diff --git a/bitlen_amd64.go b/bitlen_amd64.go
new file mode 100644
index 0000000..99ef323
--- /dev/null
+++ b/bitlen_amd64.go
@@ -0,0 +1,11 @@
+//go:build amd64 && !gccgo
+// +build amd64,!gccgo
+
+package encoding
+
+import "math/bits"
+
+// bitlen returns the number of bits required to represent x.
+func bitlen(x uint64) int {
+	return bits.Len64(x)
+}
diff --git a/bitlen_amd64.s b/bitlen_amd64.s
deleted file mode 100644
index 5e26858..0000000
--- a/bitlen_amd64.s
+++ /dev/null
@@ -1,12 +0,0 @@
-// +build !gccgo
-
-// func bitlen(x Word) (n int)
-TEXT ·bitlen(SB),4,$0
-	BSRQ x+0(FP), AX
-	JZ Z1
-	ADDQ $1, AX
-	MOVQ AX, n+8(FP)
-	RET
-
-Z1:	MOVQ $0, n+8(FP)
-	RET
diff --git a/bitlen_decl.go b/bitlen_decl.go
index 8641ac4..5fc5503 100644
--- a/bitlen_decl.go
+++ b/bitlen_decl.go
@@ -1,5 +1,6 @@
+//go:build !gccgo && (386 || arm)
 // +build !gccgo
-// +build amd64 386 arm
+// +build 386 arm
 
 package encoding