Index: sys/amd64/amd64/support.S
===================================================================
--- sys/amd64/amd64/support.S
+++ sys/amd64/amd64/support.S
@@ -302,6 +302,8 @@
 
 	ALIGN_TEXT
 1256:
+	testq	$15,%rdi
+	jnz	100f
 .if \erms == 1
 	rep
 	movsb
@@ -315,6 +317,34 @@
 .endif
 	\end
 	ret
+100:
+	movq	(%rsi),%r8
+	movq	8(%rsi),%r9
+	movq	%rdi,%r10
+	movq	%rdi,%rcx
+	andq	$15,%rcx
+	leaq	-16(%rdx,%rcx),%rdx
+	neg	%rcx
+	leaq	16(%rdi,%rcx),%rdi
+	leaq	16(%rsi,%rcx),%rsi
+	movq	%rdx,%rcx
+.if \erms == 1
+	rep
+	movsb
+	movq	%r8,(%r10)
+	movq	%r9,8(%r10)
+.else
+	shrq	$3,%rcx                         /* copy by 64-bit words */
+	rep
+	movsq
+	movq	%r8,(%r10)
+	movq	%r9,8(%r10)
+	movq	%rdx,%rcx
+	andl	$7,%ecx                         /* any bytes left? */
+	jne	100408b
+.endif
+	\end
+	ret
 
 .if \overlap == 1
 	/*