add abridged spec, split out vblock format to own file

[libreriscv.git] / simple_v_extension / abridged_spec.mdwn

# Simple-V (Parallelism Extension Proposal) Specification (Abridged)

* Copyright (C) 2017, 2018, 2019 Luke Kenneth Casson Leighton
* Status: DRAFTv0.6
* Last edited: 25 jun 2019

[[!toc ]]

# Introduction

Simple-V is a uniform parallelism API for RISC-V hardware that allows
the Program Counter to enter "sub-contexts" in which, ultimately, standard
RISC-V scalar opcodes are executed.

The sub-context execution is "nested" in "re-entrant" form, in the
following order:

* Main standard RISC-V Program Counter (PC)
* VBLOCK sub-execution context (PCVBLK increments whilst PC is paused)
* VL element loops (STATE srcoffs and destoffs increment, PC and PCVBLK pause)
* SUBVL element loops (STATE svsrcoffs/svdestoffs increment, VL pauses)

Note: **there are *no* new opcodes**. The scheme works *entirely*
on hidden context that augments *scalar* RISCV instructions.

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

There are five CSRs, available in any privilege level:

* MVL (the Maximum Vector Length)
* VL (which has different characteristics from standard CSRs)
* SUBVL (effectively a kind of SIMD)
* STATE (containing copies of MVL, VL and SUBVL as well as context information)
* PCVBLK (the current operation being executed within a VBLOCK Group)

For Privilege Levels (trap handling) there are the following CSRs,
where x may be u, m, s or h for User, Machine, Supervisor or Hypervisor
Modes respectively:

* (x)ePCVBLK (a copy of the sub-execution Program Counter, that is relative
  to the start of the current VBLOCK Group, set on a trap).
* (x) eSTATE (useful for saving and restoring during context switch,
  and for providing fast transitions)

The u/m/s CSRs are treated and handled exactly like their (x)epc
equivalents.  On entry to or exit from a privilege level, the contents
of its (x)eSTATE are swapped with STATE.

(x)EPCVBLK CSRs must be treated exactly like their corresponding (x)epc
equivalents. See VBLOCK section for details.

## MAXVECTORLENGTH (MVL) <a name="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).

## Vector Length (VL) <a name="vl" />

VSETVL is slightly different from RVV.  Similar to 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

## SUBVL - Sub Vector Length

This is a "group by quantity" that effectivrly asks each iteration
of the hardware loop to load SUBVL elements of width elwidth at a
time. Effectively, SUBVL is like a SIMD multiplier: instead of just 1
operation issued, SUBVL operations are issued.

The main effect of SUBVL is that predication bits are applied per
**group**, rather than by individual element.

## STATE

This is a standard CSR that contains sufficient information for a
full context save/restore.  It contains (and permits setting of):

* MVL
* VL
* destoffs - the destination element offset of the current parallel
  instruction being executed
* srcoffs - for twin-predication, the source element offset as well.
* SUBVL
* svdestoffs - the subvector destination element offset of the current
  parallel instruction being executed
* svsrcoffs - for twin-predication, the subvector source element offset
  as well.

The format of the STATE CSR is as follows:

| (29..28 | (27..26) | (25..24) | (23..18) | (17..12) | (11..6) | (5...0) |
| ------- | -------- | -------- | -------- | -------- | ------- | ------- |
| dsvoffs | ssvoffs  | subvl    | destoffs | srcoffs  | vl      | maxvl   |

Notes:

* The entries are truncated to be within range.  Attempts to set VL to
  greater than MAXVL will truncate VL.
* Both VL and MAXVL are stored offset by one.  0b000000 represents VL=1,
  0b000001 represents VL=2.  This allows the full range 1 to XLEN instead
  of 0 to only 63.

