[apple/xnu.git] / osfmk / arm / strlen.s

/*
 * Copyright (c) 2011 Apple, Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 *
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */

#include <arm/arch.h>
.syntax unified
.code 32
.globl _strlen

#define addr r0
#define word r1
#define temp r2
#define mask r3
#define save ip
#define indx r0

.macro IfWordDoesntContainNUL_SetZ
#if defined _ARM_ARCH_6
//  In each word of the string, we check for NUL bytes via a saturating
//  unsigned subtraction of each byte from 0x1.  The result of this is
//  non-zero if and only if the corresponding byte in the string is NUL.
//  Simply using a TST instruction checks all four bytes for NULs in one
//  go.
    uqsub8  temp,   mask,   word
    tst     temp,           temp
#else
//  If we're on armv5, we do not have the uqsub8 instruction, so we need
//  to use a different test for NUL.  Instead, we compute:
//
//      byte - 0x1 & ~byte
//
//  and test the high-order bit.  If it is set, then byte is NUL.  Just
//  as with the other test, this can be applied simultaneously to all
//  bytes in a word.
    sub     temp,   word,   mask
    bic     temp,   temp,   word
    tst     temp,           mask, lsl #7
#endif
.endm

.text
.align 4
.long 0x0           // padding
.long 0x01010101    // mask for use in finding NULs
_strlen:
//  Establish stack frame, load mask that we will use to find NUL bytes,
//  and set aside a copy of the pointer to the string.
    push    {r7,lr}
    mov     r7,     sp
    ldr     mask,   (_strlen-4)
    add		save,   addr,   #4

//  Load the aligned word that contains the start of the string, then OR
//  0x01 into any bytes that preceed the start of the string to prevent
//  false positives when we check for NUL bytes.
    and     temp,   addr,   #3
    bic     addr,   addr,   #3
    lsl     temp,   temp,   #3
    ldr     word,  [addr],  #4
    rsb     temp,   temp,   #32
    orr     word,   word,   mask, lsr temp

//  Check if the string ends in the first word.  If so, don't load any
//  more of the string; instead jump directly to the cleanup code.
    IfWordDoesntContainNUL_SetZ
    bne     1f

.align 4
//  Load one word of the string on each iteration, and check it for NUL
//  bytes.  If a NUL is found, fall through into the cleanup code.
0:  ldr     word,  [addr],  #4
    IfWordDoesntContainNUL_SetZ
    beq		0b

