glibc/sysdeps/aarch64/strlen.S

/* Copyright (C) 2012-2014 Free Software Foundation, Inc.

   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library.  If not, see
   <http://www.gnu.org/licenses/>.  */

#include <sysdep.h>

/* Assumptions:
 *
 * ARMv8-a, AArch64
 */

/* Arguments and results.  */
#define srcin		x0
#define len		x0

/* Locals and temporaries.  */
#define src		x1
#define data1		x2
#define data2		x3
#define data2a		x4
#define has_nul1	x5
#define has_nul2	x6
#define tmp1		x7
#define tmp2		x8
#define tmp3		x9
#define tmp4		x10
#define zeroones	x11
#define pos		x12

#define REP8_01 0x0101010101010101
#define REP8_7f 0x7f7f7f7f7f7f7f7f
#define REP8_80 0x8080808080808080

	/* Start of critial section -- keep to one 64Byte cache line.  */
ENTRY_ALIGN (strlen, 6)
	mov	zeroones, #REP8_01
	bic	src, srcin, #15
	ands	tmp1, srcin, #15
	b.ne	L(misaligned)
	/* 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.  */
L(loop):
	ldp	data1, data2, [src], #16
L(realigned):
	sub	tmp1, data1, zeroones
	orr	tmp2, data1, #REP8_7f
	sub	tmp3, data2, zeroones
	orr	tmp4, data2, #REP8_7f
	bic	has_nul1, tmp1, tmp2
	bics	has_nul2, tmp3, tmp4
	ccmp	has_nul1, #0, #0, eq	/* NZCV = 0000  */
	b.eq	L(loop)
	/* End of critical section -- keep to one 64Byte cache line.  */

	sub	len, src, srcin
	cbz	has_nul1, L(nul_in_data2)
#ifdef __AARCH64EB__
	mov	data2, data1
#endif
	sub	len, len, #8
	mov	has_nul2, has_nul1
L(nul_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.  */
	RET

L(misaligned):
	cmp	tmp1, #8
	neg	tmp1, tmp1
	ldp	data1, data2, [src], #16
	lsl	tmp1, tmp1, #3		/* Bytes beyond alignment -> bits.  */
	mov	tmp2, #~0
#ifdef __AARCH64EB__
	/* Big-endian.  Early bytes are at MSB.  */
	lsl	tmp2, tmp2, tmp1	/* Shift (tmp1 & 63).  */
#else
	/* Little-endian.  Early bytes are at LSB.  */
	lsr	tmp2, tmp2, tmp1	/* Shift (tmp1 & 63).  */
#endif
	orr	data1, data1, tmp2
	orr	data2a, data2, tmp2
	csinv	data1, data1, xzr, le
	csel	data2, data2, data2a, le
	b	L(realigned)
END (strlen)
libc_hidden_builtin_def (strlen)