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sfsub.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/sfsub.c       $Revision: 1.1 $
 *
 *  Purpose:
 *    Single_subtract: subtract two single precision values.
 *
 *  External Interfaces:
 *    sgl_fsub(leftptr, rightptr, 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_subtract: subtract two single precision values.
 */
int
sgl_fsub(
          sgl_floating_point *leftptr,
          sgl_floating_point *rightptr,
          sgl_floating_point *dstptr,
          unsigned int *status)
    {
    register unsigned int left, right, result, extent;
    register unsigned int signless_upper_left, signless_upper_right, save;
    
    register int result_exponent, right_exponent, diff_exponent;
    register int sign_save, jumpsize;
    register boolean inexact = FALSE, underflowtrap;
        
    /* Create local copies of the numbers */
    left = *leftptr;
    right = *rightptr;

    /* A zero "save" helps discover equal operands (for later),  *
     * and is used in swapping operands (if needed).             */
    Sgl_xortointp1(left,right,/*to*/save);

    /*
     * check first operand for NaN's or infinity
     */
    if ((result_exponent = Sgl_exponent(left)) == SGL_INFINITY_EXPONENT)
      {
      if (Sgl_iszero_mantissa(left)) 
          {
          if (Sgl_isnotnan(right)) 
            {
            if (Sgl_isinfinity(right) && save==0) 
                {
                /* 
                 * invalid since operands are same signed infinity's
                 */
                if (Is_invalidtrap_enabled()) return(INVALIDEXCEPTION);
                    Set_invalidflag();
                    Sgl_makequietnan(result);
                *dstptr = result;
                return(NOEXCEPTION);
                }
            /*
             * return infinity
             */
            *dstptr = left;
            return(NOEXCEPTION);
            }
          }
      else 
          {
            /*
             * is NaN; signaling or quiet?
             */
            if (Sgl_isone_signaling(left)) 
            {
                  /* trap if INVALIDTRAP enabled */
            if (Is_invalidtrap_enabled()) return(INVALIDEXCEPTION);
            /* make NaN quiet */
            Set_invalidflag();
            Sgl_set_quiet(left);
            }
          /* 
           * is second operand a signaling NaN? 
           */
          else if (Sgl_is_signalingnan(right)) 
            {
            /* trap if INVALIDTRAP enabled */
                  if (Is_invalidtrap_enabled()) return(INVALIDEXCEPTION);
            /* make NaN quiet */
            Set_invalidflag();
            Sgl_set_quiet(right);
            *dstptr = right;
            return(NOEXCEPTION);
            }
          /*
           * return quiet NaN
           */
          *dstptr = left;
          return(NOEXCEPTION);
          }
      } /* End left NaN or Infinity processing */
    /*
     * check second operand for NaN's or infinity
     */
    if (Sgl_isinfinity_exponent(right)) 
      {
      if (Sgl_iszero_mantissa(right)) 
          {
          /* return infinity */
          Sgl_invert_sign(right);
          *dstptr = right;
          return(NOEXCEPTION);
          }
        /*
         * is NaN; signaling or quiet?
         */
        if (Sgl_isone_signaling(right)) 
          {
            /* trap if INVALIDTRAP enabled */
          if (Is_invalidtrap_enabled()) return(INVALIDEXCEPTION);
          /* make NaN quiet */
          Set_invalidflag();
          Sgl_set_quiet(right);
          }
      /*
       * return quiet NaN
       */
      *dstptr = right;
      return(NOEXCEPTION);
      } /* End right NaN or Infinity processing */

    /* Invariant: Must be dealing with finite numbers */

    /* Compare operands by removing the sign */
    Sgl_copytoint_exponentmantissa(left,signless_upper_left);
    Sgl_copytoint_exponentmantissa(right,signless_upper_right);

    /* sign difference selects sub or add operation. */
    if(Sgl_ismagnitudeless(signless_upper_left,signless_upper_right))
      {
      /* Set the left operand to the larger one by XOR swap *
       *  First finish the first word using "save"          */
      Sgl_xorfromintp1(save,right,/*to*/right);
      Sgl_xorfromintp1(save,left,/*to*/left);
      result_exponent = Sgl_exponent(left);
      Sgl_invert_sign(left);
      }
    /* Invariant:  left is not smaller than right. */ 

    if((right_exponent = Sgl_exponent(right)) == 0)
        {
      /* Denormalized operands.  First look for zeroes */
      if(Sgl_iszero_mantissa(right)) 
          {
          /* right is zero */
          if(Sgl_iszero_exponentmantissa(left))
            {
            /* Both operands are zeros */
            Sgl_invert_sign(right);
            if(Is_rounding_mode(ROUNDMINUS))
                {
                Sgl_or_signs(left,/*with*/right);
                }
            else
                {
                Sgl_and_signs(left,/*with*/right);
                }
            }
          else 
            {
            /* Left is not a zero and must be the result.  Trapped
             * underflows are signaled if left is denormalized.  Result
             * is always exact. */
            if( (result_exponent == 0) && Is_underflowtrap_enabled() )
                {
                /* need to normalize results mantissa */
                sign_save = Sgl_signextendedsign(left);
                Sgl_leftshiftby1(left);
                Sgl_normalize(left,result_exponent);
                Sgl_set_sign(left,/*using*/sign_save);
                    Sgl_setwrapped_exponent(left,result_exponent,unfl);
                *dstptr = left;
                /* inexact = FALSE */
                return(UNDERFLOWEXCEPTION);
                }
            }
          *dstptr = left;
          return(NOEXCEPTION);
          }

