[Libre-soc-isa] [Bug 1077] evaluate removing /vec234 from instructions, move subvl to SVSTATE

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=1077

--- Comment #6 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Jacob Lifshay from comment #5)
> 3. move subvl from the prefix to SVSTATE -- imho this renders subvl largely
> useless because GPU compilers are likely to need to follow option #2 due to
> the large number of setvl ops that would be required. This is also imho the
> worst option from the HW perspective since it needs to propagate extra state
> in the decode pipeline from setvl to instructions afterward because subvl
> would be constantly changing.

also this makes reserving registers even worse because it's now dynamic state
instead of easily predictable from the instruction encoding and doesn't even
eliminate the large register allocations possibly needed for subvl=4

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