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swab.c(libc): use a simplified version of byte swapping

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This version of swab function simplifies the logic of swapping adjacent
bytes. Previous version of swab() used an arbitrary unrolling, which was
relevant back in the day but unnecessary for modern compilers, as if the
input size is known at compile time, they can do it automatically.

This version of swab() is inspired by musl.
A similar version can be found at: https://github.com/openbsd/src/blob/master/lib/libc/string/swab.c

Signed-off-by: rilysh <nightquick@proton.me>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1086
This commit is contained in:
rilysh 2024-04-22 15:52:25 -06:00 committed by Warner Losh
parent f578d01997
commit bac2eea13a
1 changed files with 11 additions and 47 deletions
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58
lib/libc/string/swab.c
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@ -1,35 +1,6 @@
/*-
* SPDX-License-Identifier: BSD-3-Clause
*
* Copyright (c) 1988, 1993
* The Regents of the University of California. All rights reserved.
*
* This code is derived from software contributed to Berkeley by
* Jeffrey Mogul.
*
* 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.
* 3. Neither the name of the University nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
* SPDX-License-Identifier: BSD-2-Clause
* Copyright (c) 2024 rilysh <nightquick@proton.me>
*/
#include <unistd.h>
@ -37,22 +8,15 @@
void
swab(const void * __restrict from, void * __restrict to, ssize_t len)
{
unsigned char temp;
size_t n;
const unsigned char *fp;
unsigned char *tp;
const unsigned char *f = from;
unsigned char *t = to;
if (len <= 0)
return;
n = (size_t)len >> 1;
fp = (const unsigned char *)from;
tp = (unsigned char *)to;
#define STEP temp = *fp++,*tp++ = *fp++,*tp++ = temp
/* round to multiple of 8 */
for (; n & 0x7; --n)
STEP;
for (n >>= 3; n > 0; --n) {
STEP; STEP; STEP; STEP;
STEP; STEP; STEP; STEP;
while (len > 1) {
t[0] = f[1];
t[1] = f[0];
f += 2;
t += 2;
len -= 2;
}
}