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/*
 *
 * Optmized version of the standard do_csum() function
 *
 * Return: a 64bit quantity containing the 16bit Internet checksum
 *
 * Inputs:
 *      in0: address of buffer to checksum (char *)
 *      in1: length of the buffer (int)
 *
 * Copyright (C) 1999, 2001-2002 Hewlett-Packard Co
 *      Stephane Eranian <eranian@hpl.hp.com>
 *
 * 02/04/22     Ken Chen <kenneth.w.chen@intel.com>
 *              Data locality study on the checksum buffer.
 *              More optimization cleanup - remove excessive stop bits.
 * 02/04/08     David Mosberger <davidm@hpl.hp.com>
 *              More cleanup and tuning.
 * 01/04/18     Jun Nakajima <jun.nakajima@intel.com>
 *              Clean up and optimize and the software pipeline, loading two
 *              back-to-back 8-byte words per loop. Clean up the initialization
 *              for the loop. Support the cases where load latency = 1 or 2.
 *              Set CONFIG_IA64_LOAD_LATENCY to 1 or 2 (default).
 */

#include <asm/asmmacro.h>

//
// Theory of operations:
//      The goal is to go as quickly as possible to the point where
//      we can checksum 16 bytes/loop. Before reaching that point we must
//      take care of incorrect alignment of first byte.
//
//      The code hereafter also takes care of the "tail" part of the buffer
//      before entering the core loop, if any. The checksum is a sum so it
//      allows us to commute operations. So we do the "head" and "tail"
//      first to finish at full speed in the body. Once we get the head and
//      tail values, we feed them into the pipeline, very handy initialization.
//
//      Of course we deal with the special case where the whole buffer fits
//      into one 8 byte word. In this case we have only one entry in the pipeline.
//
//      We use a (LOAD_LATENCY+2)-stage pipeline in the loop to account for
//      possible load latency and also to accomodate for head and tail.
//
//      The end of the function deals with folding the checksum from 64bits
//      down to 16bits taking care of the carry.
//
//      This version avoids synchronization in the core loop by also using a
//      pipeline for the accumulation of the checksum in resultx[] (x=1,2).
//
//       wordx[] (x=1,2)
//      |---|
//      |   | 0                 : new value loaded in pipeline
//      |---|
//      |   | -                 : in transit data
//      |---|
//      |   | LOAD_LATENCY      : current value to add to checksum
//      |---|
//      |   | LOAD_LATENCY+1    : previous value added to checksum
//      |---|                   (previous iteration)
//
//      resultx[] (x=1,2)
//      |---|
//      |   | 0                 : initial value
//      |---|
//      |   | LOAD_LATENCY-1    : new checksum
//      |---|
//      |   | LOAD_LATENCY      : previous value of checksum
//      |---|
//      |   | LOAD_LATENCY+1    : final checksum when out of the loop
//      |---|
//
//
//      See RFC1071 "Computing the Internet Checksum" for various techniques for
//      calculating the Internet checksum.
//
// NOT YET DONE:
//      - Maybe another algorithm which would take care of the folding at the
//        end in a different manner
//      - Work with people more knowledgeable than me on the network stack
//        to figure out if we could not split the function depending on the
//        type of packet or alignment we get. Like the ip_fast_csum() routine
//        where we know we have at least 20bytes worth of data to checksum.
//      - Do a better job of handling small packets.
//      - Note on prefetching: it was found that under various load, i.e. ftp read/write,
//        nfs read/write, the L1 cache hit rate is at 60% and L2 cache hit rate is at 99.8%
//        on the data that buffer points to (partly because the checksum is often preceded by
//        a copy_from_user()).  This finding indiate that lfetch will not be beneficial since
//        the data is already in the cache.
//

#define saved_pfs       r11
#define hmask           r16
#define tmask           r17
#define first1          r18
#define firstval        r19
#define firstoff        r20
#define last            r21
#define lastval         r22
#define lastoff         r23
#define saved_lc        r24
#define saved_pr        r25
#define tmp1            r26
#define tmp2            r27
#define tmp3            r28
#define carry1          r29
#define carry2          r30
#define first2          r31

#define buf             in0
#define len             in1

#define LOAD_LATENCY    2       // XXX fix me

#if (LOAD_LATENCY != 1) && (LOAD_LATENCY != 2)
# error "Only 1 or 2 is supported/tested for LOAD_LATENCY."
#endif

