internal/deephash: re-use map iteration values

This requires changes to the Go toolchain.
The changes are upstream at https://golang.org/cl/320929.
They haven't been pulled into our fork yet.

No need to allocate new iteration scratch values for every map.

name              old time/op    new time/op    delta
Hash-8              13.6µs ± 0%    13.5µs ± 0%   -1.01%  (p=0.008 n=5+5)
HashMapAcyclic-8    21.2µs ± 1%    21.1µs ± 2%     ~     (p=0.310 n=5+5)

name              old alloc/op   new alloc/op   delta
Hash-8              1.58kB ± 0%    1.46kB ± 0%   -7.60%  (p=0.008 n=5+5)
HashMapAcyclic-8      152B ± 0%      128B ± 0%  -15.79%  (p=0.008 n=5+5)

name              old allocs/op  new allocs/op  delta
Hash-8                49.0 ± 0%      43.0 ± 0%  -12.24%  (p=0.008 n=5+5)
HashMapAcyclic-8      4.00 ± 0%      2.00 ± 0%  -50.00%  (p=0.008 n=5+5)

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

internal/deephash/deephash.go

@@ -200,6 +200,7 @@ type mapHasher struct {
 	ebuf [sha256.Size]byte // scratch buffer
 	s256 hash.Hash         // sha256 hash.Hash
 	bw   *bufio.Writer     // to hasher into ebuf
+	val  valueCache        // re-usable values for map iteration
 }
 
 func (mh *mapHasher) Reset() {
@@ -228,10 +229,22 @@ var mapHasherPool = &sync.Pool{
 		mh := new(mapHasher)
 		mh.s256 = sha256.New()
 		mh.bw = bufio.NewWriter(mh.s256)
+		mh.val = make(valueCache)
 		return mh
 	},
 }
 
+type valueCache map[reflect.Type]reflect.Value
+
+func (c valueCache) get(t reflect.Type) reflect.Value {
+	v, ok := c[t]
+	if !ok {
+		v = reflect.New(t).Elem()
+		c[t] = v
+	}
+	return v
+}
+
 // hashMapAcyclic is the faster sort-free version of map hashing. If
 // it detects a cycle it returns false and guarantees that nothing was
 // written to w.
@@ -240,8 +253,8 @@ func hashMapAcyclic(w *bufio.Writer, v reflect.Value, visited map[uintptr]bool)
 	defer mapHasherPool.Put(mh)
 	mh.Reset()
 	iter := v.MapRange()
-	k := reflect.New(v.Type().Key()).Elem()
+	k := mh.val.get(v.Type().Key())
-	e := reflect.New(v.Type().Elem()).Elem()
+	e := mh.val.get(v.Type().Elem())
 	for iter.Next() {
 		key := iterKey(iter, k)
 		val := iterVal(iter, e)