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>

diff --git a/src/runtime/iface.go b/src/runtime/iface.go
index 243e51fc480021955e6a6abfd9f8fa20258ff82d..05de282aa758eab136e5fd7c88caf15ee675a19b 100644 (file)
--- a/src/runtime/iface.go
+++ b/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 ""
 }