/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2005 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

        .file   "strlen.s"

/
/ strlen(s)
/
/ Returns the number of non-NULL bytes in string argument.
/
/
/ Fast assembly language version of the following C-program strlen
/ which represents the `standard' for the C-library.
/
/       size_t
/       strlen(const char *s)
/       {
/               const char      *s0 = s + 1;
/
/               while (*s++ != '\0')
/                       ;
/               return (s - s0);
/       }
/
/ In this assembly language version, the following expression is used
/ to check if a 32-bit word data contains a null byte or not:
/       (((A & 0x7f7f7f7f) + 0x7f7f7f7f) | A) & 0x80808080
/ If the above expression geneates a value other than 0x80808080,
/ that means the 32-bit word data contains a null byte.
/

#include "SYS.h"

        ENTRY(strlen)
        mov     4(%esp), %edx           / src in %edx
        mov     %edx, %eax              / cpy src to %eax

        and     $3, %edx                / is src aligned?
        jz      countbytes
                                        / work byte-wise until aligned
        cmpb    $0, (%eax)              / is *src == 0 ?
        jz      done
        inc     %eax                    / increment src
        cmp     $3, %edx                / if aligned, jump to word-wise check
        jz      countbytes
        cmpb    $0, (%eax)
        jz      done
        inc     %eax
        cmp     $2, %edx
        jz      countbytes
        cmpb    $0, (%eax)
        jz      done
        inc     %eax

        .align    16

countbytes:
        mov     (%eax), %ecx            / load wrd
        add     $4, %eax                / increment src by 4 (bytes in word)
        lea     -0x01010101(%ecx), %edx / (wrd - 0x01010101)
        not     %ecx                    / ~wrd
        and     $0x80808080, %ecx       / ~wrd & 0x80808080
        and     %edx, %ecx              / (wrd - 0x01010101) & ~wrd & 0x80808080
        jz      countbytes              / if zero, no null byte found -- cont

has_zero_byte:
        bsfl    %ecx, %ecx              / find first set bit (null byte)
        shr     $3, %ecx                / switch bit position to byte posn
        lea     -4(%eax, %ecx, 1), %eax / undo pre-increment and count bytes
done:
        sub     4(%esp), %eax           / return (src - old_src)
        ret
        SET_SIZE(strlen)