Félix Saparelli

(about me)

a.k.a. passcod

Wasm bits and bobs

Posted on Mar 17 ‘20

Background

I’ve been dabbling with Wasm for several years, but only really started going at it in the past month, and for the purposes of this post, for the past two weeks. I had a bad idea and I’ve been working to make it real.

I’m not coming from the JS-and-Wasm perspective. Some of the things here might be relevant, but here I’m mostly talking from the point of view of writing a Wasm-engine-powered integration, not writing Wasm for the web and not particularly writing Wasm at all even.

For those who don’t know me, I work (as a preference) primarily in Rust, and I work (for money) primarily in PHP, JS, Ruby, Linux, etc. Currently I’m in the telecommunication industry in New Zealand.

The wasm text and bytecode format

One very interesting thing that I like about wasm is that the text format, and to a certain extent the bytecode, is an s-expression. Instructions are the usual stack machine as seen e.g. in assembly. But the structure is all s-expressions. Perhaps that’s surprising and interesting to me because I’m not intimately familiar with other binary library and executable formats… fasterthanlime’s ELF exploration is still on my to-read list.

The standard wasm tools come with wat2wasm and wasm2wat, which translate between the bytecode (wasm) and text (wat) formats. wat2wasm will produce simple yet nice errors if you write wrong wat.

My preferred way of writing small wasm programs is to write the wat directly instead of using a language on top. I am fairly comfortable with stack languages (I have a lingering fondness for dc) and a lot of the work involves more interacting with wasm structure than it does the behaviour of a module. To write larger programs, especially those dealing with allocations, I use Rust with wee_alloc, optionally in no_std mode. I do not use wasm rust frameworks such as wasm-pack or wasm-bindgen.

I have tried AssemblyScript, I am not interested in C and family, and that’s pretty much the extent of my options as much of everything else either embeds an entire runtime or is too high level or is too eldritch, wildly annoying, or unfamiliar.

Even more useful is wat’s ability to write stack instructions in s-expressions… or not, as the need may be. For example, this:

i32.const 31
call $addOne
i32.const 8
i32.mul

Can equally (and more clearly) be written:

(i32.mul
   (call $addOne
      (i32.const 31))
   (i32.const 8))

Strictly more verbose, but helpful where following along with a stack notation can be confusing.

The wasm module system

There is an assymmetry in the module system that… makes sense to anyone who’s used language-level module systems but might not be immediately obvious when approaching this in the context of dynamic libraries.

There are four types of exports and imports: functions (bread and butter), globals (i.e. constants and statics, but see later), memories (generally only one), tables (for dispatch and the like, which I don’t much deal with).

While engines do support all types, as per spec, languages targetting Wasm often only support functions well. It’s not uncommon to initially start with an integration that expects an exported global, only to then change it to a function that’s read on init and documented to need a constant output, because some desired language doesn’t support making wasm globals.

Wasm has the potential concept of multiple linear memories, and of exportable and importable memories. Currently, the spec only supports one memory, which can either be defined in the module or imported (defined elsewhere, including some other module). In theory and/or experiments, most languages also only support a single memory, or only support additional memories as addressable blobs of data. C &co, with manual memory management, can in theory allocate anywhere, and so may be better off… Rust’s AllocRef nightly feature shows promise to be able to specify the allocator for some data, and therefore be able to configure multiple allocators each targeted at a different memory. However, that will require multiple memory support at the (spec and then) language level in the first place. For now, designing integrations to handle more than one memories is not required but a good future-proofing step.

Exports are straightforward: each export has a name and maps to some entry in the module’s index spaces. Once you compile a module from bytecode you can look up all of its exports and get the indices for the names. This is important later.

Imports have two-level names: a namespace and a name. The idea is for integrations to both be able to provide multiple libraries of imports without clashes, and to support plugging one module’s exports directly to another module’s imports, presumably namespaced under the first module’s name, version, some random string, etc.

In practice there are two namespaces worth knowing about: env is the de-facto default namespace, and js is the de-facto namespace for web APIs.

In Rust, to specify the import namespace (defaults to env), you need to use the #[link(wasm_import_namespace = “foo”)] attribute on the extern block like so:

#[link(wasm_import_namespace = “log”)]
extern {
   fn trace(ptr: i32, len: i32);
   fn debug(ptr: i32, len: i32);
   fn info(ptr: i32, len: i32);
   fn warn(ptr: i32, len: i32);
   fn error(ptr: i32, len: i32);
}

Functions calls

In the wasmer runtime, which is what I’ve most experience with, there are two contexts to call exported functions in: on an Instance, that is, once a compiled module is instantiated (we’ll come back to that), and from a Ctx, that is, from inside an imported function call. The first is highly ergonomic, the other not very (this will probably improve going forward, there’s no reason not to).

let func: Func<(i32, i32)> = instance.func(“foo_functer”)?;
let res = func.call(42, 43)?;

To call from a Ctx, the best way currently is to pre-emptively (before instantiating) obtain the indices of the exported functions you want to call from the compiled module, and then call into the Ctx using those indices:

// after compiling, with a Module
let export_index = module
    .info()
    .exports
    .get(“foo_functer”)
    .unwrap();

let func_index = if let ExportIndex::Func(func_index) = export_index {
    unsafe { std::mem::transmute(*func_index) }
} else {
    panic!(“aaah”);
}

// inside an imported function, with a Ctx
let foo = 42;
let fun = 43;

let res = ctx.call_with_table_index(
    func_index,
    &[WasmValue::I32(foo as _), WasmValue::I32(fun as _)],
)?;

Multi-value

Something that is not obvious at first glance is that multi-value returns in wasm is comparatively young and not very well supported, which presents nasty surprises when trying to use it in all but the most trivial cases.

