strnlen.S

  1/* SPDX-License-Identifier: GPL-2.0-only */
  2/*
  3 * Copyright (C) 2013 ARM Ltd.
  4 * Copyright (C) 2013 Linaro.
  5 *
  6 * This code is based on glibc cortex strings work originally authored by Linaro
  7 * be found @
  8 *
  9 * http://bazaar.launchpad.net/~linaro-toolchain-dev/cortex-strings/trunk/
 10 * files/head:/src/aarch64/
 11 */
 12
 13#include <linux/linkage.h>
 14#include <asm/assembler.h>
 15
 16/*
 17 * determine the length of a fixed-size string
 18 *
 19 * Parameters:
 20 *	x0 - const string pointer
 21 *	x1 - maximal string length
 22 * Returns:
 23 *	x0 - the return length of specific string
 24 */
 25
 26/* Arguments and results.  */
 27srcin		.req	x0
 28len		.req	x0
 29limit		.req	x1
 30
 31/* Locals and temporaries.  */
 32src		.req	x2
 33data1		.req	x3
 34data2		.req	x4
 35data2a		.req	x5
 36has_nul1	.req	x6
 37has_nul2	.req	x7
 38tmp1		.req	x8
 39tmp2		.req	x9
 40tmp3		.req	x10
 41tmp4		.req	x11
 42zeroones	.req	x12
 43pos		.req	x13
 44limit_wd	.req	x14
 45
 46#define REP8_01 0x0101010101010101
 47#define REP8_7f 0x7f7f7f7f7f7f7f7f
 48#define REP8_80 0x8080808080808080
 49
 50SYM_FUNC_START(__pi_strnlen)
 51	cbz	limit, .Lhit_limit
 52	mov	zeroones, #REP8_01
 53	bic	src, srcin, #15
 54	ands	tmp1, srcin, #15
 55	b.ne	.Lmisaligned
 56	/* Calculate the number of full and partial words -1.  */
 57	sub	limit_wd, limit, #1 /* Limit != 0, so no underflow.  */
 58	lsr	limit_wd, limit_wd, #4  /* Convert to Qwords.  */
 59
 60	/*
 61	* NUL detection works on the principle that (X - 1) & (~X) & 0x80
 62	* (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
 63	* can be done in parallel across the entire word.
 64	*/
 65	/*
 66	* The inner loop deals with two Dwords at a time.  This has a
 67	* slightly higher start-up cost, but we should win quite quickly,
 68	* especially on cores with a high number of issue slots per
 69	* cycle, as we get much better parallelism out of the operations.
 70	*/
 71.Lloop:
 72	ldp	data1, data2, [src], #16
 73.Lrealigned:
 74	sub	tmp1, data1, zeroones
 75	orr	tmp2, data1, #REP8_7f
 76	sub	tmp3, data2, zeroones
 77	orr	tmp4, data2, #REP8_7f
 78	bic	has_nul1, tmp1, tmp2
 79	bic	has_nul2, tmp3, tmp4
 80	subs	limit_wd, limit_wd, #1
 81	orr	tmp1, has_nul1, has_nul2
 82	ccmp	tmp1, #0, #0, pl    /* NZCV = 0000  */
 83	b.eq	.Lloop
 84
 85	cbz	tmp1, .Lhit_limit   /* No null in final Qword.  */
 86
 87	/*
 88	* We know there's a null in the final Qword. The easiest thing
 89	* to do now is work out the length of the string and return
 90	* MIN (len, limit).
 91	*/
 92	sub	len, src, srcin
 93	cbz	has_nul1, .Lnul_in_data2
 94CPU_BE( mov	data2, data1 )	/*perpare data to re-calculate the syndrome*/
 95
 96	sub	len, len, #8
 97	mov	has_nul2, has_nul1
 98.Lnul_in_data2:
 99	/*
100	* For big-endian, carry propagation (if the final byte in the
101	* string is 0x01) means we cannot use has_nul directly.  The
102	* easiest way to get the correct byte is to byte-swap the data
103	* and calculate the syndrome a second time.
104	*/
105CPU_BE( rev	data2, data2 )
106CPU_BE( sub	tmp1, data2, zeroones )
107CPU_BE( orr	tmp2, data2, #REP8_7f )
108CPU_BE( bic	has_nul2, tmp1, tmp2 )
109
110	sub	len, len, #8
111	rev	has_nul2, has_nul2
112	clz	pos, has_nul2
113	add	len, len, pos, lsr #3       /* Bits to bytes.  */
114	cmp	len, limit
115	csel	len, len, limit, ls     /* Return the lower value.  */
116	ret
117
118.Lmisaligned:
119	/*
120	* Deal with a partial first word.
121	* We're doing two things in parallel here;
122	* 1) Calculate the number of words (but avoiding overflow if
123	* limit is near ULONG_MAX) - to do this we need to work out
124	* limit + tmp1 - 1 as a 65-bit value before shifting it;
125	* 2) Load and mask the initial data words - we force the bytes
126	* before the ones we are interested in to 0xff - this ensures
127	* early bytes will not hit any zero detection.
128	*/
129	ldp	data1, data2, [src], #16
130
131	sub	limit_wd, limit, #1
132	and	tmp3, limit_wd, #15
133	lsr	limit_wd, limit_wd, #4
134
135	add	tmp3, tmp3, tmp1
136	add	limit_wd, limit_wd, tmp3, lsr #4
137
138	neg	tmp4, tmp1
139	lsl	tmp4, tmp4, #3  /* Bytes beyond alignment -> bits.  */
140
141	mov	tmp2, #~0
142	/* Big-endian.  Early bytes are at MSB.  */
143CPU_BE( lsl	tmp2, tmp2, tmp4 )	/* Shift (tmp1 & 63).  */
144	/* Little-endian.  Early bytes are at LSB.  */
145CPU_LE( lsr	tmp2, tmp2, tmp4 )	/* Shift (tmp1 & 63).  */
146
147	cmp	tmp1, #8
148
149	orr	data1, data1, tmp2
150	orr	data2a, data2, tmp2
151
152	csinv	data1, data1, xzr, le
153	csel	data2, data2, data2a, le
154	b	.Lrealigned
155
156.Lhit_limit:
157	mov	len, limit
158	ret
159SYM_FUNC_END(__pi_strnlen)
160
161SYM_FUNC_ALIAS_WEAK(strnlen, __pi_strnlen)
162EXPORT_SYMBOL_NOKASAN(strnlen)