Simple-V Vectorisation for the OpenPOWER ISA

SV is in DRAFT STATUS. SV has not yet been submitted to the OpenPOWER Foundation ISA WG for review.

https://bugs.libre-soc.org/show_bug.cgi?id=213

Fundamental design principles:

• Simplicity of introduction and implementation on the existing OpenPOWER ISA
• Effectively a hardware for-loop, pausing PC, issuing multiple scalar operations
• Preserving the underlying scalar execution dependencies as if the for-loop had been expanded as actual scalar instructions (termed "preserving Program Order")
• Augments ("tags") existing instructions, providing Vectorisation "context" rather than adding new ones.
• Does not modify or deviate from the underlying scalar OpenPOWER ISA unless it provides significant performance or other advantage to do so in the Vector space (dropping XER.SO and OE=1 for example)

Advantages of these design principles:

• It is therefore easy to create a first (and sometimes only) implementation as literally a for-loop in hardware, simulators, and compilers.
• More complex HDL can be done by repeating existing scalar ALUs and pipelines as blocks.
• As (mostly) a high-level "context" that does not (significantly) deviate from scalar OpenPOWER ISA and, in its purest form being "a for loop around scalar instructions", it is minimally-disruptive and consequently stands a reasonable chance of broad community adoption and acceptance
• Completely wipes not just SIMD opcode proliferation off the map (SIMD is O(N⁶) opcode proliferation) but off of Vectorisation ISAs as well. No more separate Vector instructions.

Pages being developed and examples

• overview explaining the basics.
• implementation implementation planning and coordination
• svp64 contains the packet-format only
• setvl the Cray-style "Vector Length" instruction
• predication discussion on predication concepts
• cr int predication instructions needed for effective predication
• masked vector chaining
• discussion
• example dep matrices
• major opcode allocation
• opcode regs deduped
• vector swizzle
• mv.swizzle
• mv.x
• fcvt FP Conversion (due to OpenPOWER Scalar FP32)
• fclass detect class of FP numbers
• int fp mv Move and convert GPR <-> FPR, needed for !VSX
• mv.vec move to and from vec2/3/4
• 16 bit compressed experimental
• toc data pointer experimental
• ldst Load and Store
• sprs SPRs
• bitmanip
• remap "Remapping" for Matrix Multiply and RGB "Structure Packing"
• propagation Context propagation including svp64, swizzle and remap
• vector ops Vector ops needed to make a "complete" Vector ISA
• av opcodes scalar opcodes for Audio/Video
• byteswap
• TODO: OpenPOWER transcendentals

Additional links:

• https://www.sigarch.org/simd-instructions-considered-harmful/
• simple v extension old (deprecated) version
• llvm