#define PIPE_DEPTH                      (LOAD_LATENCY+2)
#define ELD     p[LOAD_LATENCY]         // end of load
#define ELD_1   p[LOAD_LATENCY+1]       // and next stage

// unsigned long do_csum(unsigned char *buf,long len)

GLOBAL_ENTRY(do_csum)
        .prologue
        .save ar.pfs, saved_pfs
        alloc saved_pfs=ar.pfs,2,16,0,16
        .rotr word1[4], word2[4],result1[LOAD_LATENCY+2],result2[LOAD_LATENCY+2]
        .rotp p[PIPE_DEPTH], pC1[2], pC2[2]
        mov ret0=r0             // in case we have zero length
        cmp.lt p0,p6=r0,len     // check for zero length or negative (32bit len)
        ;;
        add tmp1=buf,len        // last byte's address
        .save pr, saved_pr
        mov saved_pr=pr         // preserve predicates (rotation)
(p6)    br.ret.spnt.many rp     // return if zero or negative length

        mov hmask=-1            // intialize head mask
        tbit.nz p15,p0=buf,0    // is buf an odd address?
        and first1=-8,buf       // 8-byte align down address of first1 element

        and firstoff=7,buf      // how many bytes off for first1 element
        mov tmask=-1            // initialize tail mask

        ;;
        adds tmp2=-1,tmp1       // last-1
        and lastoff=7,tmp1      // how many bytes off for last element
        ;;
        sub tmp1=8,lastoff      // complement to lastoff
        and last=-8,tmp2        // address of word containing last byte
        ;;
        sub tmp3=last,first1    // tmp3=distance from first1 to last
        .save ar.lc, saved_lc
        mov saved_lc=ar.lc      // save lc
        cmp.eq p8,p9=last,first1        // everything fits in one word ?

        ld8 firstval=[first1],8 // load, ahead of time, "first1" word
        and tmp1=7, tmp1        // make sure that if tmp1==8 -> tmp1=0
        shl tmp2=firstoff,3     // number of bits
        ;;
(p9)    ld8 lastval=[last]      // load, ahead of time, "last" word, if needed
        shl tmp1=tmp1,3         // number of bits
(p9)    adds tmp3=-8,tmp3       // effectively loaded
        ;;
(p8)    mov lastval=r0          // we don't need lastval if first1==last
        shl hmask=hmask,tmp2    // build head mask, mask off [0,first1off[
        shr.u tmask=tmask,tmp1  // build tail mask, mask off ]8,lastoff]
        ;;
        .body
#define count tmp3

(p8)    and hmask=hmask,tmask   // apply tail mask to head mask if 1 word only
(p9)    and word2[0]=lastval,tmask      // mask last it as appropriate
        shr.u count=count,3     // how many 8-byte?
        ;;
        // If count is odd, finish this 8-byte word so that we can
        // load two back-to-back 8-byte words per loop thereafter.
        and word1[0]=firstval,hmask     // and mask it as appropriate
        tbit.nz p10,p11=count,0         // if (count is odd)
        ;;
(p8)    mov result1[0]=word1[0]
(p9)    add result1[0]=word1[0],word2[0]
        ;;
        cmp.ltu p6,p0=result1[0],word1[0]       // check the carry
        cmp.eq.or.andcm p8,p0=0,count           // exit if zero 8-byte
        ;;
(p6)    adds result1[0]=1,result1[0]
(p8)    br.cond.dptk .do_csum_exit      // if (within an 8-byte word)
(p11)   br.cond.dptk .do_csum16         // if (count is even)

