svn commit: r187700 - head/lib/libc/string

[Ben Kaduk]

On Sun, Jan 25, 2009 at 6:08 PM, Xin LI <delphij at freebsd.org> wrote:
> Author: delphij
> Date: Sun Jan 25 23:08:47 2009
> New Revision: 187700
> URL: http://svn.freebsd.org/changeset/base/187700
>
> Log:
>  Rewrite of MI strlen(3) in a way that can better utilize modern hardware by
>  reducing branches and doing word-sized operation.
>
>  The idea is taken from J.T. Conklin's x86_64 optimized version of strlen(3)
>  for NetBSD, and reimplemented in C by me.
>
>  Discussed on: -arch@
>
> Modified:
>  head/lib/libc/string/strlen.c
>
> Modified: head/lib/libc/string/strlen.c
> ==============================================================================
> --- head/lib/libc/string/strlen.c       Sun Jan 25 21:27:31 2009        (r187699)
> +++ head/lib/libc/string/strlen.c       Sun Jan 25 23:08:47 2009        (r187700)
> @@ -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 +24,87 @@
>  * 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>
>
> +/*
> + * 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 for strings longer than word length.

The grammar is not quite right, here.  Did you mean to say "more than 5.2
times as fast as the raw implementation [...]"

-Ben Kaduk