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April 25, 2017 18:07
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/* | |
COPIED FROM MUSL | |
http://git.musl-libc.org/cgit/musl/tree/src/string/memset.c | |
*/ | |
#include <string.h> | |
#include <stdint.h> | |
void *memset(void *dest, int c, size_t n) | |
{ | |
unsigned char *s = dest; | |
size_t k; | |
/* Fill head and tail with minimal branching. Each | |
* conditional ensures that all the subsequently used | |
* offsets are well-defined and in the dest region. */ | |
if (!n) return dest; | |
s[0] = s[n-1] = c; | |
if (n <= 2) return dest; | |
s[1] = s[n-2] = c; | |
s[2] = s[n-3] = c; | |
if (n <= 6) return dest; | |
s[3] = s[n-4] = c; | |
if (n <= 8) return dest; | |
/* Advance pointer to align it at a 4-byte boundary, | |
* and truncate n to a multiple of 4. The previous code | |
* already took care of any head/tail that get cut off | |
* by the alignment. */ | |
k = -(uintptr_t)s & 3; | |
s += k; | |
n -= k; | |
n &= -4; | |
#ifdef __GNUC__ | |
typedef uint32_t __attribute__((__may_alias__)) u32; | |
typedef uint64_t __attribute__((__may_alias__)) u64; | |
u32 c32 = ((u32)-1)/255 * (unsigned char)c; | |
/* In preparation to copy 32 bytes at a time, aligned on | |
* an 8-byte bounary, fill head/tail up to 28 bytes each. | |
* As in the initial byte-based head/tail fill, each | |
* conditional below ensures that the subsequent offsets | |
* are valid (e.g. !(n<=24) implies n>=28). */ | |
*(u32 *)(s+0) = c32; | |
*(u32 *)(s+n-4) = c32; | |
if (n <= 8) return dest; | |
*(u32 *)(s+4) = c32; | |
*(u32 *)(s+8) = c32; | |
*(u32 *)(s+n-12) = c32; | |
*(u32 *)(s+n-8) = c32; | |
if (n <= 24) return dest; | |
*(u32 *)(s+12) = c32; | |
*(u32 *)(s+16) = c32; | |
*(u32 *)(s+20) = c32; | |
*(u32 *)(s+24) = c32; | |
*(u32 *)(s+n-28) = c32; | |
*(u32 *)(s+n-24) = c32; | |
*(u32 *)(s+n-20) = c32; | |
*(u32 *)(s+n-16) = c32; | |
/* Align to a multiple of 8 so we can fill 64 bits at a time, | |
* and avoid writing the same bytes twice as much as is | |
* practical without introducing additional branching. */ | |
k = 24 + ((uintptr_t)s & 4); | |
s += k; | |
n -= k; | |
/* If this loop is reached, 28 tail bytes have already been | |
* filled, so any remainder when n drops below 32 can be | |
* safely ignored. */ | |
u64 c64 = c32 | ((u64)c32 << 32); | |
for (; n >= 32; n-=32, s+=32) { | |
*(u64 *)(s+0) = c64; | |
*(u64 *)(s+8) = c64; | |
*(u64 *)(s+16) = c64; | |
*(u64 *)(s+24) = c64; | |
} | |
#else | |
/* Pure C fallback with no aliasing violations. */ | |
for (; n; n--, s++) *s = c; | |
#endif | |
return dest; | |
} |
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