## VL, MVL and SUBVL instruction aliases

This table contains pseudo-assembly instruction aliases. Note the
subtraction of 1 from the CSRRWI pseudo variants, to compensate for the
reduced range of the 5 bit immediate.

| alias           | CSR                  |
| -               | -                    |
| SETVL rd, rs    | CSRRW  VL, rd, rs    |
| SETVLi rd, #n   | CSRRWI VL, rd, #n-1  |
| GETVL rd        | CSRRW  VL, rd, x0    |
| SETMVL rd, rs   | CSRRW  MVL, rd, rs   |
| SETMVLi rd, #n  | CSRRWI MVL,rd, #n-1  |
| GETMVL rd       | CSRRW  MVL, rd, x0   |

Note: CSRRC and other bitsetting may still be used, they are however not particularly useful (very obscure).

## Register key-value (CAM) table <a name="regcsrtable" />

The purpose of the Register table is to mark which registers change behaviour
if used in a "Standard" (normally scalar) opcode.

16 bit format:

| RegCAM | 15       | (14..8)  | 7   | (6..5) | (4..0)  |
| ------ | -        | -        | -   | ------ | ------- |
| 0      | isvec0   | regidx0  | i/f | vew0   | regkey  |
| 1      | isvec1   | regidx1  | i/f | vew1   | regkey  |
| 2      | isvec2   | regidx2  | i/f | vew2   | regkey  |
| 3      | isvec3   | regidx3  | i/f | vew3   | regkey  |

8 bit format:

| RegCAM | | 7   | (6..5) | (4..0)  |
| ------ | | -   | ------ | ------- |
| 0      | | i/f | vew0   | regnum  |

Mapping the 8-bit to 16-bit format:

| RegCAM | 15      | (14..8)    | 7   | (6..5) | (4..0)  |
| ------ | -       | -          | -   | ------ | ------- |
| 0      | isvec=1 | regnum0<<2 | i/f | vew0   | regnum0 |
| 1      | isvec=1 | regnum1<<2 | i/f | vew1   | regnum1 |
| 2      | isvec=1 | regnum2<<2 | i/f | vew2   | regnum2 |
| 3      | isvec=1 | regnum2<<2 | i/f | vew3   | regnum3 |

Fields:

* 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.
* isvec indicates that the register (whether a src or dest) is to progress
  incrementally forward on each loop iteration.  this gives the "effect"
  of vectorisation.  isvec is zero indicates "do not progress", giving
  the "effect" of that register being scalar.
* vew overrides the operation's default width.  See table below
* regkey is the register which, if encountered in an op (as src or dest)
  is to be "redirected"
* in the 16-bit format, regidx is the *actual* register to be used
  for the operation (note that it is 7 bits wide)

| vew | bitwidth            |
| --- | ------------------- |
| 00  | default (XLEN/FLEN) |
| 01  | 8 bit               |
| 10  | 16 bit              |
| 11  | 32 bit              |

A useful way to view the above table (and not have it as a CAM):

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

    struct vectorised {
        bool isvector:1; 
        int  vew:2; 
        bool enabled:1;
        int  predidx:7;  
    }

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

    for (i = 0; i < len; i++) // from VBLOCK Format
       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].enabled  = true;

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

The Predication Table 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. Like the Register table, it
is an indirect lookup that allows the RV opcodes to not need modification.

* regidx is the register that in combination with the
  i/f flag, if that integer or floating-point register is referred to in a
  (standard RV) instruction results in the lookup table being referenced
  to find the predication mask to use for this operation.
* predidx is the *actual* (full, 7 bit) register to be used for the
  predication mask.
* inv indicates that the predication mask bits are to be inverted
  prior to use *without* actually modifying the contents of the
  registerfrom which those bits originated.
* 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)
* ffirst is a special mode that stops sequential element processing when
  a data-dependent condition occurs, whether a trap or a conditional test.
  The handling of each (trap or conditional test) is slightly different:
  see Instruction sections for further details

16 bit format:

| PrCSR | (15..11) | 10     | 9     | 8   | (7..1)  | 0       |
| ----- | -        | -      | -     | -   | ------- | ------- |
| 0     | predidx  | zero0  | inv0  | i/f | regidx  | ffirst0 |
| 1     | predidx  | zero1  | inv1  | i/f | regidx  | ffirst1 |
| 2     | predidx  | zero2  | inv2  | i/f | regidx  | ffirst2 |
| 3     | predidx  | zero3  | inv3  | i/f | regidx  | ffirst3 |

Note: predidx=x0, zero=1, inv=1 is a RESERVED encoding.  Its use must
generate an illegal instruction trap.

