[Libre-soc-isa] [Bug 213] SimpleV Standard writeup needed

bugzilla-daemon at libre-soc.org bugzilla-daemon at libre-soc.org
Tue Oct 20 15:45:08 BST 2020


--- Comment #82 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #51)

> 5. It's ok to deviate from how Power's scalar ISA does things when there's a
> better way.

just to come back to this, i wrote up the fundamental design principles at the
top here:

these are extremely important because, the opposite of the advantages is: if we
deviate too far from the idea of vectorising scalar OpenPOWER in a pure "block
repetition of scalar" form it not only becomes far too complex to implement,
OPF which has members who have far more experience than we do will reject it.

things like cutting out XER.SO (and its propagation into CRs) have always been
a massive performance problem for PowerISA implementations: Paul told me that
the only places it's really used as a result is in test suites!  therefore
removing XER.SO from the vectorisation is easily justifiable.

likewise we *might* be able to make a good case for putting CA into CRs (in
place of the SO bit) because this would allow massive chains of big integers to
be dropped into the OoO pipelines, to create 1024-long adds and beyond.

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