add Stride LOAD example and encoding

[libreriscv.git] / simple_v_extension.mdwn

# Variable-width Variable-packed SIMD / Simple-V / Parallelism Extension Proposal

This proposal exists so as to be able to satisfy several disparate
requirements: power-conscious, area-conscious, and performance-conscious
designs all pull an ISA and its implementation in different conflicting
directions, as do the specific intended uses for any given implementation.

Additionally, the existing P (SIMD) proposal and the V (Vector) proposals,
whilst each extremely powerful in their own right and clearly desirable,
are also:

* Clearly independent in their origins (Cray and AndeStar v3 respectively)
  so need work to adapt to the RISC-V ethos and paradigm
* Are sufficiently large so as to make adoption (and exploration for
  analysis and review purposes) prohibitively expensive
* Both contain partial duplication of pre-existing RISC-V instructions
  (an undesirable characteristic)
* Both have independent and disparate methods for introducing parallelism
  at the instruction level.
* Both require that their respective parallelism paradigm be implemented
  along-side and integral to their respective functionality *or not at all*.
* Both independently have methods for introducing parallelism that
  could, if separated, benefit
  *other areas of RISC-V not just DSP or Floating-point respectively*.

Therefore it makes a huge amount of sense to have a means and method
of introducing instruction parallelism in a flexible way that provides
implementors with the option to choose exactly where they wish to offer
performance improvements and where they wish to optimise for power
and/or area (and if that can be offered even on a per-operation basis that
would provide even more flexibility).

Additionally it makes sense to *split out* the parallelism inherent within
each of P and V, and to see if each of P and V then, in *combination* with
a "best-of-both" parallelism extension, would work well.

**TODO**: reword this to better suit this document:

Having looked at both P and V as they stand, they're _both_ very much
"separate engines" that, despite both their respective merits and
extremely powerful features, don't really cleanly fit into the RV design
ethos (or the flexible extensibility) and, as such, are both in danger
of not being widely adopted.  I'm inclined towards recommending:

* splitting out the DSP aspects of P-SIMD to create a single-issue DSP
* splitting out the polymorphism, esoteric data types (GF, complex
  numbers) and unusual operations of V to create a single-issue "Esoteric
  Floating-Point" extension
* splitting out the loop-aspects, vector aspects and data-width aspects
  of both P and V to a *new* "P-SIMD / Simple-V" and requiring that they
  apply across *all* Extensions, whether those be DSP, M, Base, V, P -
  everything.

**TODO**: propose overflow registers be actually one of the integer regs
(flowing to multiple regs).

**TODO**: propose "mask" (predication) registers likewise.  combination with
standard RV instructions and overflow registers extremely powerful

## Stride

**TODO**: propose two LOAD/STORE offset CSRs, which mark a particular
register as being "if you use this reg in LOAD/STORE, use the offset
amount CSRoffsN (N=0,1) instead of treating LOAD/STORE as contiguous".
can be used for matrix spanning.

> For LOAD/STORE, could a better option be to interpret the offset in the 
> opcode as a stride instead, so "LOAD t3, 12(t2)" would, if t3 is 
> configured as a length-4 vector base, result in t3 = *t2, t4 = *(t2+12), 
> t5 = *(t2+24), t6 = *(t2+32)?  Perhaps include a bit in the 
> vector-control CSRs to select between offset-as-stride and unit-stride 
> memory accesses? 

So there would be an instruction like this:

| SETOFF | On=rN | OBank={float|int} | Smode={offs|unit} | OFFn=rM |
| opcode | 5 bit | 1 bit             | 1 bit             | 5 bit, OFFn=XLEN |


which would mean:

* CSR-Offset register n <= (float|int) register number N
* CSR-Offset Stride-mode = offset or unit
* CSR-Offset amount register n = contents of register M

LOAD rN, ldoffs(rM) would then be (assuming packed bit-width not set):

> offs = 0
> stride = 1
> vector-len = CSR-Vector-length register N
>
> for (o = 0, o < 2, o++)
>   if (CSR-Offset register o == M)
>       offs = CSR-Offset amount register o
>       if CSR-Offset Stride-mode == offset:
>           stride = ldoffs
>       break
>
> for (i = 0, i < vector-len; i++)
>   r[N+i] = mem[(offs*i + r[M+i])*stride]

# Analysis and discussion of Vector vs SIMD

There are four combined areas between the two proposals that help with
parallelism without over-burdening the ISA with a huge proliferation of
instructions:

* Fixed vs variable parallelism (fixed or variable "M" in SIMD)
* Implicit vs fixed instruction bit-width (integral to instruction or not)
* Implicit vs explicit type-conversion (compounded on bit-width)
* Implicit vs explicit inner loops.
* Masks / tagging (selecting/preventing certain indexed elements from execution)

The pros and cons of each are discussed and analysed below.

