clarify Reg CSR table

[libreriscv.git] / simple_v_extension / specification.mdwn

# Simple-V (Parallelism Extension Proposal) Specification

* Status: DRAFTv0.1
* Last edited: 30 sep 2018

With thanks to:

* Allen Baum
* Jacob Bachmeyer
* Guy Lemurieux
* Jacob Lifshay
* The RISC-V Founders, without whom this all would not be possible.

[[!toc ]]

# Summary and Background: Rationale

Simple-V is a uniform parallelism API for RISC-V hardware that has several
unplanned side-effects including code-size reduction, expansion of
HINT space and more.  The reason for
creating it is to provide a manageable way to turn a pre-existing design
into a parallel one, in a step-by-step incremental fashion, allowing
the implementor to focus on adding hardware where it is needed and necessary.

Critically: **No new instructions are added**.  The parallelism (if any
is implemented) is implicitly added by tagging *standard* scalar registers
for redirection.  When such a tagged register is used in any instruction,
it indicates that the PC shall **not** be incremented; instead a loop
is activated where *multiple* instructions are issued to the pipeline
(as determined by a length CSR), with contiguously incrementing register
numbers starting from the tagged register.  When the last "element"
has been reached, only then is the PC permitted to move on.  Thus
Simple-V effectively sits (slots) *in between* the instruction decode phase
and the ALU(s).

The barrier to entry with SV is therefore very low.  The minimum
compliantt implementation is software-emulation (traps), requiring
only the CSRs and CSR tables, and that an exception be thrown if an
instruction's registers are detected to have been tagged.  The looping
that would otherwise be done in hardware is thus carried out in software,
instead.  Whilst much slower, it is "compliant" with the SV specification,
and may be suited for implementation in RV32E and also in situations
where the implementor wishes to focus on certain aspects of SV, without
unnecessary time and resources into the silicon, whilst also conforming
strictly with the API.  A good area to punt to software would be the
polymorphic element width capability for example.

Hardware Parallelism, if any, is therefore added at the implementor's
discretion to turn what would otherwise be a sequential loop into a
parallel one.

To emphasise that clearly: Simple-V (SV) is *not*:

* A SIMD system
* A SIMT system
* A Vectorisation Microarchitecture
* A microarchitecture of any specific kind
* A mandary parallel processor microarchitecture of any kind
* A supercomputer extension

SV does **not** tell implementors how or even if they should implement
parallelism: it is a hardware "API" (Application Programming Interface)
that, if implemented, presents a uniform and consistent way to *express*
parallelism, at the same time leaving the choice of if, how, how much,
when and whether to parallelise operations **entirely to the implementor**.

# CSRs <a name="csrs"></a>

For U-Mode there are two CSR key-value stores needed to create lookup
tables which are used at the register decode phase.

* A register CSR key-value table (typically 8 32-bit CSRs of 2 16-bits each)
* A predication CSR key-value table (again, 8 32-bit CSRs of 2 16-bits each)
* Small U-Mode and S-Mode register and predication CSR key-value tables
  (2 32-bit CSRs of 2x 16-bit entries each).
* An optional "reshaping" CSR key-value table which remaps from a 1D
  linear shape to 2D or 3D, including full transposition.

There are also four additional CSRs for User-Mode:

* CFG subsets the CSR tables
* MVL (the Maximum Vector Length)
* VL (which has different characteristics from standard CSRs)
* STATE (useful for saving and restoring during context switch,
  and for providing fast transitions)

There are also three additional CSRs for Supervisor-Mode:

* SMVL
* SVL
* SSTATE

And likewise for M-Mode:

* MMVL
* MVL
* MSTATE

Both Supervisor and M-Mode have their own (small) CSR register and
predication tables of only 4 entries each.

## CFG

This CSR may be used to switch between subsets of the CSR Register and
Predication Tables: it is kept to 5 bits so that a single CSRRWI instruction
can be used.  A setting of all ones is reserved to indicate that SimpleV
is disabled.

| (4..3) | (2...0) |
| ------ | ------- |
| size   | bank    |

Bank is 3 bits in size, and indicates the starting index of the CSR
Register and Predication Table entries that are "enabled".  Given that
each CSR table row is 16 bits and contains 2 CAM entries each, there
are only 8 CSRs to cover in each table, so 8 bits is sufficient.

Size is 2 bits.  With the exception of when bank == 7 and size == 3,
the number of elements enabled is taken by right-shifting 2 by size:

| size   | elements |
| ------ | -------- |
| 0      | 2        |
| 1      | 4        |
| 2      | 8        |
| 3      | 16       |

Given that there are 2 16-bit CAM entries per CSR table row, this
may also be viewed as the number of CSR rows to enable, by raising size to
the power of 2.

Examples:

* When bank = 0 and size = 3, SVREGCFG0 through to SVREGCFG7 are
  enabled, and SVPREDCFG0 through to SVPREGCFG7 are enabled.
* When bank = 1 and size = 3, SVREGCFG1 through to SVREGCFG7 are
  enabled, and SVPREDCFG1 through to SVPREGCFG7 are enabled.
* When bank = 3 and size = 0, SVREGCFG3 and SVPREDCFG3 are enabled.
* When bank = 3 and size = 0, SVREGCFG3 and SVPREDCFG3 are enabled.
* When bank = 7 and size = 1, SVREGCFG7 and SVPREDCFG7 are enabled.
* When bank = 7 and size = 3, SimpleV is entirely disabled.

In this way it is possible to enable and disable SimpleV with a
single instruction, and, furthermore, on context-switching the quantity
of CSRs to be saved and restored is greatly reduced.

## MAXVECTORLENGTH (MVL)

MAXVECTORLENGTH is the same concept as MVL in RVV, except that it
is variable length and may be dynamically set.  MVL is
however limited to the regfile bitwidth XLEN (1-32 for RV32,
1-64 for RV64 and so on).

The reason for setting this limit is so that predication registers, when
marked as such, may fit into a single register as opposed to fanning out
over several registers.  This keeps the implementation a little simpler.

The other important factor to note is that the actual MVL is **offset
by one**, so that it can fit into only 6 bits (for RV64) and still cover
a range up to XLEN bits.  So, when setting the MVL CSR to 0, this actually
means that MVL==1.  When setting the MVL CSR to 3, this actually means
that MVL==4, and so on.  This is expressed more clearly in the "pseudocode"
section, where there are subtle differences between CSRRW and CSRRWI.

