runtime: map iterators: always use intrabucket randomess

Fixes #8688

LGTM=rsc
R=golang-codereviews, bradfitz, rsc, khr
CC=golang-codereviews
https://golang.org/cl/135660043

src/runtime/hashmap.go

@@ -59,7 +59,8 @@ import (
 
 const (
 	// Maximum number of key/value pairs a bucket can hold.
-	bucketCnt = 8
+	bucketCntBits = 3
+	bucketCnt     = 1 << bucketCntBits
 
 	// Maximum average load of a bucket that triggers growth.
 	loadFactor = 6.5
@@ -138,7 +139,7 @@ type hiter struct {
 	offset      uint8          // intra-bucket offset to start from during iteration (should be big enough to hold bucketCnt-1)
 	wrapped     bool           // already wrapped around from end of bucket array to beginning
 	B           uint8
-	i	    uint8
+	i           uint8
 	bucket      uintptr
 	checkBucket uintptr
 }
@@ -562,17 +563,12 @@ func mapiterinit(t *maptype, h *hmap, it *hiter) {
 	it.buckets = h.buckets
 
 	// decide where to start
-	switch {
-	case h.B == 0:
-		it.startBucket = 0
-		it.offset = uint8(fastrand1()) & (bucketCnt - 1)
-	case h.B <= 31:
-		it.startBucket = uintptr(fastrand1()) & (uintptr(1)<<h.B-1)
-		it.offset = 0
-	default:
-		it.startBucket = (uintptr(fastrand1()) + uintptr(fastrand1())<<31) & (uintptr(1)<<h.B-1)
-		it.offset = 0
+	r := uintptr(fastrand1())
+	if h.B > 31-bucketCntBits {
+		r += uintptr(fastrand1()) << 31
 	}
+	it.startBucket = r & (uintptr(1)<<h.B - 1)
+	it.offset = uint8(r >> h.B & (bucketCnt - 1))
 
 	// iterator state
 	it.bucket = it.startBucket

src/runtime/map_test.go

@@ -412,30 +412,33 @@ func TestMapNanGrowIterator(t *testing.T) {
 
 func TestMapIterOrder(t *testing.T) {
 	for _, n := range [...]int{3, 7, 9, 15} {
-		// Make m be {0: true, 1: true, ..., n-1: true}.
-		m := make(map[int]bool)
-		for i := 0; i < n; i++ {
-			m[i] = true
-		}
-		// Check that iterating over the map produces at least two different orderings.
-		ord := func() []int {
-			var s []int
-			for key := range m {
-				s = append(s, key)
+		for i := 0; i < 1000; i++ {
+			// Make m be {0: true, 1: true, ..., n-1: true}.
+			m := make(map[int]bool)
+			for i := 0; i < n; i++ {
+				m[i] = true
 			}
-			return s
-		}
-		first := ord()
-		ok := false
-		for try := 0; try < 100; try++ {
-			if !reflect.DeepEqual(first, ord()) {
-				ok = true
+			// Check that iterating over the map produces at least two different orderings.
+			ord := func() []int {
+				var s []int
+				for key := range m {
+					s = append(s, key)
+				}
+				return s
+			}
+			first := ord()
+			ok := false
+			for try := 0; try < 100; try++ {
+				if !reflect.DeepEqual(first, ord()) {
+					ok = true
+					break
+				}
+			}
+			if !ok {
+				t.Errorf("Map with n=%d elements had consistent iteration order: %v", n, first)
 				break
 			}
 		}
-		if !ok {
-			t.Errorf("Map with n=%d elements had consistent iteration order: %v", n, first)
-		}
 	}
 }