source: trunk/packages/xen-3.1/xen-3.1/xen/arch/ia64/linux/strlen.S @ 34

/*
 *
 * Optimized version of the standard strlen() function
 *
 *
 * Inputs:
 *      in0     address of string
 *
 * Outputs:
 *      ret0    the number of characters in the string (0 if empty string)
 *      does not count the \0
 *
 * Copyright (C) 1999, 2001 Hewlett-Packard Co
 *      Stephane Eranian <eranian@hpl.hp.com>
 *
 * 09/24/99 S.Eranian add speculation recovery code
 */

#include <asm/asmmacro.h>

//
//
// This is an enhanced version of the basic strlen. it includes a combination
// of compute zero index (czx), parallel comparisons, speculative loads and
// loop unroll using rotating registers.
//
// General Ideas about the algorithm:
//        The goal is to look at the string in chunks of 8 bytes.
//        so we need to do a few extra checks at the beginning because the
//        string may not be 8-byte aligned. In this case we load the 8byte
//        quantity which includes the start of the string and mask the unused
//        bytes with 0xff to avoid confusing czx.
//        We use speculative loads and software pipelining to hide memory
//        latency and do read ahead safely. This way we defer any exception.
//
//        Because we don't want the kernel to be relying on particular
//        settings of the DCR register, we provide recovery code in case
//        speculation fails. The recovery code is going to "redo" the work using
//        only normal loads. If we still get a fault then we generate a
//        kernel panic. Otherwise we return the strlen as usual.
//
//        The fact that speculation may fail can be caused, for instance, by
//        the DCR.dm bit being set. In this case TLB misses are deferred, i.e.,
//        a NaT bit will be set if the translation is not present. The normal
//        load, on the other hand, will cause the translation to be inserted
//        if the mapping exists.
//
//        It should be noted that we execute recovery code only when we need
//        to use the data that has been speculatively loaded: we don't execute
//        recovery code on pure read ahead data.
//
// Remarks:
//      - the cmp r0,r0 is used as a fast way to initialize a predicate
//        register to 1. This is required to make sure that we get the parallel
//        compare correct.
//
//      - we don't use the epilogue counter to exit the loop but we need to set
//        it to zero beforehand.
//
//      - after the loop we must test for Nat values because neither the
//        czx nor cmp instruction raise a NaT consumption fault. We must be
//        careful not to look too far for a Nat for which we don't care.
//        For instance we don't need to look at a NaT in val2 if the zero byte
//        was in val1.
//
//      - Clearly performance tuning is required.
//
//
//
#define saved_pfs       r11
#define tmp             r10
#define base            r16
#define orig            r17
#define saved_pr        r18
#define src             r19
#define mask            r20
#define val             r21
#define val1            r22
#define val2            r23

GLOBAL_ENTRY(strlen)
        .prologue
        .save ar.pfs, saved_pfs
        alloc saved_pfs=ar.pfs,11,0,0,8 // rotating must be multiple of 8

        .rotr v[2], w[2]        // declares our 4 aliases

        extr.u tmp=in0,0,3      // tmp=least significant 3 bits
        mov orig=in0            // keep trackof initial byte address
        dep src=0,in0,0,3       // src=8byte-aligned in0 address
        .save pr, saved_pr
        mov saved_pr=pr         // preserve predicates (rotation)
        ;;

        .body

        ld8 v[1]=[src],8        // must not speculate: can fail here
        shl tmp=tmp,3           // multiply by 8bits/byte
        mov mask=-1             // our mask
        ;;
        ld8.s w[1]=[src],8      // speculatively load next
        cmp.eq p6,p0=r0,r0      // sets p6 to true for cmp.and
        sub tmp=64,tmp          // how many bits to shift our mask on the right
        ;;
        shr.u   mask=mask,tmp   // zero enough bits to hold v[1] valuable part
        mov ar.ec=r0            // clear epilogue counter (saved in ar.pfs)
        ;;
        add base=-16,src        // keep track of aligned base
        or v[1]=v[1],mask       // now we have a safe initial byte pattern
        ;;
1:
        ld8.s v[0]=[src],8      // speculatively load next
        czx1.r val1=v[1]        // search 0 byte from right
        czx1.r val2=w[1]        // search 0 byte from right following 8bytes
        ;;
        ld8.s w[0]=[src],8      // speculatively load next to next
        cmp.eq.and p6,p0=8,val1 // p6 = p6 and val1==8
        cmp.eq.and p6,p0=8,val2 // p6 = p6 and mask==8
(p6)    br.wtop.dptk 1b         // loop until p6 == 0
        ;;
        //
        // We must return try the recovery code iff
        // val1_is_nat || (val1==8 && val2_is_nat)
        //
        // XXX Fixme
        //      - there must be a better way of doing the test
        //
        cmp.eq  p8,p9=8,val1    // p6 = val1 had zero (disambiguate)
        tnat.nz p6,p7=val1      // test NaT on val1
(p6)    br.cond.spnt .recover   // jump to recovery if val1 is NaT
        ;;
        //
        // if we come here p7 is true, i.e., initialized for // cmp
        //
        cmp.eq.and  p7,p0=8,val1// val1==8?
        tnat.nz.and p7,p0=val2  // test NaT if val2
(p7)    br.cond.spnt .recover   // jump to recovery if val2 is NaT
        ;;
(p8)    mov val1=val2           // the other test got us out of the loop
(p8)    adds src=-16,src        // correct position when 3 ahead
(p9)    adds src=-24,src        // correct position when 4 ahead
        ;;
        sub ret0=src,orig       // distance from base
        sub tmp=8,val1          // which byte in word
        mov pr=saved_pr,0xffffffffffff0000
        ;;
        sub ret0=ret0,tmp       // adjust
        mov ar.pfs=saved_pfs    // because of ar.ec, restore no matter what
        br.ret.sptk.many rp     // end of normal execution

        //
        // Outlined recovery code when speculation failed
        //
        // This time we don't use speculation and rely on the normal exception
        // mechanism. that's why the loop is not as good as the previous one
        // because read ahead is not possible
        //
        // IMPORTANT:
        // Please note that in the case of strlen() as opposed to strlen_user()
        // we don't use the exception mechanism, as this function is not
        // supposed to fail. If that happens it means we have a bug and the
        // code will cause of kernel fault.
        //
        // XXX Fixme
        //      - today we restart from the beginning of the string instead
        //        of trying to continue where we left off.
        //
.recover:
        ld8 val=[base],8        // will fail if unrecoverable fault
        ;;
        or val=val,mask         // remask first bytes
        cmp.eq p0,p6=r0,r0      // nullify first ld8 in loop
        ;;
        //
        // ar.ec is still zero here
        //
2:
(p6)    ld8 val=[base],8        // will fail if unrecoverable fault
        ;;
        czx1.r val1=val         // search 0 byte from right
        ;;
        cmp.eq p6,p0=8,val1     // val1==8 ?
(p6)    br.wtop.dptk 2b         // loop until p6 == 0
        ;;                      // (avoid WAW on p63)
        sub ret0=base,orig      // distance from base
        sub tmp=8,val1
        mov pr=saved_pr,0xffffffffffff0000
        ;;
        sub ret0=ret0,tmp       // length=now - back -1
        mov ar.pfs=saved_pfs    // because of ar.ec, restore no matter what
        br.ret.sptk.many rp     // end of successful recovery code
END(strlen)