8 bit format:

| PrCSR | 7     | 6     | 5   | (4..0)  |
| ----- | -     | -     | -   | ------- |
| 0     | zero0 | inv0  | i/f | regnum  |

Mapping from 8 to 16 bit format, the table becomes:

| PrCSR | (15..11) | 10     | 9     | 8   | (7..1)  | 0       |
| ----- | -        | -      | -     | -   | ------- | ------- |
| 0     | x9       | zero0  | inv0  | i/f | regnum  | ff=0    |
| 1     | x10      | zero1  | inv1  | i/f | regnum  | ff=0    |
| 2     | x11      | zero2  | inv2  | i/f | regnum  | ff=0    |
| 3     | x12      | zero3  | inv3  | i/f | regnum  | ff=0    |

Pseudocode for predication:

    struct pred {
        bool zero;    // zeroing
        bool inv;     // register at predidx is inverted
        bool ffirst;  // fail-on-first
        bool enabled; // use this to tell if the table-entry is active
        int predidx;  // redirection: actual int register to use
    }

    struct pred fp_pred_reg[32];
    struct pred int_pred_reg[32];

    for (i = 0; i < len; i++) // number of Predication entries in VBLOCK
      tb = int_pred_reg if PredicateTable[i].type == 0 else fp_pred_reg;
      idx = VBLOCKPredicateTable[i].regidx
      tb[idx].zero     = CSRpred[i].zero
      tb[idx].inv      = CSRpred[i].inv
      tb[idx].ffirst   = CSRpred[i].ffirst
      tb[idx].predidx  = CSRpred[i].predidx
      tb[idx].enabled  = true

    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

## Fail-on-First Mode <a name="ffirst-mode"></a>

ffirst is a special data-dependent predicate mode.  There are two
variants: one is for faults: typically for LOAD/STORE operations,
which may encounter end of page faults during a series of operations.
The other variant is comparisons such as FEQ (or the augmented behaviour
of Branch), and any operation that returns a result of zero (whether
integer or floating-point).  In the FP case, this includes negative-zero.

Note that the execution order must "appear" to be sequential for ffirst
mode to work correctly.  An in-order architecture must execute the element
operations in sequence, whilst an out-of-order architecture must *commit*
the element operations in sequence (giving the appearance of in-order
execution).

Note also, that if ffirst mode is needed without predication, a special
"always-on" Predicate Table Entry may be constructed by setting
inverse-on and using x0 as the predicate register.  This
will have the effect of creating a mask of all ones, allowing ffirst
to be set.

### Fail-on-first traps

Except for the first element, ffault stops sequential element processing
when a trap occurs.  The first element is treated normally (as if ffirst
is clear).  Should any subsequent element instruction require a trap,
instead it and subsequent indexed elements are ignored (or cancelled in
out-of-order designs), and VL is set to the *last* instruction that did
not take the trap.

Note that predicated-out elements (where the predicate mask bit is zero)
are clearly excluded (i.e. the trap will not occur).  However, note that
the loop still had to test the predicate bit: thus on return,
VL is set to include elements that did not take the trap *and* includes
the elements that were predicated (masked) out (not tested up to the
point where the trap occurred).

If SUBVL is being used (SUBVL!=1), the first *sub-group* of elements
will cause a trap as normal (as if ffirst is not set); subsequently,
the trap must not occur in the *sub-group* of elements.  SUBVL will **NOT**
be modified.

Given that predication bits apply to SUBVL groups, the same rules apply
to predicated-out (masked-out) sub-groups in calculating the value that VL
is set to.

