internal/runtime/maps: make clear also erase tombstones

This will make future uses of the map faster because the probe
sequences will likely be shorter.

Change-Id: If10f3af49a5feaff7d1b82337bbbfb93bcd9dcb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/633076
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>

src/cmd/compile/internal/reflectdata/map_swiss.go

@@ -159,6 +159,7 @@ func SwissMapType() *types.Type {
 	//     globalShift uint8
 	//
 	//     writing uint8
+	//     tombstonePossible bool
 	//     // N.B Padding
 	//
 	//     clearSeq uint64
@@ -172,6 +173,7 @@ func SwissMapType() *types.Type {
 		makefield("globalDepth", types.Types[types.TUINT8]),
 		makefield("globalShift", types.Types[types.TUINT8]),
 		makefield("writing", types.Types[types.TUINT8]),
+		makefield("tombstonePossible", types.Types[types.TBOOL]),
 		makefield("clearSeq", types.Types[types.TUINT64]),
 	}
 

src/internal/runtime/maps/map.go

@@ -191,6 +191,7 @@ func h2(h uintptr) uintptr {
 	return h & 0x7f
 }
 
+// Note: changes here must be reflected in cmd/compile/internal/reflectdata/map_swiss.go:SwissMapType.
 type Map struct {
 	// The number of filled slots (i.e. the number of elements in all
 	// tables). Excludes deleted slots.
@@ -235,6 +236,10 @@ type Map struct {
 	// that both sides will detect the race.
 	writing uint8
 
+	// tombstonePossible is false if we know that no table in this map
+	// contains a tombstone.
+	tombstonePossible bool
+
 	// clearSeq is a sequence counter of calls to Clear. It is used to
 	// detect map clears during iteration.
 	clearSeq uint64
@@ -657,7 +662,9 @@ func (m *Map) Delete(typ *abi.SwissMapType, key unsafe.Pointer) {
 		m.deleteSmall(typ, hash, key)
 	} else {
 		idx := m.directoryIndex(hash)
-		m.directoryAt(idx).Delete(typ, m, hash, key)
+		if m.directoryAt(idx).Delete(typ, m, hash, key) {
+			m.tombstonePossible = true
+		}
 	}
 
 	if m.used == 0 {
@@ -722,7 +729,7 @@ func (m *Map) deleteSmall(typ *abi.SwissMapType, hash uintptr, key unsafe.Pointe
 
 // Clear deletes all entries from the map resulting in an empty map.
 func (m *Map) Clear(typ *abi.SwissMapType) {
-	if m == nil || m.Used() == 0 {
+	if m == nil || m.Used() == 0 && !m.tombstonePossible {
 		return
 	}
 
@@ -744,9 +751,10 @@ func (m *Map) Clear(typ *abi.SwissMapType) {
 			lastTab = t
 		}
 		m.used = 0
-		m.clearSeq++
+		m.tombstonePossible = false
 		// TODO: shrink directory?
 	}
+	m.clearSeq++
 
 	// Reset the hash seed to make it more difficult for attackers to
 	// repeatedly trigger hash collisions. See https://go.dev/issue/25237.
@@ -767,7 +775,6 @@ func (m *Map) clearSmall(typ *abi.SwissMapType) {
 	g.ctrls().setEmpty()
 
 	m.used = 0
-	m.clearSeq++
 }
 
 func (m *Map) Clone(typ *abi.SwissMapType) *Map {

src/internal/runtime/maps/table.go

@@ -103,7 +103,7 @@ func (t *table) reset(typ *abi.SwissMapType, capacity uint16) {
 	groupCount := uint64(capacity) / abi.SwissMapGroupSlots
 	t.groups = newGroups(typ, groupCount)
 	t.capacity = capacity
-	t.resetGrowthLeft()
+	t.growthLeft = t.maxGrowthLeft()
 
 	for i := uint64(0); i <= t.groups.lengthMask; i++ {
 		g := t.groups.group(typ, i)
@@ -111,9 +111,9 @@ func (t *table) reset(typ *abi.SwissMapType, capacity uint16) {
 	}
 }
 
-// Preconditions: table must be empty.
-func (t *table) resetGrowthLeft() {
-	var growthLeft uint16
+// maxGrowthLeft is the number of inserts we can do before
+// resizing, starting from an empty table.
+func (t *table) maxGrowthLeft() uint16 {
 	if t.capacity == 0 {
 		// No real reason to support zero capacity table, since an
 		// empty Map simply won't have a table.
@@ -125,15 +125,15 @@ func (t *table) resetGrowthLeft() {
 		//
 		// TODO(go.dev/issue/54766): With a special case in probing for
 		// single-group tables, we could fill all slots.
-		growthLeft = t.capacity - 1
+		return t.capacity - 1
 	} else {
 		if t.capacity*maxAvgGroupLoad < t.capacity {
 			// TODO(prattmic): Do something cleaner.
 			panic("overflow")
 		}
-		growthLeft = (t.capacity * maxAvgGroupLoad) / abi.SwissMapGroupSlots
+		return (t.capacity * maxAvgGroupLoad) / abi.SwissMapGroupSlots
 	}
-	t.growthLeft = growthLeft
+
 }
 
 func (t *table) Used() uint64 {
@@ -417,7 +417,8 @@ func (t *table) uncheckedPutSlot(typ *abi.SwissMapType, hash uintptr, key, elem
 	}
 }
 
-func (t *table) Delete(typ *abi.SwissMapType, m *Map, hash uintptr, key unsafe.Pointer) {
+// Delete returns true if it put a tombstone in t.
+func (t *table) Delete(typ *abi.SwissMapType, m *Map, hash uintptr, key unsafe.Pointer) bool {
 	seq := makeProbeSeq(h1(hash), t.groups.lengthMask)
 	for ; ; seq = seq.next() {
 		g := t.groups.group(typ, seq.offset)
@@ -466,15 +467,17 @@ func (t *table) Delete(typ *abi.SwissMapType, m *Map, hash uintptr, key unsafe.P
 				// full now, we can simply remove the element.
 				// Otherwise, we create a tombstone to mark the
 				// slot as deleted.
+				var tombstone bool
 				if g.ctrls().matchEmpty() != 0 {
 					g.ctrls().set(i, ctrlEmpty)
 					t.growthLeft++
 				} else {
 					g.ctrls().set(i, ctrlDeleted)
+					tombstone = true
 				}
 
 				t.checkInvariants(typ, m)
-				return
+				return tombstone
 			}
 			match = match.removeFirst()
 		}
@@ -483,7 +486,7 @@ func (t *table) Delete(typ *abi.SwissMapType, m *Map, hash uintptr, key unsafe.P
 		if match != 0 {
 			// Finding an empty slot means we've reached the end of
 			// the probe sequence.
-			return
+			return false
 		}
 	}
 }
@@ -593,14 +596,19 @@ func (t *table) tombstones() uint16 {
 
 // Clear deletes all entries from the map resulting in an empty map.
 func (t *table) Clear(typ *abi.SwissMapType) {
+	mgl := t.maxGrowthLeft()
+	if t.used == 0 && t.growthLeft == mgl { // no current entries and no tombstones
+		return
+	}
 	for i := uint64(0); i <= t.groups.lengthMask; i++ {
 		g := t.groups.group(typ, i)
-		typedmemclr(typ.Group, g.data)
+		if g.ctrls().matchFull() != 0 {
+			typedmemclr(typ.Group, g.data)
+		}
 		g.ctrls().setEmpty()
 	}
-
 	t.used = 0
-	t.resetGrowthLeft()
+	t.growthLeft = mgl
 }
 
 type Iter struct {