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dfsqrt.c

/*
 * Linux/PA-RISC Project (http://www.parisc-linux.org/)
 *
 * Floating-point emulation code
 *  Copyright (C) 2001 Hewlett-Packard (Paul Bame) <bame@debian.org>
 *
 *    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);
}

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