Stream: wasm

Topic: ✔ wasmlink import export difference

Scott Waye (Aug 05 2021 at 16:05):

The example for wasmlink with the renderer and the markdown. In the markdown module the export is

 (export "render" (func $render.command_export))

with a type of

(func $render.command_export (type 7) (param i32 i32) (result i32)

In the renderer the import is

(import "markdown" "render" (func $renderer::markdown::render::witx_import::h556dc1094c69443f (type 8)))

with a type of

 (type (;8;) (func (param i32 i32 i32)))

Why does the import take the result from a param and not a result?. Is the result a pointer to a block of 2 i64s?

Scott Waye (Aug 05 2021 at 16:06):

I mean the last parameter is that a pointer to 2 i64s that are populated by the markdown module, or is it a pointer to a pointer to 2 i64s and the pointer is supplied by the markdown/linker?

Scott Waye (Aug 05 2021 at 16:14):

Forgot the online tool which is super useful, thanks for that. I see its the first option

      #[link(wasm_import_module = "input")]
      extern "C" {
        #[cfg_attr(target_arch = "wasm32", link_name = "hello")]
        #[cfg_attr(not(target_arch = "wasm32"), link_name = "input_hello")]
        fn witx_import(_: i32, _: i32, _: i32, );
      }
      witx_import(ptr0, len0, ptr1);
      let len2 = *((ptr1 + 8) as *const i32) as usize;
      String::from_utf8(Vec::from_raw_parts(*((ptr1 + 0) as *const i32) as *mut _, len2, len2)).unwrap()
    }
  }
  static mut RET_AREA: [i64; 2] = [0; 2];

Peter Huene (Aug 05 2021 at 17:03):

Hi @Scott Waye. wasmlink is expecting the canonical ABI that witx-bindgen uses for its imports and exports. For functions that return "multiple values" (i.e. anything that can't fit in a single wasm value type, which a list is considered to be because it returns a pointer-length pair), the importer (caller) passes in an additional argument that is a pointer to an array of 64-bit values and the exporter (callee) returns a pointer to an array of 64-bit values of the same length. This facilitates the adapter code that sits between the caller and callee as it does not need to explicitly allocate any space in either linear memory; the code generated by witx-bindgen handles allocating the return value space and the code generated by wasmlink ensures that what's returned by the callee is marshaled correctly to the caller.

Scott Waye (Aug 05 2021 at 17:06):

and the utf8 array, is that memcpy'ed between the 2?

Scott Waye (Aug 05 2021 at 17:06):

no wait, scrap that.

Peter Huene (Aug 05 2021 at 17:06):

yes, the adapter code that wasmlink generates calls into each module to allocate space from their linear memories and uses memory.copy instructions to memcpy the memory between the two linear memories

Scott Waye (Aug 05 2021 at 17:07):

ah, ok, that's why it needs canonical_abi_realloc (which I'll need to create on the c# side)

Peter Huene (Aug 05 2021 at 17:07):

that's right

Scott Waye (Aug 05 2021 at 17:07):

thanks

Notification Bot (Aug 05 2021 at 17:08):

Scott Waye has marked this topic as resolved.

Till Schneidereit (Aug 05 2021 at 17:44):

one thing to note is that there'll be a bit more churn on the ABI before this is all fully stabilized. For example, we already know that the way short reads/writes are facilitated will change a bit, from the push/pull buffers right now to a streams based interface. We're aiming to wrap this phase of churn up as quickly as we can, though

Scott Waye (Aug 05 2021 at 17:45):

thanks, no problem, I'm just playing. wasmlink is completing without errors so made some progress.

Scott Waye (Aug 05 2021 at 23:09):

Thanks for the help. I've successfully started with a dotnet string, sent it to rust, reversed it, and passed it back. Ship it!
image.png

Peter Huene (Aug 05 2021 at 23:10):

super cool work! :)

Peter Huene (Aug 05 2021 at 23:11):

is this using mono to target wasm?

Scott Waye (Aug 05 2021 at 23:12):

No, this is NativeAOT-LLVM

Peter Huene (Aug 05 2021 at 23:12):

ah, nice

Scott Waye (Aug 05 2021 at 23:13):

Its an experimental backend to https://github.com/dotnet/runtimelab/tree/feature/NativeAOT

Peter Huene (Aug 05 2021 at 23:15):

as a fan of C# myself, it's really exciting to see work in this space

Last updated: Dec 23 2024 at 12:05 UTC