memcmp.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 memory areas(when two memory areas' offset are different,
* alignment handled by the hardware)
*
* Parameters:
*  x0 - const memory area 1 pointer
*  x1 - const memory area 2 pointer
*  x2 - the maximal compare byte length
* Returns:
*  x0 - a compare result, maybe less than, equal to, or greater than ZERO
*/

/* Parameters and result.  */
src1		.req	x0
src2		.req	x1
limit		.req	x2
result		.req	x0

/* Internal variables.  */
data1		.req	x3
data1w		.req	w3
data2		.req	x4
data2w		.req	w4
has_nul		.req	x5
diff		.req	x6
endloop		.req	x7
tmp1		.req	x8
tmp2		.req	x9
tmp3		.req	x10
pos		.req	x11
limit_wd	.req	x12
mask		.req	x13

WEAK(memcmp)
	cbz	limit, .Lret0
	eor	tmp1, src1, src2
	tst	tmp1, #7
	b.ne	.Lmisaligned8
	ands	tmp1, src1, #7
	b.ne	.Lmutual_align
	sub	limit_wd, limit, #1 /* limit != 0, so no underflow.  */
	lsr	limit_wd, limit_wd, #3 /* Convert to Dwords.  */
	/*
	* The input source addresses are at alignment boundary.
	* Directly compare eight bytes each time.
	*/
.Lloop_aligned:
	ldr	data1, [src1], #8
	ldr	data2, [src2], #8
.Lstart_realigned:
	subs	limit_wd, limit_wd, #1
	eor	diff, data1, data2	/* Non-zero if differences found.  */
	csinv	endloop, diff, xzr, cs	/* Last Dword or differences.  */
	cbz	endloop, .Lloop_aligned

	/* Not reached the limit, must have found a diff.  */
	tbz	limit_wd, #63, .Lnot_limit

	/* Limit % 8 == 0 => the diff is in the last 8 bytes. */
	ands	limit, limit, #7
	b.eq	.Lnot_limit
	/*
	* The remained bytes less than 8. It is needed to extract valid data
	* from last eight bytes of the intended memory range.
	*/
	lsl	limit, limit, #3	/* bytes-> bits.  */
	mov	mask, #~0
CPU_BE( lsr	mask, mask, limit )
CPU_LE( lsl	mask, mask, limit )
	bic	data1, data1, mask
	bic	data2, data2, mask

	orr	diff, diff, mask
	b	.Lnot_limit

.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 precede the start point.
	*/
	bic	src1, src1, #7
	bic	src2, src2, #7
	ldr	data1, [src1], #8
	ldr	data2, [src2], #8
	/*
	* We can not add limit with alignment offset(tmp1) here. Since the
	* addition probably make the limit overflown.
	*/
	sub	limit_wd, limit, #1/*limit != 0, so no underflow.*/
	and	tmp3, limit_wd, #7
	lsr	limit_wd, limit_wd, #3
	add	tmp3, tmp3, tmp1
	add	limit_wd, limit_wd, tmp3, lsr #3
	add	limit, limit, tmp1/* Adjust the limit for the extra.  */

	lsl	tmp1, tmp1, #3/* Bytes beyond alignment -> bits.*/
	neg	tmp1, tmp1/* Bits to alignment -64.  */
	mov	tmp2, #~0
	/*mask off the non-intended bytes before the start address.*/
CPU_BE( lsl	tmp2, tmp2, tmp1 )/*Big-endian.Early bytes are at MSB*/
	/* Little-endian.  Early bytes are at LSB.  */
CPU_LE( lsr	tmp2, tmp2, tmp1 )

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

	/*src1 and src2 have different alignment offset.*/
.Lmisaligned8:
	cmp	limit, #8
	b.lo	.Ltiny8proc /*limit < 8: compare byte by byte*/

	and	tmp1, src1, #7
	neg	tmp1, tmp1
	add	tmp1, tmp1, #8/*valid length in the first 8 bytes of src1*/
	and	tmp2, src2, #7
	neg	tmp2, tmp2
	add	tmp2, tmp2, #8/*valid length in the first 8 bytes of src2*/
	subs	tmp3, tmp1, tmp2
	csel	pos, tmp1, tmp2, hi /*Choose the maximum.*/

	sub	limit, limit, pos
	/*compare the proceeding bytes in the first 8 byte segment.*/
.Ltinycmp:
	ldrb	data1w, [src1], #1
	ldrb	data2w, [src2], #1
	subs	pos, pos, #1
	ccmp	data1w, data2w, #0, ne  /* NZCV = 0b0000.  */
	b.eq	.Ltinycmp
	cbnz	pos, 1f /*diff occurred before the last byte.*/
	cmp	data1w, data2w
	b.eq	.Lstart_align
1:
	sub	result, data1, data2
	ret

.Lstart_align:
	lsr	limit_wd, limit, #3
	cbz	limit_wd, .Lremain8

	ands	xzr, src1, #7
	b.eq	.Lrecal_offset
	/*process more leading bytes to make src1 aligned...*/
	add	src1, src1, tmp3 /*backwards src1 to alignment boundary*/
	add	src2, src2, tmp3
	sub	limit, limit, tmp3
	lsr	limit_wd, limit, #3
	cbz	limit_wd, .Lremain8
	/*load 8 bytes from aligned SRC1..*/
	ldr	data1, [src1], #8
	ldr	data2, [src2], #8

	subs	limit_wd, limit_wd, #1
	eor	diff, data1, data2  /*Non-zero if differences found.*/
	csinv	endloop, diff, xzr, ne
	cbnz	endloop, .Lunequal_proc
	/*How far is the current SRC2 from the alignment boundary...*/
	and	tmp3, tmp3, #7

.Lrecal_offset:/*src1 is aligned now..*/
	neg	pos, tmp3
.Lloopcmp_proc:
	/*
	* Divide the eight bytes into two parts. First,backwards the src2
	* to an alignment boundary,load eight bytes and compare from
	* the SRC2 alignment boundary. 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]
	eor	diff, data1, data2  /* Non-zero if differences found.  */
	cbnz	diff, .Lnot_limit

	/*The second part process*/
	ldr	data1, [src1], #8
	ldr	data2, [src2], #8
	eor	diff, data1, data2  /* Non-zero if differences found.  */
	subs	limit_wd, limit_wd, #1
	csinv	endloop, diff, xzr, ne/*if limit_wd is 0,will finish the cmp*/
	cbz	endloop, .Lloopcmp_proc
.Lunequal_proc:
	cbz	diff, .Lremain8

/* There is difference occurred in the latest comparison. */
.Lnot_limit:
/*
* For little endian,reverse the low significant equal bits into MSB,then
* following CLZ can find how many equal bits exist.
*/
CPU_LE( rev	diff, diff )
CPU_LE( rev	data1, data1 )
CPU_LE( rev	data2, data2 )

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

.Lremain8:
	/* Limit % 8 == 0 =>. all data are equal.*/
	ands	limit, limit, #7
	b.eq	.Lret0

.Ltiny8proc:
	ldrb	data1w, [src1], #1
	ldrb	data2w, [src2], #1
	subs	limit, limit, #1

	ccmp	data1w, data2w, #0, ne  /* NZCV = 0b0000. */
	b.eq	.Ltiny8proc
	sub	result, data1, data2
	ret
.Lret0:
	mov	result, #0
	ret
ENDPIPROC(memcmp)
EXPORT_SYMBOL_NOKASAN(memcmp)