libc/arch-arm64/generic/bionic/strcmp.S

/* Copyright (c) 2012, 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>

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

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

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

	/* Start of performance-critical section  -- one 64B cache line.  */
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
	/* End of performance-critical section  -- one 64B cache line.  */

#ifndef	__AARCH64EB__
	rev	syndrome, syndrome
	rev	data1, data1
	/* 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.  */
	clz	pos, syndrome
	rev	data2, data2
	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
#else
	/* 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 can't just subtract the bytes as the
	   MSB might be significant.  */
	cbnz	has_nul, 1f
	cmp	data1, data2
	cset	result, ne
	cneg	result, result, lo
	ret
1:
	/* Re-compute the NUL-byte detection, using a byte-reversed value.  */
	rev	tmp3, data1
	sub	tmp1, tmp3, zeroones
	orr	tmp2, tmp3, #REP8_7f
	bic	has_nul, tmp1, tmp2
	rev	has_nul, has_nul
	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
#endif

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

.Lmisaligned8:
	/* We can do better than this.  */
	ldrb	data1w, [src1], #1
	ldrb	data2w, [src2], #1
	cmp	data1w, #1
	ccmp	data1w, data2w, #0, cs	/* NZCV = 0b0000.  */
	b.eq	.Lmisaligned8
	sub	result, data1, data2
	ret
END(strcmp)