fusl/src/string/memset.c

#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] = 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 = -(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;
}