tun: unwind summing loop in checksumNoFold()

$ benchstat old.txt new.txt
goos: linux
goarch: amd64
pkg: golang.zx2c4.com/wireguard/tun
cpu: 12th Gen Intel(R) Core(TM) i5-12400
                 │   old.txt    │               new.txt               │
                 │    sec/op    │   sec/op     vs base                │
Checksum/64-12     10.670n ± 2%   4.769n ± 0%  -55.30% (p=0.000 n=10)
Checksum/128-12    19.665n ± 2%   8.032n ± 0%  -59.16% (p=0.000 n=10)
Checksum/256-12     37.68n ± 1%   16.06n ± 0%  -57.37% (p=0.000 n=10)
Checksum/512-12     76.61n ± 3%   32.13n ± 0%  -58.06% (p=0.000 n=10)
Checksum/1024-12   160.55n ± 4%   64.25n ± 0%  -59.98% (p=0.000 n=10)
Checksum/1500-12   231.05n ± 7%   94.12n ± 0%  -59.26% (p=0.000 n=10)
Checksum/2048-12    309.5n ± 3%   128.5n ± 0%  -58.48% (p=0.000 n=10)
Checksum/4096-12    603.8n ± 4%   257.2n ± 0%  -57.41% (p=0.000 n=10)
Checksum/8192-12   1185.0n ± 3%   515.5n ± 0%  -56.50% (p=0.000 n=10)
Checksum/9000-12   1328.5n ± 5%   564.8n ± 0%  -57.49% (p=0.000 n=10)
Checksum/9001-12   1340.5n ± 3%   564.8n ± 0%  -57.87% (p=0.000 n=10)
geomean             185.3n        77.99n       -57.92%

Reviewed-by: Adrian Dewhurst <adrian@tailscale.com>
Signed-off-by: Jordan Whited <jordan@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

2 files changed, 123 insertions, 12 deletions

diff --git a/tun/checksum.go b/tun/checksum.go
index f4f8471..29a8fc8 100644
--- a/tun/checksum.go
+++ b/tun/checksum.go
@@ -3,23 +3,99 @@ package tun
 import "encoding/binary"
 
