csum-partial_64.c

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/*
 * arch/x86_64/lib/csum-partial.c
 *
 * This file contains network checksum routines that are better done
 * in an architecture-specific manner due to speed.
 */
 
#include <linux/compiler.h>
#include <linux/module.h>
#include <asm/checksum.h>

static inline unsigned short from32to16(unsigned a) 
{
    unsigned short b = a >> 16; 
    asm("addw %w2,%w0\n\t"
        "adcw $0,%w0\n" 
        : "=r" (b)
        : "0" (b), "r" (a));
    return b;
}

/*
 * Do a 64-bit checksum on an arbitrary memory area.
 * Returns a 32bit checksum.
 *
 * This isn't as time critical as it used to be because many NICs
 * do hardware checksumming these days.
 * 
 * Things tried and found to not make it faster:
 * Manual Prefetching
 * Unrolling to an 128 bytes inner loop.
 * Using interleaving with more registers to break the carry chains.
 */
static unsigned do_csum(const unsigned char *buff, unsigned len)
{
    unsigned odd, count;
    unsigned long result = 0;

    if (unlikely(len == 0))
        return result; 
    odd = 1 & (unsigned long) buff;
    if (unlikely(odd)) {
        result = *buff << 8;
        len--;
        buff++;
    }
    count = len >> 1;       /* nr of 16-bit words.. */
    if (count) {
        if (2 & (unsigned long) buff) {
            result += *(unsigned short *)buff;
            count--;
            len -= 2;
            buff += 2;
        }
        count >>= 1;        /* nr of 32-bit words.. */
        if (count) {
            unsigned long zero;
            unsigned count64;
            if (4 & (unsigned long) buff) {
                result += *(unsigned int *) buff;
                count--;
                len -= 4;
                buff += 4;
            }
            count >>= 1;    /* nr of 64-bit words.. */

            /* main loop using 64byte blocks */
            zero = 0;
            count64 = count >> 3;
            while (count64) { 
                asm("addq 0*8(%[src]),%[res]\n\t"
                    "adcq 1*8(%[src]),%[res]\n\t"
                    "adcq 2*8(%[src]),%[res]\n\t"
                    "adcq 3*8(%[src]),%[res]\n\t"
                    "adcq 4*8(%[src]),%[res]\n\t"
                    "adcq 5*8(%[src]),%[res]\n\t"
                    "adcq 6*8(%[src]),%[res]\n\t"
                    "adcq 7*8(%[src]),%[res]\n\t"
                    "adcq %[zero],%[res]"
                    : [res] "=r" (result)
                    : [src] "r" (buff), [zero] "r" (zero),
                    "[res]" (result));
                buff += 64;
                count64--;
            }

            /* last up to 7 8byte blocks */
            count %= 8; 
            while (count) { 
                asm("addq %1,%0\n\t"
                    "adcq %2,%0\n" 
                        : "=r" (result)
                    : "m" (*(unsigned long *)buff), 
                    "r" (zero),  "0" (result));
                --count; 
                    buff += 8;
            }
            result = add32_with_carry(result>>32,
                          result&0xffffffff); 

            if (len & 4) {
                result += *(unsigned int *) buff;
                buff += 4;
            }
        }
        if (len & 2) {
            result += *(unsigned short *) buff;
            buff += 2;
        }
    }
    if (len & 1)
        result += *buff;
    result = add32_with_carry(result>>32, result & 0xffffffff); 
    if (unlikely(odd)) { 
        result = from32to16(result);
        result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
    }
    return result;
}

/*
 * computes the checksum of a memory block at buff, length len,
 * and adds in "sum" (32-bit)
 *
 * returns a 32-bit number suitable for feeding into itself
 * or csum_tcpudp_magic
 *
 * this function must be called with even lengths, except
 * for the last fragment, which may be odd
 *
 * it's best to have buff aligned on a 64-bit boundary
 */
__wsum csum_partial(const void *buff, int len, __wsum sum)
{
    return (__force __wsum)add32_with_carry(do_csum(buff, len),
                        (__force u32)sum);
}

/*
 * this routine is used for miscellaneous IP-like checksums, mainly
 * in icmp.c
 */
__sum16 ip_compute_csum(const void *buff, int len)
{
    return csum_fold(csum_partial(buff,len,0));
}
EXPORT_SYMBOL(ip_compute_csum);