/cowsay/3.04_1/bin/cowsay

Well, I initially thought, but then I wondered and:

otool -L /opt/homebrew/Cellar/cowsay/3.04_1/bin/cowsay
/opt/homebrew/Cellar/cowsay/3.04_1/bin/cowsay: is not an object file

head -n 4 /opt/homebrew/Cellar/cowsay/3.04_1/bin/cowsay
#!/usr/bin/perl

##
## Cowsay 3.03

🙈

So let's try this cowsay.c:

curl -o cowsay.c https://raw.githubusercontent.com/0xAether/ccowsay/master/cowsay.c
clang -O3 cowsay.c -o cowsay
ls -lh cowsay 
-rwxr-xr-x  1 helge  wheel    50K Apr  7 12:21 cowsay

But still 712k vs 50k for the native binary, aka 14⨯ the size.

Compile Own Rust Lib

helge@M2ni /tmp % cargo new --lib sum
     Created library `sum` package
helge@M2ni /tmp % cd sum

Add this to Cargo.toml:

[lib]
crate-type = ["cdylib"]

And replace lib.rs w/ the sum function:

#[no_mangle]
extern "C" fn sum(a: i32, b: i32) -> i32 {
  let s = a + b;
  println!("From WASM: Sum is: {:?}", s);
  s
}

Release Build yields a stunning 1.6MB:

cargo build --release --target wasm32-wasi
du -sh target/wasm32-wasi/release/sum.wasm
1.6M	target/wasm32-wasi/release/sum.wasm

OK, maybe it is just what "print" adds? Let's see:

#[no_mangle]
extern "C" fn sum(a: i32, b: i32) -> i32 {
  let s = a + b;
  // println removed
  s
}

cargo build --release --target wasm32-wasi
du -sh target/wasm32-wasi/release/sum.wasm
1.6M	target/wasm32-wasi/release/sum.wasm

OK, as shown by @codertrix the thing has to be explicitly stripped, adding this to the Cargo.toml yields to the expected result:

[profile.release]
opt-level = "z"
strip = "symbols"
lto = true
panic = "abort"
codegen-units = 1

Gives:

ls -lh target/wasm32-wasi/release/sum.wasm
-rwxr-xr-x  1 helge  wheel   108B Apr  7 15:31 target/wasm32-wasi/release/sum.wasm

108 bytes, no we are talking!

Still a little much for what is adding two numbers, but let's assume this is just a common .wasm framing overhead. Adding an identical second function sum2 adds 19 bytes.