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