memcpy.c

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#include <stdint.h>
#include <string.h>
 
void* memcpy(void* _RESTRICT s1, const void* _RESTRICT s2, size_t n)
{
    uint8_t* d = s1;
    const uint8_t* s = s2;
 
    while (((uintptr_t)d & 7) && n)
    {
        *d++ = *s++;
        n--;
    }
 
    while (n >= 64)
    {
        *(uint64_t*)(d + 0) = *(const uint64_t*)(s + 0);
        *(uint64_t*)(d + 8) = *(const uint64_t*)(s + 8);
        *(uint64_t*)(d + 16) = *(const uint64_t*)(s + 16);
        *(uint64_t*)(d + 24) = *(const uint64_t*)(s + 24);
        *(uint64_t*)(d + 32) = *(const uint64_t*)(s + 32);
        *(uint64_t*)(d + 40) = *(const uint64_t*)(s + 40);
        *(uint64_t*)(d + 48) = *(const uint64_t*)(s + 48);
        *(uint64_t*)(d + 56) = *(const uint64_t*)(s + 56);
        d += 64;
        s += 64;
        n -= 64;
    }
 
    while (n >= 8)
    {
        *(uint64_t*)d = *(const uint64_t*)s;
        d += 8;
        s += 8;
        n -= 8;
    }
 
    while (n--)
    {
        *d++ = *s++;
    }
 
    return s1;
}