strchr_32.c

Go to the documentation of this file.

/*
 * Copyright 2010 Tilera Corporation. All Rights Reserved.
 *
 *   This program is free software; you can redistribute it and/or
 *   modify it under the terms of the GNU General Public License
 *   as published by the Free Software Foundation, version 2.
 *
 *   This program is distributed in the hope that it will be useful, but
 *   WITHOUT ANY WARRANTY; without even the implied warranty of
 *   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
 *   NON INFRINGEMENT.  See the GNU General Public License for
 *   more details.
 */

#include <linux/types.h>
#include <linux/string.h>
#include <linux/module.h>

#undef strchr

char *strchr(const char *s, int c)
{
    int z, g;

    /* Get an aligned pointer. */
    const uintptr_t s_int = (uintptr_t) s;
    const uint32_t *p = (const uint32_t *)(s_int & -4);

    /* Create four copies of the byte for which we are looking. */
    const uint32_t goal = 0x01010101 * (uint8_t) c;

    /* Read the first aligned word, but force bytes before the string to
     * match neither zero nor goal (we make sure the high bit of each
     * byte is 1, and the low 7 bits are all the opposite of the goal
     * byte).
     *
     * Note that this shift count expression works because we know shift
     * counts are taken mod 32.
     */
    const uint32_t before_mask = (1 << (s_int << 3)) - 1;
    uint32_t v = (*p | before_mask) ^ (goal & __insn_shrib(before_mask, 1));

    uint32_t zero_matches, goal_matches;
    while (1) {
        /* Look for a terminating '\0'. */
        zero_matches = __insn_seqb(v, 0);

        /* Look for the goal byte. */
        goal_matches = __insn_seqb(v, goal);

        if (__builtin_expect(zero_matches | goal_matches, 0))
            break;

        v = *++p;
    }

    z = __insn_ctz(zero_matches);
    g = __insn_ctz(goal_matches);

    /* If we found c before '\0' we got a match. Note that if c == '\0'
     * then g == z, and we correctly return the address of the '\0'
     * rather than NULL.
     */
    return (g <= z) ? ((char *)p) + (g >> 3) : NULL;
}
EXPORT_SYMBOL(strchr);