Obligatory Dilbert:

Links:

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


Scalable Vectors for the Power ISA

SV is designed as a strict RISC-paradigm Scalable Vector ISA for Hybrid 3D CPU GPU VPU workloads. As such it brings features normally only found in Cray Supercomputers (Cray-1, NEC SX-Aurora) and in GPUs, but keeps strictly to a Simple RISC principle of leveraging a Scalar ISA, exclusively using "Prefixing". Not one single actual explicit Vector opcode exists in SV, at all. It is suitable for low-power Embedded and DSP Workloads as much as it is for power-efficient Supercomputing.

Fundamental design principles:

  • Taking the simplicity of the RISC paradigm and applying it strictly and uniformly to create a Scalable Vector 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")
  • Specifically designed to be Precise-Interruptible at all times (many Vector ISAs have operations which, due to higher internal accuracy or other complexity, must be effectively atomic only for the full Vector operation's duration, adversely affecting interrupt response latency, or be abandoned and started again)
  • Augments ("tags") existing instructions, providing Vectorisation "context" rather than adding new instructions.
  • Strictly does not interfere with or alter the non-Scalable Power ISA in any way
  • In the Prefix space, does not modify or deviate from the underlying scalar Power ISA unless it provides significant performance or other advantage to do so in the Vector space (dropping the "sticky" characteristics of XER.SO and CR0.SO for example)
  • Designed for Supercomputing: avoids creating significant sequential dependency hazards, allowing standard high performance superscalar multi-issue micro-architectures to be leveraged.
  • Divided into Compliancy Levels to reduce cost of implementation for specific needs.

Advantages of these design principles:

  • Simplicity of introduction and implementation on top of the existing Power ISA without disruption.
  • It is therefore easy to create a first (and sometimes only) implementation as literally a for-loop in hardware, simulators, and compilers.
  • Hardware Architects may understand and implement SV as being an extra pipeline stage, inserted between decode and issue, that is a simple for-loop issuing element-level sub-instructions.
  • More complex HDL can be done by repeating existing scalar ALUs and pipelines as blocks and leveraging existing Multi-Issue Infrastructure
  • As (mostly) a high-level "context" that does not (significantly) deviate from scalar Power 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.

Comparative instruction count:

  • ARM NEON SIMD: around 2,000 instructions, prerequisite: ARM Scalar.
  • ARM SVE: around 4,000 instructions, prerequisite: NEON and ARM Scalar
  • ARM SVE2: around 1,000 instructions, prerequisite: SVE, NEON, and ARM Scalar
  • Intel AVX-512: around 4,000 instructions, prerequisite AVX, AVX2, AVX-128 and AVX-256 which in turn critically rely on the rest of x86, for a grand total of well over 10,000 instructions.
  • RISV-V RVV: 192 instructions, prerequisite 96 Scalar RV64GC instructions
  • SVP64: six instructions, two of which are in the same space (svshape, svshape2), with 24-bit prefixing of prerequisite SFS (150) or SFFS (214) Compliancy Subsets. There are no dedicated Vector instructions, only Scalar-prefixed.

Comparative Basic Design Principle:

  • ARM NEON and VSX: PackedSIMD. No instruction-overloaded meaning (every instruction is unique for a given register bitwidth, guaranteeing binary interoperability)
  • Intel AVX-512 (and below): Hybrid Packed-Predicated SIMD with no instruction-overloading, guaranteeing binary interoperability but at the same time penalising the ISA with runaway opcode proliferation.
  • ARM SVE/SVE2: Hybrid Packed-Predicated SIMD with instruction-overloading that destroys binary interoperability. This is hidden behind the misuse of the word "Scalable" and is permitted under License by "Silicon Partners".
  • RISC-V RVV: Cray-style Scalable Vector but with instruction-overloading permitted by the specification that destroys binary interoperability.
  • SVP64: Cray-style Scalable Vector with no instruction-overloaded meanings. The regfile numbers and bitwidths shall not change in a future revision (for the same instruction encoding): "Silicon Partner" Scaling is prohibited, in order to guarantee binary interoperability. Future revisions of SVP64 may extend VSX instructions to achieve larger regfiles, and non-interoperability on the same will likewise be prohibited.

SV comprises several compliancy levels suited to Embedded, Energy efficient High-Performance Compute, Distributed Computing and Advanced Computational Supercomputing. The Compliancy Levels are arranged such that even at the bare minimum Level, full Soft-Emulation of all optional and future features is possible.