      /* Neither are zeroes */
      Sgl_clear_sign(right);  /* Exponent is already cleared */
      if(result_exponent == 0 )
          {
          /* Both operands are denormalized.  The result must be exact
           * and is simply calculated.  A sum could become normalized and a
           * difference could cancel to a true zero. */
          if( (/*signed*/int) save >= 0 )
            {
            Sgl_subtract(left,/*minus*/right,/*into*/result);
            if(Sgl_iszero_mantissa(result))
                {
                if(Is_rounding_mode(ROUNDMINUS))
                  {
                  Sgl_setone_sign(result);
                  }
                else
                  {
                  Sgl_setzero_sign(result);
                  }
                *dstptr = result;
                return(NOEXCEPTION);
                }
            }
          else
            {
            Sgl_addition(left,right,/*into*/result);
            if(Sgl_isone_hidden(result))
                {
                *dstptr = result;
                return(NOEXCEPTION);
                }
            }
          if(Is_underflowtrap_enabled())
            {
            /* need to normalize result */
            sign_save = Sgl_signextendedsign(result);
            Sgl_leftshiftby1(result);
            Sgl_normalize(result,result_exponent);
            Sgl_set_sign(result,/*using*/sign_save);
                Sgl_setwrapped_exponent(result,result_exponent,unfl);
            *dstptr = result;
            /* inexact = FALSE */
            return(UNDERFLOWEXCEPTION);
            }
          *dstptr = result;
          return(NOEXCEPTION);
          }
      right_exponent = 1;     /* Set exponent to reflect different bias
                         * with denomalized numbers. */
      }
    else
      {
      Sgl_clear_signexponent_set_hidden(right);
      }
    Sgl_clear_exponent_set_hidden(left);
    diff_exponent = result_exponent - right_exponent;

    /* 
     * Special case alignment of operands that would force alignment 
     * beyond the extent of the extension.  A further optimization
     * could special case this but only reduces the path length for this
     * infrequent case.
     */
    if(diff_exponent > SGL_THRESHOLD)
      {
      diff_exponent = SGL_THRESHOLD;
      }
    
    /* Align right operand by shifting to right */
    Sgl_right_align(/*operand*/right,/*shifted by*/diff_exponent,
      /*and lower to*/extent);

    /* Treat sum and difference of the operands separately. */
    if( (/*signed*/int) save >= 0 )
      {
      /*
       * Difference of the two operands.  Their can be no overflow.  A
       * borrow can occur out of the hidden bit and force a post
       * normalization phase.
       */
      Sgl_subtract_withextension(left,/*minus*/right,/*with*/extent,/*into*/result);
      if(Sgl_iszero_hidden(result))
          {
          /* Handle normalization */
          /* A straight foward algorithm would now shift the result
           * and extension left until the hidden bit becomes one.  Not
           * all of the extension bits need participate in the shift.
           * Only the two most significant bits (round and guard) are
           * needed.  If only a single shift is needed then the guard
           * bit becomes a significant low order bit and the extension
           * must participate in the rounding.  If more than a single 
           * shift is needed, then all bits to the right of the guard 
           * bit are zeros, and the guard bit may or may not be zero. */
          sign_save = Sgl_signextendedsign(result);
            Sgl_leftshiftby1_withextent(result,extent,result);

            /* Need to check for a zero result.  The sign and exponent
           * fields have already been zeroed.  The more efficient test
           * of the full object can be used.
           */
          if(Sgl_iszero(result))
            /* Must have been "x-x" or "x+(-x)". */
            {
            if(Is_rounding_mode(ROUNDMINUS)) Sgl_setone_sign(result);
            *dstptr = result;
            return(NOEXCEPTION);
            }
          result_exponent--;
          /* Look to see if normalization is finished. */
          if(Sgl_isone_hidden(result))
            {
            if(result_exponent==0)
                {
                /* Denormalized, exponent should be zero.  Left operand *
                 * was normalized, so extent (guard, round) was zero    */
                goto underflow;
                }
            else
                {
                /* No further normalization is needed. */
                Sgl_set_sign(result,/*using*/sign_save);
                Ext_leftshiftby1(extent);
                goto round;
                }
            }

          /* Check for denormalized, exponent should be zero.  Left    *
           * operand was normalized, so extent (guard, round) was zero */
          if(!(underflowtrap = Is_underflowtrap_enabled()) &&
             result_exponent==0) goto underflow;

          /* Shift extension to complete one bit of normalization and
           * update exponent. */
          Ext_leftshiftby1(extent);