### Fail-on-first conditional tests

ffault stops sequential element conditional testing on the first element result
being zero.  VL is set to the number of elements that were processed before
the fail-condition was encountered.

Note that just as with traps, if SUBVL!=1, the first of any of the *sub-group*
will cause the processing to end, and, even if there were elements within
the *sub-group* that passed the test, that sub-group is still (entirely)
excluded from the count (from setting VL).  i.e. VL is set to the total
number of *sub-groups* that had no fail-condition up until execution was
stopped.

Note again that, just as with traps, predicated-out (masked-out) elements
are included in the count leading up to the fail-condition, even though they
were not tested.

The pseudo-code for Predication makes this clearer and simpler than it is
in words (the loop ends, VL is set to the current element index, "i").

# Instructions <a name="instructions" />

To illustrate how Scalar operations are turned "vector" and "predicated",
simplified example pseudo-code for an integer ADD operation is shown below.
Floating-point would use the FP Register Table.

    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++)
        xSTATE.srcoffs = i # save context
        if (predval & 1<<i) # predication uses intregs
           ireg[rd+id] <= ireg[rs1+irs1] + ireg[rs2+irs2];
           if (!int_vec[rd ].isvector) break;
        if (int_vec[rd ].isvector)  { id += 1; }
        if (int_vec[rs1].isvector)  { irs1 += 1; }
        if (int_vec[rs2].isvector)  { irs2 += 1; }

Note that for simplicity there is quite a lot missing from the above
pseudo-code.

## SUBVL Pseudocode <a name="subvl-pseudocode"></a>

Adding in support for SUBVL is a matter of adding in an extra inner
for-loop, where register src and dest are still incremented inside the
inner part. Not that the predication is still taken from the VL index.

So whilst elements are indexed by "(i * SUBVL + s)", predicate bits are
indexed by "(i)"

    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++)
       xSTATE.srcoffs = i # save context
       for (s = 0; s < SUBVL; s++)
        xSTATE.ssvoffs = s # save context
        if (predval & 1<<i) # predication uses intregs
           # actual add is here (at last)
           ireg[rd+id] <= ireg[rs1+irs1] + ireg[rs2+irs2];
           if (!int_vec[rd ].isvector) break;
        if (int_vec[rd ].isvector)  { id += 1; }
        if (int_vec[rs1].isvector)  { irs1 += 1; }
        if (int_vec[rs2].isvector)  { irs2 += 1; }
        if (id == VL or irs1 == VL or irs2 == VL) {
          # end VL hardware loop
          xSTATE.srcoffs = 0; # reset
          xSTATE.ssvoffs = 0; # reset
          return;
        }

NOTE: pseudocode simplified greatly: zeroing, proper predicate handling,
elwidth handling etc. all left out.

## Instruction Format

It is critical to appreciate that there are
**no operations added to SV, at all**.

Examples are given below where "standard" RV scalar behaviour is augmented.

## Branch Instructions

Branch operations are augmented slightly to be a little more like FP
Compares (FEQ, FNE etc.), by permitting the cumulation (and storage)
of multiple comparisons into a register (taken indirectly from the predicate
table).  As such, "ffirst" - fail-on-first - condition mode can be enabled.
See ffirst mode in the Predication Table section.

### 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 the predication register to use (if one is enabled) is taken from
the *first* src register, and that this is used, just as with predicated
arithmetic operations, to mask whether the comparison operations take
place or not.  If the second register is also marked as predicated,
that (scalar) predicate register is used as a **destination** to store
the results of all the comparisons.

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).

