sfsqrt.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/sfsqrt.c      $Revision: 1.1 $
 *
 *  Purpose:
 *  Single Floating-point Square Root
 *
 *  External Interfaces:
 *  sgl_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 "sgl_float.h"

/*
 *  Single Floating-point Square Root
 */

/*ARGSUSED*/
unsigned int
sgl_fsqrt(
    sgl_floating_point *srcptr,
    unsigned int *nullptr,
    sgl_floating_point *dstptr,
    unsigned int *status)
{
    register unsigned int src, result;
    register int src_exponent;
    register unsigned int newbit, sum;
    register boolean guardbit = FALSE, even_exponent;

    src = *srcptr;
        /*
         * check source operand for NaN or infinity
         */
        if ((src_exponent = Sgl_exponent(src)) == SGL_INFINITY_EXPONENT) {
                /*
                 * is signaling NaN?
                 */
                if (Sgl_isone_signaling(src)) {
                        /* trap if INVALIDTRAP enabled */
                        if (Is_invalidtrap_enabled()) return(INVALIDEXCEPTION);
                        /* make NaN quiet */
                        Set_invalidflag();
                        Sgl_set_quiet(src);
                }
                /*
                 * Return quiet NaN or positive infinity.
         *  Fall through to negative test if negative infinity.
                 */
        if (Sgl_iszero_sign(src) || Sgl_isnotzero_mantissa(src)) {
                    *dstptr = src;
                    return(NOEXCEPTION);
        }
        }

        /*
         * check for zero source operand
         */
    if (Sgl_iszero_exponentmantissa(src)) {
        *dstptr = src;
        return(NOEXCEPTION);
    }

        /*
         * check for negative source operand 
         */
    if (Sgl_isone_sign(src)) {
        /* trap if INVALIDTRAP enabled */
        if (Is_invalidtrap_enabled()) return(INVALIDEXCEPTION);
        /* make NaN quiet */
        Set_invalidflag();
        Sgl_makequietnan(src);
        *dstptr = src;
        return(NOEXCEPTION);
    }

    /*
     * Generate result
     */
    if (src_exponent > 0) {
        even_exponent = Sgl_hidden(src);
        Sgl_clear_signexponent_set_hidden(src);
    }
    else {
        /* normalize operand */
        Sgl_clear_signexponent(src);
        src_exponent++;
        Sgl_normalize(src,src_exponent);
        even_exponent = src_exponent & 1;
    }
    if (even_exponent) {
        /* exponent is even */
        /* Add comment here.  Explain why odd exponent needs correction */
        Sgl_leftshiftby1(src);
    }
    /*
     * Add comment here.  Explain following algorithm.
     * 
     * Trust me, it works.
     *
     */
    Sgl_setzero(result);
    newbit = 1 << SGL_P;
    while (newbit && Sgl_isnotzero(src)) {
        Sgl_addition(result,newbit,sum);
        if(sum <= Sgl_all(src)) {
            /* update result */
            Sgl_addition(result,(newbit<<1),result);
            Sgl_subtract(src,sum,src);
        }
        Sgl_rightshiftby1(newbit);
        Sgl_leftshiftby1(src);
    }
    /* correct exponent for pre-shift */
    if (even_exponent) {
        Sgl_rightshiftby1(result);
    }

    /* check for inexact */
    if (Sgl_isnotzero(src)) {
        if (!even_exponent && Sgl_islessthan(result,src)) 
            Sgl_increment(result);
        guardbit = Sgl_lowmantissa(result);
        Sgl_rightshiftby1(result);

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

        if (Is_inexacttrap_enabled()) {
            Sgl_set_exponent(result,
             ((src_exponent-SGL_BIAS)>>1)+SGL_BIAS);
            *dstptr = result;
            return(INEXACTEXCEPTION);
        }
        else Set_inexactflag();
    }
    else {
        Sgl_rightshiftby1(result);
    }
    Sgl_set_exponent(result,((src_exponent-SGL_BIAS)>>1)+SGL_BIAS);
    *dstptr = result;
    return(NOEXCEPTION);
}