wasm32-unknown-emscripten programs crash with codegen-units > 1

[tlively]

When libstd is built with codegen-units > 1, programs built for wasm32-unknown-emscripten can crash.

I tried this code:

hello.rs

fn main() {
    println!("Hello, WebAssembly!");
}

rustc --target wasm32-unknown-emscripten hello.rs
node hello.js

I expected to see this happen:

Hello, WebAssembly!

Instead, this happened:

exception thrown: RuntimeError: invalid function,RuntimeError: invalid function
    at  (<WASM>[804]+7)
    at Object.Module.dynCall_vii (~/tlively/hello.js:5928:92)
    at invoke_vii (~/tlively/hello.js:6022:26)
    at  (<WASM>[550]+121)
    at  (<WASM>[804]+7)
    at Object.Module.dynCall_vii (~/tlively/hello.js:5928:92)
    at invoke_vii (~/tlively/hello.js:6022:26)
    at  (<WASM>[256]+130)
    at  (<WASM>[114]+50)
    at  (<WASM>[50]+7)
failed to asynchronously prepare wasm: RuntimeError: invalid function
(node:151136) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): RuntimeError: invalid function
(node:151136) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

However, compiling libstd with codegen-units = 1 gives the expected behavior.

Meta

rustc:

rustc 1.20.0-dev (3610a70ce 2017-07-05)
binary: rustc
commit-hash: 3610a70ce488953c5b0379fece70f2baad30a825
commit-date: 2017-07-05
host: x86_64-unknown-linux-gnu
release: 1.20.0-dev
LLVM version: 4.0

emcc: 1.37.15 (commit e99b50793a81b97a2559ec93fa49c7a0a8aaa4cc)
node: v8.1.2

config.toml:

[build]
submodules = false
target = ["x86_64-unknown-linux-gnu", "asmjs-unknown-emscripten", "wasm32-unknown-emscripten"]
 
[rust]
codegen-units = 48
debug-assertions = true
 
[llvm]
assertions = true
targets = "X86;JSBackend"
clean-rebuild = true
optimize = false
ccache = true

[tlively]

Update: codegen-units = 2 is sufficient to trigger this bug.

[tlively]

@kripken The emscripten-visible effect of having multiple codegen units is that the resulting rlib contains one object file for each codegen unit. Do you have a sense for whether this would be a Rust bug or an emscripten bug?

[CryZe]

Considering there's another breakage caused by codegen-units the issue before this one, I think it may be the same cause: #43095

[kripken]

Looks like this uses wasm32-unknown-emscripten, does that mean it uses the wasm backend (and not asm.js)?

[CryZe]

Nah, this right here is the old one that goes through asm.js

[kripken]

Then the target looks wrong, does changing it to the asm.js target (asmjs-unknown-emscripten) fix things?

[tlively]

yikes, my setup was all screwed up and was always using the wasm backend. Using emsdk incoming this works as expected for both wasm32-unknown-emscripten and asmjs-uknown-emscripten.

[tlively]

This is still an issue when using wasm32-experimental-emscripten. If I don't fix it in the next couple days I'll make a new issue.