fef
904584ccc0
This is just a minor overhaul of several utility functions, in part because it kept bothering me and in part because i was bored.
134 lines
4.3 KiB
C
134 lines
4.3 KiB
C
/* See the end of this file for copyright and license terms. */
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#include <gay/types.h>
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#include <limits.h>
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#include <string.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 if 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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* Note: this leaves performance on the table and each architecture
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* would be best served with a tailor made routine instead, even if
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* using the same trick.
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*/
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/* Magic numbers for the algorithm */
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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(unsigned long) - 1)
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usize strlen(const char *str)
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{
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const char *p;
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const unsigned long *lp;
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unsigned long va, vb;
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/*
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* Before trying the hard (unaligned byte-by-byte access) way
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* to figure out whether there is a nul character, try to see
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* if there is a nul character is within this accessible word
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* first.
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*
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* p and (p & ~LONGPTR_MASK) must be equally accessible since
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* they always fall in the same memory page, as long as page
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* boundaries is integral multiple of word size.
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*/
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lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK);
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va = *lp - mask01;
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vb = (~*lp) & mask80;
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lp++;
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if (va & vb) {
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/* Check if we have \0 in the first part */
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for (p = str; p < (const char *)lp; p++) {
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if (*p == '\0')
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return p - str;
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}
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}
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/* Scan the rest of the string using word sized operation */
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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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/*
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* The original version from FreeBSD uses 4 (or 8) if
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* branches querying every byte sequentially here.
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* I was bored and rewrote it branchless, but that
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* probably doesn't help in terms of performance
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* because clang's optimizer is better anyway.
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*/
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p = (const char *)lp;
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unsigned long l = *lp;
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unsigned long add = 1;
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#pragma clang loop unroll(full)
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for (int i = 0; i < sizeof(l); i++) {
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add &= (l & 0xffu) != 0;
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p += add;
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l >>= 8;
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}
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return p - str;
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}
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}
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}
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/*
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* This file is part of GayBSD.
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* Copyright (c) 2021 fef <owo@fef.moe>.
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*
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* GayBSD is nonviolent software: you may only use, redistribute, and/or
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* modify it under the terms of the Cooperative Nonviolent Public License
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* (CNPL) as found in the LICENSE file in the source code root directory
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* or at <https://git.pixie.town/thufie/npl-builder>; either version 7
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* of the license, or (at your option) any later version.
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*
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* GayBSD comes with ABSOLUTELY NO WARRANTY, to the extent
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* permitted by applicable law. See the CNPL for details.
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*
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* ==========================================================================
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*
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* This file is derived from FreeBSD source code.
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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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* $FreeBSD$
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*/
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