## Vector Length (VL)

VSETVL is slightly different from RVV.  Like RVV, VL is set to be within
the range 1 <= VL <= MVL (where MVL in turn is limited to 1 <= MVL <= XLEN)

    VL = rd = MIN(vlen, MVL)

where 1 <= MVL <= XLEN

However just like MVL it is important to note that the range for VL has
subtle design implications, covered in the "CSR pseudocode" section

The fixed (specific) setting of VL allows vector LOAD/STORE to be used
to switch the entire bank of registers using a single instruction (see
Appendix, "Context Switch Example").  The reason for limiting VL to XLEN
is down to the fact that predication bits fit into a single register of
length XLEN bits.

The second change is that when VSETVL is requested to be stored
into x0, it is *ignored* silently (VSETVL x0, x5)

The third and most important change is that, within the limits set by
MVL, the value passed in **must** be set in VL (and in the
destination register).

This has implication for the microarchitecture, as VL is required to be
set (limits from MVL notwithstanding) to the actual value
requested.  RVV has the option to set VL to an arbitrary value that suits
the conditions and the micro-architecture: SV does *not* permit this.

The reason is so that if SV is to be used for a context-switch or as a
substitute for LOAD/STORE-Multiple, the operation can be done with only
2-3 instructions (setup of the CSRs, VSETVL x0, x0, #{regfilelen-1},
single LD/ST operation).  If VL does *not* get set to the register file
length when VSETVL is called, then a software-loop would be needed.
To avoid this need, VL *must* be set to exactly what is requested
(limits notwithstanding).

Therefore, in turn, unlike RVV, implementors *must* provide
pseudo-parallelism (using sequential loops in hardware) if actual
hardware-parallelism in the ALUs is not deployed.  A hybrid is also
permitted (as used in Broadcom's VideoCore-IV) however this must be
*entirely* transparent to the ISA.

The fourth change is that VSETVL is implemented as a CSR, where the
behaviour of CSRRW (and CSRRWI) must be changed to specifically store
the *new* value in the destination register, **not** the old value.
Where context-load/save is to be implemented in the usual fashion
by using a single CSRRW instruction to obtain the old value, the
*secondary* CSR must be used (SVSTATE).  This CSR behaves
exactly as standard CSRs, and contains more than just VL.

One interesting side-effect of using CSRRWI to set VL is that this
may be done with a single instruction, useful particularly for a
context-load/save.  There are however limitations: CSRWWI's immediate
is limited to 0-31.

## STATE

This is a standard CSR that contains sufficient information for a
full context save/restore.  It contains (and permits setting of)
MVL, VL, CFG, the destination element offset of the current parallel
instruction being executed, and, for twin-predication, the source
element offset as well.  Interestingly it may hypothetically
also be used to make the immediately-following instruction to skip a
certain number of elements, however the recommended method to do
this is predication.

Setting destoffs and srcoffs is realistically intended for saving state
so that exceptions (page faults in particular) may be serviced and the
hardware-loop that was being executed at the time of the trap, from
user-mode (or Supervisor-mode), may be returned to and continued from
where it left off.  The reason why this works is because setting
User-Mode STATE will not change (not be used) in M-Mode or S-Mode
(and is entirely why M-Mode and S-Mode have their own STATE CSRs).

The format of the STATE CSR is as follows:

| (28..26) | (25..24) | (23..18) | (17..12) | (11..6) | (5...0) |
| -------- | -------- | -------- | -------- | ------- | ------- |
| size     | bank     | destoffs | srcoffs  | vl      | maxvl   |

When setting this CSR, the following characteristics will be enforced:

* **MAXVL** will be truncated (after offset) to be within the range 1 to XLEN
* **VL** will be truncated (after offset) to be within the range 1 to MAXVL
* **srcoffs** will be truncated to be within the range 0 to VL-1
* **destoffs** will be truncated to be within the range 0 to VL-1

## MVL, VL and CSR Pseudocode

The pseudo-code for get and set of VL and MVL are as follows:

    set_mvl_csr(value, rd):
        regs[rd] = MVL
        MVL = MIN(value, MVL)

    get_mvl_csr(rd):
        regs[rd] = VL

    set_vl_csr(value, rd):
        VL = MIN(value, MVL)
        regs[rd] = VL # yes returning the new value NOT the old CSR

    get_vl_csr(rd):
        regs[rd] = VL

Note that where setting MVL behaves as a normal CSR, unlike standard CSR
behaviour, setting VL will return the **new** value of VL **not** the old
one.

For CSRRWI, the range of the immediate is restricted to 5 bits.  In order to
maximise the effectiveness, an immediate of 0 is used to set VL=1,
an immediate of 1 is used to set VL=2 and so on:

    CSRRWI_Set_MVL(value):
        set_mvl_csr(value+1, x0)

    CSRRWI_Set_VL(value):
        set_vl_csr(value+1, x0)

However for CSRRW the following pseudocide is used for MVL and VL,
where setting the value to zero will cause an exception to be raised.
The reason is that if VL or MVL are set to zero, the STATE CSR is
not capable of returning that value.

    CSRRW_Set_MVL(rs1, rd):
        value = regs[rs1]
        if value == 0:
            raise Exception
        set_mvl_csr(value, rd)

    CSRRW_Set_VL(rs1, rd):
        value = regs[rs1]
        if value == 0:
            raise Exception
        set_vl_csr(value, rd)

In this way, when CSRRW is utilised with a loop variable, the value
that goes into VL (and into the destination register) may be used
in an instruction-minimal fashion:

     CSRvect1 = {type: F, key: a3, val: a3, elwidth: dflt}
     CSRvect2 = {type: F, key: a7, val: a7, elwidth: dflt}
     CSRRWI MVL, 3          # sets MVL == **4** (not 3)
     j zerotest             # in case loop counter a0 already 0
    loop:
     CSRRW VL, t0, a0       # vl = t0 = min(mvl, a0)
     ld     a3, a1          # load 4 registers a3-6 from x
     slli   t1, t0, 3       # t1 = vl * 8 (in bytes)
     ld     a7, a2          # load 4 registers a7-10 from y
     add    a1, a1, t1      # increment pointer to x by vl*8
     fmadd a7, a3, fa0, a7 # v1 += v0 * fa0 (y = a * x + y)
     sub    a0, a0, t0      # n -= vl (t0)
     st     a7, a2          # store 4 registers a7-10 to y
     add    a2, a2, t1      # increment pointer to y by vl*8
    zerotest:
     bnez   a0, loop        # repeat if n != 0

With the STATE CSR, just like with CSRRWI, in order to maximise the
utilisation of the limited bitspace, "000000" in binary represents
VL==1, "00001" represents VL==2 and so on (likewise for MVL):

    CSRRW_Set_SV_STATE(rs1, rd):
        value = regs[rs1]
        get_state_csr(rd)
        MVL = set_mvl_csr(value[11:6]+1)
        VL = set_vl_csr(value[5:0]+1)
        CFG = value[28:24]>>24
        destoffs = value[23:18]>>18
        srcoffs = value[23:18]>>12

    get_state_csr(rd):
        regs[rd] = (MVL-1) | (VL-1)<<6 | (srcoffs)<<12 |
                   (destoffs)<<18 | (CFG)<<24
        return regs[rd]

In both cases, whilst CSR read of VL and MVL return the exact values
of VL and MVL respectively, reading and writing the STATE CSR returns
those values **minus one**.  This is absolutely critical to implement
if the STATE CSR is to be used for fast context-switching.

