expvar: make BenchmarkAdd{Same,Different} comparable to 1.8

bradfitz noted in change 36717 that the new behavior was no longer
comparable with the old.  This change restores comparable behavior
for -cpu=1.

BenchmarkMapAddSame                 909           909           +0.00%
BenchmarkMapAddSame-6               1309          262           -79.98%
BenchmarkMapAddDifferent            2856          3030          +6.09%
BenchmarkMapAddDifferent-6          3803          581           -84.72%

updates #18177

Change-Id: Ifaff5a1f48be92002d86c296220313b7efdc81d6
Reviewed-on: https://go-review.googlesource.com/36723
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/expvar/expvar_test.go

@@ -221,11 +221,12 @@ func BenchmarkMapSet(b *testing.B) {
 }
 
 func BenchmarkMapAddSame(b *testing.B) {
-	m := new(Map).Init()
-	b.ResetTimer()
-
 	b.RunParallel(func(pb *testing.PB) {
 		for pb.Next() {
+			m := new(Map).Init()
+			m.Add("red", 1)
+			m.Add("red", 1)
+			m.Add("red", 1)
 			m.Add("red", 1)
 		}
 	})
@@ -241,19 +242,17 @@ func BenchmarkMapAddDifferent(b *testing.B) {
 		procKeys[i] = keys
 	}
 
-	m := new(Map).Init()
 	b.ResetTimer()
 
 	var n int32
 	b.RunParallel(func(pb *testing.PB) {
 		i := int(atomic.AddInt32(&n, 1)-1) % len(procKeys)
 		keys := procKeys[i]
-		j := 0
 
 		for pb.Next() {
-			m.Add(keys[j], 1)
-			if j++; j == len(keys) {
-				j = 0
+			m := new(Map).Init()
+			for _, k := range keys {
+				m.Add(k, 1)
 			}
 		}
 	})