WebAssembly

--target wasm32-none and --target wasm32-wasi build a program into a WebAssembly module. As on bare metal, nothing in the build is C: voltc compiles the program and std through LLVM and links the module with wasm-ld, which comes with LLVM’s lld.

voltc build hello.volt --target wasm32-wasi -o hello.wasm   # for a WASI host
voltc build hello.volt --target wasm32-none -o hello.wasm   # for a web page

Both export _start, which runs the program’s main and then exits with its result, and the module’s memory. usize is 32 bits, as a pointer is. std’s runtime is the bare-metal one: printing goes to the console, a panic prints its message there and exits with 101, and the heap starts where wasm-ld puts __heap_base and grows the memory a 64 KiB page at a time when it needs more. A program that defines volt_console_write or volt_exit itself (as an export fn) is given that instead of the host’s.

In both, @cfg("os") is none (std has no OS layer there yet), @cfg("arch") is wasm32 and @cfg("target") is the --target name, so a library can tell them apart.

Under node, WASI’s module runs through node:wasi:

import { readFile } from "node:fs/promises";
import { WASI } from "node:wasi";

const wasi = new WASI({ version: "preview1", args: [], env: {}, returnOnExit: true });
const { instance } = await WebAssembly.instantiate(await readFile("hello.wasm"), {
    wasi_snapshot_preview1: wasi.wasiImport,
});
process.exitCode = wasi.start(instance);

A page gives a wasm32-none module its console and exit:

let memory;
const text = new TextDecoder();
const env = {
    volt_console_write: (p, n) => console.log(text.decode(new Uint8Array(memory.buffer, p, n))),
    volt_exit: (code) => { throw new Error(`exited with ${code}`); },
};
const { instance } = await WebAssembly.instantiateStreaming(fetch("hello.wasm"), { env });
memory = instance.exports.memory;
instance.exports._start();

examples/wasm/run.mjs runs either kind under node. The test suite builds examples/wasm/hello.volt, 128-bit integer arithmetic and every f128 operation (std::softfloat’s quad routines: the module has no compiler-rt) for both targets, in debug and release builds, and runs them there.

• WASI’s files, arguments, environment and clock: std’s fs, process and time are bare metal’s there, so std::process::args() is empty and opening a file fails.
• Threads, and calling JavaScript functions other than the console and exit from Volt.