Wasmer: Can't handle multiple output using wasmer.

[AchalaSB]

Hi,
It would be nice if we are able to return multiple output from source code through wasmer.
(wasm is currenlty accepting single output and they are working on multi-value output)

Tryed to return multiple result using go-wasmer. Here is sample code

#[no_mangle]
pub extern fn test(x: i32, y: i32, z: i32) -> (i32, i32, i32) {
    (x, y, z) 
}

// try this since test function returning 0
#[no_mangle]
pub extern fn sub_main() -> (i32, i32, i32) {
   test(5, 1, 3)
}

fn main() {}

When I try to call test function from wasmer

sum := instance.Exports["test"] 
result, _ := sum(4, 5, 3)

It is returning result as 0. And if I call sub_main function

sum := instance.Exports["sub_main"] 
result, _ := sum()

It is returning error

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4a2f15]

goroutine 1 [running]:
main.main()
        /go-ext-plugin/wasmer-ext.go:21 +0x125
exit status 2

[danaugrs]

Looking forward to Wasmer integrating the multi-value proposal! Any updates on this?

[Hywan]

We are working on it. It should land soon :-).

[AchalaSB]

@syrusakbary @Hywan
Any update on this? as Multi value return reached to phase4

[diegobes]

In which branch is the work in progress ?

[nlewycky]

feature/multi-value-return

[syrusakbary]

The main reason this is taking a bit more than expected is because one of the backends that we are using (cranelift) doesn't handle well the System-V ABI for doing transparent native calls between the host and the wasm (without trampolines).

Here's the link if you would like to get more info:

cranelift #1178 (comment).

[syrusakbary]

This should be solved in the refactor :)

[AchalaSB]

@syrusakbary I assume this issue has been resolved in the refactor. But I wanted to know
How rust to wasm compilation done for this?
cargo build --target=wasm32-unknown-unknown is the regular command for compilation. What is the command for this multi-value returns?
PN: with rustc it may possible but there I can't use any crates like i do in cargo build

[michaelvoronov]

@AchalaSB According to the docs: the environment variable RUSTFLAGS can be used to pass options for code generation to the Rust compiler. These flags will affect all compiled crates.

So

RUSTFLAGS="-C target-feature=+multivalue" cargo build --target=wasm32-unknown-unknown

can help you.

P.S. Also it may be usefull to use build.rustflags

[AchalaSB]

@michaelvoronov Nope this command won't work.
I ran this for simple code

#[no_mangle]
pub extern fn test(x: i32, y:i32) -> (i32, i32){
    (x+y , x-y)
}

but wasm file is broken, when I tries to open wasm file its shown errors

Unable to open 'testing.wasm': Unable to resolve text model content for resource wasm-preview:/home/achala/testing/target/wasm32-unknown-unknown/debug/testing.wasm.
Error while reading the Wasm: readWasm failed: 0000030: error: expected valid result type (got -0x20)

[syrusakbary]

Closing issue since multivalues is now fully supported in Wasmer :)