RFC: MI strlen()

[Alfred Perlstein]

* Xin LI <delphij at delphij.net> [090108 18:27] wrote:
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> 
> Hi,
> 
> Here is a new implementation of strlen() which employed the bitmask
> skill in order to achieve better performance on modern hardware.  For
> common case, this would be a 5.2x boost on FreeBSD/amd64.  The code is
> intended for MI use when there is no hand-optimized assembly.
> 
> 	http://people.freebsd.org/~delphij/for_review/strlen.diff
> 
> Note that this version of strlen() has suboptimal performance if there
> are a lot of characters that has their highest bit set (we can change it
> to have uniform performance at the expense of about ~30% performance
> penalty).
> 
> Comments?

It's pretty cool, what does valgrind say when you walk past
the end of a string, can it figure that out?

> 
> Cheers,
> - --
> Xin LI <delphij at delphij.net>	http://www.delphij.net/
> FreeBSD - The Power to Serve!
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> Index: strlen.c
> ===================================================================
> --- strlen.c	(revision 186910)
> +++ strlen.c	(working copy)
> @@ -1,6 +1,6 @@
>  /*-
> - * Copyright (c) 1990, 1993
> - *	The Regents of the University of California.  All rights reserved.
> + * Copyright (c) 2009 Xin LI <delphij at 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)
> @@ -27,21 +24,93 @@
>   * SUCH DAMAGE.
>   */
>  
> -#if defined(LIBC_SCCS) && !defined(lint)
> -static char sccsid[] = "@(#)strlen.c	8.1 (Berkeley) 6/4/93";
> -#endif /* LIBC_SCCS and not lint */
>  #include <sys/cdefs.h>
>  __FBSDID("$FreeBSD$");
>  
> +#include <sys/limits.h>
> +#include <sys/types.h>
>  #include <string.h>
>  
> +#ifndef CTASSERT
> +#define CTASSERT(x)		_CTASSERT(x, __LINE__)
> +#define _CTASSERT(x, y)		__CTASSERT(x, y)
> +#define __CTASSERT(x, y)	typedef char __assert_ ## y [(x) ? 1 : -1]
> +#endif
> +
> +CTASSERT(LONG_BIT == 32 || LONG_BIT == 64);
> +
> +/*
> + * 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 compared to the raw implementation
> + * on Intel T7300 under EM64T mode.
> + */
> +
> +/* 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;
> +#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)
>  {
> -	const char *s;
> +	const char *p;
> +	const unsigned long *lp;
>  
> -	for (s = str; *s; ++s);
> -	return(s - str);
> +	/* 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;
>  }
>  

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-- 
- Alfred Perlstein