runtime: fix hashmap load factor computation

overLoadFactor wasn't really doing what it says it does.
It was reporting overOrEqualToLoadFactor.  That's actually what we
want when adding an entry to a map, but it isn't what we want when
constructing a map in the first place.

The impetus for this change is that if you make a map with a hint
of exactly 8 (which happens, for example, with the unitMap in
time/format.go), we allocate 2 buckets for it instead of 1.

Instead, make overLoadFactor really report when it is > the max
allowed load factor, not >=.  Adjust the callers who want to ensure
that the map is no more than the max load factor after an insertion
by adding a +1 to the current (pre-addition) size.

Change-Id: Ie8d85344800a9a870036b637b1031ddd9e4b93f9
Reviewed-on: https://go-review.googlesource.com/61053
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>

src/runtime/export_test.go

@@ -376,3 +376,8 @@ func (rw *RWMutex) Lock() {
 func (rw *RWMutex) Unlock() {
 	rw.rw.unlock()
 }
+
+func MapBuckets(m map[int]int) int {
+	h := *(**hmap)(unsafe.Pointer(&m))
+	return 1 << h.B
+}

src/runtime/hashmap.go

@@ -573,7 +573,7 @@ again:
 
 	// If we hit the max load factor or we have too many overflow buckets,
 	// and we're not already in the middle of growing, start growing.
-	if !h.growing() && (overLoadFactor(h.count, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
+	if !h.growing() && (overLoadFactor(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
 		hashGrow(t, h)
 		goto again // Growing the table invalidates everything, so try again
 	}
@@ -904,7 +904,7 @@ func hashGrow(t *maptype, h *hmap) {
 	// Otherwise, there are too many overflow buckets,
 	// so keep the same number of buckets and "grow" laterally.
 	bigger := uint8(1)
-	if !overLoadFactor(h.count, h.B) {
+	if !overLoadFactor(h.count+1, h.B) {
 		bigger = 0
 		h.flags |= sameSizeGrow
 	}
@@ -944,7 +944,7 @@ func hashGrow(t *maptype, h *hmap) {
 
 // overLoadFactor reports whether count items placed in 1<<B buckets is over loadFactor.
 func overLoadFactor(count int, B uint8) bool {
-	return count >= bucketCnt && uintptr(count) >= loadFactorNum*(bucketShift(B)/loadFactorDen)
+	return count > bucketCnt && uintptr(count) > loadFactorNum*(bucketShift(B)/loadFactorDen)
 }
 
 // tooManyOverflowBuckets reports whether noverflow buckets is too many for a map with 1<<B buckets.

src/runtime/hashmap_fast.go

@@ -406,7 +406,7 @@ again:
 
 	// If we hit the max load factor or we have too many overflow buckets,
 	// and we're not already in the middle of growing, start growing.
-	if !h.growing() && (overLoadFactor(h.count, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
+	if !h.growing() && (overLoadFactor(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
 		hashGrow(t, h)
 		goto again // Growing the table invalidates everything, so try again
 	}
@@ -495,7 +495,7 @@ again:
 
 	// If we hit the max load factor or we have too many overflow buckets,
 	// and we're not already in the middle of growing, start growing.
-	if !h.growing() && (overLoadFactor(h.count, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
+	if !h.growing() && (overLoadFactor(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
 		hashGrow(t, h)
 		goto again // Growing the table invalidates everything, so try again
 	}
@@ -596,7 +596,7 @@ again:
 
 	// If we hit the max load factor or we have too many overflow buckets,
 	// and we're not already in the middle of growing, start growing.
-	if !h.growing() && (overLoadFactor(h.count, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
+	if !h.growing() && (overLoadFactor(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
 		hashGrow(t, h)
 		goto again // Growing the table invalidates everything, so try again
 	}

src/runtime/map_test.go

@@ -596,6 +596,35 @@ func TestIgnoreBogusMapHint(t *testing.T) {
 	}
 }
 
+func TestMapBuckets(t *testing.T) {
+	// Test that maps of different sizes have the right number of buckets.
+	// These tests depend on bucketCnt and loadFactor* in hashmap.go.
+	for _, tt := range [...]struct {
+		n, b int
+	}{
+		{8, 1},
+		{9, 2},
+		{13, 2},
+		{14, 4},
+		{26, 4},
+	} {
+		m := map[int]int{}
+		for i := 0; i < tt.n; i++ {
+			m[i] = i
+		}
+		if got := runtime.MapBuckets(m); got != tt.b {
+			t.Errorf("no hint n=%d want %d buckets, got %d", tt.n, tt.b, got)
+		}
+		m = make(map[int]int, tt.n)
+		for i := 0; i < tt.n; i++ {
+			m[i] = i
+		}
+		if got := runtime.MapBuckets(m); got != tt.b {
+			t.Errorf("hint n=%d want %d buckets, got %d", tt.n, tt.b, got)
+		}
+	}
+}
+
 func benchmarkMapPop(b *testing.B, n int) {
 	m := map[int]int{}
 	for i := 0; i < b.N; i++ {