Sub-pages

Pages being developed and examples

SVP64 "Modes":

  • For condition register operations see cr ops - SVP64 Condition Register ops: Guidelines on Vectorisation of any v3.0B base operations which return or modify a Condition Register bit or field.
  • For LD/ST Modes, see ldst.
  • For Branch modes, see branches - SVP64 Conditional Branch behaviour: All/Some Vector CRs
  • For arithmetic and logical, see normal
  • mv.vec pack/unpack move to and from vec2/3/4, actually an RM.EXTRA Mode and a remap mode

Core SVP64 instructions:

  • setvl the Cray-style "Vector Length" instruction
  • svremap, svindex and svshape: part of remap "Remapping" for Matrix Multiply, DCT/FFT and RGB-style "Structure Packing" as well as general-purpose Indexing. Also describes associated SPRs.
  • svstep Key stepping instruction, primarily for Vertical-First Mode and also providing traditional "Vector Iota" capability.

Please note: there are only six instructions in the whole of SV. Beyond this point are additional Scalar instructions related to specific workloads that have nothing to do with the SV Specification

Stability Guarantees in Simple-V

Providing long-term stability in an ISA is extremely challenging but critically important. It requires certain guarantees to be provided.

  • Firstly: that instructions will never be ambiguously-defined.
  • Secondly, that no instruction shall change meaning to produce different results on different hardware (present or future).
  • Thirdly, that Scalar "defined words" (32 bit instruction encodings) if Vwctorised will also always be implemented as identical Scalar instructions (the sole semi-exception being Vevtorised Branch-Conditional)
  • Fourthly, that implementors are not permitted to either add arbitrary features nor implement features in an incompatible way. (Performance may differ, but differing results are not permitted).
  • Fifthly, that any part of Simple-V not implemented by a lower Compliancy Level is required to raise an illegal instruction trap (allowing soft-emulation), including if Simple-V is not implemented at all.
  • Sixthly, that any UNDEFINED behaviour for practical implementation reasons is clearly documented for both programmers and hardware implementors.

In particular, given the strong recent emphasis and interest in "Scalable Vector" ISAs, it is most unfortunate that both ARM SVE and RISC-V RVV permit the exact same instruction to produce different results on different hardware depending on a "Silicon Partner" hardware choice. This choice catastrophically and irrevocably causes binary non-interoperability despite being a "feature". Explained in https://m.youtube.com/watch?v=HNEm8zmkjBU it is the exact same binary-incompatibility issue faced by Power ISA on its 32- to 64-bit transition: 32-bit hardware was unable to trap-and-emulate 64-bit binaries because the opcodes were (are) the same.

It is therefore guaranteed that extensions to the register file width and quantity in Simple-V shall only be made in future by explicit means, ensuring binary compatibility.

Optional Scalar instructions

Additional Instructions for specific purposes (not SVP64)

All of these instructions below have nothing to do with SV. They are all entirely designed as Scalar instructions that, as Scalar instructions, stand on their own merit. Considerable lengths have been made to provide justifications for each of these Scalar instructions in a Scalar context, completely independently of SVP64.

Some of these Scalar instructions happen also designed to make Scalable Vector binaries more efficient, such as the crweird group. Others are to bring the Scalar Power ISA up-to-date within specific workloads, such as a Javascript Rounding instruction (which saves 35 instructions including 5 branches). None of them are strictly necessary but performance and power consumption may be (or, is already) compromised in certain workloads and use-cases without them.

Vector-related but still Scalar:

  • mv.swizzle vec2/3/4 Swizzles (RGBA, XYZW) for 3D and CUDA. designed as a Scalar instruction.
  • vector ops scalar operations needed for supporting vectors
  • cr int predication scalar instructions needed for effective predication

Stand-alone Scalar Instructions:

Twin targetted instructions (two registers out, one implicit, just like Load-with-Update).

Explanation of the rules for twin register targets (implicit RS, FRS) explained in SVP64 appendix

Architectural Note

This section is primarily for the ISA Working Group and for IBM in their capacity and responsibility for allocating "Architectural Resources" (opcodes), but it is also useful for general understanding of Simple-V.

Simple-V is effectively a type of "Zero-Overhead Loop Control" to which an entire 24 bits are exclusively dedicated in a fully RISC-abstracted manner. Within those 24-bits there are no Scalar instructions, and no Vector instructions: there is only "Loop Control".

