flonum-mult.c

早期freebsd实现

/* flonum_multip.c - multiply two flonums
   Copyright (C) 1987 Free Software Foundation, Inc.

This file is part of Gas, the GNU Assembler.

The GNU assembler is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY.  No author or distributor
accepts responsibility to anyone for the consequences of using it
or for whether it serves any particular purpose or works at all,
unless he says so in writing.  Refer to the GNU Assembler General
Public License for full details.

Everyone is granted permission to copy, modify and redistribute
the GNU Assembler, but only under the conditions described in the
GNU Assembler General Public License.  A copy of this license is
supposed to have been given to you along with the GNU Assembler
so you can know your rights and responsibilities.  It should be
in a file named COPYING.  Among other things, the copyright
notice and this notice must be preserved on all copies.  */

#include "flonum.h"

/*	plan for a . b => p(roduct)


	+-------+-------+-/   /-+-------+-------+
	| a	| a	|  ...	| a	| a	|
	|  A	|  A-1	|	|  1	|  0	|
	+-------+-------+-/   /-+-------+-------+


	+-------+-------+-/   /-+-------+-------+
	| b	| b	|  ...	| b	| b	|
	|  B	|  B-1	|	|  1	|  0	|
	+-------+-------+-/   /-+-------+-------+


	+-------+-------+-/   /-+-------+-/   /-+-------+-------+
	| p	| p	|  ...	| p	|  ...	| p	| p	|
	|  A+B+1|  A+B	|	|  N	|	|  1	|  0	|
	+-------+-------+-/   /-+-------+-/   /-+-------+-------+

				   /^\
	 (carry) a .b	   ...	    |	   ...	 a .b	 a .b
		  A  B 		    |		  0  1	  0  0
				    |
			   ...	    |	   ...	 a .b
				    |		  1  0
				    |
				    |	   ...
				    |
				    |
				    |
				    |		  ___
				    |		  \
				    +-----  P  =   >  a .b
					     N	  /__  i  j

					N = 0 ... A+B

					for all i,j where i+j=N
					[i,j integers > 0]

a[], b[], p[] may not intersect.
Zero length factors signify 0 significant bits: treat as 0.0.
0.0 factors do the right thing.
Zero length product OK.

I chose the ForTran accent "foo[bar]" instead of the C accent "*garply"
because I felt the ForTran way was more intuitive. The C way would
probably yield better code on most C compilers. Dean Elsner.
(C style also gives deeper insight [to me] ... oh well ...)
*/

void
flonum_multip (a, b, product)
     FLONUM_TYPE *	a,
		 *	b,
		 *	product;
{
  int			size_of_a;		/* 0 origin */
  int			size_of_b;		/* 0 origin */
  int			size_of_product;	/* 0 origin */
  int			size_of_sum;		/* 0 origin */
  int			extra_product_positions;/* 1 origin */
  unsigned long int	work;
  unsigned long int	carry;
  long int		exponent;
  LITTLENUM_TYPE *	q;
  long int		significant;		/* TRUE when we emit a non-0 littlenum  */
				/* ForTran accent follows. */
  int			P;	/* Scan product low-order -> high. */
  int			N;	/* As in sum above.  */
  int			A;	/* Which [] of a? */
  int			B;	/* Which [] of b? */

  if((a->sign!='-' && a->sign!='+') || (b->sign!='-' && b->sign!='+')) {
    /* ...
    Got to fail somehow.  Any suggestions? */
    product->sign=0;
    return;
  }
  product -> sign = (a->sign == b->sign) ? '+' : '-';
  size_of_a		= a       -> leader	-  a       -> low;
  size_of_b		= b       -> leader	-  b       -> low;
  exponent		= a	  -> exponent	+  b	   -> exponent;
  size_of_product	= product -> high	-  product -> low;
  size_of_sum		= size_of_a		+  size_of_b;
  extra_product_positions  =  size_of_product  -  size_of_sum;
  if (extra_product_positions < 0)
    {
      P = extra_product_positions; /* P < 0 */
      exponent -= extra_product_positions; /* Increases exponent. */
    }
  else
    {
      P = 0;
    }
  carry = 0;
  significant = 0;
  for (N = 0;
       N <= size_of_sum;
       N++)
    {
      work = carry;
      carry = 0;
      for (A = 0;
	   A <= N;
	   A ++)
	{
	  B = N - A;
	  if (A <= size_of_a   &&   B <= size_of_b  &&  B >= 0)
	    {
#ifdef TRACE
printf("a:low[%d.]=%04x b:low[%d.]=%04x work_before=%08x\n", A, a->low[A], B, b->low[B], work);
#endif
	      work += a -> low [A]   *   b -> low [B];
	      carry += work >> LITTLENUM_NUMBER_OF_BITS;
	      work &= LITTLENUM_MASK;
#ifdef TRACE
printf("work=%08x carry=%04x\n", work, carry);
#endif
	    }
	}
      significant |= work;
      if (significant || P<0)
	{
	  if (P >= 0)
	    {
	      product -> low [P] = work;
#ifdef TRACE
printf("P=%d. work[p]:=%04x\n", P, work);
#endif
	    }
	  P ++;
	}
      else
	{
	  extra_product_positions ++;
	  exponent ++;
	}
    }
  /*
   * [P]-> position # size_of_sum + 1.
   * This is where 'carry' should go.
   */
#ifdef TRACE
printf("final carry =%04x\n", carry);
#endif
  if (carry)
    {
      if (extra_product_positions > 0)
	{
	  product -> low [P] = carry;
	}
      else
	{
	  /* No room at high order for carry littlenum. */
	  /* Shift right 1 to make room for most significant littlenum. */
	  exponent ++;
	  P --;
	  for (q  = product -> low + P;
	       q >= product -> low;
	       q --)
	    {
	      work = * q;
	      * q = carry;
	      carry = work;
	    }
	}
    }
  else
    {
      P --;
    }
  product -> leader	= product -> low + P;
  product -> exponent	= exponent;
}

/* end: flonum_multip.c */