Pseudo-code for branch:

    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) or zeroing:
        result = 0
    else
        result = preg[rd]

    for (int i = 0; i < VL; ++i)
      if (zeroing)
        if not (ps & (1<<i))
           result &= ~(1<<i);
      else if (ps & (1<<i))
          if (cmp(s1 ? reg[src1+i]:reg[src1],
                               s2 ? reg[src2+i]:reg[src2])
              result |= 1<<i;
          else
              result &= ~(1<<i);

     if not exists(rd)
        if result == ps
            goto branch
     else
        preg[rd] = result # store in destination
        if preg[rd] == ps
            goto branch

Notes:

* 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.
* The execution of "parallelised" instructions **must** be implemented
  as "re-entrant" (to use a term from software).  If an exception (trap)
  occurs during the middle of a vectorised
  Branch (now a SV predicated compare) operation, the partial results
  of any comparisons must be written out to the destination
  register before the trap is permitted to begin.  If however there
  is no predicate, the **entire** set of comparisons must be **restarted**,
  with the offset loop indices set back to zero.  This is because
  there is no place to store the temporary result during the handling
  of traps.

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.

As RV Scalar does not have "FNE", predication inversion must be used.
Also: note that FP Compare may be predicated, using the destination
integer register (rd) to determine the predicate.  FP Compare is **not**
a twin-predication operation, as, again, just as with SV Branches,
there are three registers involved: FP src1, FP src2 and INT rd.

Also: note that ffirst (fail first mode) applies directly to this operation.

### 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++;
        xSTATE.srcoffs = i # save context
        xSTATE.destoffs = j # save context
        reg[rd+j] = SCALAR_OPERATION_ON(reg[rs+i])
        if (int_csr[rs].isvec) i++;
        if (int_csr[rd].isvec) j++; else break

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.

### 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.

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.

### 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.

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

In vectorised architectures there are usually at least two different modes
for LOAD/STORE:

* Read (or write for STORE) from sequential locations, where one
  register specifies the address, and the one address is incremented
  by a fixed amount.  This is usually known as "Unit Stride" mode.
* Read (or write) from multiple indirected addresses, where the
  vector elements each specify separate and distinct addresses.

To support these different addressing modes, the CSR Register "isvector"
bit is used.  So, for a LOAD, when the src register is set to
scalar, the LOADs are sequentially incremented by the src register
element width, and when the src register is set to "vector", the
elements are treated as indirection addresses.  Simplified
pseudo-code would look like this:

    function op_ld(rd, rs) # LD not VLD!
      rdv = int_csr[rd].active ? int_csr[rd].regidx : rd;
      rsv = 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++;
        if (int_csr[rd].isvec)
          # indirect mode (multi mode)
          srcbase = ireg[rsv+i];
        else
          # unit stride mode
          srcbase = ireg[rsv] + i * XLEN/8; # offset in bytes
        ireg[rdv+j] <= mem[srcbase + imm_offs];
        if (!int_csr[rs].isvec &&
            !int_csr[rd].isvec) break # scalar-scalar LD
        if (int_csr[rs].isvec) i++;
        if (int_csr[rd].isvec) j++;

## 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 etc. 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.

**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 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.  Again: setting scalar or vector mode
on the src for LOAD and dest for STORE switches mode from "Unit Stride"
to "Multi-indirection", respectively.

# Element bitwidth polymorphism <a name="elwidth"></a>

Element bitwidth is best covered as its own special section, as it
is quite involved and applies uniformly across-the-board.  SV restricts
bitwidth polymorphism to default, 8-bit, 16-bit and 32-bit.

The effect of setting an element bitwidth is to re-cast each entry
in the register table, and for all memory operations involving
load/stores of certain specific sizes, to a completely different width.
Thus In c-style terms, on an RV64 architecture, effectively each register
now looks like this:

    typedef union {
        uint8_t  actual_bytes[8]; // 8 for RV64, 4 for RV32, 16 for RV128
        uint8_t  b[0]; // array of type uint8_t
        uint16_t s[0];
        uint32_t i[0];
        uint64_t l[0];
        uint128_t d[0];
    } reg_t;

    reg_t int_regfile[128];

where when accessing any individual regfile[n].b entry it is permitted
(in c) to arbitrarily over-run the *declared* length of the array (zero),
and thus "overspill" to consecutive register file entries in a fashion
that is completely transparent to a greatly-simplified software / pseudo-code
representation.
It is however critical to note that it is clearly the responsibility of
the implementor to ensure that, towards the end of the register file,
an exception is thrown if attempts to access beyond the "real" register
bytes is ever attempted.

