map: use correct load factor for deciding when to grow

The correct load factor is 6.5, not 6.
This got broken by accident in CL 462115.

Fixes #63438

Change-Id: Ib07bb6ab6103aec87cb775bc06bd04362a64e489
Reviewed-on: https://go-review.googlesource.com/c/go/+/533279
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/export_test.go

@@ -583,6 +583,10 @@ func MapBucketsPointerIsNil(m map[int]int) bool {
 	return h.buckets == nil
 }
 
+func OverLoadFactor(count int, B uint8) bool {
+	return overLoadFactor(count, B)
+}
+
 func LockOSCounts() (external, internal uint32) {
 	gp := getg()
 	if gp.m.lockedExt+gp.m.lockedInt == 0 {

src/runtime/map.go

@@ -70,7 +70,7 @@ const (
 	// Because of minimum alignment rules, bucketCnt is known to be at least 8.
 	// Represent as loadFactorNum/loadFactorDen, to allow integer math.
 	loadFactorDen = 2
-	loadFactorNum = (bucketCnt * 13 / 16) * loadFactorDen
+	loadFactorNum = loadFactorDen * bucketCnt * 13 / 16
 
 	// Maximum key or elem size to keep inline (instead of mallocing per element).
 	// Must fit in a uint8.

src/runtime/map_test.go

@@ -1419,3 +1419,18 @@ func TestEmptyMapWithInterfaceKey(t *testing.T) {
 		_ = mi[panicStructKey]
 	})
 }
+
+func TestLoadFactor(t *testing.T) {
+	for b := uint8(0); b < 20; b++ {
+		count := 13 * (1 << b) / 2 // 6.5
+		if b == 0 {
+			count = 8
+		}
+		if runtime.OverLoadFactor(count, b) {
+			t.Errorf("OverLoadFactor(%d,%d)=true, want false", count, b)
+		}
+		if !runtime.OverLoadFactor(count+1, b) {
+			t.Errorf("OverLoadFactor(%d,%d)=false, want true", count+1, b)
+		}
+	}
+}