## Register CSR key-value (CAM) table

The purpose of the Register CSR table is four-fold:

* To mark integer and floating-point registers as requiring "redirection"
  if it is ever used as a source or destination in any given operation.
  This involves a level of indirection through a 5-to-7-bit lookup table,
  such that **unmodified** operands with 5 bit (3 for Compressed) may
  access up to **64** registers.
* To indicate whether, after redirection through the lookup table, the
  register is a vector (or remains a scalar).
* To over-ride the implicit or explicit bitwidth that the operation would
  normally give the register.

| RgCSR | | 15       | (14..8)  | 7   | (6..5) | (4..0)  |
| ----- | | -        | -        | -   | ------ | ------- |
| 0     | | isvec0   | regidx0  | i/f | vew0   | regkey  |
| 1     | | isvec1   | regidx1  | i/f | vew1   | regkey  |
| ..    | | isvec..  | regidx.. | i/f | vew..  | regkey  |
| 15    | | isvec15  | regidx15 | i/f | vew15  | regkey  |

i/f is set to "1" to indicate that the redirection/tag entry is to be applied
to integer registers; 0 indicates that it is relevant to floating-point
registers.  vew has the following meanings, indicating that the instruction's
operand size is "over-ridden" in a polymorphic fashion:

| vew | bitwidth   |
| --- | ---------- |
| 00  | default    |
| 01  | default/2  |
| 10  | default\*2 |
| 11  | 8          |

As the above table is a CAM (key-value store) it may be appropriate
(faster, implementation-wise) to expand it as follows:

    struct vectorised fp_vec[32], int_vec[32];

    for (i = 0; i < 16; i++) // 16 CSRs?
       tb = int_vec if CSRvec[i].type == 0 else fp_vec
       idx = CSRvec[i].regkey // INT/FP src/dst reg in opcode
       tb[idx].elwidth  = CSRvec[i].elwidth
       tb[idx].regidx   = CSRvec[i].regidx  // indirection
       tb[idx].isvector = CSRvec[i].isvector // 0=scalar
       tb[idx].packed   = CSRvec[i].packed  // SIMD or not

The actual size of the CSR Register table depends on the platform
and on whether other Extensions are present (RV64G, RV32E, etc.).
For details see "Subsets" section.

16-bit CSR Register CAM entries are mapped directly into 32-bit
on any RV32-based system, however RV64 (XLEN=64) and RV128 (XLEN=128)
are slightly different: the 16-bit entries appear (and can be set)
multiple times, in an overlapping fashion.  Here is the table for RV64:

| CSR#  | 63..48  | 47..32  | 31..16  | 15..0   |
| 0x4c0 | RgCSR3  | RgCSR2  | RgCSR1  | RgCSR0  |
| 0x4c1 | RgCSR5  | RgCSR4  | RgCSR3  | RgCSR2  |
| 0x4c2 | ...     | ...     | ...     | ...     |
| 0x4c1 | RgCSR15 | RgCSR14 | RgCSR13 | RgCSR12 |
| 0x4c8 | n/a     | n/a     | RgCSR15 | RgCSR4  |

The rules for writing to these CSRs are that any entries above the ones
being set will be automatically wiped (to zero), so to fill several entries
they must be written in a sequentially increasing manner.  This functionality
was in an early draft of RVV and it means that, firstly, compilers do not have
to spend time zero-ing out CSRs unnecessarily, and secondly, that on
context-switching (and function calls) the number of CSRs that may need
saving is implicitly known.

The reason for the overlapping entries is that in the worst-case on an
RV64 system, only 4 64-bit CSR reads/writes are required for a full
context-switch (and an RV128 system, only 2 128-bit CSR reads/writes).

--

TODO: move elsewhere

    # TODO: use elsewhere (retire for now)
    vew = CSRbitwidth[rs1]
    if (vew == 0)
        bytesperreg = (XLEN/8) # or FLEN as appropriate
    elif (vew == 1)
        bytesperreg = (XLEN/4) # or FLEN/2 as appropriate
    else:
        bytesperreg = bytestable[vew] # 8 or 16
    simdmult = (XLEN/8) / bytesperreg # or FLEN as appropriate
    vlen = CSRvectorlen[rs1] * simdmult
    CSRvlength = MIN(MIN(vlen, MAXVECTORLENGTH), rs2)

The reason for multiplying the vector length by the number of SIMD elements
(in each individual register) is so that each SIMD element may optionally be
predicated.

An example of how to subdivide the register file when bitwidth != default
is given in the section "Bitwidth Virtual Register Reordering".

## Predication CSR <a name="predication_csr_table"></a>

TODO: update CSR tables, now 7-bit for regidx

The Predication CSR is a key-value store indicating whether, if a given
destination register (integer or floating-point) is referred to in an
instruction, it is to be predicated.  Tt is particularly important to note
that the *actual* register used can be *different* from the one that is
in the instruction, due to the redirection through the lookup table.

