wasm: use wasi ABI for basic startup/stdout

This allows TinyGo-built binaries to run under wasmtime, for example:

    tinygo build -o test.wasm -no-debug -target=wasm examples/test
    wasmtime run test.wasm 0

src/runtime/runtime_wasm.go

@@ -2,35 +2,43 @@
 
 package runtime
 
+import "unsafe"
+
 type timeUnit float64 // time in milliseconds, just like Date.now() in JavaScript
 
 const tickMicros = 1000000
 
-//go:export io_get_stdout
-func io_get_stdout() int32
-
-//go:export resource_write
-func resource_write(id int32, ptr *uint8, len int32) int32
-
-var stdout int32
-
-func init() {
-	stdout = io_get_stdout()
+// Implements __wasi_ciovec_t and __wasi_iovec_t.
+type wasiIOVec struct {
+	buf    unsafe.Pointer
+	bufLen uint
 }
 
-//go:export _start
+//go:wasm-module wasi_unstable
+//export fd_write
+func fd_write(id uint32, iovs *wasiIOVec, iovs_len uint, nwritten *uint) (errno uint)
+
+//export _start
 func _start() {
 	initAll()
-}
-
-//go:export cwa_main
-func cwa_main() {
-	initAll() // _start is not called by olin/cwa so has to be called here
 	callMain()
 }
 
+// Using global variables to avoid heap allocation.
+var (
+	putcharBuf   = byte(0)
+	putcharIOVec = wasiIOVec{
+		buf:    unsafe.Pointer(&putcharBuf),
+		bufLen: 1,
+	}
+)
+
 func putchar(c byte) {
-	resource_write(stdout, &c, 1)
+	// write to stdout
+	const stdout = 1
+	var nwritten uint
+	putcharBuf = c
+	fd_write(stdout, &putcharIOVec, 1, &nwritten)
 }
 
 var handleEvent func()

targets/wasm_exec.js

@@ -181,13 +181,15 @@
 
 			const timeOrigin = Date.now() - performance.now();
 			this.importObject = {
-				env: {
-					io_get_stdout: function() {
-						return 1;
-					},
-
-					resource_write: function(fd, ptr, len) {
+				wasi_unstable: {
+					// https://github.com/bytecodealliance/wasmtime/blob/master/docs/WASI-api.md#__wasi_fd_write
+					fd_write: function(fd, iovs_ptr, iovs_len, nwritten_ptr) {
+						let nwritten = 0;
 						if (fd == 1) {
+							for (let iovs_i=0; iovs_i<iovs_len;iovs_i++) {
+								let iov_ptr = iovs_ptr+iovs_i*8; // assuming wasm32
+								let ptr = mem().getUint32(iov_ptr + 0, true);
+								let len = mem().getUint32(iov_ptr + 4, true);
 								for (let i=0; i<len; i++) {
 									let c = mem().getUint8(ptr+i);
 									if (c == 13) { // CR
@@ -201,11 +203,15 @@
 										logLine.push(c);
 									}
 								}
+							}
 						} else {
 							console.error('invalid file descriptor:', fd);
 						}
+						mem().setUint32(nwritten_ptr, nwritten, true);
+						return 0;
 					},
-
+				},
+				env: {
 					// func ticks() float64
 					"runtime.ticks": () => {
 						return timeOrigin + performance.now();
@@ -344,7 +350,7 @@
 						setTimeout(resolve, 0); // make sure it is asynchronous
 					};
 				});
-				this._inst.exports.cwa_main();
+				this._inst.exports._start();
 				if (this.exited) {
 					break;
 				}