Re: 64-bit subtract from vector unsigned int
On Tue, Apr 7, 2020 at 8:27 AM Lennart Sorensen
<lsorense@csclub.uwaterloo.ca> wrote:
>
> On Tue, Apr 07, 2020 at 05:51:54AM -0400, Jeffrey Walton wrote:
> > Hi Everyone,
> >
> > I'm porting a 64-bit algorithm to 32-bit PowerPC (an old PowerMac).
> > The algorithm is simple when 64-bit is available, but it gets a little
> > ugly under 32-bit.
> >
> > PowerPC has a "Vector Subtract Carryout Unsigned Word" (vsubcuw),
> > https://www.nxp.com/docs/en/reference-manual/ALTIVECPEM.pdf. The
> > altivec intrinsics are vec_vsubcuw and vec_subc.
> >
> > The problem is, I don't know how to use it. I've been experimenting
> > with it but I don't see the use (yet).
> >
> > How does one use vsubcuw to implement a subtract with borrow?
>
> Does your 32 bit powerpc have altivec? A lot do not. It is certainly
> not a universal feature. As far as I remember, G4 and G5 powermacs have
> it, but nothing older.
Yes, this is an old PowerMac G4 with Power4. It has a Altivec unit,
but it is only 32-bit. Add, subtract, shift and rotate (and friends)
on 64-bit values are missing.
As old as the hardware is (circa 2000), that old PowerPC chip
outperforms some modern hardware, like Atoms, Celerons and low-end ARM
cpu's in modern gadgets.
Testing some algorithms, like Simon-128 and Speck-128, show a need for
Altivec. For example, Integer-based Speck-128 was running at about 70
cpb. Altivec-based Speck-128 dropped to 10 cpb even with me doing all
the 64-bit fixups. (Speck-128 runs around 2.5 cpb when the native
hardware supports 64-bit operations, like on Power8).
Jeff
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