          /* Discover first one bit to determine shift amount.  Use a
           * modified binary search.  We have already shifted the result
           * one position right and still not found a one so the remainder
           * of the extension must be zero and simplifies rounding. */
          /* Scan bytes */
          while(Sgl_iszero_hiddenhigh7mantissa(result))
            {
            Sgl_leftshiftby8(result);
            if((result_exponent -= 8) <= 0  && !underflowtrap)
                goto underflow;
            }
          /* Now narrow it down to the nibble */
          if(Sgl_iszero_hiddenhigh3mantissa(result))
            {
            /* The lower nibble contains the normalizing one */
            Sgl_leftshiftby4(result);
            if((result_exponent -= 4) <= 0 && !underflowtrap)
                goto underflow;
            }
          /* Select case were first bit is set (already normalized)
           * otherwise select the proper shift. */
          if((jumpsize = Sgl_hiddenhigh3mantissa(result)) > 7)
            {
            /* Already normalized */
            if(result_exponent <= 0) goto underflow;
            Sgl_set_sign(result,/*using*/sign_save);
            Sgl_set_exponent(result,/*using*/result_exponent);
            *dstptr = result;
            return(NOEXCEPTION);
            }
          Sgl_sethigh4bits(result,/*using*/sign_save);
          switch(jumpsize) 
            {
            case 1:
                {
                Sgl_leftshiftby3(result);
                result_exponent -= 3;
                break;
                }
            case 2:
            case 3:
                {
                Sgl_leftshiftby2(result);
                result_exponent -= 2;
                break;
                }
            case 4:
            case 5:
            case 6:
            case 7:
                {
                Sgl_leftshiftby1(result);
                result_exponent -= 1;
                break;
                }
            }
          if(result_exponent > 0) 
            {
            Sgl_set_exponent(result,/*using*/result_exponent);
            *dstptr = result; /* Sign bit is already set */
            return(NOEXCEPTION);
            }
          /* Fixup potential underflows */
        underflow:
          if(Is_underflowtrap_enabled())
            {
            Sgl_set_sign(result,sign_save);
                Sgl_setwrapped_exponent(result,result_exponent,unfl);
            *dstptr = result;
            /* inexact = FALSE */
            return(UNDERFLOWEXCEPTION);
            }
          /*
           * Since we cannot get an inexact denormalized result,
           * we can now return.
           */
          Sgl_right_align(result,/*by*/(1-result_exponent),extent);
          Sgl_clear_signexponent(result);
          Sgl_set_sign(result,sign_save);
          *dstptr = result;
          return(NOEXCEPTION);
          } /* end if(hidden...)... */
      /* Fall through and round */
      } /* end if(save >= 0)... */
    else 
      {
      /* Add magnitudes */
      Sgl_addition(left,right,/*to*/result);
      if(Sgl_isone_hiddenoverflow(result))
          {
          /* Prenormalization required. */
          Sgl_rightshiftby1_withextent(result,extent,extent);
          Sgl_arithrightshiftby1(result);
          result_exponent++;
          } /* end if hiddenoverflow... */
      } /* end else ...sub magnitudes... */
    
    /* Round the result.  If the extension is all zeros,then the result is
     * exact.  Otherwise round in the correct direction.  No underflow is
     * possible. If a postnormalization is necessary, then the mantissa is
     * all zeros so no shift is needed. */
  round:
    if(Ext_isnotzero(extent))
      {
      inexact = TRUE;
      switch(Rounding_mode())
          {
          case ROUNDNEAREST: /* The default. */
          if(Ext_isone_sign(extent))
            {
            /* at least 1/2 ulp */
            if(Ext_isnotzero_lower(extent)  ||
              Sgl_isone_lowmantissa(result))
                {
                /* either exactly half way and odd or more than 1/2ulp */
                Sgl_increment(result);
                }
            }
          break;

          case ROUNDPLUS:
          if(Sgl_iszero_sign(result))
            {
            /* Round up positive results */
            Sgl_increment(result);
            }
          break;
          
          case ROUNDMINUS:
          if(Sgl_isone_sign(result))
            {
            /* Round down negative results */
            Sgl_increment(result);
            }
          
          case ROUNDZERO:;
          /* truncate is simple */
          } /* end switch... */
      if(Sgl_isone_hiddenoverflow(result)) result_exponent++;
      }
    if(result_exponent == SGL_INFINITY_EXPONENT)
        {
        /* Overflow */
        if(Is_overflowtrap_enabled())
          {
          Sgl_setwrapped_exponent(result,result_exponent,ovfl);
          *dstptr = result;
          if (inexact)
            if (Is_inexacttrap_enabled())
                return(OVERFLOWEXCEPTION | INEXACTEXCEPTION);
            else Set_inexactflag();
          return(OVERFLOWEXCEPTION);
          }
        else
          {
          Set_overflowflag();
          inexact = TRUE;
          Sgl_setoverflow(result);
          }
      }
    else Sgl_set_exponent(result,result_exponent);
    *dstptr = result;
    if(inexact) 
      if(Is_inexacttrap_enabled()) return(INEXACTEXCEPTION);
      else Set_inexactflag();
    return(NOEXCEPTION);
    }

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