1#include <string.h>
2#include <stdint.h>
3
4void *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