xref: /bionic/libc/arch-arm64/generic/bionic/memcmp.S

/* Copyright (c) 2014, Linaro Limited
   All rights reserved.

   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions are met:
       * Redistributions of source code must retain the above copyright
         notice, this list of conditions and the following disclaimer.
       * Redistributions in binary form must reproduce the above copyright
         notice, this list of conditions and the following disclaimer in the
         documentation and/or other materials provided with the distribution.
       * Neither the name of the Linaro nor the
         names of its contributors may be used to endorse or promote products
         derived from this software without specific prior written permission.

   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
   HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

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

#include <private/bionic_asm.h>

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

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

ENTRY(memcmp)
	cbz	limit, .Lret0
	eor	tmp1, src1, src2
	tst	tmp1, #7
	b.ne	.Lmisaligned8
	ands	tmp1, src1, #7
	b.ne	.Lmutual_align
	add	limit_wd, limit, #7
	lsr	limit_wd, limit_wd, #3
	/* Start of performance-critical section  -- one 64B cache line.  */
.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, ne	/* Last Dword or differences.  */
	cbz	endloop, .Lloop_aligned
	/* End of performance-critical section  -- one 64B cache line.  */

	/* Not reached the limit, must have found a diff.  */
	cbnz	limit_wd, .Lnot_limit

	/* Limit % 8 == 0 => all bytes significant.  */
	ands	limit, limit, #7
	b.eq	.Lnot_limit

	lsl	limit, limit, #3	/* Bits -> bytes.  */
	mov	mask, #~0
#ifdef __AARCH64EB__
	lsr	mask, mask, limit
#else
	lsl	mask, mask, limit
#endif
	bic	data1, data1, mask
	bic	data2, data2, mask

	orr	diff, diff, mask
.Lnot_limit:

#ifndef	__AARCH64EB__
	rev	diff, diff
	rev	data1, data1
	rev	data2, data2
#endif
	/* 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
	/* 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

.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
	add	limit, limit, tmp1	/* Adjust the limit for the extra.  */
	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
#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
	add	limit_wd, limit, #7
	orr	data1, data1, tmp2
	orr	data2, data2, tmp2
	lsr	limit_wd, limit_wd, #3
	b	.Lstart_realigned

.Lret0:
	mov	result, #0
	ret

	.p2align 6
.Lmisaligned8:
	sub	limit, limit, #1
1:
	/* Perhaps we can do better than this.  */
	ldrb	data1w, [src1], #1
	ldrb	data2w, [src2], #1
	subs	limit, limit, #1
	ccmp	data1w, data2w, #0, cs	/* NZCV = 0b0000.  */
	b.eq	1b
	sub	result, data1, data2
	ret
END(memcmp)