* regidx is the actual register that in combination with the
  i/f flag, if that integer or floating-point register is referred to,
  results in the lookup table being referenced to find the predication
  mask to use on the operation in which that (regidx) register has
  been used
* predidx (in combination with the bank bit in the future) is the
  *actual* register to be used for the predication mask.  Note:
  in effect predidx is actually a 6-bit register address, as the bank
  bit is the MSB (and is nominally set to zero for now).
* inv indicates that the predication mask bits are to be inverted
  prior to use *without* actually modifying the contents of the
  register itself.
* zeroing is either 1 or 0, and if set to 1, the operation must
  place zeros in any element position where the predication mask is
  set to zero.  If zeroing is set to 0, unpredicated elements *must*
  be left alone.  Some microarchitectures may choose to interpret
  this as skipping the operation entirely.  Others which wish to
  stick more closely to a SIMD architecture may choose instead to
  interpret unpredicated elements as an internal "copy element"
  operation (which would be necessary in SIMD microarchitectures
  that perform register-renaming)
* "packed" indicates if the register is to be interpreted as SIMD
  i.e. containing multiple contiguous elements of size equal to "bitwidth".
  (Note: in earlier drafts this was in the Register CSR table.
  However after extending to 7 bits there was not enough space.
  To use "unpredicated" packed SIMD, set the predicate to x0 and
  set "invert".  This has the effect of setting a predicate of all 1s)

| PrCSR | 13     | 12     | 11    | 10  | (9..5)  | (4..0)  |
| ----- | -      | -      | -     | -   | ------- | ------- |
| 0     | bank0  | zero0  | inv0  | i/f | regidx  | predkey |
| 1     | bank1  | zero1  | inv1  | i/f | regidx  | predkey |
| ..    | bank.. | zero.. | inv.. | i/f | regidx  | predkey |
| 15    | bank15 | zero15 | inv15 | i/f | regidx  | predkey |

The Predication CSR Table is a key-value store, so implementation-wise
it will be faster to turn the table around (maintain topologically
equivalent state):

    struct pred {
        bool zero;
        bool inv;
        bool enabled;
        int predidx; // redirection: actual int register to use
    }

    struct pred fp_pred_reg[32];   // 64 in future (bank=1)
    struct pred int_pred_reg[32];  // 64 in future (bank=1)

    for (i = 0; i < 16; i++)
      tb = int_pred_reg if CSRpred[i].type == 0 else fp_pred_reg;
      idx = CSRpred[i].regidx
      tb[idx].zero = CSRpred[i].zero
      tb[idx].inv  = CSRpred[i].inv
      tb[idx].predidx  = CSRpred[i].predidx
      tb[idx].enabled  = true

So when an operation is to be predicated, it is the internal state that
is used.  In Section 6.4.2 of Hwacha's Manual (EECS-2015-262) the following
pseudo-code for operations is given, where p is the explicit (direct)
reference to the predication register to be used:

    for (int i=0; i<vl; ++i)
        if ([!]preg[p][i])
           (d ? vreg[rd][i] : sreg[rd]) =
            iop(s1 ? vreg[rs1][i] : sreg[rs1],
                s2 ? vreg[rs2][i] : sreg[rs2]); // for insts with 2 inputs

This instead becomes an *indirect* reference using the *internal* state
table generated from the Predication CSR key-value store, which iwws used
as follows.

    if type(iop) == INT:
        preg = int_pred_reg[rd]
    else:
        preg = fp_pred_reg[rd]

    for (int i=0; i<vl; ++i)
        predicate, zeroing = get_pred_val(type(iop) == INT, rd):
        if (predicate && (1<<i))
           (d ? regfile[rd+i] : regfile[rd]) =
            iop(s1 ? regfile[rs1+i] : regfile[rs1],
                s2 ? regfile[rs2+i] : regfile[rs2]); // for insts with 2 inputs
        else if (zeroing)
           (d ? regfile[rd+i] : regfile[rd]) = 0

Note:

* d, s1 and s2 are booleans indicating whether destination,
  source1 and source2 are vector or scalar
* key-value CSR-redirection of rd, rs1 and rs2 have NOT been included
  above, for clarity.  rd, rs1 and rs2 all also must ALSO go through
  register-level redirection (from the Register CSR table) if they are
  vectors.

If written as a function, obtaining the predication mask (and whether
zeroing takes place) may be done as follows:

    def get_pred_val(bool is_fp_op, int reg):
       tb = int_reg if is_fp_op else fp_reg
       if (!tb[reg].enabled):
          return ~0x0, False       // all enabled; no zeroing
       tb = int_pred if is_fp_op else fp_pred
       if (!tb[reg].enabled):
          return ~0x0, False       // all enabled; no zeroing
       predidx = tb[reg].predidx   // redirection occurs HERE
       predicate = intreg[predidx] // actual predicate HERE
       if (tb[reg].inv):
          predicate = ~predicate   // invert ALL bits
       return predicate, tb[reg].zero

Note here, critically, that **only** if the register is marked
in its CSR **register** table entry as being "active" does the testing
proceed further to check if the CSR **predicate** table entry is
also active.

Note also that this is in direct contrast to branch operations
for the storage of comparisions: in these specific circumstances
the requirement for there to be an active CSR *register* entry
is removed.

## REMAP CSR

(Note: both the REMAP and SHAPE sections are best read after the
 rest of the document has been read)

There is one 32-bit CSR which may be used to indicate which registers,
if used in any operation, must be "reshaped" (re-mapped) from a linear
form to a 2D or 3D transposed form.  The 32-bit REMAP CSR may reshape
up to 3 registers:

| 29..28 | 27..26 | 25..24 | 23 | 22..16  | 15 | 14..8   | 7  | 6..0    |
| ------ | ------ | ------ | -- | ------- | -- | ------- | -- | ------- |
| shape2 | shape1 | shape0 | 0  | regidx2 | 0  | regidx1 | 0  | regidx0 |

regidx0-2 refer not to the Register CSR CAM entry but to the underlying
*real* register (see regidx, the value) and consequently is 7-bits wide.
shape0-2 refers to one of three SHAPE CSRs.  A value of 0x3 is reserved.
Bits 7, 15, 23, 30 and 31 are also reserved, and must be set to zero.

## SHAPE 1D/2D/3D vector-matrix remapping CSRs

(Note: both the REMAP and SHAPE sections are best read after the
 rest of the document has been read)

There are three "shape" CSRs, SHAPE0, SHAPE1, SHAPE2, 32-bits in each,
which have the same format.  When each SHAPE CSR is set entirely to zeros,
remapping is disabled: the register's elements are a linear (1D) vector.

| 26..24  | 23 | 22..16  | 15 | 14..8   | 7  | 6..0    |
| ------- | -- | ------- | -- | ------- | -- | ------- |
| permute | 0  | zdimsz  | 0  | ydimsz  | 0  | xdimsz  |

xdimsz, ydimsz and zdimsz are offset by 1, such that a value of 0 indicates
that the array dimensionality for that dimension is 1.  A value of xdimsz=2
would indicate that in the first dimension there are 3 elements in the
array.  The format of the array is therefore as follows:

    array[xdim+1][ydim+1][zdim+1]