        // Here count is odd.
        ld8 word1[1]=[first1],8         // load an 8-byte word
        cmp.eq p9,p10=1,count           // if (count == 1)
        adds count=-1,count             // loaded an 8-byte word
        ;;
        add result1[0]=result1[0],word1[1]
        ;;
        cmp.ltu p6,p0=result1[0],word1[1]
        ;;
(p6)    adds result1[0]=1,result1[0]
(p9)    br.cond.sptk .do_csum_exit      // if (count == 1) exit
        // Fall through to caluculate the checksum, feeding result1[0] as
        // the initial value in result1[0].
        //
        // Calculate the checksum loading two 8-byte words per loop.
        //
.do_csum16:
        add first2=8,first1
        shr.u count=count,1     // we do 16 bytes per loop
        ;;
        adds count=-1,count
        mov carry1=r0
        mov carry2=r0
        brp.loop.imp 1f,2f
        ;;
        mov ar.ec=PIPE_DEPTH
        mov ar.lc=count // set lc
        mov pr.rot=1<<16
        // result1[0] must be initialized in advance.
        mov result2[0]=r0
        ;;
        .align 32
1:
(ELD_1) cmp.ltu pC1[0],p0=result1[LOAD_LATENCY],word1[LOAD_LATENCY+1]
(pC1[1])adds carry1=1,carry1
(ELD_1) cmp.ltu pC2[0],p0=result2[LOAD_LATENCY],word2[LOAD_LATENCY+1]
(pC2[1])adds carry2=1,carry2
(ELD)   add result1[LOAD_LATENCY-1]=result1[LOAD_LATENCY],word1[LOAD_LATENCY]
(ELD)   add result2[LOAD_LATENCY-1]=result2[LOAD_LATENCY],word2[LOAD_LATENCY]
2:
(p[0])  ld8 word1[0]=[first1],16
(p[0])  ld8 word2[0]=[first2],16
        br.ctop.sptk 1b
        ;;
        // Since len is a 32-bit value, carry cannot be larger than a 64-bit value.
(pC1[1])adds carry1=1,carry1    // since we miss the last one
(pC2[1])adds carry2=1,carry2
        ;;
        add result1[LOAD_LATENCY+1]=result1[LOAD_LATENCY+1],carry1
        add result2[LOAD_LATENCY+1]=result2[LOAD_LATENCY+1],carry2
        ;;
        cmp.ltu p6,p0=result1[LOAD_LATENCY+1],carry1
        cmp.ltu p7,p0=result2[LOAD_LATENCY+1],carry2
        ;;
(p6)    adds result1[LOAD_LATENCY+1]=1,result1[LOAD_LATENCY+1]
(p7)    adds result2[LOAD_LATENCY+1]=1,result2[LOAD_LATENCY+1]
        ;;
        add result1[0]=result1[LOAD_LATENCY+1],result2[LOAD_LATENCY+1]
        ;;
        cmp.ltu p6,p0=result1[0],result2[LOAD_LATENCY+1]
        ;;
(p6)    adds result1[0]=1,result1[0]
        ;;
.do_csum_exit:
        //
        // now fold 64 into 16 bits taking care of carry
        // that's not very good because it has lots of sequentiality
        //
        mov tmp3=0xffff
        zxt4 tmp1=result1[0]
        shr.u tmp2=result1[0],32
        ;;
        add result1[0]=tmp1,tmp2
        ;;
        and tmp1=result1[0],tmp3
        shr.u tmp2=result1[0],16
        ;;
        add result1[0]=tmp1,tmp2
        ;;
        and tmp1=result1[0],tmp3
        shr.u tmp2=result1[0],16
        ;;
        add result1[0]=tmp1,tmp2
        ;;
        and tmp1=result1[0],tmp3
        shr.u tmp2=result1[0],16
        ;;
        add ret0=tmp1,tmp2
        mov pr=saved_pr,0xffffffffffff0000
        ;;
        // if buf was odd then swap bytes
        mov ar.pfs=saved_pfs            // restore ar.ec
(p15)   mux1 ret0=ret0,@rev             // reverse word
        ;;
        mov ar.lc=saved_lc
(p15)   shr.u ret0=ret0,64-16   // + shift back to position = swap bytes
        br.ret.sptk.many rp

//      I (Jun Nakajima) wrote an equivalent code (see below), but it was
//      not much better than the original. So keep the original there so that
//      someone else can challenge.
//
//      shr.u word1[0]=result1[0],32
//      zxt4 result1[0]=result1[0]
//      ;;
//      add result1[0]=result1[0],word1[0]
//      ;;
//      zxt2 result2[0]=result1[0]
//      extr.u word1[0]=result1[0],16,16
//      shr.u carry1=result1[0],32
//      ;;
//      add result2[0]=result2[0],word1[0]
//      ;;
//      add result2[0]=result2[0],carry1
//      ;;
//      extr.u ret0=result2[0],16,16
//      ;;
//      add ret0=ret0,result2[0]
//      ;;
//      zxt2 ret0=ret0
//      mov ar.pfs=saved_pfs             // restore ar.ec
//      mov pr=saved_pr,0xffffffffffff0000
//      ;;
//      // if buf was odd then swap bytes
//      mov ar.lc=saved_lc
//(p15) mux1 ret0=ret0,@rev             // reverse word
//      ;;
//(p15) shr.u ret0=ret0,64-16   // + shift back to position = swap bytes
//      br.ret.sptk.many rp

END(do_csum)