[Libre-soc-isa] [Bug 1183] add /mrr mode (reverse mode) to Data-Dependent Fail-First CR_ops

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=1183

--- Comment #4 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #3)

> imo SNZ is quite low priority for simple and mapreduce modes since all it
> can do is set the output to 1,

no: it inverts the *predicate* bit, otherwise set to zero,
so that it is TRUE rather than false.

again i reiterate: see sv.bc

> it has no effect like in fail-first. 

this is not true.

> I think higher priority is supporting zz=0 

your opinion here is invalid as you have misunderstood SNZ.

> Another thing I realized while working on divmod, it would be really handy
> if stuff with a scalar destination would run through all elements instead of
> stopping at the first, I didn't check if we do that.

that's exactly what /mr does. jacob this has been in the spec for
well over 18 months, possibly as long as 2 years. i am a little
alarmed that you are suggesting things that have been in the spec
that long.

although i will need to evaluate below to check...

>  This is useful for not
> overwriting all the CRs that you could be storing some other variable in.
> 
> e.g.:
> sv.cmpi/ff=lt 0, 1, *10, 5
> is:
> i = 0
> while i < VL:
>     CR0 = cmpd(gpr[10 + i], 5)
>     if CR0.lt:
>         break
>     i += 1
> VL = i

yep that's ddffirst with mapreduce.

> note how only CR0 is ever written and yet the whole vector loop is run

that will - should - be sv.cmpi/ff=lt/mr
(this is another good unit test to have, will
 add it to the list)

/mr simply switches off the normal "terminate at first scalar"
(which with no predication is the first element).

mr can be *used* for reduction because a scalar may be used as
src and dest and therefore as an accumulator. but it has many
more uses.

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