However whilst illustrative of the dimensionality, that does not take the
"permute" setting into account.  "permute" may be any one of six values
(0-5, with values of 6 and 7 being reserved, and not legal).  The table
below shows how the permutation dimensionality order works:

| permute | order | array format             |
| ------- | ----- | ------------------------ |
| 000     | 0,1,2 | (xdim+1)(ydim+1)(zdim+1) |
| 001     | 0,2,1 | (xdim+1)(zdim+1)(ydim+1) |
| 010     | 1,0,2 | (ydim+1)(xdim+1)(zdim+1) |
| 011     | 1,2,0 | (ydim+1)(zdim+1)(xdim+1) |
| 100     | 2,0,1 | (zdim+1)(xdim+1)(ydim+1) |
| 101     | 2,1,0 | (zdim+1)(ydim+1)(xdim+1) |

In other words, the "permute" option changes the order in which
nested for-loops over the array would be done.  The algorithm below
shows this more clearly, and may be executed as a python program:

    # mapidx = REMAP.shape2
    xdim = 3 # SHAPE[mapidx].xdim_sz+1
    ydim = 4 # SHAPE[mapidx].ydim_sz+1
    zdim = 5 # SHAPE[mapidx].zdim_sz+1

    lims = [xdim, ydim, zdim]
    idxs = [0,0,0] # starting indices
    order = [1,0,2] # experiment with different permutations, here

    for idx in range(xdim * ydim * zdim):
        new_idx = idxs[0] + idxs[1] * xdim + idxs[2] * xdim * ydim
        print new_idx,
        for i in range(3):
            idxs[order[i]] = idxs[order[i]] + 1
            if (idxs[order[i]] != lims[order[i]]):
                break
            print
            idxs[order[i]] = 0

Here, it is assumed that this algorithm be run within all pseudo-code
throughout this document where a (parallelism) for-loop would normally
run from 0 to VL-1 to refer to contiguous register
elements; instead, where REMAP indicates to do so, the element index
is run through the above algorithm to work out the **actual** element
index, instead.  Given that there are three possible SHAPE entries, up to
three separate registers in any given operation may be simultaneously
remapped:

    function op_add(rd, rs1, rs2) # add not VADD!
      ...
      ...
      for (i = 0; i < VL; i++)
        if (predval & 1<<i) # predication uses intregs
           ireg[rd+remap(id)] <= ireg[rs1+remap(irs1)] +
                                 ireg[rs2+remap(irs2)];
        if (int_vec[rd ].isvector)  { id += 1; }
        if (int_vec[rs1].isvector)  { irs1 += 1; }
        if (int_vec[rs2].isvector)  { irs2 += 1; }

By changing remappings, 2D matrices may be transposed "in-place" for one
operation, followed by setting a different permutation order without
having to move the values in the registers to or from memory.  Also,
the reason for having REMAP separate from the three SHAPE CSRs is so
that in a chain of matrix multiplications and additions, for example,
the SHAPE CSRs need only be set up once; only the REMAP CSR need be
changed to target different registers.

Note that:

* If permute option 000 is utilised, the actual order of the
  reindexing does not change!
* If two or more dimensions are set to zero, the actual order does not change!
* The above algorithm is pseudo-code **only**.  Actual implementations
  will need to take into account the fact that the element for-looping
  must be **re-entrant**, due to the possibility of exceptions occurring.
  See MSTATE CSR, which records the current element index.
* Twin-predicated operations require **two** separate and distinct
  element offsets.  The above pseudo-code algorithm will be applied
  separately and independently to each, should each of the two
  operands be remapped.  *This even includes C.LDSP* and other operations
  in that category, where in that case it will be the **offset** that is
  remapped (see Compressed Stack LOAD/STORE section).
* Setting the total elements (xdim+1) times (ydim+1) times (zdim+1) to
  less than MVL is **perfectly legal**, albeit very obscure.  It permits
  entries to be regularly presented to operands **more than once**, thus
  allowing the same underlying registers to act as an accumulator of
  multiple vector or matrix operations, for example.

Clearly here some considerable care needs to be taken as the remapping
could hypothetically create arithmetic operations that target the
exact same underlying registers, resulting in data corruption due to
pipeline overlaps.  Out-of-order / Superscalar micro-architectures with
register-renaming will have an easier time dealing with this than
DSP-style SIMD micro-architectures.

# Instruction Execution Order

Simple-V behaves as if it is a hardware-level "macro expansion system",
substituting and expanding a single instruction into multiple sequential
instructions with contiguous and sequentially-incrementing registers.
As such, it does **not** modify - or specify - the behaviour and semantics of
the execution order: that may be deduced from the **existing** RV
specification in each and every case.

So for example if a particular micro-architecture permits out-of-order
execution, and it is augmented with Simple-V, then wherever instructions
may be out-of-order then so may the "post-expansion" SV ones.

If on the other hand there are memory guarantees which specifically
prevent and prohibit certain instructions from being re-ordered
(such as the Atomicity Axiom, or FENCE constraints), then clearly
those constraints **MUST** also be obeyed "post-expansion".

It should be absolutely clear that SV is **not** about providing new
functionality or changing the existing behaviour of a micro-architetural
design, or about changing the RISC-V Specification.
It is **purely** about compacting what would otherwise be contiguous
instructions that use sequentially-increasing register numbers down
to the **one** instruction.

# Instructions

Despite being a 98% complete and accurate topological remap of RVV
concepts and functionality, no new instructions are needed.
Compared to RVV: *All* RVV instructions can be re-mapped, however xBitManip
becomes a critical dependency for efficient manipulation of predication
masks (as a bit-field).  Despite the removal of all operations,
with the exception of CLIP and VSELECT.X
*all instructions from RVV Base are topologically re-mapped and retain their
complete functionality, intact*.  Note that if RV64G ever had
a MV.X added as well as FCLIP, the full functionality of RVV-Base would
be obtained in SV.

Three instructions, VSELECT, VCLIP and VCLIPI, do not have RV Standard
equivalents, so are left out of Simple-V.  VSELECT could be included if
there existed a MV.X instruction in RV (MV.X is a hypothetical
non-immediate variant of MV that would allow another register to
specify which register was to be copied).  Note that if any of these three
instructions are added to any given RV extension, their functionality
will be inherently parallelised.

With some exceptions, where it does not make sense or is simply too
challenging, all RV-Base instructions are parallelised:

* CSR instructions, whilst a case could be made for fast-polling of
  a CSR into multiple registers, would require guarantees of strict
  sequential ordering that SV does not provide.  Therefore, CSRs are
  not really suitable and are left out.
* LUI, C.J, C.JR, WFI, AUIPC are not suitable for parallelising so are
  left as scalar.
* LR/SC could hypothetically be parallelised however their purpose is
  single (complex) atomic memory operations where the LR must be followed
  up by a matching SC.  A sequence of parallel LR instructions followed
  by a sequence of parallel SC instructions therefore is guaranteed to
  not be useful. Not least: the guarantees of LR/SC
  would be impossible to provide if emulated in a trap.
* EBREAK, NOP, FENCE and others do not use registers so are not inherently
  paralleliseable anyway.

