device: remove code using unsafe

There is no performance impact.

name                             old time/op  new time/op  delta
TrieIPv4Peers100Addresses1000-8  78.6ns ± 1%  79.4ns ± 3%    ~     (p=0.604 n=10+9)
TrieIPv4Peers10Addresses10-8     29.1ns ± 2%  28.8ns ± 1%  -1.12%  (p=0.014 n=10+9)
TrieIPv6Peers100Addresses1000-8  78.9ns ± 1%  78.6ns ± 1%    ~     (p=0.492 n=10+10)
TrieIPv6Peers10Addresses10-8     29.3ns ± 2%  28.6ns ± 2%  -2.16%  (p=0.000 n=10+10)

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

device/allowedips.go

@@ -7,6 +7,7 @@ package device
 
 import (
 	"container/list"
+	"encoding/binary"
 	"errors"
 	"math/bits"
 	"net"
@@ -32,45 +33,24 @@ type trieEntry struct {
 	perPeerElem *list.Element
 }
 
-func isLittleEndian() bool {
-	one := uint32(1)
-	return *(*byte)(unsafe.Pointer(&one)) != 0
-}
-
-func swapU32(i uint32) uint32 {
-	if !isLittleEndian() {
-		return i
-	}
-
-	return bits.ReverseBytes32(i)
-}
-
-func swapU64(i uint64) uint64 {
-	if !isLittleEndian() {
-		return i
-	}
-
-	return bits.ReverseBytes64(i)
-}
-
 func commonBits(ip1, ip2 []byte) uint8 {
 	size := len(ip1)
 	if size == net.IPv4len {
-		a := (*uint32)(unsafe.Pointer(&ip1[0]))
-		b := (*uint32)(unsafe.Pointer(&ip2[0]))
-		x := *a ^ *b
-		return uint8(bits.LeadingZeros32(swapU32(x)))
+		a := binary.BigEndian.Uint32(ip1)
+		b := binary.BigEndian.Uint32(ip2)
+		x := a ^ b
+		return uint8(bits.LeadingZeros32(x))
 	} else if size == net.IPv6len {
-		a := (*uint64)(unsafe.Pointer(&ip1[0]))
-		b := (*uint64)(unsafe.Pointer(&ip2[0]))
-		x := *a ^ *b
+		a := binary.BigEndian.Uint64(ip1)
+		b := binary.BigEndian.Uint64(ip2)
+		x := a ^ b
 		if x != 0 {
-			return uint8(bits.LeadingZeros64(swapU64(x)))
+			return uint8(bits.LeadingZeros64(x))
 		}
-		a = (*uint64)(unsafe.Pointer(&ip1[8]))
-		b = (*uint64)(unsafe.Pointer(&ip2[8]))
-		x = *a ^ *b
-		return 64 + uint8(bits.LeadingZeros64(swapU64(x)))
+		a = binary.BigEndian.Uint64(ip1[8:])
+		b = binary.BigEndian.Uint64(ip2[8:])
+		x = a ^ b
+		return 64 + uint8(bits.LeadingZeros64(x))
 	} else {
 		panic("Wrong size bit string")
 	}