[Libre-soc-dev] [RFC] svp64 "source zeroing" makes no sense

[Richard Wilbur]

On Sun, Mar 21, 2021 at 2:22 PM Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:
>
> On Sun, Mar 21, 2021 at 8:11 PM Richard Wilbur <richard.wilbur at gmail.com>
> wrote:
>
> > The original post made no mention of the file or the language
>
>
> soorryyy :)

I forgive you and thank you for supplying the context.

> ok so does it make more sense, now?

It definitely makes more sense now.  I still have some comments:

What is the maximum value of VL (or similarly, the maximum length of
the predication masks)?  I have in mind an optimization that avoids
iterating through the 0 bits of the predication masks with logic that
will generate the {src|dst}step pointing to the next non-zero bit in
the mask provided there is at least one bit set.

       if not src_zeroing:
            srcstep += skip_zeros(srcmask, srcstep, vl)
            if (srcstep == vl):  break

       if not dest_zeroing:
            dststep += skip_zeros(dstmask, dststep, vl)
            if (dststep == vl):  break

       if dest_zeroing and ((1<<dststep) & dstmask) == 0):
            result = 0
            Condition_Register = EQzero
       else:
            if src_zeroing and ((1<<srcstep) & srcmask) == 0:
                RA = 0
                RB = 0
            else:
                RA = get_register_RA
                RB = get_register_RB
            result, Condition_Register = calc_operation(RA, RB)

       store_result(result)
       if Rc=1: store_cr(Condition_Register)

1.  This changes the loops that skip zeros in predication masks into
an update of {src|dst}step by a fast logic block represented by
skip_zeros().
2.  Here I have compared 1<<dststep to dstmask instead of srcmask.
3.  It seems likely that we would like to perform the operation with
src arguments 0 if the flag src_zeroing is set and the srcmask
specifies.
4.  If there are un-enumerated side effects that we wish to reproduce
from calc_operation() (aside from [result, Condition_Register]) then
we will likely want to perform the operation even though we are
zeroing the destination.