[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 #5 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Jacob Lifshay from comment #2)
> imho it is not sensible to remove subvl from the prefix because GPU code
> often has subvl different in each instruction
> 
> All of that is unless we do what some other GPU compilers do,
> <snip>essentially
> removing subvl completely from generated code.

because of that, I think we have 3 valid choices:
1. leave subvl as-is

2. remove subvl completely since it's mostly only useful for GPU code and we
can do what other GPU compilers do to not need it at all (see the second
paragraph of comment #2)

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.

I prefer either 1 or 2.

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