[Libre-soc-dev] Simple-V svp64 draft spec

Luke Kenneth Casson Leighton lkcl at lkcl.net
Sat Jan 23 22:09:03 GMT 2021

we've a draft of the SVP64 encoding completed, which introduces not
just vector-predication for OpenPOWER (a feature entirely missing from
VSX and extremely challenging if not impractical to retro-fit), but a
whole stack of modern Vector features last seen in Cray supercomputers
(reintroduced with RVV and SVE2) and a whole stack of innovations not
seen before in any Vector ISA, ever.  these include:

* Twin-predication (effectively an ordered sequential multiple VINSERT)
* Predicate-result (turns every single arithmetic operation into a type of cmp)
* Data-dependent fail-on-first (SVE2 and RVV only have LD/ST fail-first)

i'll be speaking about this at FOSDEM2021 through a rapid-fire
"overview" which, if you'd like to read it in advance, it's here:

the actual encoding requires some context to understand (the overview, above):

i've kicked things off with a python program that can "understand"
SV-augmented assembly opcodes and turn them into an EXT001-plus-v3.0B
64-bit encoding:

we will be following a step-by-step implementation procedure which is
documented as-it-happens, here:

the idea being for that to become a page where other implementors can
obtain information and guidance about how to implement SV in a
step-by-step straightforward fashion.

this is a *massive* amount of work and a huge upgrade to OpenPOWER
which turns it into a modern-day 3D GPU/VPU ISA in ways that VSX
cannot be.  we are also adding features such as:

* Swizzle (10 to 30% of GPU Shader applications use vec2/3/4 Swizzle)
* transcendental opcodes planned (also absolutely critical for 3D GPUs)
* Galois Field and bit-manipulation opcodes suitable for cryptographic
  applications, Audio/Video encode and decode.
* REMAP capability (aka ARM NEON "structure packing") suitable also
  for Matrix Multiply with a single fma or madd instruction to perform up
  to a full 4x4 matrix multiply or Rijndael MixColums with one opcode.

and a stack more.  the "top-level" page which ties it all together is here:

the work is critically dependent on review, and approval by the OPF
ISA WG.  assistance in review, general questions, always appreciated.


crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68

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