/*
   * 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 straightforward 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);
      }