src/string/memset.c

   1 #include <string.h>
   2 #include <stdint.h>
   3
   4 void *memset(void *dest, int c, size_t n)
   5 {
   6         unsigned char *s = dest;
   7         size_t k;
   8
   9         /* Fill head and tail with minimal branching. Each
  10          * conditional ensures that all the subsequently used
  11          * offsets are well-defined and in the dest region. */
  12
  13         if (!n) return dest;
  14         s[0] = c;
  15         s[n-1] = c;
  16         if (n <= 2) return dest;
  17         s[1] = c;
  18         s[2] = c;
  19         s[n-2] = c;
  20         s[n-3] = c;
  21         if (n <= 6) return dest;
  22         s[3] = c;
  23         s[n-4] = c;
  24         if (n <= 8) return dest;
  25
  26         /* Advance pointer to align it at a 4-byte boundary,
  27          * and truncate n to a multiple of 4. The previous code
  28          * already took care of any head/tail that get cut off
  29          * by the alignment. */
  30
  31         k = -(uintptr_t)s & 3;
  32         s += k;
  33         n -= k;
  34         n &= -4;
  35
  36 #ifdef __GNUC__
  37         typedef uint32_t __attribute__((__may_alias__)) u32;
  38         typedef uint64_t __attribute__((__may_alias__)) u64;
  39
  40         u32 c32 = ((u32)-1)/255 * (unsigned char)c;
  41
  42         /* In preparation to copy 32 bytes at a time, aligned on
  43          * an 8-byte bounary, fill head/tail up to 28 bytes each.
  44          * As in the initial byte-based head/tail fill, each
  45          * conditional below ensures that the subsequent offsets
  46          * are valid (e.g. !(n<=24) implies n>=28). */
  47
  48         *(u32 *)(s+0) = c32;
  49         *(u32 *)(s+n-4) = c32;
  50         if (n <= 8) return dest;
  51         *(u32 *)(s+4) = c32;
  52         *(u32 *)(s+8) = c32;
  53         *(u32 *)(s+n-12) = c32;
  54         *(u32 *)(s+n-8) = c32;
  55         if (n <= 24) return dest;
  56         *(u32 *)(s+12) = c32;
  57         *(u32 *)(s+16) = c32;
  58         *(u32 *)(s+20) = c32;
  59         *(u32 *)(s+24) = c32;
  60         *(u32 *)(s+n-28) = c32;
  61         *(u32 *)(s+n-24) = c32;
  62         *(u32 *)(s+n-20) = c32;
  63         *(u32 *)(s+n-16) = c32;
  64
  65         /* Align to a multiple of 8 so we can fill 64 bits at a time,
  66          * and avoid writing the same bytes twice as much as is
  67          * practical without introducing additional branching. */
  68
  69         k = 24 + ((uintptr_t)s & 4);
  70         s += k;
  71         n -= k;
  72
  73         /* If this loop is reached, 28 tail bytes have already been
  74          * filled, so any remainder when n drops below 32 can be
  75          * safely ignored. */
  76
  77         u64 c64 = c32 | ((u64)c32 << 32);
  78         for (; n >= 32; n-=32, s+=32) {
  79                 *(u64 *)(s+0) = c64;
  80                 *(u64 *)(s+8) = c64;
  81                 *(u64 *)(s+16) = c64;
  82                 *(u64 *)(s+24) = c64;
  83         }
  84 #else
  85         /* Pure C fallback with no aliasing violations. */
  86         for (; n; n--, s++) *s = c;
  87 #endif
  88
  89         return dest;
  90 }