memchr_32.c

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/*
 * 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>

void *memchr(const void *s, int c, size_t n)
{
    const uint32_t *last_word_ptr;
    const uint32_t *p;
    const char *last_byte_ptr;
    uintptr_t s_int;
    uint32_t goal, before_mask, v, bits;
    char *ret;

    if (__builtin_expect(n == 0, 0)) {
        /* Don't dereference any memory if the array is empty. */
        return NULL;
    }

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

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

    /* Read the first word, but munge it so that bytes before the array
     * will not match goal.
     *
     * Note that this shift count expression works because we know
     * shift counts are taken mod 32.
     */
    before_mask = (1 << (s_int << 3)) - 1;
    v = (*p | before_mask) ^ (goal & before_mask);

    /* Compute the address of the last byte. */
    last_byte_ptr = (const char *)s + n - 1;

    /* Compute the address of the word containing the last byte. */
    last_word_ptr = (const uint32_t *)((uintptr_t) last_byte_ptr & -4);

    while ((bits = __insn_seqb(v, goal)) == 0) {
        if (__builtin_expect(p == last_word_ptr, 0)) {
            /* We already read the last word in the array,
             * so give up.
             */
            return NULL;
        }
        v = *++p;
    }

    /* We found a match, but it might be in a byte past the end
     * of the array.
     */
    ret = ((char *)p) + (__insn_ctz(bits) >> 3);
    return (ret <= last_byte_ptr) ? ret : NULL;
}
EXPORT_SYMBOL(memchr);