diff --git a/libc/arch-arm64/default/bionic/strnlen.S b/libc/arch-arm64/default/bionic/strnlen.S
new file mode 100644
index 0000000..1694532
--- /dev/null
+++ b/libc/arch-arm64/default/bionic/strnlen.S
@@ -0,0 +1,174 @@
+/* Copyright (c) 2014, Linaro Limited
+   All rights reserved.
+
+   Redistribution and use in source and binary forms, with or without
+   modification, are permitted provided that the following conditions are met:
+       * Redistributions of source code must retain the above copyright
+         notice, this list of conditions and the following disclaimer.
+       * Redistributions in binary form must reproduce the above copyright
+         notice, this list of conditions and the following disclaimer in the
+         documentation and/or other materials provided with the distribution.
+       * Neither the name of the Linaro nor the
+         names of its contributors may be used to endorse or promote products
+         derived from this software without specific prior written permission.
+
+   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+   HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*/
+
+/* Assumptions:
+ *
+ * ARMv8-a, AArch64
+ */
+
+#include <private/bionic_asm.h>
+
+/* Arguments and results.  */
+#define srcin		x0
+#define len		x0
+#define limit		x1
+
+/* Locals and temporaries.  */
+#define src		x2
+#define data1		x3
+#define data2		x4
+#define data2a		x5
+#define has_nul1	x6
+#define has_nul2	x7
+#define tmp1		x8
+#define tmp2		x9
+#define tmp3		x10
+#define tmp4		x11
+#define zeroones	x12
+#define pos		x13
+#define limit_wd	x14
+
+#define REP8_01 0x0101010101010101
+#define REP8_7f 0x7f7f7f7f7f7f7f7f
+#define REP8_80 0x8080808080808080
+
+	.text
+	.p2align	6
+.Lstart:
+	/* Pre-pad to ensure critical loop begins an icache line.  */
+	.rep 7
+	nop
+	.endr
+	/* Put this code here to avoid wasting more space with pre-padding.  */
+.Lhit_limit:
+	mov	len, limit
+	ret
+
+ENTRY(strnlen_default)
+	cbz	limit, .Lhit_limit
+	mov	zeroones, #REP8_01
+	bic	src, srcin, #15
+	ands	tmp1, srcin, #15
+	b.ne	.Lmisaligned
+	/* Calculate the number of full and partial words -1.  */
+	sub	limit_wd, limit, #1	/* Limit != 0, so no underflow.  */
+	lsr	limit_wd, limit_wd, #4	/* Convert to Qwords.  */
+
+	/* NUL detection works on the principle that (X - 1) & (~X) & 0x80
+	   (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
+	   can be done in parallel across the entire word.  */
+	/* The inner loop deals with two Dwords at a time.  This has a
+	   slightly higher start-up cost, but we should win quite quickly,
+	   especially on cores with a high number of issue slots per
+	   cycle, as we get much better parallelism out of the operations.  */
+
+	/* Start of critial section -- keep to one 64Byte cache line.  */
+.Lloop:
+	ldp	data1, data2, [src], #16
+.Lrealigned:
+	sub	tmp1, data1, zeroones
+	orr	tmp2, data1, #REP8_7f
+	sub	tmp3, data2, zeroones
+	orr	tmp4, data2, #REP8_7f
+	bic	has_nul1, tmp1, tmp2
+	bic	has_nul2, tmp3, tmp4
+	subs	limit_wd, limit_wd, #1
+	orr	tmp1, has_nul1, has_nul2
+	ccmp	tmp1, #0, #0, pl	/* NZCV = 0000  */
+	b.eq	.Lloop
+	/* End of critical section -- keep to one 64Byte cache line.  */
+
+	orr	tmp1, has_nul1, has_nul2
+	cbz	tmp1, .Lhit_limit	/* No null in final Qword.  */
+
+	/* We know there's a null in the final Qword.  The easiest thing
+	   to do now is work out the length of the string and return
+	   MIN (len, limit).  */
+
+	sub	len, src, srcin
+	cbz	has_nul1, .Lnul_in_data2
+#ifdef __AARCH64EB__
+	mov	data2, data1
+#endif
+	sub	len, len, #8
+	mov	has_nul2, has_nul1
+.Lnul_in_data2:
+#ifdef __AARCH64EB__
+	/* For big-endian, carry propagation (if the final byte in the
+	   string is 0x01) means we cannot use has_nul directly.  The
+	   easiest way to get the correct byte is to byte-swap the data
+	   and calculate the syndrome a second time.  */
+	rev	data2, data2
+	sub	tmp1, data2, zeroones
+	orr	tmp2, data2, #REP8_7f
+	bic	has_nul2, tmp1, tmp2
+#endif
+	sub	len, len, #8
+	rev	has_nul2, has_nul2
+	clz	pos, has_nul2
+	add	len, len, pos, lsr #3		/* Bits to bytes.  */
+	cmp	len, limit
+	csel	len, len, limit, ls		/* Return the lower value.  */
+	ret
+
+.Lmisaligned:
+	/* Deal with a partial first word.
+	   We're doing two things in parallel here;
+	   1) Calculate the number of words (but avoiding overflow if
+	      limit is near ULONG_MAX) - to do this we need to work out
+	      limit + tmp1 - 1 as a 65-bit value before shifting it;
+	   2) Load and mask the initial data words - we force the bytes
+	      before the ones we are interested in to 0xff - this ensures
+	      early bytes will not hit any zero detection.  */
+	sub	limit_wd, limit, #1
+	neg	tmp4, tmp1
+	cmp	tmp1, #8
+
+	and	tmp3, limit_wd, #15
+	lsr	limit_wd, limit_wd, #4
+	mov	tmp2, #~0
+
+	ldp	data1, data2, [src], #16
+	lsl	tmp4, tmp4, #3		/* Bytes beyond alignment -> bits.  */
+	add	tmp3, tmp3, tmp1
+
+#ifdef __AARCH64EB__
+	/* Big-endian.  Early bytes are at MSB.  */
+	lsl	tmp2, tmp2, tmp4	/* Shift (tmp1 & 63).  */
+#else
+	/* Little-endian.  Early bytes are at LSB.  */
+	lsr	tmp2, tmp2, tmp4	/* Shift (tmp1 & 63).  */
+#endif
+	add	limit_wd, limit_wd, tmp3, lsr #4
+
+	orr	data1, data1, tmp2
+	orr	data2a, data2, tmp2
+
+	csinv	data1, data1, xzr, le
+	csel	data2, data2, data2a, le
+	b	.Lrealigned
+END(strnlen_default)