memchr.c

#define SYSCLIB_DISABLE_BUILTINS
#include <sysclib.h>
#include <stdint.h>
#include <limits.h>
 
#define SS (sizeof(size_t))
#define ALIGN (sizeof(size_t)-1)
#define ONES ((size_t)-1/UCHAR_MAX)
#define HIGHS (ONES * (UCHAR_MAX/2+1))
#define HASZERO(x) (((x)-ONES) & ~(x) & HIGHS)
 
void *memchr(const void *src, int c, size_t n)
{
    const unsigned char *s = src;
    c = (unsigned char)c;
#ifdef __GNUC__
    for (; ((uintptr_t)s & ALIGN) && n && *s != c; s++, n--);
    if (n && *s != c) {
        typedef size_t __attribute__((__may_alias__)) word;
        const word *w;
        size_t k = ONES * c;
        for (w = (const void *)s; n>=SS && !HASZERO(*w^k); w++, n-=SS);
        s = (const void *)w;
    }
#endif
    for (; n && *s != c; s++, n--);
    return n ? (void *)s : 0;
}