source: azure_iot_hub/trunk/musl-1.1.18/src/string/memset.c@ 388

#include <string.h>
#include <stdint.h>

void *memset(void *dest, int c, size_t n)
{
        unsigned char *s = dest;
        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] = c;
        s[n-1] = c;
        if (n <= 2) return dest;
        s[1] = c;
        s[2] = c;
        s[n-2] = c;
        s[n-3] = c;
        if (n <= 6) return dest;
        s[3] = c;
        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 = -(uintptr_t)s & 3;
        s += k;
        n -= k;
        n &= -4;

#ifdef __GNUC__
        typedef uint32_t __attribute__((__may_alias__)) u32;
        typedef 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 + ((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;
}