diff --git a/winsup/cygwin/Makefile.am b/winsup/cygwin/Makefile.am
index f8c249f52..0200f6e2a 100644
--- a/winsup/cygwin/Makefile.am
+++ b/winsup/cygwin/Makefile.am
@@ -58,6 +58,7 @@ TARGET_FILES= \
 	x86_64/memmove.S \
 	x86_64/mempcpy.S \
 	x86_64/memset.S \
+	x86_64/swab.S \
 	x86_64/wmemcpy.S \
 	x86_64/wmemmove.S \
 	x86_64/wmempcpy.S
diff --git a/winsup/cygwin/x86_64/swab.S b/winsup/cygwin/x86_64/swab.S
new file mode 100644
index 000000000..ae73d1124
--- /dev/null
+++ b/winsup/cygwin/x86_64/swab.S
@@ -0,0 +1,47 @@
+/*
+ * Written by J.T. Conklin <jtc@NetBSD.org>.
+ * Public domain.
+ */
+
+#include <machine/asm.h>
+
+#if defined(LIBC_SCCS)
+	RCSID("$NetBSD: swab.S,v 1.4 2014/05/22 15:01:57 uebayasi Exp $")
+#endif
+
+#define LOAD_SWAP_STORE_WORD \
+	lodsw	; \
+	xchgb	%al,%ah ; \
+	stosw
+
+ENTRY3(swab)
+	xchgq	%rdi,%rsi
+	cld				# set direction forward
+
+	shrq	$1,%rdx
+	testq	$7,%rdx			# copy first group of 1 to 7 words
+	jz	L2			# while swapping alternate bytes.
+L1:	lodsw
+	rorw	$8,%ax
+	stosw
+	decq	%rdx
+	testq	$7,%rdx
+	jnz	L1
+
+L2:	shrq	$3,%rdx			# copy remainder 8 words at a time
+	jz	L4			# while swapping alternate bytes.
+L3:
+	LOAD_SWAP_STORE_WORD
+	LOAD_SWAP_STORE_WORD
+	LOAD_SWAP_STORE_WORD
+	LOAD_SWAP_STORE_WORD
+	LOAD_SWAP_STORE_WORD
+	LOAD_SWAP_STORE_WORD
+	LOAD_SWAP_STORE_WORD
+	LOAD_SWAP_STORE_WORD
+
+	decq	%rdx
+	jnz	L3
+L4:
+	ret
+END(swab)