text: /*-
   * Copyright (c) 2009 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
   * 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 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 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)
   * 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.
   */
  
  #include <sys/cdefs.h>
  __FBSDID("$FreeBSD: src/lib/libc/string/strlen.c,v 1.7 2009/01/26 07:31:28 delphij Exp $");
  
  #include <limits.h>
  #include <sys/types.h>
  #include <string.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.  However, we can further reduce ~1/3 of
   * time if we consider that strlen() usually operate on 7-bit ASCII
   * by employing the following expression, which allows false positive
   * when high bit of 1 and use the tail case to catch these case:
   *
   *	((x - 0x01....01) & 0x80....80)
   *
   * This is more than 5.2 times as fast as the raw implementation on
   * Intel T7300 under long mode for strings longer than word length.
   */
  
  /* 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(const char *str)
  {
  	const char *p;
  	const unsigned long *lp;
  
  	/* Skip the first few bytes until we have an aligned p */
  	for (p = str; (uintptr_t)p & LONGPTR_MASK; p++)
  	    if (*p == '\0')
  		return (p - str);
  
  	/* Scan the rest of the string using word sized operation */
  	for (lp = (const unsigned long *)p; ; lp++)
  	    if ((*lp - mask01) & mask80) {
  		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);
  }
  
  