misc/wasm: fix passing large negative integers from JS to Go

This commit addresses a FIXME left in the code of wasm_exec.js to
properly get the upper 32 bit of a JS number to be stored as an
64-bit integer. A bitshift operation is not possible, because in
JavaScript bitshift operations only operate on the lower 32 bits.

Change-Id: I8f627fd604e592682d9d322942a4852db64a7f66
Reviewed-on: https://go-review.googlesource.com/113076Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/wasm/wasm_exec.js

@@ -77,11 +77,7 @@
 
 	const setInt64 = (addr, v) => {
 		mem().setUint32(addr + 0, v, true);
-		if (v >= 0) {
-			mem().setUint32(addr + 4, v / 4294967296, true);
-		} else {
-			mem().setUint32(addr + 4, -1, true); // FIXME
-		}
+		mem().setUint32(addr + 4, Math.floor(v / 4294967296), true);
 	}
 
 	const getInt64 = (addr) => {

src/syscall/js/js_test.go

@@ -58,6 +58,24 @@ func TestInt(t *testing.T) {
 	}
 }
 
+func TestIntConversion(t *testing.T) {
+	testIntConversion(t, 0)
+	testIntConversion(t, 1)
+	testIntConversion(t, -1)
+	testIntConversion(t, 1<<20)
+	testIntConversion(t, -1<<20)
+	testIntConversion(t, 1<<40)
+	testIntConversion(t, -1<<40)
+	testIntConversion(t, 1<<60)
+	testIntConversion(t, -1<<60)
+}
+
+func testIntConversion(t *testing.T, want int) {
+	if got := js.ValueOf(want).Int(); got != want {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
+}
+
 func TestFloat(t *testing.T) {
 	want := 42.123
 	o := dummys.Get("someFloat")