[Libre-soc-isa] [Bug 535] setvl/setvli encoding & future reg file expansion

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=535

--- Comment #2 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
i took another look at RVV setvl/i and some example loops: a critical aspect of
setvl/i is that there is an RT and an RA.

* RA is the "number of times that we would *like* to run the hw-loop"
* RT contains a copy of VL that is set to the *actual* hw-loop quantity
* at the end of the loop, the actual amount can be subtracted.

loop:
    vsetvli a3, a0   #  update a3 with vl (# of elements this iteration)
    sub a0, a0, a3   # Decrement count by vl
    bnez a0, loop    # Any more?

if we do not have both RT and RA the "lack" of the same will require an
additional mr (register copy) instruction.

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