cmd/link: support wasmexport on js/wasm

Add export functions to the wasm module on GOOS=js. (Other parts
work the same way as wasip1.)

Add a test.

Fixes #65199.

Change-Id: Ia22580859fe40631d487f70ee293c12867e0c988
Reviewed-on: https://go-review.googlesource.com/c/go/+/606855
Reviewed-by: Zxilly Chou <zxilly@outlook.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

misc/wasm/wasm_exec.js

@@ -216,10 +216,15 @@
 				return decoder.decode(new DataView(this._inst.exports.mem.buffer, saddr, len));
 			}
 
+			const testCallExport = (a, b) => {
+				return this._inst.exports.testExport(a, b);
+			}
+
 			const timeOrigin = Date.now() - performance.now();
 			this.importObject = {
 				_gotest: {
 					add: (a, b) => a + b,
+					callExport: testCallExport,
 				},
 				gojs: {
 					// Go's SP does not change as long as no Go code is running. Some operations (e.g. calls, getters and setters)

src/cmd/link/internal/wasm/asm.go

@@ -446,7 +446,7 @@ func writeExportSec(ctxt *ld.Link, ldr *loader.Loader, lenHostImports int) {
 		ctxt.Out.WriteByte(0x02)      // mem export
 		writeUleb128(ctxt.Out, 0)     // memidx
 	case "js":
-		writeUleb128(ctxt.Out, 4) // number of exports
+		writeUleb128(ctxt.Out, uint64(4+len(ldr.WasmExports))) // number of exports
 		for _, name := range []string{"run", "resume", "getsp"} {
 			s := ldr.Lookup("wasm_export_"+name, 0)
 			if s == 0 {
@@ -457,6 +457,12 @@ func writeExportSec(ctxt *ld.Link, ldr *loader.Loader, lenHostImports int) {
 			ctxt.Out.WriteByte(0x00)            // func export
 			writeUleb128(ctxt.Out, uint64(idx)) // funcidx
 		}
+		for _, s := range ldr.WasmExports {
+			idx := uint32(lenHostImports) + uint32(ldr.SymValue(s)>>16) - funcValueOffset
+			writeName(ctxt.Out, ldr.SymName(s))
+			ctxt.Out.WriteByte(0x00)            // func export
+			writeUleb128(ctxt.Out, uint64(idx)) // funcidx
+		}
 		writeName(ctxt.Out, "mem") // inst.exports.mem in wasm_exec.js
 		ctxt.Out.WriteByte(0x02)   // mem export
 		writeUleb128(ctxt.Out, 0)  // memidx

src/syscall/js/js_test.go

@@ -56,6 +56,46 @@ func TestWasmImport(t *testing.T) {
 	}
 }
 
+// testCallExport is imported from host (wasm_exec.js), which calls testExport.
+//
+//go:wasmimport _gotest callExport
+func testCallExport(a int32, b int64) int64
+
+//go:wasmexport testExport
+func testExport(a int32, b int64) int64 {
+	testExportCalled = true
+	// test stack growth
+	growStack(1000)
+	// force a goroutine switch
+	ch := make(chan int64)
+	go func() {
+		ch <- int64(a)
+		ch <- b
+	}()
+	return <-ch + <-ch
+}
+
+var testExportCalled bool
+
+func growStack(n int64) {
+	if n > 0 {
+		growStack(n - 1)
+	}
+}
+
+func TestWasmExport(t *testing.T) {
+	testExportCalled = false
+	a := int32(123)
+	b := int64(456)
+	want := int64(a) + b
+	if got := testCallExport(a, b); got != want {
+		t.Errorf("got %v, want %v", got, want)
+	}
+	if !testExportCalled {
+		t.Error("testExport not called")
+	}
+}
+
 func TestBool(t *testing.T) {
 	want := true
 	o := dummys.Get("someBool")
@@ -613,7 +653,7 @@ func TestCallAllocations(t *testing.T) {
 		tmpArray := js.Global().Get("Array").New(0)
 		numAllocs := testing.AllocsPerRun(100, func() {
 			tmpArray.Call("concat", args...)
-		});
+		})
 
 		if numAllocs != float64(test.expected) {
 			t.Errorf("got numAllocs %#v, want %#v", numAllocs, test.expected)
@@ -630,7 +670,7 @@ func TestInvokeAllocations(t *testing.T) {
 		concatFunc := tmpArray.Get("concat").Call("bind", tmpArray)
 		numAllocs := testing.AllocsPerRun(100, func() {
 			concatFunc.Invoke(args...)
-		});
+		})
 
 		if numAllocs != float64(test.expected) {
 			t.Errorf("got numAllocs %#v, want %#v", numAllocs, test.expected)
@@ -647,7 +687,7 @@ func TestNewAllocations(t *testing.T) {
 
 		numAllocs := testing.AllocsPerRun(100, func() {
 			arrayConstructor.New(args...)
-		});
+		})
 
 		if numAllocs != float64(test.expected) {
 			t.Errorf("got numAllocs %#v, want %#v", numAllocs, test.expected)