## Fixed vs variable parallelism length

In David Patterson and Andrew Waterman's analysis of SIMD and Vector
ISAs, the analysis comes out clearly in favour of (effectively) variable
length SIMD.  As SIMD is a fixed width, typically 4, 8 or in extreme cases
16 or 32 simultaneous operations, the setup, teardown and corner-cases of SIMD
are extremely burdensome except for applications whose requirements
*specifically* match the *precise and exact* depth of the SIMD engine.

Thus, SIMD, no matter what width is chosen, is never going to be acceptable
for general-purpose computation, and in the context of developing a
general-purpose ISA, is never going to satisfy 100 percent of implementors.

That basically leaves "variable-length vector" as the clear *general-purpose*
winner, at least in terms of greatly simplifying the instruction set,
reducing the number of instructions required for any given task, and thus
reducing power consumption for the same.

## Implicit vs fixed instruction bit-width

SIMD again has a severe disadvantage here, over Vector: huge proliferation
of specialist instructions that target 8-bit, 16-bit, 32-bit, 64-bit, and
have to then have operations *for each and between each*.  It gets very
messy, very quickly.

The V-Extension on the other hand proposes to set the bit-width of
future instructions on a per-register basis, such that subsequent instructions
involving that register are *implicitly* of that particular bit-width until
otherwise changed or reset.

This has some extremely useful properties, without being particularly
burdensome to implementations, given that instruction decode already has
to direct the operation to a correctly-sized width ALU engine, anyway.

Not least: in places where an ISA was previously constrained (due for
whatever reason, including limitations of the available operand spcace),
implicit bit-width allows the meaning of certain operations to be
type-overloaded *without* pollution or alteration of frozen and immutable
instructions, in a fully backwards-compatible fashion.

## Implicit and explicit type-conversion

The Draft 2.3 V-extension proposal has (deprecated) polymorphism to help
deal with over-population of instructions, such that type-casting from
integer (and floating point) of various sizes is automatically inferred
due to "type tagging" that is set with a special instruction.  A register
will be *specifically* marked as "16-bit Floating-Point" and, if added
to an operand that is specifically tagged as "32-bit Integer" an implicit
type-conversion will take placce *without* requiring that type-conversion
to be explicitly done with its own separate instruction.

However, implicit type-conversion is not only quite burdensome to
implement (explosion of inferred type-to-type conversion) but also is
never really going to be complete.  It gets even worse when bit-widths
also have to be taken into consideration.

Overall, type-conversion is generally best to leave to explicit
type-conversion instructions, or in definite specific use-cases left to
be part of an actual instruction (DSP or FP)

## Zero-overhead loops vs explicit loops

The initial Draft P-SIMD Proposal by Chuanhua Chang of Andes Technology
contains an extremely interesting feature: zero-overhead loops.  This
proposal would basically allow an inner loop of instructions to be
repeated indefinitely, a fixed number of times.

Its specific advantage over explicit loops is that the pipeline in a
DSP can potentially be kept completely full *even in an in-order
implementation*.  Normally, it requires a superscalar architecture and
out-of-order execution capabilities to "pre-process" instructions in order
to keep ALU pipelines 100% occupied.

This very simple proposal offers a way to increase pipeline activity in the
one key area which really matters: the inner loop.

## Mask and Tagging

*TODO: research masks as they can be superb and extremely powerful.
If B-Extension is implemented and provides Bit-Gather-Scatter it
becomes really cool and easy to switch out certain indexed values
from an array of data, but actually BGS **on its own** might be
sufficient.  Bottom line, this is complex, and needs a proper analysis.
The other sections are pretty straightforward.*

## Conclusions

In the above sections the four different ways where parallel instruction
execution has closely and loosely inter-related implications for the ISA and
for implementors, were outlined.  The pluses and minuses came out as
follows:

* Fixed vs variable parallelism: <b>variable</b>
* Implicit (indirect) vs fixed (integral) instruction bit-width: <b>indirect</b>
* Implicit vs explicit type-conversion: <b>explicit</b>
* Implicit vs explicit inner loops: <b>implicit</b>
* Tag or no-tag: <b>TODO</b>

In particular: variable-length vectors came out on top because of the
high setup, teardown and corner-cases associated with the fixed width
of SIMD.  Implicit bit-width helps to extend the ISA to escape from
former limitations and restrictions (in a backwards-compatible fashion),
and implicit (zero-overhead) loops provide a means to keep pipelines
potentially 100% occupied *without* requiring a super-scalar or out-of-order
architecture.