This is why there are no actual Vector operations in Simple-V: all suitable Scalar Operations are Vectorised or not at all. This has some extremely important implications when considering adding new instructions, and especially when allocating the Opcode Space for them. To protect SVP64 from damage, a "Hard Rule" has to be set:

 Scalar Instructions must be simultaneously added in the corresponding
 SVP64 opcode space with the exact same 32-bit "Defined Word" or they
 must not be added at all.  Likewise, instructions planned for addition
 in what is considered (wrongly) to be the exclusive "Vector" domain
 must correspondingly be added in the Scalar space with the exact same
 32-bit "Defined Word", or they must not be added at all.

Some explanation of the above is needed. Firstly, "Defined Word" is a term used in Section 1.6.3 of the Power ISA v3 1 Book I: it means, in short, "a 32 bit instruction", which can then be Prefixed by EXT001 to extend it to 64-bit (named EXT100-163). Prefixed-Prefixed (96-bit Variable-Length) encodings are prohibited in v3.1 and they are just as prohibited in Simple-V: it's too complex in hardware. This means that only 32-bit "Defined Words" may be Vectorised, and in particular it means that no 64-bit instruction (EXT100-163) may ever be Vectorised.

Secondly, the term "Vectoriseable" was used. This refers to "instructions which if SVP64-Prefixed are actually meaningful". sc is meaningless to Vectorise, for example, as is sync and mtmsr (there is only ever going to be one MSR).

The problem comes if the rationale is applied, "if unused, Unvectoriseable opcodes can therefore be allocated to alternative instructions mixing inside the SVP64 Opcode space", which unfortunately results in huge inadviseable complexity in HDL at the Decode Phase, attempting to discern between the two types. Worse than that, if the alternate 64-bit instruction is Vectoriseable but the 32-bit Scalar "Defined Word" is already allocated, how can there ever be a Scalar version of the alternate instruction? It would have to be added as a completely different 32-bit "Defined Word", and things go rapidly downhill in the Decoder as well as the ISA from there.

Therefore to avoid risk and long-term damage to the Power ISA:

  • even Unvectoriseable "Defined Words" (mtmsr) must have the corresponding SVP64 Prefixed Space RESERVED, permanently requiring Illegal Instruction to be raised (the 64-bit encoding corresponding to an illegal sv.mtmsr if ever incorrectly attempted must be defined to raise an Exception)
  • Even instructions that may not be Scalar (although for various practical reasons this is extremely rare if not impossible, if not just generally "strongly discouraged") which have no meaning or use as a 32-bit Scalar "Defined Word", must still have the Scalar "Defined Word" RESERVED in the scalar opcode space, as an Illegal Instruction.

A good example of the former is mtmsr because there is only one MSR register (sv.mtmsr is meaningless, as is sv.sc), and a good example of the latter is mv.x which is so deeply problematic to add to any Scalar ISA that it was rejected outright and an alternative route taken (Indexed REMAP).

Another good example would be Cross Product which has no meaning at all in a Scalar ISA (Cross Product as a concept only applies to Mathematical Vectors). If any such Vector operation were ever added, it would be critically important to reserve the exact same Scalar opcode with the exact same "Defined Word" in the Scalar Power ISA opcode space, as an Illegal Instruction. There are good reasons why Cross Product has not been proposed, but it serves to illustrate the point as far as Architectural Resource Allocation is concerned.

Bottom line is that whilst this seems wasteful the alternatives are a destabilisation of the Power ISA and impractically-complex Hardware Decoders. With the Scalar Power ISA (v3.0, v3.1) already being comprehensive in the number of instructions, keeping further Decode complexity down is a high priority.

Other Scalable Vector ISAs

These Scalable Vector ISAs are listed to aid in understanding and context of what is involved.

A comprehensive list of 3D GPU, Packed SIMD, Predicated-SIMD and true Scalable Vector ISAs may be found at the vector isa comparison page. Note: AVX-512 and SVE2 are not Vector ISAs, they are Predicated-SIMD. Public discussions have taken place at Conferences attended by both Intel and ARM on adding a setvl instruction which would easily make both AVX-512 and SVE2 truly "Scalable". comparison table in tabular form.

