internal/runtime/maps: don't hash twice when deleting

│  baseline   │             experiment              │
                     │   sec/op    │   sec/op     vs base                │
MapDeleteLargeKey-24   312.0n ± 6%   162.3n ± 5%  -47.97% (p=0.000 n=10)

Change-Id: I31f1f8e3c344cf8abf2e9eb4b51b78fcd67b93c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/625906
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

src/internal/runtime/maps/map.go

@@ -663,7 +663,7 @@ 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, key)
+		m.directoryAt(idx).Delete(typ, m, hash, key)
 	}
 
 	if m.used == 0 {

src/internal/runtime/maps/table.go

@@ -409,9 +409,7 @@ func (t *table) uncheckedPutSlot(typ *abi.SwissMapType, hash uintptr, key unsafe
 	}
 }
 
-func (t *table) Delete(typ *abi.SwissMapType, m *Map, key unsafe.Pointer) {
-	hash := typ.Hasher(key, m.seed)
-
+func (t *table) Delete(typ *abi.SwissMapType, m *Map, hash uintptr, key unsafe.Pointer) {
 	seq := makeProbeSeq(h1(hash), t.groups.lengthMask)
 	for ; ; seq = seq.next() {
 		g := t.groups.group(typ, seq.offset)

src/runtime/map_benchmark_test.go

@@ -1090,3 +1090,14 @@ func BenchmarkMapPop(b *testing.B) {
 	b.Run("Key=*int32/Elem=int32", benchSizes(benchmarkMapPop[*int32, int32]))
 	b.Run("Key=int32/Elem=*int32", benchSizes(benchmarkMapPop[int32, *int32]))
 }
+
+func BenchmarkMapDeleteLargeKey(b *testing.B) {
+	m := map[string]int{}
+	for i := range 9 {
+		m[fmt.Sprintf("%d", i)] = i
+	}
+	key := strings.Repeat("*", 10000)
+	for range b.N {
+		delete(m, key)
+	}
+}