util/deephash: optimize hashing of byte arrays, reduce allocs in Hash

name              old time/op    new time/op    delta
Hash-6               173µs ± 4%     101µs ± 3%   -41.69%  (p=0.000 n=10+9)
HashMapAcyclic-6     101µs ± 5%     105µs ± 3%    +3.52%  (p=0.001 n=9+10)
TailcfgNode-6       29.4µs ± 2%    16.4µs ± 3%   -44.25%  (p=0.000 n=8+10)

name              old alloc/op   new alloc/op   delta
Hash-6              3.60kB ± 0%    1.13kB ± 0%   -68.70%  (p=0.000 n=10+10)
HashMapAcyclic-6    2.53kB ± 0%    2.53kB ± 0%      ~     (p=0.137 n=10+8)
TailcfgNode-6         528B ± 0%        0B       -100.00%  (p=0.000 n=10+10)

name              old allocs/op  new allocs/op  delta
Hash-6                84.0 ± 0%      40.0 ± 0%   -52.38%  (p=0.000 n=10+10)
HashMapAcyclic-6       202 ± 0%       202 ± 0%      ~     (all equal)
TailcfgNode-6         11.0 ± 0%       0.0       -100.00%  (p=0.000 n=10+10)

Updates tailscale/corp#2130

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/deephash/deephash_test.go

@@ -169,6 +169,29 @@ func TestHashMapAcyclic(t *testing.T) {
 	}
 }
 
+func TestPrintArray(t *testing.T) {
+	type T struct {
+		X [32]byte
+	}
+	x := &T{X: [32]byte{1: 1, 31: 31}}
+	var got bytes.Buffer
+	bw := bufio.NewWriter(&got)
+	h := &hasher{
+		bw:      bw,
+		visited: map[uintptr]bool{},
+	}
+	h.print(reflect.ValueOf(x))
+	bw.Flush()
+	const want = "struct" +
+		"\x00\x00\x00\x00\x00\x00\x00\x01" + // 1 field
+		"\x00\x00\x00\x00\x00\x00\x00\x00" + // 0th field
+		// the 32 bytes:
+		"\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x1f"
+	if got := got.Bytes(); string(got) != want {
+		t.Errorf("wrong:\n got: %q\nwant: %q\n", got, want)
+	}
+}
+
 func BenchmarkHashMapAcyclic(b *testing.B) {
 	b.ReportAllocs()
 	m := map[int]string{}
@@ -238,3 +261,28 @@ func TestMapCyclicFallback(t *testing.T) {
 	v.M["m"] = v.M
 	Hash(v)
 }
+
+func TestArrayAllocs(t *testing.T) {
+	type T struct {
+		X [32]byte
+	}
+	x := &T{X: [32]byte{1: 1, 2: 2, 3: 3, 4: 4}}
+	n := int(testing.AllocsPerRun(1000, func() {
+		sink = Hash(x)
+	}))
+	if n > 0 {
+		t.Errorf("allocs = %v; want 0", n)
+	}
+}
+
+func BenchmarkHashArray(b *testing.B) {
+	b.ReportAllocs()
+	type T struct {
+		X [32]byte
+	}
+	x := &T{X: [32]byte{1: 1, 2: 2, 3: 3, 4: 4}}
+
+	for i := 0; i < b.N; i++ {
+		sink = Hash(x)
+	}
+}