[Libre-soc-dev] new svp64 page

[Lauri Kasanen]

On Thu, 10 Dec 2020 16:27:33 +0000
Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:

> lauri, jacob, what's your thoughts on using 2 bits for clamping mode?
> this is *not* the same as elwidth itself, which is the "chop" in VSX
> ops pseudocode.
>
> or: another idea:
>
> * extsb, extsh, extsw specify one type of width
> * twin predication specifies 2 more (src elwidth, dest elwidth)
> * 1 bit says "operation is to be clamped" (not to which range, that's implicit)

I can't come up with a use case for having different clamping to dst
elwidth. If you want 8-bit unsigned saturation, there's no reason for
you to write that to 16-bit elements. So I would take the clamp width
from the dst elwidth.

I would simply have two bits to enable clamping, unsigned and signed.
16 and 32 bit do need both, not just 8-bit.

- Lauri