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 }
91