strcmp.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>

/*
 * compare two strings
 *
 * Parameters:
 *	x0 - const string 1 pointer
 *    x1 - const string 2 pointer
 * Returns:
 * x0 - an integer less than, equal to, or greater than zero
 * if  s1  is  found, respectively, to be less than, to match,
 * or be greater than s2.
 */

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

/* Parameters and result.  */
src1		.req	x0
src2		.req	x1
result		.req	x0

/* Internal variables.  */
data1		.req	x2
data1w		.req	w2
data2		.req	x3
data2w		.req	w3
has_nul		.req	x4
diff		.req	x5
syndrome	.req	x6
tmp1		.req	x7
tmp2		.req	x8
tmp3		.req	x9
zeroones	.req	x10
pos		.req	x11

ENTRY(strcmp)
	eor	tmp1, src1, src2
	mov	zeroones, #REP8_01
	tst	tmp1, #7
	b.ne	.Lmisaligned8
	ands	tmp1, src1, #7
	b.ne	.Lmutual_align

	/*
	* 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.
	*/
.Lloop_aligned:
	ldr	data1, [src1], #8
	ldr	data2, [src2], #8
.Lstart_realigned:
	sub	tmp1, data1, zeroones
	orr	tmp2, data1, #REP8_7f
	eor	diff, data1, data2	/* Non-zero if differences found.  */
	bic	has_nul, tmp1, tmp2	/* Non-zero if NUL terminator.  */
	orr	syndrome, diff, has_nul
	cbz	syndrome, .Lloop_aligned
	b	.Lcal_cmpresult

.Lmutual_align:
	/*
	* Sources are mutually aligned, but are not currently at an
	* alignment boundary.  Round down the addresses and then mask off
	* the bytes that preceed the start point.
	*/
	bic	src1, src1, #7
	bic	src2, src2, #7
	lsl	tmp1, tmp1, #3		/* Bytes beyond alignment -> bits.  */
	ldr	data1, [src1], #8
	neg	tmp1, tmp1		/* Bits to alignment -64.  */
	ldr	data2, [src2], #8
	mov	tmp2, #~0
	/* Big-endian.  Early bytes are at MSB.  */
CPU_BE( lsl	tmp2, tmp2, tmp1 )	/* Shift (tmp1 & 63).  */
	/* Little-endian.  Early bytes are at LSB.  */
CPU_LE( lsr	tmp2, tmp2, tmp1 )	/* Shift (tmp1 & 63).  */

	orr	data1, data1, tmp2
	orr	data2, data2, tmp2
	b	.Lstart_realigned

.Lmisaligned8:
	/*
	* Get the align offset length to compare per byte first.
	* After this process, one string's address will be aligned.
	*/
	and	tmp1, src1, #7
	neg	tmp1, tmp1
	add	tmp1, tmp1, #8
	and	tmp2, src2, #7
	neg	tmp2, tmp2
	add	tmp2, tmp2, #8
	subs	tmp3, tmp1, tmp2
	csel	pos, tmp1, tmp2, hi /*Choose the maximum. */
.Ltinycmp:
	ldrb	data1w, [src1], #1
	ldrb	data2w, [src2], #1
	subs	pos, pos, #1
	ccmp	data1w, #1, #0, ne  /* NZCV = 0b0000.  */
	ccmp	data1w, data2w, #0, cs  /* NZCV = 0b0000.  */
	b.eq	.Ltinycmp
	cbnz	pos, 1f /*find the null or unequal...*/
	cmp	data1w, #1
	ccmp	data1w, data2w, #0, cs
	b.eq	.Lstart_align /*the last bytes are equal....*/
1:
	sub	result, data1, data2
	ret

.Lstart_align:
	ands	xzr, src1, #7
	b.eq	.Lrecal_offset
	/*process more leading bytes to make str1 aligned...*/
	add	src1, src1, tmp3
	add	src2, src2, tmp3
	/*load 8 bytes from aligned str1 and non-aligned str2..*/
	ldr	data1, [src1], #8
	ldr	data2, [src2], #8

	sub	tmp1, data1, zeroones
	orr	tmp2, data1, #REP8_7f
	bic	has_nul, tmp1, tmp2
	eor	diff, data1, data2 /* Non-zero if differences found.  */
	orr	syndrome, diff, has_nul
	cbnz	syndrome, .Lcal_cmpresult
	/*How far is the current str2 from the alignment boundary...*/
	and	tmp3, tmp3, #7
.Lrecal_offset:
	neg	pos, tmp3
.Lloopcmp_proc:
	/*
	* Divide the eight bytes into two parts. First,backwards the src2
	* to an alignment boundary,load eight bytes from the SRC2 alignment
	* boundary,then compare with the relative bytes from SRC1.
	* If all 8 bytes are equal,then start the second part's comparison.
	* Otherwise finish the comparison.
	* This special handle can garantee all the accesses are in the
	* thread/task space in avoid to overrange access.
	*/
	ldr	data1, [src1,pos]
	ldr	data2, [src2,pos]
	sub	tmp1, data1, zeroones
	orr	tmp2, data1, #REP8_7f
	bic	has_nul, tmp1, tmp2
	eor	diff, data1, data2  /* Non-zero if differences found.  */
	orr	syndrome, diff, has_nul
	cbnz	syndrome, .Lcal_cmpresult

	/*The second part process*/
	ldr	data1, [src1], #8
	ldr	data2, [src2], #8
	sub	tmp1, data1, zeroones
	orr	tmp2, data1, #REP8_7f
	bic	has_nul, tmp1, tmp2
	eor	diff, data1, data2  /* Non-zero if differences found.  */
	orr	syndrome, diff, has_nul
	cbz	syndrome, .Lloopcmp_proc

.Lcal_cmpresult:
	/*
	* reversed the byte-order as big-endian,then CLZ can find the most
	* significant zero bits.
	*/
CPU_LE( rev	syndrome, syndrome )
CPU_LE( rev	data1, data1 )
CPU_LE( rev	data2, data2 )

	/*
	* For big-endian we cannot use the trick with the syndrome value
	* as carry-propagation can corrupt the upper bits if the trailing
	* bytes in the string contain 0x01.
	* However, if there is no NUL byte in the dword, we can generate
	* the result directly.  We ca not just subtract the bytes as the
	* MSB might be significant.
	*/
CPU_BE( cbnz	has_nul, 1f )
CPU_BE( cmp	data1, data2 )
CPU_BE( cset	result, ne )
CPU_BE( cneg	result, result, lo )
CPU_BE( ret )
CPU_BE( 1: )
	/*Re-compute the NUL-byte detection, using a byte-reversed value. */
CPU_BE(	rev	tmp3, data1 )
CPU_BE(	sub	tmp1, tmp3, zeroones )
CPU_BE(	orr	tmp2, tmp3, #REP8_7f )
CPU_BE(	bic	has_nul, tmp1, tmp2 )
CPU_BE(	rev	has_nul, has_nul )
CPU_BE(	orr	syndrome, diff, has_nul )

	clz	pos, syndrome
	/*
	* The MS-non-zero bit of the syndrome marks either the first bit
	* that is different, or the top bit of the first zero byte.
	* Shifting left now will bring the critical information into the
	* top bits.
	*/
	lsl	data1, data1, pos
	lsl	data2, data2, pos
	/*
	* But we need to zero-extend (char is unsigned) the value and then
	* perform a signed 32-bit subtraction.
	*/
	lsr	data1, data1, #56
	sub	result, data1, data2, lsr #56
	ret
ENDPIPROC(strcmp)