//  The last word that we loaded contained a NUL.  Subtracting the saved
//  pointer from the current pointer gives us the number of bytes from
//  the start of the string to the word containing the NUL.
1:  sub     indx,   addr,   save
#if defined _ARM_ARCH_6
//  To that we add the index of the first NUL byte in the word, computed
//  using REV and CLZ followed by a shift.
    rev     temp,           temp
    clz     temp,           temp
    add     indx,   indx,   temp, lsr #3
#else
//  armv5 does not have the REV instruction, so instead we find the
//  index of the NUL byte in word with a linear search.
    tst     word,           #0x000000ff
    addne   indx,           #1
    tstne   word,           #0x0000ff00
    addne   indx,           #1
    tstne   word,           #0x00ff0000
    addne   indx,           #1
#endif
    pop     {r7,pc}
Commit	Line	Data
5ba3f43e A	1	/*
	2	* Copyright (c) 2011 Apple, Inc. All rights reserved.
	3	*
	4	* @APPLE_OSREFERENCE_LICENSE_HEADER_START@
	5	*
	6	* This file contains Original Code and/or Modifications of Original Code
	7	* as defined in and that are subject to the Apple Public Source License
	8	* Version 2.0 (the 'License'). You may not use this file except in
	9	* compliance with the License. The rights granted to you under the License
	10	* may not be used to create, or enable the creation or redistribution of,
	11	* unlawful or unlicensed copies of an Apple operating system, or to
	12	* circumvent, violate, or enable the circumvention or violation of, any
	13	* terms of an Apple operating system software license agreement.
	14	*
	15	* Please obtain a copy of the License at
	16	* http://www.opensource.apple.com/apsl/ and read it before using this file.
	17	*
	18	* The Original Code and all software distributed under the License are
	19	* distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
	20	* EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
	21	* INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
	22	* FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
	23	* Please see the License for the specific language governing rights and
	24	* limitations under the License.
	25	*
	26	* @APPLE_OSREFERENCE_LICENSE_HEADER_END@
	27	*/
	28
	29	#include <arm/arch.h>
	30	.syntax unified
	31	.code 32
	32	.globl _strlen
	33
	34	#define addr r0
	35	#define word r1
	36	#define temp r2
	37	#define mask r3
	38	#define save ip
	39	#define indx r0
	40
	41	.macro IfWordDoesntContainNUL_SetZ
	42	#if defined _ARM_ARCH_6
	43	// In each word of the string, we check for NUL bytes via a saturating
	44	// unsigned subtraction of each byte from 0x1. The result of this is
	45	// non-zero if and only if the corresponding byte in the string is NUL.
	46	// Simply using a TST instruction checks all four bytes for NULs in one
	47	// go.
	48	uqsub8 temp, mask, word
	49	tst temp, temp
	50	#else
	51	// If we're on armv5, we do not have the uqsub8 instruction, so we need
	52	// to use a different test for NUL. Instead, we compute:
	53	//
	54	// byte - 0x1 & ~byte
	55	//
	56	// and test the high-order bit. If it is set, then byte is NUL. Just
	57	// as with the other test, this can be applied simultaneously to all
	58	// bytes in a word.
	59	sub temp, word, mask
	60	bic temp, temp, word
	61	tst temp, mask, lsl #7
	62	#endif
	63	.endm
	64
65	.text
66	.align 4
67	.long 0x0 // padding
68	.long 0x01010101 // mask for use in finding NULs
69	_strlen:
70	// Establish stack frame, load mask that we will use to find NUL bytes,
71	// and set aside a copy of the pointer to the string.
72	push {r7,lr}
73	mov r7, sp
74	ldr mask, (_strlen-4)
75	add save, addr, #4
76
77	// Load the aligned word that contains the start of the string, then OR
78	// 0x01 into any bytes that preceed the start of the string to prevent
79	// false positives when we check for NUL bytes.
80	and temp, addr, #3
81	bic addr, addr, #3
82	lsl temp, temp, #3
83	ldr word, [addr], #4
84	rsb temp, temp, #32
85	orr word, word, mask, lsr temp
86
87	// Check if the string ends in the first word. If so, don't load any
88	// more of the string; instead jump directly to the cleanup code.
89	IfWordDoesntContainNUL_SetZ
90	bne 1f
91
92	.align 4
93	// Load one word of the string on each iteration, and check it for NUL
94	// bytes. If a NUL is found, fall through into the cleanup code.
95	0: ldr word, [addr], #4
96	IfWordDoesntContainNUL_SetZ
97	beq 0b
98
99	// The last word that we loaded contained a NUL. Subtracting the saved
100	// pointer from the current pointer gives us the number of bytes from
101	// the start of the string to the word containing the NUL.
102	1: sub indx, addr, save
103	#if defined _ARM_ARCH_6
104	// To that we add the index of the first NUL byte in the word, computed
105	// using REV and CLZ followed by a shift.
106	rev temp, temp
107	clz temp, temp
108	add indx, indx, temp, lsr #3
109	#else
110	// armv5 does not have the REV instruction, so instead we find the
111	// index of the NUL byte in word with a linear search.
112	tst word, #0x000000ff
113	addne indx, #1
114	tstne word, #0x0000ff00
115	addne indx, #1
116	tstne word, #0x00ff0000
117	addne indx, #1
118	#endif
119	pop {r7,pc}