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/*

 * 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: in_cksum_i386.c,v 1.2 2001-09-27 12:01:55 chris Exp $

 */

#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("addl " #n "(%2), %0" : "=r" (sum) : "0" (sum), "r" (w))

#define ADDC(n) asm("adcl " #n "(%2), %0" : "=r" (sum) : "0" (sum), "r" (w))

#define LOAD(n) asm volatile("movb " #n "(%1), %0" : "=r" (junk) : "r" (w))

#define MOP     asm("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);

}