Constructing a SIMD/Simple-Vector proposal based around even only these four
(five?) requirements would therefore seem to be a logical thing to do.

# Instruction Format

**TODO** *basically borrow from both P and V, which should be quite simple
to do, with the exception of Tag/no-tag, which needs a bit more
thought.  V's Section 17.19 of Draft V2.3 spec is reminiscent of B's BGS
gather-scatterer, and, if implemented, could actually be a really useful
way to span 8-bit up to 64-bit groups of data, where BGS as it stands
and described by Clifford does **bits** of up to 16 width.  Lots to
look at and investigate!*

# Note on implementation of parallelism

One extremely important aspect of this proposal is to respect and support
implementors desire to focus on power, area or performance.  In that regard,
it is proposed that implementors be free to choose whether to implement
the Vector (or variable-width SIMD) parallelism as sequential operations
with a single ALU, fully parallel (if practical) with multiple ALUs, or
a hybrid combination of both.

In Broadcom's Videocore-IV, they chose hybrid, and called it "Virtual
Parallelism".  They achieve a 16-way SIMD at an **instruction** level
by providing a combination of a 4-way parallel ALU *and* an externally
transparent loop that feeds 4 sequential sets of data into each of the
4 ALUs.

Also in the same core, it is worth noting that particularly uncommon
but essential operations (Reciprocal-Square-Root for example) are
*not* part of the 4-way parallel ALU but instead have a *single* ALU.
Under the proposed Vector (varible-width SIMD) implementors would
be free to do precisely that: i.e. free to choose *on a per operation
basis* whether and how much "Virtual Parallelism" to deploy.

It is absolutely critical to note that it is proposed that such choices MUST
be **entirely transparent** to the end-user and the compiler.  Whilst
a Vector (varible-width SIM) may not precisely match the width of the
parallelism within the implementation, the end-user **should not care**
and in this way the performance benefits are gained but the ISA remains
simple.  All that happens at the end of an instruction run is: some
parallel units (if there are any) would remain offline, completely
transparently to the ISA, the program, and the compiler.

The "SIMD considered harmful" trap of having huge complexity and extra
instructions to deal with corner-cases is thus avoided, and implementors
get to choose precisely where to focus and target the benefits of their
implementationefforts..

# V-Extension to Simple-V Comparative Analysis

This section covers the ways in which Simple-V is comparable
to, or more flexible than, V-Extension (V2.3-draft).  Also covered is
one major weak-point (register files are fixed size, where V is
arbitrary length), and how best to deal with that, should V be adapted
to be on top of Simple-V.

The first stages of this section go over each of the sections of V2.3-draft V
where appropriate

## 17.3 Shape Encoding

Simple-V's proposed means of expressing whether a register (from the
standard integer or the standard floating-point file) is a scalar or
a vector is to simply set the vector length to 1.  The instruction
would however have to specify which register file (integer or FP) that
the vector-length was to be applied to.

Extended shapes (2-D etc) would not be part of Simple-V at all.

## 17.4 Representation Encoding

Simple-V would not have representation-encoding.  This is part of
polymorphism, which is considered too complex to implement (TODO: confirm?)

## 17.5 Element Bitwidth

This is directly equivalent to Simple-V's "Packed", and implies that
integer (or floating-point) are divided down into vector-indexable
chunks of size Bitwidth.

In this way it becomes possible to have ADD effectively and implicitly
turn into ADDb (8-bit add), ADDw (16-bit add) and so on, and where
vector-length has been set to greater than 1, it becomes a "Packed"
(SIMD) instruction.

It remains to be decided what should be done when RV32 / RV64 ADD (sized)
opcodes are used.  One useful idea would be, on an RV64 system where
a 32-bit-sized ADD was performed, to simply use the least significant
32-bits of the register (exactly as is currently done) but at the same
time to *respect the packed bitwidth as well*.

