runtime/internal/atomic: add TestUnaligned64

Add a variant of sync/atomic's TestUnaligned64 to
runtime/internal/atomic.

Skips the test on arm for now where it's currently failing.

Updates #17786

Change-Id: If63f9c1243e9db7b243a95205b2d27f7d1dc1e6e
Reviewed-on: https://go-review.googlesource.com/33159
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/internal/atomic/atomic_test.go

@@ -66,3 +66,39 @@ func TestXadduintptrOnUint64(t *testing.T) {
 		t.Fatalf("xadduintptr should increase lower-order bits, want %d, got %d", inc, val)
 	}
 }
+
+func shouldPanic(t *testing.T, name string, f func()) {
+	defer func() {
+		if recover() == nil {
+			t.Errorf("%s did not panic", name)
+		}
+	}()
+	f()
+}
+
+// Variant of sync/atomic's TestUnaligned64:
+func TestUnaligned64(t *testing.T) {
+	// Unaligned 64-bit atomics on 32-bit systems are
+	// a continual source of pain. Test that on 32-bit systems they crash
+	// instead of failing silently.
+
+	switch runtime.GOARCH {
+	default:
+		if unsafe.Sizeof(int(0)) != 4 {
+			t.Skip("test only runs on 32-bit systems")
+		}
+	case "arm":
+		t.Skipf("TODO: implement. golang.org/issue/17786")
+	case "amd64p32", "mips", "mipsle":
+		// amd64p32 and mips can handle unaligned atomics.
+		t.Skipf("test not needed on %v", runtime.GOARCH)
+	}
+
+	x := make([]uint32, 4)
+	up64 := (*uint64)(unsafe.Pointer(&x[1])) // misaligned
+	p64 := (*int64)(unsafe.Pointer(&x[1]))   // misaligned
+
+	shouldPanic(t, "Load64", func() { atomic.Load64(up64) })
+	shouldPanic(t, "Loadint64", func() { atomic.Loadint64(p64) })
+	shouldPanic(t, "Store64", func() { atomic.Store64(up64, 0) })
+}