memchr.c

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// Imported from musl Libc

#include <limits.h>
#include <stdint.h>
#include <string.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 * (size_t)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*)(uintptr_t)s : 0;
}