src/memset.c

/*
 * @TAG(OTHER_MIT)
 */
/*
 * Copyright © 2005-2014 Rich Felker, et al.
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
 * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
 * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
 * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

#include "util.h"

void *__sel4runtime_memset(void *dest, int c, sel4runtime_size_t n)
{
    unsigned char *s = dest;
    sel4runtime_size_t k;

    /* Fill head and tail with minimal branching. Each
     * conditional ensures that all the subsequently used
     * offsets are well-defined and in the dest region. */

    if (!n) {
        return dest;
    }
    s[0] = s[n - 1] = c;
    if (n <= 2) {
        return dest;
    }
    s[1] = s[n - 2] = c;
    s[2] = s[n - 3] = c;
    if (n <= 6) {
        return dest;
    }
    s[3] = s[n - 4] = c;
    if (n <= 8) {
        return dest;
    }

    /* Advance pointer to align it at a 4-byte boundary,
     * and truncate n to a multiple of 4. The previous code
     * already took care of any head/tail that get cut off
     * by the alignment. */

    k = -(sel4runtime_uintptr_t)s & 3;
    s += k;
    n -= k;
    n &= -4;

#ifdef __GNUC__
    typedef sel4runtime_uint32_t __attribute__((__may_alias__)) u32;
    typedef sel4runtime_uint64_t __attribute__((__may_alias__)) u64;

    u32 c32 = ((u32) - 1) / 255 * (unsigned char)c;

    /* In preparation to copy 32 bytes at a time, aligned on
     * an 8-byte bounary, fill head/tail up to 28 bytes each.
     * As in the initial byte-based head/tail fill, each
     * conditional below ensures that the subsequent offsets
     * are valid (e.g. !(n<=24) implies n>=28). */

    *(u32 *)(s + 0) = c32;
    *(u32 *)(s + n - 4) = c32;
    if (n <= 8) {
        return dest;
    }
    *(u32 *)(s + 4) = c32;
    *(u32 *)(s + 8) = c32;
    *(u32 *)(s + n - 12) = c32;
    *(u32 *)(s + n - 8) = c32;
    if (n <= 24) {
        return dest;
    }
    *(u32 *)(s + 12) = c32;
    *(u32 *)(s + 16) = c32;
    *(u32 *)(s + 20) = c32;
    *(u32 *)(s + 24) = c32;
    *(u32 *)(s + n - 28) = c32;
    *(u32 *)(s + n - 24) = c32;
    *(u32 *)(s + n - 20) = c32;
    *(u32 *)(s + n - 16) = c32;

    /* Align to a multiple of 8 so we can fill 64 bits at a time,
     * and avoid writing the same bytes twice as much as is
     * practical without introducing additional branching. */

    k = 24 + ((sel4runtime_uintptr_t)s & 4);
    s += k;
    n -= k;

    /* If this loop is reached, 28 tail bytes have already been
     * filled, so any remainder when n drops below 32 can be
     * safely ignored. */

    u64 c64 = c32 | ((u64)c32 << 32);
    for (; n >= 32; n -= 32, s += 32) {
        *(u64 *)(s + 0) = c64;
        *(u64 *)(s + 8) = c64;
        *(u64 *)(s + 16) = c64;
        *(u64 *)(s + 24) = c64;
    }
#else
    /* Pure C fallback with no aliasing violations. */
    for (; n; n--, s++) {
        *s = c;
    }
#endif

    return dest;
}