linux/arch/arm64/lib/strnlen.S

/*
 * Copyright (C) 2013 ARM Ltd.
 * Copyright (C) 2013 Linaro.
 *
 * This code is based on glibc cortex strings work originally authored by Linaro
 * and re-licensed under GPLv2 for the Linux kernel. The original code can
 * be found @
 *
 * http://bazaar.launchpad.net/~linaro-toolchain-dev/cortex-strings/trunk/
 * files/head:/src/aarch64/
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 * This program 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <linux/linkage.h>
#include <asm/assembler.h>

/*
 * determine the length of a fixed-size string
 *
 * Parameters:
 *	x0 - const string pointer
 *	x1 - maximal string length
 * Returns:
 *	x0 - the return length of specific string
 */

/* Arguments and results.  */
srcin		.req	x0
len		.req	x0
limit		.req	x1

/* Locals and temporaries.  */
src		.req	x2
data1		.req	x3
data2		.req	x4
data2a		.req	x5
has_nul1	.req	x6
has_nul2	.req	x7
tmp1		.req	x8
tmp2		.req	x9
tmp3		.req	x10
tmp4		.req	x11
zeroones	.req	x12
pos		.req	x13
limit_wd	.req	x14

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

ENTRY(strnlen)
	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.
	*/
.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

	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
CPU_BE( mov	data2, data1 )	/*perpare data to re-calculate the syndrome*/

	sub	len, len, #8
	mov	has_nul2, has_nul1
.Lnul_in_data2:
	/*
	* 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.
	*/
CPU_BE( rev	data2, data2 )
CPU_BE( sub	tmp1, data2, zeroones )
CPU_BE( orr	tmp2, data2, #REP8_7f )
CPU_BE( bic	has_nul2, tmp1, tmp2 )

	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.
	*/
	ldp	data1, data2, [src], #16

	sub	limit_wd, limit, #1
	and	tmp3, limit_wd, #15
	lsr	limit_wd, limit_wd, #4

	add	tmp3, tmp3, tmp1
	add	limit_wd, limit_wd, tmp3, lsr #4

	neg	tmp4, tmp1
	lsl	tmp4, tmp4, #3  /* Bytes beyond alignment -> bits.  */

	mov	tmp2, #~0
	/* Big-endian.  Early bytes are at MSB.  */
CPU_BE( lsl	tmp2, tmp2, tmp4 )	/* Shift (tmp1 & 63).  */
	/* Little-endian.  Early bytes are at LSB.  */
CPU_LE( lsr	tmp2, tmp2, tmp4 )	/* Shift (tmp1 & 63).  */

	cmp	tmp1, #8

	orr	data1, data1, tmp2
	orr	data2a, data2, tmp2

	csinv	data1, data1, xzr, le
	csel	data2, data2, data2a, le
	b	.Lrealigned

.Lhit_limit:
	mov	len, limit
	ret
ENDPIPROC(strnlen)