/*
* Copyright (c) 2012 Apple Computer, 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@
*
* This file implements strnlen( ) for the x86_64 architecture.
*/
.globl _strnlen
/*****************************************************************************
* Macros *
*****************************************************************************/
.macro EstablishFrame
push %rbp
mov %rsp, %rbp
.endm
.macro ClearFrameAndReturn
pop %rbp
ret
.endm
/*****************************************************************************
* Entrypoint *
*****************************************************************************/
.text
.align 4
_strnlen:
// size_t strnlen(char *s, size_t maxlen)// If maxlen is larger than any object that can be allocated, we know a priori
// that it does not effect the operation of the function in any way// edge cases here much cleaner.
test %rsi, %rsi
js _strlen
// The strnlen() function attempts to compute the length of s, but never
// scans beyond the first maxlen bytes of s.
//
// Thus, we need to early-out without doing any reads at all if maxlen == 0.
EstablishFrame
mov %rsi, %rax
jz L_maxlenExhausted
// We are going to check the string in aligned 16-byte blocks. The first such
// block may contain characters that preceed the start of the string, so we
// construct a mask based on the string's alignment to use in processing this
// initial block. We also need to account for these characters in maxlen.
mov %rdi, %rcx
and $0xf, %rcx
or $-1, %rdx
shl %cl, %rdx // mask
add %rcx, %rsi // adjust maxlen
// Load the 16-byte block containing the start of the string. If any NUL
// bytes are present in this block, the corresponding *bit* in ecx will be 1.
// We check only the bits that are set in the mask, to avoid detecting NULs
// that preceed the start of the string.
and $-16, %rdi
pxor %xmm0, %xmm0
pcmpeqb (%rdi), %xmm0
pmovmskb %xmm0, %ecx
and %rdx, %rcx
jnz L_foundNUL
// Now subtract 16 from maxlen. If this causes a borrow, then we exhausted
// maxlen somewhere in this 16-byte block (formally, we have read past maxlen
// bytes, but that is not a problem// behavior is not different from if we had stopped at maxlen). If the result
// is exactly zero, we need to stop before reading the *next* 16 bytes.
sub $16, %rsi
jbe L_maxlenExhausted
L_loop:
add $16, %rdi
pxor %xmm0, %xmm0
pcmpeqb (%rdi), %xmm0
pmovmskb %xmm0, %ecx
test %rcx, %rcx
jnz L_foundNUL
sub $16, %rsi
ja L_loop
L_maxlenExhausted:
// If we exhaust maxlen bytes without finding a NUL, we return maxlen.
ClearFrameAndReturn
L_foundNUL:
// The last 16-byte block that we searched contained at least one NUL.
// We use bsf to identify the first NUL.
bsf %rcx, %rdx
// Handle the case where the NUL that we found is preceeded by the maxlen'th
// byte of the string, returning maxlen.
cmp %rdx, %rsi
jb L_maxlenExhausted
// Otherwise, return the length of the string.
sub %rsi, %rax
add %rdx, %rax
ClearFrameAndReturn