All other operations using registers are automatically parallelised.
This includes AMOMAX, AMOSWAP and so on, where particular care and
attention must be paid.

Example pseudo-code for an integer ADD operation (including scalar operations).
Floating-point uses fp csrs.

    function op_add(rd, rs1, rs2) # add not VADD!
      int i, id=0, irs1=0, irs2=0;
      predval = get_pred_val(FALSE, rd);
      rd  = int_vec[rd ].isvector ? int_vec[rd ].regidx : rd;
      rs1 = int_vec[rs1].isvector ? int_vec[rs1].regidx : rs1;
      rs2 = int_vec[rs2].isvector ? int_vec[rs2].regidx : rs2;
      for (i = 0; i < VL; i++)
        if (predval & 1<<i) # predication uses intregs
           ireg[rd+id] <= ireg[rs1+irs1] + ireg[rs2+irs2];
        if (int_vec[rd ].isvector)  { id += 1; }
        if (int_vec[rs1].isvector)  { irs1 += 1; }
        if (int_vec[rs2].isvector)  { irs2 += 1; }

## Instruction Format

There are **no operations added to SV, at all**.
Instead SV  *overloads* pre-existing branch operations into predicated
variants, and implicitly overloads arithmetic operations, MV,
FCVT, and LOAD/STORE
depending on CSR configurations for bitwidth and
predication.  **Everything** becomes parallelised.  *This includes
Compressed instructions* as well as any
future instructions and Custom Extensions.

## Branch Instructions

### Standard Branch <a name="standard_branch"></a>

Branch operations use standard RV opcodes that are reinterpreted to
be "predicate variants" in the instance where either of the two src
registers are marked as vectors (active=1, vector=1).

Note that he predication register to use (if one is enabled) is taken from
the *first* src register.  The target (destination) predication register
to use (if one is enabled) is taken from the *second* src register.

If either of src1 or src2 are scalars (whether by there being no
CSR register entry or whether by the CSR entry specifically marking
the register as "scalar") the comparison goes ahead as vector-scalar
or scalar-vector.

In instances where no vectorisation is detected on either src registers
the operation is treated as an absolutely standard scalar branch operation.
Where vectorisation is present on either or both src registers, the
branch may stil go ahead if any only if *all* tests succeed (i.e. excluding
those tests that are predicated out).

Note that just as with the standard (scalar, non-predicated) branch
operations, BLE, BGT, BLEU and BTGU may be synthesised by inverting
src1 and src2.

In Hwacha EECS-2015-262 Section 6.7.2 the following pseudocode is given
for predicated compare operations of function "cmp":

    for (int i=0; i<vl; ++i)
      if ([!]preg[p][i])
         preg[pd][i] = cmp(s1 ? vreg[rs1][i] : sreg[rs1],
                           s2 ? vreg[rs2][i] : sreg[rs2]);

With associated predication, vector-length adjustments and so on,
and temporarily ignoring bitwidth (which makes the comparisons more
complex), this becomes:

    s1 = reg_is_vectorised(src1);
    s2 = reg_is_vectorised(src2);

    if not s1 && not s2
        if cmp(rs1, rs2) # scalar compare
            goto branch
        return

    preg = int_pred_reg[rd]
    reg = int_regfile

    ps = get_pred_val(I/F==INT, rs1);
    rd = get_pred_val(I/F==INT, rs2); # this may not exist

    if not exists(rd)
        temporary_result = 0
    else
        preg[rd] = 0; # initialise to zero

    for (int i = 0; i < VL; ++i)
      if (ps & (1<<i)) && (cmp(s1 ? reg[src1+i]:reg[src1],
                               s2 ? reg[src2+i]:reg[src2])
          if not exists(rd)
              temporary_result |= 1<<i;
          else
              preg[rd] |= 1<<i;  # bitfield not vector

     if not exists(rd)
        if temporary_result == ps
            goto branch
     else
        if preg[rd] == ps
            goto branch

Notes:

* zeroing has been temporarily left out of the above pseudo-code,
  for clarity
* Predicated SIMD comparisons would break src1 and src2 further down
  into bitwidth-sized chunks (see Appendix "Bitwidth Virtual Register
  Reordering") setting Vector-Length times (number of SIMD elements) bits
  in Predicate Register rd, as opposed to just Vector-Length bits.

TODO: predication now taken from src2.  also branch goes ahead
if all compares are successful.

Note also that where normally, predication requires that there must
also be a CSR register entry for the register being used in order
for the **predication** CSR register entry to also be active,
for branches this is **not** the case.  src2 does **not** have
to have its CSR register entry marked as active in order for
predication on src2 to be active.

### Floating-point Comparisons

There does not exist floating-point branch operations, only compare.
Interestingly no change is needed to the instruction format because
FP Compare already stores a 1 or a zero in its "rd" integer register
target, i.e. it's not actually a Branch at all: it's a compare.
Thus, no change is made to the floating-point comparison, so

It is however noted that an entry "FNE" (the opposite of FEQ) is missing,
and whilst in ordinary branch code this is fine because the standard
RVF compare can always be followed up with an integer BEQ or a BNE (or
a compressed comparison to zero or non-zero), in predication terms that
becomes more of an impact.  To deal with this, SV's predication has
had "invert" added to it.

### Compressed Branch Instruction

Compressed Branch instructions are, just like standard Branch instructions,
reinterpreted to be vectorised and predicated based on the source register
(rs1s) CSR entries.  As however there is only the one source register,
given that c.beqz a10 is equivalent to beqz a10,x0, the optional target
to store the results of the comparisions is taken from CSR predication
table entries for **x0**.

The specific required use of x0 is, with a little thought, quite obvious,
but is counterintuitive.  Clearly it is **not** recommended to redirect
x0 with a CSR register entry, however as a means to opaquely obtain
a predication target it is the only sensible option that does not involve
additional special CSRs (or, worse, additional special opcodes).

Note also that, just as with standard branches, the 2nd source
(in this case x0 rather than src2) does **not** have to have its CSR
register table marked as "active" in order for predication to work.

