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. 950: 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. 1021: 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} 120