runtime: faster range on empty map

benchmark                old ns/op    new ns/op    delta
BenchmarkMapIter               191          190   -0.52%
BenchmarkMapIterEmpty           22            4  -78.96%

R=golang-dev, minux.ma, dvyukov, iant, khr
CC=golang-dev
https://golang.org/cl/9637043

src/pkg/runtime/hashmap.c

@@ -1355,7 +1355,7 @@ reflect·mapassign(MapType *t, Hmap *h, uintptr key, uintptr val, bool pres)
 void
 runtime·mapiterinit(MapType *t, Hmap *h, struct hash_iter *it)
 {
-	if(h == nil) {
+	if(h == nil || h->count == 0) {
 		it->key = nil;
 		return;
 	}

src/pkg/runtime/mapspeed_test.go

@@ -233,3 +233,24 @@ func BenchmarkNewEmptyMap(b *testing.B) {
 		_ = make(map[int]int)
 	}
 }
+
+func BenchmarkMapIter(b *testing.B) {
+	m := make(map[int]bool)
+	for i := 0; i < 8; i++ {
+		m[i] = true
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for _, _ = range m {
+		}
+	}
+}
+
+func BenchmarkMapIterEmpty(b *testing.B) {
+	m := make(map[int]bool)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for _, _ = range m {
+		}
+	}
+}