root/arch/x86/include/asm/word-at-a-time.h

DEFINITIONS

1. count_masked_bytes
2. count_masked_bytes
3. has_zero
4. prep_zero_mask
5. create_zero_mask
6. find_zero
7. load_unaligned_zeropad

#ifndef _ASM_WORD_AT_A_TIME_H
#define _ASM_WORD_AT_A_TIME_H

#include <linux/kernel.h>

/*
 * This is largely generic for little-endian machines, but the
 * optimal byte mask counting is probably going to be something
 * that is architecture-specific. If you have a reliably fast
 * bit count instruction, that might be better than the multiply
 * and shift, for example.
 */
struct word_at_a_time {
        const unsigned long one_bits, high_bits;
};

#define WORD_AT_A_TIME_CONSTANTS { REPEAT_BYTE(0x01), REPEAT_BYTE(0x80) }

#ifdef CONFIG_64BIT

/*
 * Jan Achrenius on G+: microoptimized version of
 * the simpler "(mask & ONEBYTES) * ONEBYTES >> 56"
 * that works for the bytemasks without having to
 * mask them first.
 */
static inline long count_masked_bytes(unsigned long mask)
{
        return mask*0x0001020304050608ul >> 56;
}

#else   /* 32-bit case */

/* Carl Chatfield / Jan Achrenius G+ version for 32-bit */
static inline long count_masked_bytes(long mask)
{
        /* (000000 0000ff 00ffff ffffff) -> ( 1 1 2 3 ) */
        long a = (0x0ff0001+mask) >> 23;
        /* Fix the 1 for 00 case */
        return a & mask;
}

#endif

/* Return nonzero if it has a zero */
static inline unsigned long has_zero(unsigned long a, unsigned long *bits, const struct word_at_a_time *c)
{
        unsigned long mask = ((a - c->one_bits) & ~a) & c->high_bits;
        *bits = mask;
        return mask;
}

static inline unsigned long prep_zero_mask(unsigned long a, unsigned long bits, const struct word_at_a_time *c)
{
        return bits;
}

static inline unsigned long create_zero_mask(unsigned long bits)
{
        bits = (bits - 1) & ~bits;
        return bits >> 7;
}

/* The mask we created is directly usable as a bytemask */
#define zero_bytemask(mask) (mask)

static inline unsigned long find_zero(unsigned long mask)
{
        return count_masked_bytes(mask);
}

/*
 * Load an unaligned word from kernel space.
 *
 * In the (very unlikely) case of the word being a page-crosser
 * and the next page not being mapped, take the exception and
 * return zeroes in the non-existing part.
 */
static inline unsigned long load_unaligned_zeropad(const void *addr)
{
        unsigned long ret, dummy;

        asm(
                "1:\tmov %2,%0\n"
                "2:\n"
                ".section .fixup,\"ax\"\n"
                "3:\t"
                "lea %2,%1\n\t"
                "and %3,%1\n\t"
                "mov (%1),%0\n\t"
                "leal %2,%%ecx\n\t"
                "andl %4,%%ecx\n\t"
                "shll $3,%%ecx\n\t"
                "shr %%cl,%0\n\t"
                "jmp 2b\n"
                ".previous\n"
                _ASM_EXTABLE(1b, 3b)
                :"=&r" (ret),"=&c" (dummy)
                :"m" (*(unsigned long *)addr),
                 "i" (-sizeof(unsigned long)),
                 "i" (sizeof(unsigned long)-1));
        return ret;
}

#endif /* _ASM_WORD_AT_A_TIME_H */