Re: Cray style vectors

Anton Ertl wrote:

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.

Nice!
Subtracting the results of having used the sign bit as part of the multiplication.
Here you can probably schedule the fixup to happen in parallel with the actual multiplication:
;; inputs in r9 & r10, result in rdx:rax, rbx & rcx as scratch
   mov rax,r9 ;; All these can start in the first cycle
   mul r10
   mov rbx,r9 ;; The MOV can be handled by the renamer
   sar r9,63
   mov rcx,r10 ;; Ditto
   sar r10,63
   and rbx,r9 ;; Second set of ops
   and rcx,r10
   add rbx,rcx ;; Third cycle
   sub rdx,rbx ;; Do a single adjustment as soon as the MUL finishes
Terje
--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"