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