The extended encoding (Table 17.6) would not be part of Simple-V.

## 17.6 Base Vector Extension Supported Types

TODO: analyse.  probably exactly the same.

## 17.7 Maximum Vector Element Width

No equivalent in Simple-V

## 17.8 Vector Configuration Registers

TODO: analyse.

## 17.9 Legal Vector Unit Configurations

TODO: analyse

## 17.10 Vector Unit CSRs

TODO: analyse

> Ok so this is an aspect of Simple-V that I hadn't thought through,
> yet (proposal / idea only a few days old!).  in V2.3-Draft ISA Section
> 17.10 the CSRs are listed.  I note that there's some general-purpose
> CSRs (including a global/active vector-length) and 16 vcfgN CSRs.  i
> don't precisely know what those are for.

>  In the Simple-V proposal, *every* register in both the integer
> register-file *and* the floating-point register-file would have at
> least a 2-bit "data-width" CSR and probably something like an 8-bit
> "vector-length" CSR (less in RV32E, by exactly one bit).

>  What I *don't* know is whether that would be considered perfectly
> reasonable or completely insane.  If it turns out that the proposed
> Simple-V CSRs can indeed be stored in SRAM then I would imagine that
> adding somewhere in the region of 10 bits per register would be... okay? 
> I really don't honestly know.

>  Would these proposed 10-or-so-bit per-register Simple-V CSRs need to
> be multi-ported? No I don't believe they would.

## 17.11 Maximum Vector Length (MVL)

Basically implicitly this is set to the maximum size of the register
file multiplied by the number of 8-bit packed ints that can fit into
a register (4 for RV32, 8 for RV64 and 16 for RV128).

## !7.12 Vector Instruction Formats

No equivalent in Simple-V because *all* instructions of *all* Extensions
are implicitly parallelised (and packed).

## 17.13 Polymorphic Vector Instructions

Polymorphism (implicit type-casting) is deliberately not supported
in Simple-V.

## 17.14 Rapid Configuration Instructions

TODO: analyse if this is useful to have an equivalent in Simple-V

## 17.15 Vector-Type-Change Instructions

TODO: analyse if this is useful to have an equivalent in Simple-V

## 17.16 Vector Length

Has a direct corresponding equivalent.

## 17.17 Predicated Execution

Predicated Execution is another name for "masking" or "tagging".  Masked
(or tagged) implies that there is a bit field which is indexed, and each
bit associated with the corresponding indexed offset register within
the "Vector".  If the tag / mask bit is 1, when a parallel operation is
issued, the indexed element of the vector has the operation carried out.
However if the tag / mask bit is *zero*, that particular indexed element
of the vector does *not* have the requested operation carried out.

In V2.3-draft V, there is a significant (not recommended) difference:
the zero-tagged elements are *set to zero*.  This loses a *significant*
advantage of mask / tagging, particularly if the entire mask register
is itself a general-purpose register, as that general-purpose register
can be inverted, shifted, and'ed, or'ed and so on.  In other words
it becomes possible, especially if Carry/Overflow from each vector
operation is also accessible, to do conditional (step-by-step) vector
operations including things like turn vectors into 1024-bit or greater
operands with very few instructions, by treating the "carry" from
one instruction as a way to do "Conditional add of 1 to the register
next door".  If V2.3-draft V sets zero-tagged elements to zero, such
extremely powerful techniques are simply not possible.

It is noted that there is no mention of an equivalent to BEXT (element
skipping) which would be particularly fascinating and powerful to have.
In this mode, the "mask" would skip elements where its mask bit was zero
in either the source or the destination operand.

Lots to be discussed.

## 17.18 Vector Load/Store Instructions

These may not have a direct equivalent in Simple-V, except if mask/tagging
is to be deployed.

To be discussed.

## 17.19 Vector Register Gather

TODO

## TODO, sort

> However, there are also several features that go beyond simply attaching VL
> to a scalar operation and are crucial to being able to vectorize a lot of
> code.  To name a few:
> - Conditional execution (i.e., predicated operations)
> - Inter-lane data movement (e.g. SLIDE, SELECT)
> - Reductions (e.g., VADD with a scalar destination)

 Ok so the Conditional and also the Reductions is one of the reasons
 why as part of SimpleV / variable-SIMD / parallelism (gah gotta think
 of a decent name) i proposed that it be implemented as "if you say r0
 is to be a vector / SIMD that means operations actually take place on
 r0,r1,r2... r(N-1)".