Major opcodes summary

Simple-V itself only requires six instructions with 6-bit Minor XO (bits 26-31), and the SVP64 Prefix Encoding requires 25% space of the EXT001 Major Opcode. There are no Vector Instructions and consequently no further opcode space is required. Even though they are currently placed in the EXT022 Sandbox, the "Management" instructions (setvl, svstep, svremap, svshape, svindex) are designed to fit cleanly into EXT019 (exactly like addpcis) or other 5/6-bit Minor XO area (bits 25-31) that has space for Rc=1.

That said: for the target workloads for which Scalable Vectors are typically used, the Scalar ISA on which those workloads critically rely is somewhat anaemic. The Libre-SOC Team has therefore been addressing that by developing a number of Scalar instructions in specialist areas (Big Integer, Cryptography, 3D, Audio/Video, DSP) and it is these which require considerable Scalar opcode space.

Please be advised that even though SV is entirely DRAFT status, there is considerable concern that because there is not yet any two-way day-to-day communication established with the OPF ISA WG, we have no idea if any of these are conflicting with future plans by any OPF Members. The External ISA WG RFC Process has now been ratified but Libre-SOC may not join the OPF as an entity because it does not exist except in name. Even if it existed it would be a conflict of interest to join the OPF, due to our funding remit from NLnet. We therefore proceed on the basis of making public the intention to submit RFCs once the External ISA WG RFC Process is in place and, in a wholly unsatisfactory manner have to hope and trust that OPF ISA WG Members are reading this and take it into consideration.

Scalar Summary

As in above sections, it is emphasised strongly that Simple-V in no way critically depends on the 100 or so Scalar instructions also being developed by Libre-SOC.

None of these Draft opcodes are intended for private custom secret proprietary usage. They are all intended for entirely public, upstream, high-profile mass-volume day-to-day usage at the same level as add, popcnt and fld

  • bitmanip requires two major opcodes (due to 16+ bit immediates) those are currently EXT022 and EXT05.
  • brownfield encoding in one of those two major opcodes still requires multiple VA-Form operations (in greater numbers than EXT04 has spare)
  • space in EXT019 next to addpcis and crops is recommended (or any other 5-6 bit Minor XO areas)
  • many X-Form opcodes currently in EXT022 have no preference for a location at all, and may be moved to EXT059, EXT019, EXT031 or other much more suitable location.
  • even if ratified and even if the majority (mostly X-Form) is moved to other locations, the large immediate sizes of the remaining bitmanip instructions means it would be highly likely these remaining instructions would need two major opcodes. Fortuitously the v3.1 Spec states that both EXT005 and EXT009 are available.

Additional observations

Note that there is no Sandbox allocation in the published ISA Spec for v3.1 EXT01 usage, and because SVP64 is already 64-bit Prefixed, Prefixed-Prefixed-instructions (SVP64 Prefixed v3.1 Prefixed) would become a whopping 96-bit long instruction. Avoiding this situation is a high priority which in turn by necessity puts pressure on the 32-bit Major Opcode space.

SVP64 itself is already under pressure, being only 24 bits. If it is not permitted to take up 25% of EXT001 then it would have to be proposed in its own Major Opcode, which on first consideration would be beneficial for SVP64 due to the availability of 2 extra bits. However when combined with the bitmanip scalar instructions requiring two Major opcodes this would come to a grand total of 3 precious Major opcodes. On balance, then, sacrificing 25% of EXT001 is the "least difficult" choice. Alternative locations for SVP64 Prefixing include EXT006 and EXT017, with EXT006 being most favourable as there is room for future expansion.

Note also that EXT022, the Official Architectural Sandbox area available for "Custom non-approved purposes" according to the Power ISA Spec, is under severe design pressure as it is insufficient to hold the full extent of the instruction additions required to create a Hybrid 3D CPU-VPU-GPU. Although the wording of the Power ISA Specification leaves open the possibility of not needing to propose ISA Extensions to the ISA WG, it is clear that EXT022 is an inappropriate location for a large high-profile Extension intended for mass-volume product deployment. Every in-good-faith effort will therefore be made to work with the OPF ISA WG to submit SVP64 via the External RFC Process.

Whilst SVP64 is only 6 instructions the heavy focus on VSX for the past 12 years has left the SFFS Level anaemic and out-of-date compared to ARM and x86. This is very much a blessing, as the Scalar ISA has remained clean, making it highly suited to RISC-paradigm Scalable Vector Prefixing. Approximately 100 additional (optional) Scalar Instructions are up for proposal to bring SFFS up-to-date. None of them require or depend on PackedSIMD VSX (or VMX).

Other

Examples experiments future ideas discussion:

Additional links: