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Sync strlen with userland implementation.

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MFC after:	1 month
This commit is contained in:
Xin LI 2012-11-03 04:28:53 +00:00
parent 46f8c63df6
commit 6afdae4139
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=242506
1 changed files with 98 additions and 13 deletions
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111
sys/libkern/strlen.c
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@ -1,6 +1,6 @@
/*-
* Copyright (c) 1990, 1993
* The Regents of the University of California. All rights reserved.
* Copyright (c) 2009, 2010 Xin LI <delphij@FreeBSD.org>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
@ -10,14 +10,11 @@
* 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.
* 4. 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
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR 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
* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR 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)
@ -31,14 +28,102 @@
__FBSDID("$FreeBSD$");
#include <sys/libkern.h>
#include <sys/limits.h>
/*
* Portable strlen() for 32-bit and 64-bit systems.
*
* Rationale: it is generally much more efficient to do word length
* operations and avoid branches on modern computer systems, as
* compared to byte-length operations with a lot of branches.
*
* The expression:
*
* ((x - 0x01....01) & ~x & 0x80....80)
*
* would evaluate to a non-zero value iff any of the bytes in the
* original word is zero.
*
* On multi-issue processors, we can divide the above expression into:
* a) (x - 0x01....01)
* b) (~x & 0x80....80)
* c) a & b
*
* Where, a) and b) can be partially computed in parallel.
*
* The algorithm above is found on "Hacker's Delight" by
* Henry S. Warren, Jr.
*/
/* Magic numbers for the algorithm */
#if LONG_BIT == 32
static const unsigned long mask01 = 0x01010101;
static const unsigned long mask80 = 0x80808080;
#elif LONG_BIT == 64
static const unsigned long mask01 = 0x0101010101010101;
static const unsigned long mask80 = 0x8080808080808080;
#else
#error Unsupported word size
#endif
#define LONGPTR_MASK (sizeof(long) - 1)
/*
* Helper macro to return string length if we caught the zero
* byte.
*/
#define testbyte(x) \
do { \
if (p[x] == '\0') \
return (p - str + x); \
} while (0)
size_t
strlen(str)
const char *str;
strlen(const char *str)
{
register const char *s;
const char *p;
const unsigned long *lp;
long va, vb;
for (s = str; *s; ++s);
return(s - str);
/*
* Before trying the hard (unaligned byte-by-byte access) way
* to figure out whether there is a nul character, try to see
* if there is a nul character is within this accessible word
* first.
*
* p and (p & ~LONGPTR_MASK) must be equally accessible since
* they always fall in the same memory page, as long as page
* boundaries is integral multiple of word size.
*/
lp = (const unsigned long *)((uintptr_t)str & ~LONGPTR_MASK);
va = (*lp - mask01);
vb = ((~*lp) & mask80);
lp++;
if (va & vb)
/* Check if we have \0 in the first part */
for (p = str; p < (const char *)lp; p++)
if (*p == '\0')
return (p - str);
/* Scan the rest of the string using word sized operation */
for (; ; lp++) {
va = (*lp - mask01);
vb = ((~*lp) & mask80);
if (va & vb) {
p = (const char *)(lp);
testbyte(0);
testbyte(1);
testbyte(2);
testbyte(3);
#if (LONG_BIT >= 64)
testbyte(4);
testbyte(5);
testbyte(6);
testbyte(7);
#endif
}
}
/* NOTREACHED */
return (0);
}