 Consequently any parallel operation could be paused (or... more
 specifically: vectors disabled by resetting it back to a default /
 scalar / vector-length=1) yet the results would actually be in the
 *main register file* (integer or float) and so anything that wasn't
 possible to easily do in "simple" parallel terms could be done *out*
 of parallel "mode" instead.

 I do appreciate that the above does imply that there is a limit to the
 length that SimpleV (whatever) can be parallelised, namely that you
 run out of registers!  my thought there was, "leave space for the main
 V-Ext proposal to extend it to the length that V currently supports".
 Honestly i had not thought through precisely how that would work.

 Inter-lane (SELECT) i saw 17.19 in V2.3-Draft p117, I liked that,
 it reminds me of the discussion with Clifford on bit-manipulation
 (gather-scatter except not Bit Gather Scatter, *data* gather scatter): if
 applied "globally and outside of V and P" SLIDE and SELECT might become
 an extremely powerful way to do fast memory copy and reordering [2[.

 However I haven't quite got my head round how that would work: i am
 used to the concept of register "tags" (the modern term is "masks")
 and i *think* if "masks" were applied to a Simple-V-enhanced LOAD /
 STORE you would get the exact same thing as SELECT.

 SLIDE you could do simply by setting say r0 vector-length to say 16
 (meaning that if referred to in any operation it would be an implicit
 parallel operation on *all* registers r0 through r15), and temporarily
 set say.... r7 vector-length to say... 5.  Do a LOAD on r7 and it would
 implicitly mean "load from memory into r7 through r11".  Then you go
 back and do an operation on r0 and ta-daa, you're actually doing an
 operation on a SLID {SLIDED?) vector.

 The advantage of Simple-V (whatever) over V would be that you could
 actually do *operations* in the middle of vectors (not just SLIDEs)
 simply by (as above) setting r0 vector-length to 16 and r7 vector-length
 to 5.  There would be nothing preventing you from doing an ADD on r0
 (which meant do an ADD on r0 through r15) followed *immediately in the
 next instruction with no setup cost* a MUL on r7 (which actually meant
 "do a parallel MUL on r7 through r11").

 btw it's worth mentioning that you'd get scalar-vector and vector-scalar
 implicitly by having one of the source register be vector-length 1
 (the default) and one being N > 1.  but without having special opcodes
 to do it.  i *believe* (or more like "logically infer or deduce" as
 i haven't got access to the spec) that that would result in a further
 opcode reduction when comparing [draft] V-Ext to [proposed] Simple-V.

 Also, Reduction *might* be possible by specifying that the destination be
 a scalar (vector-length=1) whilst the source be a vector.  However... it
 would be an awful lot of work to go through *every single instruction*
 in *every* Extension, working out which ones could be parallelised (ADD,
 MUL, XOR) and those that definitely could not (DIV, SUB).  Is that worth
 the effort?  maybe.  Would it result in huge complexity? probably.
 Could an implementor just go "I ain't doing *that* as parallel!
 let's make it virtual-parallelism (sequential reduction) instead"?
 absolutely.  So, now that I think it through, Simple-V (whatever)
 covers Reduction as well.  huh, that's a surprise.


> - Vector-length speculation (making it possible to vectorize some loops with
> unknown trip count) - I don't think this part of the proposal is written
> down yet.

 Now that _is_ an interesting concept.  A little scary, i imagine, with
 the possibility of putting a processor into a hard infinite execution
 loop... :)


> Also, note the vector ISA consumes relatively little opcode space (all the
> arithmetic fits in 7/8ths of a major opcode).  This is mainly because data
> type and size is a function of runtime configuration, rather than of opcode.

 yes.  i love that aspect of V, i am a huge fan of polymorphism [1]
 which is why i am keen to advocate that the same runtime principle be
 extended to the rest of the RISC-V ISA [3]

 Yikes that's a lot.  I'm going to need to pull this into the wiki to
 make sure it's not lost.

