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