## Vectorised Dual-operand instructions

There is a series of 2-operand instructions involving copying (and
sometimes alteration):

* C.MV
* FMV, FNEG, FABS, FCVT, FSGNJ, FSGNJN and FSGNJX
* C.LWSP, C.SWSP, C.LDSP, C.FLWSP etc.
* LOAD(-FP) and STORE(-FP)

All of these operations follow the same two-operand pattern, so it is
*both* the source *and* destination predication masks that are taken into
account.  This is different from
the three-operand arithmetic instructions, where the predication mask
is taken from the *destination* register, and applied uniformly to the
elements of the source register(s), element-for-element.

The pseudo-code pattern for twin-predicated operations is as
follows:

    function op(rd, rs):
      rd = int_csr[rd].active ? int_csr[rd].regidx : rd;
      rs = int_csr[rs].active ? int_csr[rs].regidx : rs;
      ps = get_pred_val(FALSE, rs); # predication on src
      pd = get_pred_val(FALSE, rd); # ... AND on dest
      for (int i = 0, int j = 0; i < VL && j < VL;):
        if (int_csr[rs].isvec) while (!(ps & 1<<i)) i++;
        if (int_csr[rd].isvec) while (!(pd & 1<<j)) j++;
        reg[rd+j] = SCALAR_OPERATION_ON(reg[rs+i])
        if (int_csr[rs].isvec) i++;
        if (int_csr[rd].isvec) j++;

This pattern covers scalar-scalar, scalar-vector, vector-scalar
and vector-vector, and predicated variants of all of those.
Zeroing is not presently included (TODO).  As such, when compared
to RVV, the twin-predicated variants of C.MV and FMV cover
**all** standard vector operations: VINSERT, VSPLAT, VREDUCE,
VEXTRACT, VSCATTER, VGATHER, VCOPY, and more.

Note that:

* elwidth (SIMD) is not covered in the pseudo-code above
* ending the loop early in scalar cases (VINSERT, VEXTRACT) is also
  not covered
* zero predication is also not shown (TODO).

### C.MV Instruction <a name="c_mv"></a>

There is no MV instruction in RV however there is a C.MV instruction.
It is used for copying integer-to-integer registers (vectorised FMV
is used for copying floating-point).

If either the source or the destination register are marked as vectors
C.MV is reinterpreted to be a vectorised (multi-register) predicated
move operation.  The actual instruction's format does not change:

[[!table  data="""
15  12 | 11   7 | 6  2 | 1  0 |
funct4 | rd     | rs   | op   |
4      | 5      | 5    | 2    |
C.MV   | dest   | src  | C0   |
"""]]

A simplified version of the pseudocode for this operation is as follows:

    function op_mv(rd, rs) # MV not VMV!
      rd = int_csr[rd].active ? int_csr[rd].regidx : rd;
      rs = int_csr[rs].active ? int_csr[rs].regidx : rs;
      ps = get_pred_val(FALSE, rs); # predication on src
      pd = get_pred_val(FALSE, rd); # ... AND on dest
      for (int i = 0, int j = 0; i < VL && j < VL;):
        if (int_csr[rs].isvec) while (!(ps & 1<<i)) i++;
        if (int_csr[rd].isvec) while (!(pd & 1<<j)) j++;
        ireg[rd+j] <= ireg[rs+i];
        if (int_csr[rs].isvec) i++;
        if (int_csr[rd].isvec) j++;

There are several different instructions from RVV that are covered by
this one opcode:

[[!table  data="""
src    | dest    | predication   | op             |
scalar | vector  | none          | VSPLAT         |
scalar | vector  | destination   | sparse VSPLAT  |
scalar | vector  | 1-bit dest    | VINSERT        |
vector | scalar  | 1-bit? src    | VEXTRACT       |
vector | vector  | none          | VCOPY          |
vector | vector  | src           | Vector Gather  |
vector | vector  | dest          | Vector Scatter |
vector | vector  | src & dest    | Gather/Scatter |
vector | vector  | src == dest   | sparse VCOPY   |
"""]]

Also, VMERGE may be implemented as back-to-back (macro-op fused) C.MV
operations with inversion on the src and dest predication for one of the
two C.MV operations.

Note that in the instance where the Compressed Extension is not implemented,
MV may be used, but that is a pseudo-operation mapping to addi rd, x0, rs.
Note that the behaviour is **different** from C.MV because with addi the
predication mask to use is taken **only** from rd and is applied against
all elements: rs[i] = rd[i].

### FMV, FNEG and FABS Instructions

These are identical in form to C.MV, except covering floating-point
register copying.  The same double-predication rules also apply.
However when elwidth is not set to default the instruction is implicitly
and automatic converted to a (vectorised) floating-point type conversion
operation of the appropriate size covering the source and destination
register bitwidths.

(Note that FMV, FNEG and FABS are all actually pseudo-instructions)

### FVCT Instructions

These are again identical in form to C.MV, except that they cover
floating-point to integer and integer to floating-point.  When element
width in each vector is set to default, the instructions behave exactly
as they are defined for standard RV (scalar) operations, except vectorised
in exactly the same fashion as outlined in C.MV.

However when the source or destination element width is not set to default,
the opcode's explicit element widths are *over-ridden* to new definitions,
and the opcode's element width is taken as indicative of the SIMD width
(if applicable i.e. if packed SIMD is requested) instead.

For example FCVT.S.L would normally be used to convert a 64-bit
integer in register rs1 to a 64-bit floating-point number in rd.
If however the source rs1 is set to be a vector, where elwidth is set to
default/2 and "packed SIMD" is enabled, then the first 32 bits of
rs1 are converted to a floating-point number to be stored in rd's
first element and the higher 32-bits *also* converted to floating-point
and stored in the second.  The 32 bit size comes from the fact that
FCVT.S.L's integer width is 64 bit, and with elwidth on rs1 set to
divide that by two it means that rs1 element width is to be taken as 32.

Similar rules apply to the destination register.

## LOAD / STORE Instructions and LOAD-FP/STORE-FP <a name="load_store"></a>

An earlier draft of SV modified the behaviour of LOAD/STORE.  This
actually undermined the fundamental principle of SV, namely that there
be no modifications to the scalar behaviour (except where absolutely
necessary), in order to simplify an implementor's task if considering
converting a pre-existing scalar design to support parallelism.

So the original RISC-V scalar LOAD/STORE and LOAD-FP/STORE-FP functionality
do not change in SV, however just as with C.MV it is important to note
that dual-predication is possible.  Using the template outlined in
the section "Vectorised dual-op instructions", the pseudo-code covering
scalar-scalar, scalar-vector, vector-scalar and vector-vector applies,
where SCALAR\_OPERATION is as follows, exactly as for a standard
scalar RV LOAD operation:

        srcbase = ireg[rs+i];
        return mem[srcbase + imm];

Whilst LOAD and STORE remain as-is when compared to their scalar
counterparts, the incrementing on the source register (for LOAD)
means that pointers-to-structures can be easily implemented, and
if contiguous offsets are required, those pointers (the contents
of the contiguous source registers) may simply be set up to point
to contiguous locations.

