/*
* Copyright (c) 2011 Apple, Inc. All rights reserved.
*
* @APPLE_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. 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_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 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}