Non existing function imported when compiling for wasm

[athei]

Whenever I try to compile some specific code for wasm32-unknown-unknown with enabled overflow-checks and lto the compiler inserts a function import into the resulting wasm module that looks like this:

(import "env" "_ZN4core9panicking5panic17h80a3410ec4f43255E" (func $_ZN4core9panicking5panic17h80a3410ec4f43255E (type 0))

These are the complete contents of the lib.rs:

#![no_std]

#[panic_handler]
fn my_panic(_info: &core::panic::PanicInfo) -> ! {
    loop {}
}

#[no_mangle]
pub fn multer(a: i128, b: i128) -> i128 {
    // trigger usage of the __multi3 compiler intrinsic which seems to be necessary
    // in order to reproduce the bug
    a * b
}

What seems to trigger the bug are the following conditions:

• The --target must be wasm32-unknown-unknown
• overflow-checks = true
• lto = true
• Recompile core libs with cargo xbuild or the std aware cargo
• The call of the panic function originates in some compiler extrinsic

The minimal reproducer repo contains the exact command line that reproduces the bug.

I expected to see this happen:
The resulting wasm module does not import any function from the environment.

Instead, this happened:
The resulting wasm module imports the previously mentioned panic function from the environment.

Meta

rustc +nightly --version --verbose:

rustc 1.49.0-nightly (ffa2e7ae8 2020-10-24)
binary: rustc
commit-hash: ffa2e7ae8fbf9badc035740db949b9dae271c29f
commit-date: 2020-10-24
host: x86_64-apple-darwin
release: 1.49.0-nightly
LLVM version: 11.0

[jyn514]

@athei AFAIK wasm doesn't support unwinding, I think you need panic = abort in Cargo.toml.

[athei]

@athei AFAIK wasm doesn't support unwinding, I think you need panic = abort in Cargo.toml.

I have.
https://github.com/athei/mult-panic/blob/5ff5fd6a929581b11bea21150c28843407d6df6b/Cargo.toml#L15

That said, it appears that it is disabled automatically if the target spec disables unwinding.

[vminkov]

Does anybody have a solution for this?

[cmichi]

@athei I just tried your reproducer from https://github.com/athei/mult-panic with the latest rustc 1.56.0-nightly (2827db2b1 2021-08-01) and there is no more (import "env" …) in the Wasm. Can you confirm that the bug is no longer existent?

[athei]

I can confirm that the reproducer no longer reproduces the bug with the new rust nightly. This does not necessarily mean that the root cause is fixed, though.

[jyn514]

@athei would you be interested in adding a test for this? I doubt we'll be able to track down the exact cause, but preventing it from regressing seems just as good :)

[athei]

Yes. I would be interested. Can you toss me in the general direction where this test would go (as in which test sub directory)?

[jyn514]

@athei it depends what you need to test - it looks like the code here did compile successfully, it just didn't run, so you'd probably need a new run-make test (since there's no way to run wasm natively).