 // TODO: Explore SIMD and/or other assembly optimizations.
+// TODO: Test native endian loads. See RFC 1071 section 2 part B.
 func checksumNoFold(b []byte, initial uint64) uint64 {
 	ac := initial
-	i := 0
-	n := len(b)
-	for n >= 4 {
-		ac += uint64(binary.BigEndian.Uint32(b[i : i+4]))
-		n -= 4
-		i += 4
+
+	for len(b) >= 128 {
+		ac += uint64(binary.BigEndian.Uint32(b[:4]))
+		ac += uint64(binary.BigEndian.Uint32(b[4:8]))
+		ac += uint64(binary.BigEndian.Uint32(b[8:12]))
+		ac += uint64(binary.BigEndian.Uint32(b[12:16]))
+		ac += uint64(binary.BigEndian.Uint32(b[16:20]))
+		ac += uint64(binary.BigEndian.Uint32(b[20:24]))
+		ac += uint64(binary.BigEndian.Uint32(b[24:28]))
+		ac += uint64(binary.BigEndian.Uint32(b[28:32]))
+		ac += uint64(binary.BigEndian.Uint32(b[32:36]))
+		ac += uint64(binary.BigEndian.Uint32(b[36:40]))
+		ac += uint64(binary.BigEndian.Uint32(b[40:44]))
+		ac += uint64(binary.BigEndian.Uint32(b[44:48]))
+		ac += uint64(binary.BigEndian.Uint32(b[48:52]))
+		ac += uint64(binary.BigEndian.Uint32(b[52:56]))
+		ac += uint64(binary.BigEndian.Uint32(b[56:60]))
+		ac += uint64(binary.BigEndian.Uint32(b[60:64]))
+		ac += uint64(binary.BigEndian.Uint32(b[64:68]))
+		ac += uint64(binary.BigEndian.Uint32(b[68:72]))
+		ac += uint64(binary.BigEndian.Uint32(b[72:76]))
+		ac += uint64(binary.BigEndian.Uint32(b[76:80]))
+		ac += uint64(binary.BigEndian.Uint32(b[80:84]))
+		ac += uint64(binary.BigEndian.Uint32(b[84:88]))
+		ac += uint64(binary.BigEndian.Uint32(b[88:92]))
+		ac += uint64(binary.BigEndian.Uint32(b[92:96]))
+		ac += uint64(binary.BigEndian.Uint32(b[96:100]))
+		ac += uint64(binary.BigEndian.Uint32(b[100:104]))
+		ac += uint64(binary.BigEndian.Uint32(b[104:108]))
+		ac += uint64(binary.BigEndian.Uint32(b[108:112]))
+		ac += uint64(binary.BigEndian.Uint32(b[112:116]))
+		ac += uint64(binary.BigEndian.Uint32(b[116:120]))
+		ac += uint64(binary.BigEndian.Uint32(b[120:124]))
+		ac += uint64(binary.BigEndian.Uint32(b[124:128]))
+		b = b[128:]
+	}
+	if len(b) >= 64 {
+		ac += uint64(binary.BigEndian.Uint32(b[:4]))
+		ac += uint64(binary.BigEndian.Uint32(b[4:8]))
+		ac += uint64(binary.BigEndian.Uint32(b[8:12]))
+		ac += uint64(binary.BigEndian.Uint32(b[12:16]))
+		ac += uint64(binary.BigEndian.Uint32(b[16:20]))
+		ac += uint64(binary.BigEndian.Uint32(b[20:24]))
+		ac += uint64(binary.BigEndian.Uint32(b[24:28]))
+		ac += uint64(binary.BigEndian.Uint32(b[28:32]))
+		ac += uint64(binary.BigEndian.Uint32(b[32:36]))
+		ac += uint64(binary.BigEndian.Uint32(b[36:40]))
+		ac += uint64(binary.BigEndian.Uint32(b[40:44]))
+		ac += uint64(binary.BigEndian.Uint32(b[44:48]))
+		ac += uint64(binary.BigEndian.Uint32(b[48:52]))
+		ac += uint64(binary.BigEndian.Uint32(b[52:56]))
+		ac += uint64(binary.BigEndian.Uint32(b[56:60]))
+		ac += uint64(binary.BigEndian.Uint32(b[60:64]))
+		b = b[64:]
+	}
+	if len(b) >= 32 {
+		ac += uint64(binary.BigEndian.Uint32(b[:4]))
+		ac += uint64(binary.BigEndian.Uint32(b[4:8]))
+		ac += uint64(binary.BigEndian.Uint32(b[8:12]))
+		ac += uint64(binary.BigEndian.Uint32(b[12:16]))
+		ac += uint64(binary.BigEndian.Uint32(b[16:20]))
+		ac += uint64(binary.BigEndian.Uint32(b[20:24]))
+		ac += uint64(binary.BigEndian.Uint32(b[24:28]))
+		ac += uint64(binary.BigEndian.Uint32(b[28:32]))
+		b = b[32:]
+	}
+	if len(b) >= 16 {
+		ac += uint64(binary.BigEndian.Uint32(b[:4]))
+		ac += uint64(binary.BigEndian.Uint32(b[4:8]))
+		ac += uint64(binary.BigEndian.Uint32(b[8:12]))
+		ac += uint64(binary.BigEndian.Uint32(b[12:16]))
+		b = b[16:]
 	}
-	for n >= 2 {
-		ac += uint64(binary.BigEndian.Uint16(b[i : i+2]))
-		n -= 2
-		i += 2
+	if len(b) >= 8 {
+		ac += uint64(binary.BigEndian.Uint32(b[:4]))
+		ac += uint64(binary.BigEndian.Uint32(b[4:8]))
+		b = b[8:]
 	}
-	if n == 1 {
-		ac += uint64(b[i]) << 8
+	if len(b) >= 4 {
+		ac += uint64(binary.BigEndian.Uint32(b))
+		b = b[4:]
 	}
+	if len(b) >= 2 {
+		ac += uint64(binary.BigEndian.Uint16(b))
+		b = b[2:]
+	}
+	if len(b) == 1 {
+		ac += uint64(b[0]) << 8
+	}
+
 	return ac
 }
 
diff --git a/tun/checksum_test.go b/tun/checksum_test.go
new file mode 100644
index 0000000..c1ccff5
--- /dev/null
+++ b/tun/checksum_test.go
@@ -0,0 +1,35 @@
+package tun
+
+import (
+	"fmt"
+	"math/rand"
+	"testing"
+)
+
+func BenchmarkChecksum(b *testing.B) {
+	lengths := []int{
+		64,
+		128,
+		256,
+		512,
+		1024,
+		1500,
+		2048,
+		4096,
+		8192,
+		9000,
+		9001,
+	}
+
+	for _, length := range lengths {
+		b.Run(fmt.Sprintf("%d", length), func(b *testing.B) {
+			buf := make([]byte, length)
+			rng := rand.New(rand.NewSource(1))
+			rng.Read(buf)
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				checksum(buf, 0)
+			}
+		})
+	}
+}