Multi-value [return] is when wasm functions support multiple return values instead of just one:

(func $readTwoI32s (param $offset i32) (result i32 i32)
   (i32.load (local.get $offset))
   (i32.load (i32.add (local.get $offset) (i32.const 4)))
)

To compile that with wat2wasm, you need the --enable-multi-value flag, which should have been a… flag… that this wasn’t quite as well-supported as the current spec made it out to be.

However, wasmer supports multi-value like a champ, both for calling exports:

let func: Func<(i32), (i32, i32)> = instance.func(“read_two_i32s”)?;
let (one, two) = func.call(0)?;

and for defining imports:

imports! {
   “env” => {
      “get_two_i64s” => func!(|| -> (i64, i64) {
         (41, 42)
      }),
   },
};

That initially lulled me in a false sense of security and I went about designing APIs using multi-value and testing them with multi-value hand-written wat. All seemed great!

Then I tried using Rust to write wasm modules that used my APIs and everything fell apart because Rust does not support multi-value for Wasm… and lies to you when you try using it.

See, Rust uses some kind of “C-like” ABI to do the codegen for its imports and exports in its wasm support, such that if you write this:

extern {
   fn get_two_i64s() -> (i64, i64);
}

with multi-value you might expect this wasm:

(func (export “get_two_i64s”) (result i64 i64))

but what you actually get is this:

(func (export “get_two_i64s”) (param i32))

Uhhh???

What Rust is actually exporting is a function that would look like this:

extern {
   fn get_two_i64s(pointer_to_write_to: u32);
}

which you’d then call like:

let mut buf: [i64; 2] = [0; 2];
unsafe { get_two_i64s(buf.as_mut_ptr()); }
let [a, b] = buf;

So now both sides have to know that get_two_i64s expects to write two i64s contiguously somewhere in memory you specify, and then you retrieve that.

The wasmrust “framework” does support multi-value. It doesn’t magically activate a hidden rustc flag to enable multi-value codegen, though: it post-processes the wasm, looks for “things that look like they’re multi-value functions”, and writes them a wrapper that is multi-value, leaving the originals in place so you can use both styles. What the actual fuck.

I’m sure it works great with the limited API style that wasmrust’s bindgen macros write out, and I’m sure it was a lot easier to do this than to add multi-value support to Rustc, but it sure seems like a huge kludge.

Anyway, so: multi-value is sexy, but don’t even bother with it.

Instantiation and the start section

Wasm modules can contain a start section, which can absolutely not be thought of like a main function in C and Rust: code that runs directly, without being called via an exported function.

The start section is run during the instantiation sequence. If there’s no start section, it’s not called, simple as that.

Now, wasm people will insist that the start section is a compiler detail that should absolutely not be used by common plebeians or for programs and such, that it’s useless anyway because it runs before “the module” and “exports” are available, and that implicitely exported functions rely on the start having been run, so you really shouldn’t use this for anything…

Anyway, you can’t generate it.

And fair enough. I’m sure they know their stuff and they have good reasons.

However. The instantiation process for Wasm is precisely defined. After this process, the module is ready for use. Wonderful. The start section is called as the very last step of the instantiation process.

So while the official advice is to have some export named, e.g. main or something and then having the runtime call this export straight away, if you want to deliberately flout the guidelines, you probably can. You can totally use the instantiation of a module as a kind of glorified function call.

It’s most certainly a bad idea… but you can.

Given that nothing will generate this for you, you’ll need to post-process the wasm to add the start section in yourself. A small price to pay. (Seriously, though: don’t. It’s all fun and games until nasal daemons eat your laundry, and again, nothing supports this.)

Types

People usually start with that, but it’s kind of an implementation detail in most cases, and then they leave it at that… there’s some good bits there, though.

As a recap, Wasm at the moment has 2×2 scalar types: signed ints and floats, both in 32 and 64 bit widths, plus one 128-bit vector type for SIMD (when supported).

To start with, you can’t pass 128-bit integers in using v128. Good try!

The wasm pointer size wasm is 32 bits. Period. There’s effectively no wasm64 at this point, even though it’s specced and mentioned in a few places. If you’re writing an integration and need to store or deal with pointers from inside wasm, don’t bother with usize and perhaps-faillible casts, use u32 and cast up to usize when needed (e.g. when indexing into memories). Then pop this up in your code somewhere to be overkill in making sure that cast is always safe:

#[cfg(not(any(target_pointer_width = “32”, target_pointer_width = “64”)))]
compile_error!(“only 32 and 64 bit pointers are supported”);

When engines have magical support for unsigned and smaller width integers, that’s all convention between the two sides. u8 and i16 and u32 are cast to 8, 16, or 32 bits, padded out, given to wasm as an “i32”, and then the inner module re-interprets the bits as the right type… if it wants to. Again, it’s all convention. Make sure everything is documented, because if you pass –2079915776 (i32) and meant 2215051520 (u32), well, who could have known?

There may be more

and I’m adding on as I go.