Re: Cray style vectors

EricP <ThatWouldBeTelling@thevillage.com> writes:

Anton Ertl wrote:

DCell mmul (Cell a, Cell b) /* signed multiply, mixed precision */
{
  DCell res;
 
  res = UD2D(ummul (a, b));
  if (a < 0)
    res.hi -= b;
  if (b < 0)
    res.hi -= a;
  return res;
}
 
I have this technique from Andrew Haley.  It relies on twos-complement
representation.
 
- anton

>
Yeah, that's what Alpha does with UMULH.
I'm still trying to figure out why it works.

Let's consider the case where a>=0 and b<0, and cells are 64 bits.  ua
is a interpreted as unsigned cell, and ub is b interpreted as unsigned
cell.  The following computations are in Z (the unlimited
integers).  For the case under consideration:

ua=a
ub=b+2^64

res = ua*ub = a*(b+2^64)= a*b + a*2^64

So,

a*b = res - a*2^64

The other cases are similar.

- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>