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/* $OpenBSD: strlen.S,v 1.9 2022/12/07 18:25:33 guenther Exp $ */
/* $NetBSD: strlen.S,v 1.6 2014/03/22 19:16:34 jakllsch Exp $ */
/*-
* Copyright (c) 2009 The NetBSD Foundation, Inc.
* All rights reserved.
*
* This code is derived from software contributed to The NetBSD Foundation
* by David Laight.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. 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.
*
* THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
*/
/*
* Inspired by a version written by J.T. Conklin <jtc@acorntoolworks.com>
* (Only the long comment really remains his work!)
*/
#include <machine/asm.h>
/*
* There are many well known branch-free sequences which are used
* for determining whether a zero-byte is contained within a word.
* These sequences are generally much more efficient than loading
* and comparing each byte individually.
*
* The expression [1,2]:
*
* (1) ~(((x & 0x7f....7f) + 0x7f....7f) | (x | 0x7f....7f))
*
* evaluates to a non-zero value if any of the bytes in the
* original word is zero.
*
* It also has the useful property that bytes in the result word
* that correspond to non-zero bytes in the original word have
* the value 0x00, while bytes corresponding to zero bytes have
* the value 0x80. This allows calculation of the first (and
* last) occurrence of a zero byte within the word (useful for C's
* str* primitives) by counting the number of leading (or
* trailing) zeros and dividing the result by 8. On machines
* without (or with slow) clz() / ctz() instructions, testing
* each byte in the result word for zero is necessary.
*
* This typically takes 4 instructions (5 on machines without
* "not-or") not including those needed to load the constant.
*
*
* The expression:
*
* (2) ((x - 0x01....01) & 0x80....80 & ~x)
*
* evaluates to a non-zero value if any of the bytes in the
* original word is zero.
*
* On little endian machines, the first byte in the result word
* that corresponds to a zero byte in the original byte is 0x80,
* so clz() can be used as above. On big endian machines, and
* little endian machines without (or with a slow) clz() insn,
* testing each byte in the original for zero is necessary.
*
* This typically takes 3 instructions (4 on machines without
* "and with complement") not including those needed to load
* constants.
*
*
* The expression:
*
* (3) ((x - 0x01....01) & 0x80....80)
*
* always evaluates to a non-zero value if any of the bytes in
* the original word is zero or has the top bit set.
* For strings that are likely to only contain 7-bit ascii these
* false positives will be rare.
*
* To account for possible false positives, each byte of the
* original word must be checked when the expression evaluates to
* a non-zero value. However, because it is simpler than those
* presented above, code that uses it will be faster as long as
* the rate of false positives is low.
*
* This is likely, because the false positive can only occur
* if the most significant bit of a byte within the word is set.
* The expression will never fail for typical 7-bit ASCII strings.
*
* This typically takes 2 instructions not including those needed
* to load constants.
*
*
* [1] Henry S. Warren Jr., "Hacker's Delight", Addison-Westley 2003
*
* [2] International Business Machines, "The PowerPC Compiler Writer's
* Guide", Warthman Associates, 1996
*/
ENTRY(strlen)
RETGUARD_SETUP(strlen, r10)
movabsq $0x0101010101010101,%r8
test $7,%dil
movq %rdi,%rax /* Buffer, %rdi unchanged */
movabsq $0x8080808080808080,%r9
jnz 10f /* Jump if misaligned */
_ALIGN_TEXT
1:
movq (%rax),%rdx /* get bytes to check */
2:
addq $8,%rax
mov %rdx,%rcx /* save for later check */
subq %r8,%rdx /* alg (3) above first */
not %rcx /* Invert of data */
andq %r9,%rdx
je 1b /* jump if all 0x01-0x80 */
/* Do check from alg (2) above - loops for 0x81..0xff bytes */
andq %rcx,%rdx
je 1b
/* Since we are LE, use bit scan for first 0x80 byte */
sub %rdi,%rax /* length to next word */
bsf %rdx,%rdx /* 7, 15, 23 ... 63 */
shr $3,%rdx /* 0, 1, 2 ... 7 */
lea -8(%rax,%rdx),%rax
RETGUARD_CHECK(strlen, r10)
ret
lfence
/* Misaligned, read aligned word and make low bytes non-zero */
_ALIGN_TRAPS
10:
mov %al,%cl
mov $1,%rsi
and $7,%cl /* offset into word 1..7 */
and $~7,%al /* start of word with buffer */
shl $3,%cl /* bit count 8, 16 .. 56 */
movq (%rax),%rdx /* first data in high bytes */
shl %cl,%rsi
dec %rsi
or %rsi,%rdx /* low bytes now non-zero */
jmp 2b
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