Re: Cray style vectors

Terje Mathisen <terje.mathisen@tmsw.no> writes:

Tim Rentsch wrote:

Terje Mathisen <terje.mathisen@tmsw.no> writes:
 

If I really had to write a 64x64->128 MUL, with no widening MUL or
MULH which returns the high half, then I would punt and do it using
32-bit parts (all variables are u64):  [...]

 
I wrote some code along the same lines.  A difference is you
are considering unsigned multiplication, and I am considering
signed multiplication.
 

Signed mul is just a special case of unsigned mul, right?
>
I.e. in case of a signed widening mul, you'd first extract the signs,
convert the inputs to unsigned, then do the unsigned widening mul,
before finally resotirng the sign as the XOR of the input signs?

In Gforth we use:

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