wasm-opt: Exported global cannot be mutable

[Urhengulas]

🐛 Bug description

I am using wasm-pack and wasm-bingen to build a javascript package with rust.

Since today when I am running wasm-pack build I am getting following error:

[INFO]: Checking for the Wasm target...
[INFO]: Compiling to Wasm...
    Finished release [optimized + debuginfo] target(s) in 13.59s
[INFO]: Installing wasm-bindgen...
[INFO]: Optimizing wasm binaries with `wasm-opt`...
[wasm-validator error in module] unexpected true: Exported global cannot be mutable, on 
global$0

# --- omitted .wast output ---

Fatal: error in validating input
Error: failed to execute `wasm-opt`: exited with exit code: 1
  full command: "~/.cache/.wasm-pack/wasm-opt-4d7a65327e9363b7/wasm-opt" "$REPO/rav1e_js/pkg/rav1e_js_bg.wasm" "-o" "$REPO/rav1e_js/pkg/rav1e_js_bg.wasm-opt.wasm" "-O"
To disable `wasm-opt`, add `wasm-opt = false` to your package metadata in your `Cargo.toml`.

It was still working on master yesterday (see CI), but doesn't anymore today with the same commit (see CI). (The important step is "Build").

🤔 Expected Behavior

Build package and optimize it with wasm-opt as it did before.

(Running wasm-pack build --dev works, but I'd like to have the optimization).

👟 Steps to reproduce

Trying a few things I figured out that I can make wasm-opt pass when I am removing all exported (non-static) methods on the exported structs (which is abviously not desireable).

On urhengulas/rav1e@wasm-opt-fix I removed all the methods and it builds and optimizes fine:

➜  rav1e git:(wasm-opt-fix) ✗ cd rav1e_js && wasm-pack build
[INFO]: Checking for the Wasm target...
[INFO]: Compiling to Wasm...
    Finished release [optimized + debuginfo] target(s) in 0.05s
[INFO]: License key is set in Cargo.toml but no LICENSE file(s) were found; Please add the LICENSE file(s) to your project directory
[INFO]: Installing wasm-bindgen...
[INFO]: Optimizing wasm binaries with `wasm-opt`...
[INFO]: :-) Done in 0.88s
[INFO]: :-) Your wasm pkg is ready to publish at /home/urhengulas/Documents/github.com/xiph/rav1e/rav1e_js/pkg.

But, as soon I am adding a method (e.g. fn Frame.debug(&self) -> Self in rav1e_js/src/frame.rs) back, I am getting the error from the bug description.

🌍 Your environment

Include the relevant details of your environment.

➜  rustc --version
rustc 1.45.0 (5c1f21c3b 2020-07-13)
➜  wasm-pack --version
wasm-pack 0.9.1
➜  ~/.cache/.wasm-pack/wasm-opt-4d7a65327e9363b7/wasm-opt --version
wasm-opt version_90

Thx

Thanks to everyone taking the time and having a look at this issue!

[EverlastingBugstopper]

saw this same issue the other day in some e2e tests

[alixinne]

For me this issue showed up in CI. The last build that was correct before the failed one today was on July 21st. The nightly compiler from that version already exhibited the bug so it seems the cause is some dependency somewhere triggering this behavior which is not supported by wasm-opt, but I haven't been able to trace which one yet.

[Urhengulas]

Thanks for the input @EverlastingBugstopper, @vtavernier.

---

I've created a bit more atomic example:

// src/lib.rs
use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub struct Boat {
    length: u32,
}

#[wasm_bindgen]
impl Boat {
    // works
    pub fn new(length: u32) -> Self {
        Self { length }
    }

    // doesn't work
    pub fn float(&self) -> String {
        String::from("floating... ⛵")
    }

    // works
    pub fn volume(&self) -> u32 {
        self.length * 2
    }
}

// doesn't work
#[wasm_bindgen]
pub fn string() -> String {
    String::from("wasm-opt")
}

So it seems returning a String fails wasm-opt.

It is also failing, when returning some structs, but for others not. I will investigate what the failing ones have in common.

[alixinne]

For me the bug was introduced by wasm-bindgen 0.2.66 (0.2.65 is fine).

[Urhengulas]

The problem seems to be with the non-MVP webassembly feature mutable-global. Adding --enable-mutable-globals to the wasm-opt command seems to fix it (see WebAssembly/binaryen#3006 (comment)).

My workaround is now:

1. disable wasm-opt in your Cargo.toml ...

[package.metadata.wasm-pack.profile.release]
wasm-opt = false

2. ... run wasm-pack ...

$ wasm-pack build

3. ... and optimize manually

$ /path/to/wasm-opt pkg/repo_bg.wasm -o pkg/repo_bg.wasm -O --enable-mutable-globals

(My /path/to/wasm-opt is ~/.cache/.wasm-pack/wasm-opt-4d7a65327e9363b7/wasm-opt.)

---

Edit: Originally I recommended to use wasm-opt [...] -O -all, but this creates new errors.

[ACollectionOfAtoms]

Ran into the same issue while doing the wasm game of life tutorial: https://rustwasm.github.io/docs/book/game-of-life/implementing.html

@Urhengulas It's possible to specify wasm-opt arguments, so you should be able to do this:

[package.metadata.wasm-pack.profile.release]
wasm-opt = ["-Oz", "--enable-mutable-globals"]

Change -Oz to whatever optimization level you need.