dfsqrt.c

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/*
 * Linux/PA-RISC Project (http://www.parisc-linux.org/)
 *
 * Floating-point emulation code
 *  Copyright (C) 2001 Hewlett-Packard (Paul Bame) <[email protected]>
 *
 *    This program is free software; you can redistribute it and/or modify
 *    it under the terms of the GNU General Public License as published by
 *    the Free Software Foundation; either version 2, or (at your option)
 *    any later version.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    GNU General Public License for more details.
 *
 *    You should have received a copy of the GNU General Public License
 *    along with this program; if not, write to the Free Software
 *    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */
/*
 * BEGIN_DESC
 *
 *  File:
 *  @(#)    pa/spmath/dfsqrt.c      $Revision: 1.1 $
 *
 *  Purpose:
 *  Double Floating-point Square Root
 *
 *  External Interfaces:
 *  dbl_fsqrt(srcptr,nullptr,dstptr,status)
 *
 *  Internal Interfaces:
 *
 *  Theory:
 *  <<please update with a overview of the operation of this file>>
 *
 * END_DESC
*/


#include "float.h"
#include "dbl_float.h"

/*
 *  Double Floating-point Square Root
 */

/*ARGSUSED*/
unsigned int
dbl_fsqrt(
        dbl_floating_point *srcptr,
        unsigned int *nullptr,
        dbl_floating_point *dstptr,
        unsigned int *status)
{
    register unsigned int srcp1, srcp2, resultp1, resultp2;
    register unsigned int newbitp1, newbitp2, sump1, sump2;
    register int src_exponent;
    register boolean guardbit = FALSE, even_exponent;

    Dbl_copyfromptr(srcptr,srcp1,srcp2);
        /*
         * check source operand for NaN or infinity
         */
        if ((src_exponent = Dbl_exponent(srcp1)) == DBL_INFINITY_EXPONENT) {
                /*
                 * is signaling NaN?
                 */
                if (Dbl_isone_signaling(srcp1)) {
                        /* trap if INVALIDTRAP enabled */
                        if (Is_invalidtrap_enabled()) return(INVALIDEXCEPTION);
                        /* make NaN quiet */
                        Set_invalidflag();
                        Dbl_set_quiet(srcp1);
                }
                /*
                 * Return quiet NaN or positive infinity.
         *  Fall through to negative test if negative infinity.
                 */
        if (Dbl_iszero_sign(srcp1) || 
            Dbl_isnotzero_mantissa(srcp1,srcp2)) {
                    Dbl_copytoptr(srcp1,srcp2,dstptr);
                    return(NOEXCEPTION);
        }
        }

        /*
         * check for zero source operand
         */
    if (Dbl_iszero_exponentmantissa(srcp1,srcp2)) {
        Dbl_copytoptr(srcp1,srcp2,dstptr);
        return(NOEXCEPTION);
    }

        /*
         * check for negative source operand 
         */
    if (Dbl_isone_sign(srcp1)) {
        /* trap if INVALIDTRAP enabled */
        if (Is_invalidtrap_enabled()) return(INVALIDEXCEPTION);
        /* make NaN quiet */
        Set_invalidflag();
        Dbl_makequietnan(srcp1,srcp2);
        Dbl_copytoptr(srcp1,srcp2,dstptr);
        return(NOEXCEPTION);
    }

    /*
     * Generate result
     */
    if (src_exponent > 0) {
        even_exponent = Dbl_hidden(srcp1);
        Dbl_clear_signexponent_set_hidden(srcp1);
    }
    else {
        /* normalize operand */
        Dbl_clear_signexponent(srcp1);
        src_exponent++;
        Dbl_normalize(srcp1,srcp2,src_exponent);
        even_exponent = src_exponent & 1;
    }
    if (even_exponent) {
        /* exponent is even */
        /* Add comment here.  Explain why odd exponent needs correction */
        Dbl_leftshiftby1(srcp1,srcp2);
    }
    /*
     * Add comment here.  Explain following algorithm.
     * 
     * Trust me, it works.
     *
     */
    Dbl_setzero(resultp1,resultp2);
    Dbl_allp1(newbitp1) = 1 << (DBL_P - 32);
    Dbl_setzero_mantissap2(newbitp2);
    while (Dbl_isnotzero(newbitp1,newbitp2) && Dbl_isnotzero(srcp1,srcp2)) {
        Dbl_addition(resultp1,resultp2,newbitp1,newbitp2,sump1,sump2);
        if(Dbl_isnotgreaterthan(sump1,sump2,srcp1,srcp2)) {
            Dbl_leftshiftby1(newbitp1,newbitp2);
            /* update result */
            Dbl_addition(resultp1,resultp2,newbitp1,newbitp2,
             resultp1,resultp2);  
            Dbl_subtract(srcp1,srcp2,sump1,sump2,srcp1,srcp2);
            Dbl_rightshiftby2(newbitp1,newbitp2);
        }
        else {
            Dbl_rightshiftby1(newbitp1,newbitp2);
        }
        Dbl_leftshiftby1(srcp1,srcp2);
    }
    /* correct exponent for pre-shift */
    if (even_exponent) {
        Dbl_rightshiftby1(resultp1,resultp2);
    }

    /* check for inexact */
    if (Dbl_isnotzero(srcp1,srcp2)) {
        if (!even_exponent && Dbl_islessthan(resultp1,resultp2,srcp1,srcp2)) {
            Dbl_increment(resultp1,resultp2);
        }
        guardbit = Dbl_lowmantissap2(resultp2);
        Dbl_rightshiftby1(resultp1,resultp2);

        /*  now round result  */
        switch (Rounding_mode()) {
        case ROUNDPLUS:
             Dbl_increment(resultp1,resultp2);
             break;
        case ROUNDNEAREST:
             /* stickybit is always true, so guardbit 
              * is enough to determine rounding */
             if (guardbit) {
                Dbl_increment(resultp1,resultp2);
             }
             break;
        }
        /* increment result exponent by 1 if mantissa overflowed */
        if (Dbl_isone_hiddenoverflow(resultp1)) src_exponent+=2;

        if (Is_inexacttrap_enabled()) {
            Dbl_set_exponent(resultp1,
             ((src_exponent-DBL_BIAS)>>1)+DBL_BIAS);
            Dbl_copytoptr(resultp1,resultp2,dstptr);
            return(INEXACTEXCEPTION);
        }
        else Set_inexactflag();
    }
    else {
        Dbl_rightshiftby1(resultp1,resultp2);
    }
    Dbl_set_exponent(resultp1,((src_exponent-DBL_BIAS)>>1)+DBL_BIAS);
    Dbl_copytoptr(resultp1,resultp2,dstptr);
    return(NOEXCEPTION);
}