runtime: clarify that itab.hash of dynamic entries is unused

The hash is used in type switches. However, compiler statically generates itab's
for all interface/type pairs used in switches (which are added to itabTable
in itabsinit). The dynamically-generated itab's never participate in type switches,
and thus the hash is irrelevant.

Change-Id: I4f6e37be31b8f5605cca7a1806cb04708e948cea
Reviewed-on: https://go-review.googlesource.com/c/go/+/202448
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/iface.go

@@ -66,6 +66,12 @@ func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
 	m = (*itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.mhdr)-1)*sys.PtrSize, 0, &memstats.other_sys))
 	m.inter = inter
 	m._type = typ
+	// The hash is used in type switches. However, compiler statically generates itab's
+	// for all interface/type pairs used in switches (which are added to itabTable
+	// in itabsinit). The dynamically-generated itab's never participate in type switches,
+	// and thus the hash is irrelevant.
+	// Note: m.hash is _not_ the hash used for the runtime itabTable hash table.
+	m.hash = 0
 	m.init()
 	itabAdd(m)
 	unlock(&itabLock)
@@ -233,7 +239,6 @@ imethods:
 		return iname
 	}
 	m.fun[0] = uintptr(fun0)
-	m.hash = typ.hash
 	return ""
 }