[1] inherent data type conversion: 25 years ago i designed a hypothetical
hyper-hyper-hyper-escape-code-sequencing ISA based around 2-bit
(escape-extended) opcodes and 2-bit (escape-extended) operands that
only required a fixed 8-bit instruction length.  that relied heavily
on polymorphism and runtime size configurations as well.  At the time
I thought it would have meant one HELL of a lot of CSRs... but then I
met RISC-V and was cured instantly of that delusion^Wmisapprehension :)

[2] Interestingly if you then also add in the other aspect of Simple-V
(the data-size, which is effectively functionally orthogonal / identical
to "Packed" of Packed-SIMD), masked and packed *and* vectored LOAD / STORE
operations become byte / half-word / word augmenters of B-Ext's proposed
"BGS" i.e. where B-Ext's BGS dealt with bits, masked-packed-vectored
LOAD / STORE would deal with 8 / 16 / 32 bits at a time.  Where it
would get really REALLY interesting would be masked-packed-vectored
B-Ext BGS instructions.  I can't even get my head fully round that,
which is a good sign that the combination would be *really* powerful :)

[3] ok sadly maybe not the polymorphism, it's too complicated and I
think would be much too hard for implementors to easily "slide in" to an
existing non-Simple-V implementation.  i say that despite really *really*
wanting IEEE 704 FP Half-precision to end up somewhere in RISC-V in some
fashion, for optimising 3D Graphics.  *sigh*.

## TODO: instructions (based on Hwacha) V-Ext duplication analysis

This is partly speculative due to lack of access to an up-to-date
V-Ext Spec (V2.3-draft RVV 0.4-Draft at the time of writing).  However
basin an analysis instead on Hwacha, a cursory examination shows over
an **85%** duplication of V-Ext operand-related instructions when
compared to Simple-V on a standard RG64G base.   Even Vector Fetch
is analogous to "zero-overhead loop".

Exceptions are:

* Vector Indexed Memory Instructions (non-contiguous)
* Vector Atomic Memory Instructions.
* Some of the Vector Arithmetic ops: MADD, MSUB,
  VSRL, VSRA, VEIDX, VFIRST, VSGNJN, VFSGNJX and potentially more.
* Consensual Jump

Table of RV32V Instructions

| RV32V      |     |
| -----      | --- |
| VADD       |     |
| VSUB       |     |
| VSL        |     |
| VSR        |     |
| VAND       |     |
| VOR        |     |
| VXOR       |     |
| VSEQ       |     |
| VSNE       |     |
| VSLT       |     |
| VSGE       |     |
| VCLIP      |     |
| VCVT       |     |
| VMPOP      |     |
| VMFIRST    |     |
| VEXTRACT   |     |
| VINSERT    |     |
| VMERGE     |     |
| VSELECT    |     |
| VSLIDE     |     |
| VDIV       |     |
| VREM       |     |
| VMUL       |     |
| VMULH      |     |
| VMIN       |     |
| VMAX       |     |
| VSGNJ      |     |
| VSGNJN     |     |
| VSGNJX     |     |
| VSQRT      |     |
| VCLASS     |     |
| VPOPC      |     |
| VADDI      |     |
| VSLI       |     |
| VSRI       |     |
| VANDI      |     |
| VORI       |     |
| VXORI      |     |
| VCLIPI     |     |
| VMADD      |     |
| VMSUB      |     |
| VNMADD     |     |
| VNMSUB     |     |
| VLD        |     |
| VLDS       |     |
| VLDX       |     |
| VST        |     |
| VSTS       |     |
| VSTX       |     |
| VAMOSWAP   |     |
| VAMOADD    |     |
| VAMOAND    |     |
| VAMOOR     |     |
| VAMOXOR    |     |
| VAMOMIN    |     |
| VAMOMAX    |     |

## TODO: sort

> I suspect that the "hardware loop" in question is actually a zero-overhead
> loop unit that diverts execution from address X to address Y if a certain
> condition is met.

 not quite.  The zero-overhead loop unit interestingly would be at
an [independent] level above vector-length.  The distinctions are
as follows:

* Vector-length issues *virtual* instructions where the register
  operands are *specifically* altered (to cover a range of registers),
  whereas zero-overhead loops *specifically* do *NOT* alter the operands
  in *ANY* way.

* Vector-length-driven "virtual" instructions are driven by *one*
 and *only* one instruction (whether it be a LOAD, STORE, or pure
 one/two/three-operand opcode) whereas zero-overhead loop units
 specifically apply to *multiple* instructions.