## Compressed Stack LOAD / STORE Instructions <a name="c_ld_st"></a>

C.LWSP / C.SWSP and floating-point etc. are also source-dest twin-predicated,
where it is implicit in C.LWSP/FLWSP that x2 is the source register.
It is therefore possible to use predicated C.LWSP to efficiently
pop registers off the stack (by predicating x2 as the source), cherry-picking
which registers to store to (by predicating the destination).  Likewise
for C.SWSP.  In this way, LOAD/STORE-Multiple is efficiently achieved.

However, to do so, the behaviour of C.LWSP/C.SWSP needs to be slightly
different: where x2 is marked as vectorised, instead of incrementing
the register on each loop (x2, x3, x4...), instead it is the *immediate*
that must be incremented.  Pseudo-code follows:

    function lwsp(rd, rs):
      rd = int_csr[rd].active ? int_csr[rd].regidx : rd;
      rs = x2 # effectively no redirection on x2.
      ps = get_pred_val(FALSE, rs); # predication on src
      pd = get_pred_val(FALSE, rd); # ... AND on dest
      for (int i = 0, int j = 0; i < VL && j < VL;):
        if (int_csr[rs].isvec) while (!(ps & 1<<i)) i++;
        if (int_csr[rd].isvec) while (!(pd & 1<<j)) j++;
        reg[rd+j] = mem[x2 + ((offset+i) * 4)]
        if (int_csr[rs].isvec) i++;
        if (int_csr[rd].isvec) j++;

For C.LDSP, the offset (and loop) multiplier would be 8, and for
C.LQSP it would be 16.  Effectively this makes C.LWSP etc. a Vector
"Unit Stride" Load instruction.

**Note**: it is still possible to redirect x2 to an alternative target
register.  With care, this allows C.LWSP / C.SWSP (and C.FLWSP) to be used as
general-purpose Vector "Unit Stride" LOAD/STORE operations.

## Compressed LOAD / STORE Instructions

Compressed LOAD and STORE are again exactly the same as scalar LOAD/STORE,
where the same rules apply and the same pseudo-code apply as for
non-compressed LOAD/STORE.  This is **different** from Compressed Stack
LOAD/STORE (C.LWSP / C.SWSP), which have been augmented to become
Vector "Unit Stride" capable.

Just as with uncompressed LOAD/STORE C.LD / C.ST increment the *register*
during the hardware loop, **not** the offset.

# Element bitwidth polymorphism

Element bitwidth is best covered as its own special section, as it
is quite involved and applies uniformly across-the-board.

# Exceptions

TODO: expand.  Exceptions may occur at any time, in any given underlying
scalar operation.  This implies that context-switching (traps) may
occur, and operation must be returned to where it left off.  That in
turn implies that the full state - including the current parallel
element being processed - has to be saved and restored.  This is
what the **STATE** CSR is for.

The implications are that all underlying individual scalar operations
"issued" by the parallelisation have to appear to be executed sequentially.
The further implications are that if two or more individual element
operations are underway, and one with an earlier index causes an exception,
it may be necessary for the microarchitecture to **discard** or terminate
operations with higher indices.

This being somewhat dissatisfactory, an "opaque predication" variant
of the STATE CSR is being considered.

> And what about instructions like JALR? 

answer: they're not vectorised, so not a problem

# Hints

A "HINT" is an operation that has no effect on architectural state,
where its use may, by agreed convention, give advance notification
to the microarchitecture: branch prediction notification would be
a good example.  Usually HINTs are where rd=x0.

With Simple-V being capable of issuing *parallel* instructions where
rd=x0, the space for possible HINTs is expanded considerably.  VL
could be used to indicate different hints.  In addition, if predication
is set, the predication register itself could hypothetically be passed
in as a *parameter* to the HINT operation.

No specific hints are yet defined in Simple-V

# Subsets of RV functionality

This section describes the differences when SV is implemented on top of
different subsets of RV.

## Common options

It is permitted to limit the size of either (or both) the register files
down to the original size of the standard RV architecture.  However, below
the mandatory limits set in the RV standard will result in non-compliance
with the SV Specification.

## RV32 / RV32F

When RV32 or RV32F is implemented, XLEN is set to 32, and thus the
maximum limit for predication is also restricted to 32 bits.  Whilst not
actually specifically an "option" it is worth noting.

## RV32G

Normally in standard RV32 it does not make much sense to have
RV32G, however it is automatically implied to exist in RV32+SV due to
the option for the element width to be doubled.  This may be sufficient
for implementors, such that actually needing RV32G itself (which makes
no sense given that the RV32 integer register file is 32-bit) may be
redundant.

It is a strange combination that may make sense on closer inspection,
particularly given that under the standard RV32 system many of the opcodes
to convert and sign-extend 64-bit integers to 64-bit floating-point will
be missing, as they are assumed to only be present in an RV64 context.

## RV32 (not RV32F / RV32G) and RV64 (not RV64F / RV64G)

When floating-point is not implemented, the size of the User Register and
Predication CSR tables may be halved, to only 4 2x16-bit CSRs (8 entries
per table).

## RV32E

In embedded scenarios the User Register and Predication CSRs may be
dropped entirely, or optionally limited to 1 CSR, such that the combined
number of entries from the M-Mode CSR Register table plus U-Mode
CSR Register table is either 4 16-bit entries or (if the U-Mode is
zero) only 2 16-bit entries (M-Mode CSR table only).  Likewise for
the Predication CSR tables.

RV32E is the most likely candidate for simply detecting that registers
are marked as "vectorised", and generating an appropriate exception
for the VL loop to be implemented in software.

## RV128

RV128 has not been especially considered, here, however it has some
extremely large possibilities: double the element width implies
256-bit operands, spanning 2 128-bit registers each, and predication
of total length 128 bit given that XLEN is now 128.

# Under consideration <a name="issues"></a>

for element-grouping, if there is unused space within a register
(3 16-bit elements in a 64-bit register for example), recommend:

* For the unused elements in an integer register, the used element
  closest to the MSB is sign-extended on write and the unused elements
  are ignored on read.
* The unused elements in a floating-point register are treated as-if
  they are set to all ones on write and are ignored on read, matching the
  existing standard for storing smaller FP values in larger registers.