diff --git a/src/cmd/compile/internal/gc/inl_test.go b/src/cmd/compile/internal/gc/inl_test.go
index 7868c14aa60d7..a04869c5a3337 100644
--- a/src/cmd/compile/internal/gc/inl_test.go
+++ b/src/cmd/compile/internal/gc/inl_test.go
@@ -119,6 +119,10 @@ func TestIntendedInlining(t *testing.T) {
 			"byLiteral.Less",
 			"byLiteral.Swap",
 		},
+		"encoding/base64": {
+			"assemble32",
+			"assemble64",
+		},
 		"unicode/utf8": {
 			"FullRune",
 			"FullRuneInString",
diff --git a/src/encoding/base64/base64.go b/src/encoding/base64/base64.go
index a7da7747ef63a..082210198f380 100644
--- a/src/encoding/base64/base64.go
+++ b/src/encoding/base64/base64.go
@@ -284,6 +284,9 @@ func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err err
 	var dbuf [4]byte
 	dlen := 4
 
+	// Lift the nil check outside of the loop.
+	_ = enc.decodeMap
+
 	for j := 0; j < len(dbuf); j++ {
 		if len(src) == si {
 			switch {
@@ -467,9 +470,23 @@ func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
 		return 0, nil
 	}
 
+	// Lift the nil check outside of the loop. enc.decodeMap is directly
+	// used later in this function, to let the compiler know that the
+	// receiver can't be nil.
+	_ = enc.decodeMap
+
 	si := 0
 	for strconv.IntSize >= 64 && len(src)-si >= 8 && len(dst)-n >= 8 {
-		if dn, ok := enc.decode64(src[si:]); ok {
+		if dn, ok := assemble64(
+			enc.decodeMap[src[si+0]],
+			enc.decodeMap[src[si+1]],
+			enc.decodeMap[src[si+2]],
+			enc.decodeMap[src[si+3]],
+			enc.decodeMap[src[si+4]],
+			enc.decodeMap[src[si+5]],
+			enc.decodeMap[src[si+6]],
+			enc.decodeMap[src[si+7]],
+		); ok {
 			binary.BigEndian.PutUint64(dst[n:], dn)
 			n += 6
 			si += 8
@@ -484,7 +501,12 @@ func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
 	}
 
 	for len(src)-si >= 4 && len(dst)-n >= 4 {
-		if dn, ok := enc.decode32(src[si:]); ok {
+		if dn, ok := assemble32(
+			enc.decodeMap[src[si+0]],
+			enc.decodeMap[src[si+1]],
+			enc.decodeMap[src[si+2]],
+			enc.decodeMap[src[si+3]],
+		); ok {
 			binary.BigEndian.PutUint32(dst[n:], dn)
 			n += 3
 			si += 4
@@ -509,70 +531,40 @@ func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
 	return n, err
 }
 
-// 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 0, false
-	}
-	dn |= n << 26
-	if n = uint32(enc.decodeMap[src[1]]); n == 0xff {
-		return 0, false
-	}
-	dn |= n << 20
-	if n = uint32(enc.decodeMap[src[2]]); n == 0xff {
+// assemble32 assembles 4 base64 digits into 3 bytes.
+// Each digit comes from the decode map, and will be 0xff
+// if it came from an invalid character.
+func assemble32(n1, n2, n3, n4 byte) (dn uint32, ok bool) {
+	// Check that all the digits are valid. If any of them was 0xff, their
+	// bitwise OR will be 0xff.
+	if n1|n2|n3|n4 == 0xff {
 		return 0, false
 	}
-	dn |= n << 14
-	if n = uint32(enc.decodeMap[src[3]]); n == 0xff {
-		return 0, false
-	}
-	dn |= n << 8
-	return dn, true
+	return uint32(n1)<<26 |
+			uint32(n2)<<20 |
+			uint32(n3)<<14 |
+			uint32(n4)<<8,
+		true
 }
 
-// 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 0, false
-	}
-	dn |= n << 58
-	if n = uint64(enc.decodeMap[src[1]]); n == 0xff {
-		return 0, false
-	}
-	dn |= n << 52
-	if n = uint64(enc.decodeMap[src[2]]); n == 0xff {
-		return 0, false
-	}
-	dn |= n << 46
-	if n = uint64(enc.decodeMap[src[3]]); n == 0xff {
-		return 0, false
-	}
-	dn |= n << 40
-	if n = uint64(enc.decodeMap[src[4]]); n == 0xff {
-		return 0, false
-	}
-	dn |= n << 34
-	if n = uint64(enc.decodeMap[src[5]]); n == 0xff {
-		return 0, false
-	}
-	dn |= n << 28
-	if n = uint64(enc.decodeMap[src[6]]); n == 0xff {
-		return 0, false
-	}
-	dn |= n << 22
-	if n = uint64(enc.decodeMap[src[7]]); n == 0xff {
+// assemble64 assembles 8 base64 digits into 6 bytes.
+// Each digit comes from the decode map, and will be 0xff
+// if it came from an invalid character.
+func assemble64(n1, n2, n3, n4, n5, n6, n7, n8 byte) (dn uint64, ok bool) {
+	// Check that all the digits are valid. If any of them was 0xff, their
+	// bitwise OR will be 0xff.
+	if n1|n2|n3|n4|n5|n6|n7|n8 == 0xff {
 		return 0, false
 	}
-	dn |= n << 16
-	return dn, true
+	return uint64(n1)<<58 |
+			uint64(n2)<<52 |
+			uint64(n3)<<46 |
+			uint64(n4)<<40 |
+			uint64(n5)<<34 |
+			uint64(n6)<<28 |
+			uint64(n7)<<22 |
+			uint64(n8)<<16,
+		true
 }
 
 type newlineFilteringReader struct {