Where vector-length-driven "virtual" instructions might get conceptually
blurred with zero-overhead loops is LOAD / STORE.  In the case of LOAD /
STORE, to actually be useful, vector-length-driven LOAD / STORE should
increment the LOAD / STORE memory address to correspondingly match the
increment in the register bank.  example:

* set vector-length for r0 to 4
* issue RV32 LOAD from addr 0x1230 to r0

translates effectively to:

* RV32 LOAD from addr 0x1230 to r0
* ...
* ...
* RV32 LOAD from addr 0x123B to r3

# P-Ext ISA

## 16-bit Arithmetic

| Mnemonic           | 16-bit Instruction        | Simple-V Equivalent |
| ------------------ | ------------------------- | ------------------- |
| ADD16 rt, ra, rb   | add                       | RV ADD (bitwidth=16) |
| RADD16 rt, ra, rb  | Signed Halving add        | |
| URADD16 rt, ra, rb | Unsigned Halving add      | |
| KADD16 rt, ra, rb  | Signed Saturating add     | |
| UKADD16 rt, ra, rb | Unsigned Saturating add   | |
| SUB16 rt, ra, rb   | sub                       | RV SUB (bitwidth=16) |
| RSUB16 rt, ra, rb  | Signed Halving sub        | |
| URSUB16 rt, ra, rb | Unsigned Halving sub                | |
| KSUB16 rt, ra, rb  | Signed Saturating sub               | |
| UKSUB16 rt, ra, rb | Unsigned Saturating sub             | |
| CRAS16 rt, ra, rb  | Cross Add & Sub                     | |
| RCRAS16 rt, ra, rb | Signed Halving Cross Add & Sub      | |
| URCRAS16 rt, ra, rb| Unsigned Halving Cross Add & Sub    | |
| KCRAS16 rt, ra, rb | Signed Saturating Cross Add & Sub   | |
| UKCRAS16 rt, ra, rb| Unsigned Saturating Cross Add & Sub | |
| CRSA16 rt, ra, rb  | Cross Sub & Add                     | |
| RCRSA16 rt, ra, rb | Signed Halving Cross Sub & Add      | |
| URCRSA16 rt, ra, rb| Unsigned Halving Cross Sub & Add    | |
| KCRSA16 rt, ra, rb | Signed Saturating Cross Sub & Add   | |
| UKCRSA16 rt, ra, rb| Unsigned Saturating Cross Sub & Add | |

## 8-bit Arithmetic

| Mnemonic           | 16-bit Instruction        | Simple-V Equivalent |
| ------------------ | ------------------------- | ------------------- |
| ADD8 rt, ra, rb    | add                       | RV ADD (bitwidth=8)|
| RADD8 rt, ra, rb   | Signed Halving add        | |
| URADD8 rt, ra, rb  | Unsigned Halving add      | |
| KADD8 rt, ra, rb   | Signed Saturating add     | |
| UKADD8 rt, ra, rb  | Unsigned Saturating add   | |
| SUB8 rt, ra, rb    | sub                       | RV SUB (bitwidth=8)|
| RSUB8 rt, ra, rb   | Signed Halving sub        | |
| URSUB8 rt, ra, rb  | Unsigned Halving sub      | |



# References

* SIMD considered harmful <https://www.sigarch.org/simd-instructions-considered-harmful/>
* Link to first proposal <https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/GuukrSjgBH8>
* Recommendation by Jacob Bachmeyer to make zero-overhead loop an
  "implicit program-counter" <https://groups.google.com/a/groups.riscv.org/d/msg/isa-dev/vYVi95gF2Mo/SHz6a4_lAgAJ>
* Re-continuing P-Extension proposal <https://groups.google.com/a/groups.riscv.org/forum/#!msg/isa-dev/IkLkQn3HvXQ/SEMyC9IlAgAJ>
* First Draft P-SIMD (DSP) proposal <https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/vYVi95gF2Mo>
* B-Extension discussion <https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/zi_7B15kj6s>
* Broadcom VideoCore-IV <https://docs.broadcom.com/docs/12358545>
  Figure 2 P17 and Section 3 on P16.
* Hwacha <https://www2.eecs.berkeley.edu/Pubs/TechRpts/2015/EECS-2015-262.html>
* Hwacha <https://www2.eecs.berkeley.edu/Pubs/TechRpts/2015/EECS-2015-263.html>
* Vector Workshop <http://riscv.org/wp-content/uploads/2015/06/riscv-vector-workshop-june2015.pdf>