runtime: don't publish new itab table before growth is finished

This change could improve the hit rate on itabTable during growth.

While we are here patch comments to refer to existing functions.

Change-Id: I76f81c860a3d6107e077e7e3932550858a8b7651
Reviewed-on: https://go-review.googlesource.com/55912
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/iface.go

@@ -49,7 +49,7 @@ func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
 	// First, look in the existing table to see if we can find the itab we need.
 	// This is by far the most common case, so do it without locks.
 	// Use atomic to ensure we see any previous writes done by the thread
-	// that updates the itabTable field (with atomic.Storep in addItab).
+	// that updates the itabTable field (with atomic.Storep in itabAdd).
 	t := (*itabTableType)(atomic.Loadp(unsafe.Pointer(&itabTable)))
 	if m = t.find(inter, typ); m != nil {
 		goto finish
@@ -85,7 +85,7 @@ finish:
 	panic(&TypeAssertionError{concreteString: typ.string(), assertedString: inter.typ.string(), missingMethod: m.init()})
 }
 
-// itabFind finds the given interface/type pair in t.
+// find finds the given interface/type pair in t.
 // Returns nil if the given interface/type pair isn't present.
 func (t *itabTableType) find(inter *interfacetype, typ *_type) *itab {
 	// Implemented using quadratic probing.
@@ -115,31 +115,34 @@ func (t *itabTableType) find(inter *interfacetype, typ *_type) *itab {
 func itabAdd(m *itab) {
 	t := itabTable
 	if t.count >= 3*(t.size/4) { // 75% load factor
-		// Grow hash table. Use an atomic write: see comment in getitab.
+		// Grow hash table.
 		// t2 = new(itabTableType) + some additional entries
 		// We lie and tell malloc we want pointer-free memory because
 		// all the pointed-to values are not in the heap.
 		t2 := (*itabTableType)(mallocgc((2+2*t.size)*sys.PtrSize, nil, true))
 		t2.size = t.size * 2
-		atomicstorep(unsafe.Pointer(&itabTable), unsafe.Pointer(t2))
 
 		// Copy over entries.
 		// Note: while copying, other threads may look for an itab and
 		// fail to find it. That's ok, they will then try to get the itab lock
 		// and as a consequence wait until this copying is complete.
-		for i := uintptr(0); i < t.size; i++ {
-			if m2 := *(**itab)(add(unsafe.Pointer(&t.entries), i*sys.PtrSize)); m2 != nil {
-				itabAdd(m2)
-			}
-		}
-		if itabTable.count != t.count {
+		iterate_itabs(t2.add)
+		if t2.count != t.count {
 			throw("mismatched count during itab table copy")
 		}
+		// Publish new hash table. Use an atomic write: see comment in getitab.
+		atomicstorep(unsafe.Pointer(&itabTable), unsafe.Pointer(t2))
 		// Adopt the new table as our own.
 		t = itabTable
 		// Note: the old table can be GC'ed here.
 	}
-	// See comment in itabFind about the probe sequence.
+	t.add(m)
+}
+
+// add adds the given itab to itab table t.
+// itabLock must be held.
+func (t *itabTableType) add(m *itab) {
+	// See comment in find about the probe sequence.
 	// Insert new itab in the first empty spot in the probe sequence.
 	mask := t.size - 1
 	h := itabHashFunc(m.inter, m._type) & mask
@@ -602,7 +605,8 @@ func reflect_ifaceE2I(inter *interfacetype, e eface, dst *iface) {
 }
 
 func iterate_itabs(fn func(*itab)) {
-	// Note: only runs during stop the world, so no locks/atomics needed.
+	// Note: only runs during stop the world or with itabLock held,
+	// so no other locks/atomics needed.
 	t := itabTable
 	for i := uintptr(0); i < t.size; i++ {
 		m := *(**itab)(add(unsafe.Pointer(&t.entries), i*sys.PtrSize))