Broken WASM codegen with u128 and wasm_abi

[kostko]

It seems that the following code:

#![feature(wasm_abi)]

#[no_mangle]
pub extern "wasm" fn test() {
    format!("boom: {}", 0u128);
}

can result in invalid WASM being generated in case it is compiled with lto = true. I've created a repository with the minimal example that reproduces the issue when built with (tested on rustc 1.56.0-nightly (0035d9dce 2021-08-16)):

cargo build --target wasm32-unknown-unknown --release

Parsing the generated WASM will produce an error similar to the following:

[parse exception: attempted pop from empty stack / beyond block start boundary at 536 (at 0:536)]
Fatal: error parsing wasm

Using an argument of type u64 or disabling lto or using the C ABI instead of wasm ABI (which enables LLVM's multivalue feature) works.

Meta

rustc --version --verbose:

rustc 1.56.0-nightly (0035d9dce 2021-08-16)
binary: rustc
commit-hash: 0035d9dcecee49d1f7349932bfa52c05a6f83641
commit-date: 2021-08-16
host: x86_64-unknown-linux-gnu
release: 1.56.0-nightly
LLVM version: 12.0.1

[memoryruins]

@rustbot label: +O-wasm +A-abi +A-ffi +A-lto

[Jules-Bertholet]

@rustbot label I-unsound

[bjorn3]

extern "wasm" has been removed. Can't reproduce with

echo '#[no_mangle]
pub extern "C" fn test() {
    format!("boom: {}", 0u128);
}' | rustc +nightly - --target wasm32-unknown-unknown --crate-type cdylib -Clto=yes -Copt-level=3

[kostko]

Yeah as mentioned in the original issue:

Using an argument of type u64 or disabling lto or using the C ABI instead of wasm ABI (which enables LLVM's multivalue feature) works.

If the wasm ABI was removed, then this issue is likely no longer relevant.

[nikic]

Right, there should no longer be any way to expose this from Rust anymore. There is an upstream LLVM issue tracking i128 with multi-value here: llvm/llvm-project#98323