[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 #7 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #5)

> SNZ still has no effect in simple and map-reduce modes, because SNZ is only
> relevant when there is a test that occurs across elements, rather than only
> within each element.
> 
> please write out a loop of what you think SNZ should do in simple mode. I
> expect that you will find it to be entirely redundant.

it's not. i leave you to write it out. remember i am at total burn-out.
confrontational approaches like this are just dangerous, now.
anyone asks me to do *anything* the automatic answer is "100 i will not".
however if *you* do it i will gladly walk through it and answer questions.

it also does not help that i am having difficulty recalling tests
and spec pages i wrote 6-18 months ago.

> > > 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. 
> 
> I know that, the problem is /mr can not be set at the same time as
> fail-first because they are options in a 2-bit enum-like field

arse, you're right. this stuff has dropped out of my electrical
memory.

 (you can pick
> *only one* of simple, map-reduce, or fail-first). so either fail-first
> implies /mr behavior or not. I think it should imply /mr behavior.

let me think about it. can you please also walk through the implications
for if anyone would *want* scalar destination and terminate at first?
both HF and VF.

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