diff --git a/src/encoding/base64/base64.go b/src/encoding/base64/base64.go
index 9a99370f1e..e8afc48859 100644
--- a/src/encoding/base64/base64.go
+++ b/src/encoding/base64/base64.go
@@ -465,10 +465,9 @@ func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
 	}
 
 	si := 0
-	ilen := len(src)
-	olen := len(dst)
-	for strconv.IntSize >= 64 && ilen-si >= 8 && olen-n >= 8 {
-		if ok := enc.decode64(dst[n:], src[si:]); ok {
+	for strconv.IntSize >= 64 && len(src)-si >= 8 && len(dst)-n >= 8 {
+		if dn, ok := enc.decode64(src[si:]); ok {
+			binary.BigEndian.PutUint64(dst[n:], dn)
 			n += 6
 			si += 8
 		} else {
@@ -481,8 +480,9 @@ func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
 		}
 	}
 
-	for ilen-si >= 4 && olen-n >= 4 {
-		if ok := enc.decode32(dst[n:], src[si:]); ok {
+	for len(src)-si >= 4 && len(dst)-n >= 4 {
+		if dn, ok := enc.decode32(src[si:]); ok {
+			binary.BigEndian.PutUint32(dst[n:], dn)
 			n += 3
 			si += 4
 		} else {
@@ -506,72 +506,70 @@ func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
 	return n, err
 }
 
-// decode32 tries to decode 4 base64 char into 3 bytes.
-// len(dst) and len(src) must both be >= 4.
-// Returns true if decode succeeded.
-func (enc *Encoding) decode32(dst, src []byte) bool {
-	var dn, n uint32
+// decode32 tries to decode 4 base64 characters into 3 bytes, and returns those
+// bytes. len(src) must be >= 4.
+// Returns (0, false) if decoding failed.
+func (enc *Encoding) decode32(src []byte) (dn uint32, ok bool) {
+	var n uint32
+	_ = src[3]
 	if n = uint32(enc.decodeMap[src[0]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 26
 	if n = uint32(enc.decodeMap[src[1]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 20
 	if n = uint32(enc.decodeMap[src[2]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 14
 	if n = uint32(enc.decodeMap[src[3]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 8
-
-	binary.BigEndian.PutUint32(dst, dn)
-	return true
+	return dn, true
 }
 
-// decode64 tries to decode 8 base64 char into 6 bytes.
-// len(dst) and len(src) must both be >= 8.
-// Returns true if decode succeeded.
-func (enc *Encoding) decode64(dst, src []byte) bool {
-	var dn, n uint64
+// decode64 tries to decode 8 base64 characters into 6 bytes, and returns those
+// bytes. len(src) must be >= 8.
+// Returns (0, false) if decoding failed.
+func (enc *Encoding) decode64(src []byte) (dn uint64, ok bool) {
+	var n uint64
+	_ = src[7]
 	if n = uint64(enc.decodeMap[src[0]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 58
 	if n = uint64(enc.decodeMap[src[1]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 52
 	if n = uint64(enc.decodeMap[src[2]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 46
 	if n = uint64(enc.decodeMap[src[3]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 40
 	if n = uint64(enc.decodeMap[src[4]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 34
 	if n = uint64(enc.decodeMap[src[5]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 28
 	if n = uint64(enc.decodeMap[src[6]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 22
 	if n = uint64(enc.decodeMap[src[7]]); n == 0xff {
-		return false
+		return 0, false
 	}
 	dn |= n << 16
-
-	binary.BigEndian.PutUint64(dst, dn)
-	return true
+	return dn, true
 }
 
 type newlineFilteringReader struct {