710e45c4b8
The previous code neglected to use primitives which can find the end of the string without having to branch on every character. While here augment the somewhat misleading commentary -- strlen as implemented here leaves performance on the table, especially so for userspace. Every arch should get a dedicated variant instead. In the meantime this commit lessens the problem. Tested with glibc test suite. Naive test just calling strlen in a loop on Haswell (ops/s): $(perl -e "print 'A' x 3"): before: 211198039 after: 338626619 $(perl -e "print 'A' x 100"): before: 83151997 after: 98285919
102 lines
2.9 KiB
C
102 lines
2.9 KiB
C
/*-
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* SPDX-License-Identifier: BSD-2-Clause-FreeBSD
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*
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* Copyright (c) 2009, 2010 Xin LI <delphij@FreeBSD.org>
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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#include <sys/cdefs.h>
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__FBSDID("$FreeBSD$");
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#include <sys/libkern.h>
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#include <sys/limits.h>
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/*
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* Portable strlen() for 32-bit and 64-bit systems.
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*
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* The expression:
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*
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* ((x - 0x01....01) & ~x & 0x80....80)
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*
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* would evaluate to a non-zero value iff any of the bytes in the
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* original word is zero.
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*
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* The algorithm above is found on "Hacker's Delight" by
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* Henry S. Warren, Jr.
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*/
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#if LONG_BIT == 32
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static const unsigned long mask01 = 0x01010101;
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static const unsigned long mask80 = 0x80808080;
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#elif LONG_BIT == 64
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static const unsigned long mask01 = 0x0101010101010101;
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static const unsigned long mask80 = 0x8080808080808080;
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#else
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#error Unsupported word size
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#endif
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#define LONGPTR_MASK (sizeof(long) - 1)
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#if BYTE_ORDER == LITTLE_ENDIAN
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#define FINDZERO __builtin_ctzl
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#else
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#define FINDZERO __builtin_clzl
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#endif
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size_t
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(strlen)(const char *str)
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{
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const unsigned long *lp;
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unsigned long mask;
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long va, vb;
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long val;
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lp = (unsigned long *) (uintptr_t) str;
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if ((uintptr_t)lp & LONGPTR_MASK) {
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lp = (__typeof(lp)) ((uintptr_t)lp & ~LONGPTR_MASK);
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#if BYTE_ORDER == LITTLE_ENDIAN
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mask = ~(~0UL << (((uintptr_t)str & LONGPTR_MASK) << 3));
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#else
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mask = ~(~0UL >> (((uintptr_t)str & LONGPTR_MASK) << 3));
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#endif
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val = *lp | mask;
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va = (val - mask01);
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vb = ((~val) & mask80);
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if (va & vb) {
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return ((const char *)lp - str + (FINDZERO(va & vb) >> 3));
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}
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lp++;
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}
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for (; ; lp++) {
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va = (*lp - mask01);
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vb = ((~*lp) & mask80);
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if (va & vb) {
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return ((const char *)lp - str + (FINDZERO(va & vb) >> 3));
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}
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}
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__builtin_unreachable();
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return (0);
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}
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