source: rtems/cpukit/libnetworking/netinet/in_cksum_i386.h @ 4273e70

/*
 * Checksum routine for Internet Protocol family headers.
 *
 * This routine is very heavily used in the network
 * code and should be modified for each CPU to be as fast as possible.
 *
 * This implementation is 386 version.
 *
 *  $Id$
 */

#include <stdio.h>                /* for puts */

#undef  ADDCARRY
#define ADDCARRY(x)     if ((x) > 0xffff) (x) -= 0xffff
#define REDUCE          {sum = (sum & 0xffff) + (sum >> 16); ADDCARRY(sum);}

/*
 * Thanks to gcc we don't have to guess
 * which registers contain sum & w.
 */
#define ADD(n)  __asm__ volatile \
    ("addl " #n "(%2), %0" : "=r" (sum) : "0" (sum), "r" (w))
#define ADDC(n) __asm__ volatile \
    ("adcl " #n "(%2), %0" : "=r" (sum) : "0" (sum), "r" (w))
#define LOAD(n) __asm__ volatile \
     ("movb " #n "(%1), %0" : "=q" (junk) : "r" (w))
#define MOP     __asm__ volatile \
    ("adcl         $0, %0" : "=r" (sum) : "0" (sum))

int
in_cksum(m, len)
        register struct mbuf *m;
        register int len;
{
        register u_short *w;
        register unsigned sum = 0;
        register int mlen = 0;
        int byte_swapped = 0;
        union { char    c[2]; u_short   s; } su;

        for (;m && len; m = m->m_next) {
                if (m->m_len == 0)
                        continue;
                w = mtod(m, u_short *);
                if (mlen == -1) {
                        /*
                         * The first byte of this mbuf is the continuation
                         * of a word spanning between this mbuf and the
                         * last mbuf.
                         */

                        /* su.c[0] is already saved when scanning previous
                         * mbuf.  sum was REDUCEd when we found mlen == -1
                         */
                        su.c[1] = *(u_char *)w;
                        sum += su.s;
                        w = (u_short *)((char *)w + 1);
                        mlen = m->m_len - 1;
                        len--;
                } else
                        mlen = m->m_len;
                if (len < mlen)
                        mlen = len;
                len -= mlen;
                /*
                 * Force to long boundary so we do longword aligned
                 * memory operations
                 */
                if (3 & (int) w) {
                        REDUCE;
                        if ((1 & (int) w) && (mlen > 0)) {
                                sum <<= 8;
                                su.c[0] = *(char *)w;
                                w = (u_short *)((char *)w + 1);
                                mlen--;
                                byte_swapped = 1;
                        }
                        if ((2 & (int) w) && (mlen >= 2)) {
                                sum += *w++;
                                mlen -= 2;
                        }
                }
                /*
                 * Advance to a 486 cache line boundary.
                 */
                if (4 & (int) w && mlen >= 4) {
                        ADD(0);
                        MOP;
                        w += 2;
                        mlen -= 4;
                }
                if (8 & (int) w && mlen >= 8) {
                        ADD(0);
                        ADDC(4);
                        MOP;
                        w += 4;
                        mlen -= 8;
                }
                /*
                 * Do as much of the checksum as possible 32 bits at at time.
                 * In fact, this loop is unrolled to make overhead from
                 * branches &c small.
                 */
                mlen -= 1;
                while ((mlen -= 32) >= 0) {
                        u_char junk;
                        /*
                         * Add with carry 16 words and fold in the last
                         * carry by adding a 0 with carry.
                         *
                         * The early ADD(16) and the LOAD(32) are to load
                         * the next 2 cache lines in advance on 486's.  The
                         * 486 has a penalty of 2 clock cycles for loading
                         * a cache line, plus whatever time the external
                         * memory takes to load the first word(s) addressed.
                         * These penalties are unavoidable.  Subsequent
                         * accesses to a cache line being loaded (and to
                         * other external memory?) are delayed until the
                         * whole load finishes.  These penalties are mostly
                         * avoided by not accessing external memory for
                         * 8 cycles after the ADD(16) and 12 cycles after
                         * the LOAD(32).  The loop terminates when mlen
                         * is initially 33 (not 32) to guaranteed that
                         * the LOAD(32) is within bounds.
                         */
                        ADD(16);
                        ADDC(0);
                        ADDC(4);
                        ADDC(8);
                        ADDC(12);
                        LOAD(32);
                        ADDC(20);
                        ADDC(24);
                        ADDC(28);
                        MOP;
                        w += 16;
                }
                mlen += 32 + 1;
                if (mlen >= 32) {
                        ADD(16);
                        ADDC(0);
                        ADDC(4);
                        ADDC(8);
                        ADDC(12);
                        ADDC(20);
                        ADDC(24);
                        ADDC(28);
                        MOP;
                        w += 16;
                        mlen -= 32;
                }
                if (mlen >= 16) {
                        ADD(0);
                        ADDC(4);
                        ADDC(8);
                        ADDC(12);
                        MOP;
                        w += 8;
                        mlen -= 16;
                }
                if (mlen >= 8) {
                        ADD(0);
                        ADDC(4);
                        MOP;
                        w += 4;
                        mlen -= 8;
                }
                if (mlen == 0 && byte_swapped == 0)
                        continue;       /* worth 1% maybe ?? */
                REDUCE;
                while ((mlen -= 2) >= 0) {
                        sum += *w++;
                }
                if (byte_swapped) {
                        sum <<= 8;
                        byte_swapped = 0;
                        if (mlen == -1) {
                                su.c[1] = *(char *)w;
                                sum += su.s;
                                mlen = 0;
                        } else
                                mlen = -1;
                } else if (mlen == -1)
                        /*
                         * This mbuf has odd number of bytes.
                         * There could be a word split betwen
                         * this mbuf and the next mbuf.
                         * Save the last byte (to prepend to next mbuf).
                         */
                        su.c[0] = *(char *)w;
        }

        if (len)
                puts("cksum: out of data");
        if (mlen == -1) {
                /* The last mbuf has odd # of bytes. Follow the
                   standard (the odd byte is shifted left by 8 bits) */
                su.c[1] = 0;
                sum += su.s;
        }
        REDUCE;
        return (~sum & 0xffff);
}