Re: [9fans] emulated fp on arm

[erik quanstrom <quanstro@labs.coraid.com>]

> As ieee754 (1985):
> 
>   When the sum of two operands with opposite signs (or the
>   difference of two operands with like signs) is exactly zero,
>   the sign of that sum (or difference) shall be + in all
>   rounding modes except round toward -INFINITY, in which mode
>   that sign shall be -.

due to the emulator interface, both cases call fpisub.
i've run a number of tests on this with relatively small
numbers that are equal.  and this seems to solve the
problems without creating new ones.

- erik

; diff -c fpi.c /sys/src/9/omap
fpi.c:137,143 - /sys/src/9/omap/fpi.c:137,142
  void
  fpisub(Internal *x, Internal *y, Internal *i)
  {
- 	int exact;
  	Internal *t;
  
  	if(y->e < x->e
fpi.c:158,164 - /sys/src/9/omap/fpi.c:157,162
  			SetInfinity(i);
  		return;
  	}
- 	exact = x->e == y->e;
  	matchexponents(x, y);
  	i->e = y->e;
  	i->h = y->h - x->h;
fpi.c:167,179 - /sys/src/9/omap/fpi.c:165,172
  		i->l += CarryBit;
  		i->h--;
  	}
- 	if(i->h == 0 && i->l == 0){
+ 	if(i->h == 0 && i->l == 0)
  		SetZero(i);
- 
- 		/* canceling rule */
- 		if(x->s != y->s && exact)
- 			i->s = 0;	/* 1 in round to -∞ mode */
- 	}
  	else while(i->e > 1 && (i->h & HiddenBit) == 0)
  		shift(i);
  }