The pseudo-code is as follows, to demonstrate how the sign-extending
and width-extending works:

    typedef union {
        uint8_t  b;
        uint16_t s;
        uint32_t i;
        uint64_t l;
    } el_reg_t;

    bw(elwidth):
        if elwidth == 0:
            return xlen
        if elwidth == 1:
            return xlen / 2
        if elwidth == 2:
            return xlen * 2
        // elwidth == 3:
        return 8

    get_max_elwidth(rs1, rs2):
        return max(bw(int_csr[rs1].elwidth), # default (XLEN) if not set
                   bw(int_csr[rs2].elwidth)) # again XLEN if no entry

    get_polymorphed_reg(reg, bitwidth, offset):
        el_reg_t res;
        res.l = 0; // TODO: going to need sign-extending / zero-extending
        if bitwidth == 8:
            reg.b = int_regfile[reg].b[offset]
        elif bitwidth == 16:
            reg.s = int_regfile[reg].s[offset]
        elif bitwidth == 32:
            reg.i = int_regfile[reg].i[offset]
        elif bitwidth == 64:
            reg.l = int_regfile[reg].l[offset]
        return res

    set_polymorphed_reg(reg, bitwidth, offset, val):
        if (!int_csr[reg].isvec):
            # sign/zero-extend depending on opcode requirements, from
            # the reg's bitwidth out to the full bitwidth of the regfile
            val = sign_or_zero_extend(val, bitwidth, xlen)
            int_regfile[reg].l[0] = val
        elif bitwidth == 8:
            int_regfile[reg].b[offset] = val
        elif bitwidth == 16:
            int_regfile[reg].s[offset] = val
        elif bitwidth == 32:
            int_regfile[reg].i[offset] = val
        elif bitwidth == 64:
            int_regfile[reg].l[offset] = val

      maxsrcwid =  get_max_elwidth(rs1, rs2) # source element width(s)
      destwid = int_csr[rs1].elwidth         # destination element width
      for (i = 0; i < VL; i++)
        if (predval & 1<<i) # predication uses intregs
           // TODO, calculate if over-run occurs, for each elwidth
           src1 = get_polymorphed_reg(rs1, maxsrcwid, irs1)
           // TODO, sign/zero-extend src1 and src2 as operation requires
           if (op_requires_sign_extend_src1)
              src1 = sign_extend(src1, maxsrcwid)
           src2 = get_polymorphed_reg(rs2, maxsrcwid, irs2)
           result = src1 + src2 # actual add here
           // TODO, sign/zero-extend result, as operation requires
           if (op_requires_sign_extend_dest)
              result = sign_extend(result, maxsrcwid)
           set_polymorphed_reg(rd, destwid, ird, result)
           if (!int_vec[rd].isvector) break
        if (int_vec[rd ].isvector)  { id += 1; }
        if (int_vec[rs1].isvector)  { irs1 += 1; }
        if (int_vec[rs2].isvector)  { irs2 += 1; }

## Polymorphic floating-point operation exceptions and error-handling

For floating-point operations, conversion takes place without
raising any kind of exception.  Exactly as specified in the standard
RV specification, NAN (or appropriate) is stored if the result
is beyond the range of the destination, and, again, exactly as
with the standard RV specification just as with scalar
operations, the floating-point flag is raised (FCSR).  And, again, just as
with scalar operations, it is software's responsibility to check this flag.
Given that the FCSR flags are "accrued", the fact that multiple element
operations could have occurred is not a problem.

Note that it is perfectly legitimate for floating-point bitwidths of
only 8 to be specified.  However whilst it is possible to apply IEEE 754
principles, no actual standard yet exists.  Implementors wishing to
provide hardware-level 8-bit support rather than throw a trap to emulate
in software should contact the author of this specification before
proceeding.

## Polymorphic shift operators

A special note is needed for changing the element width of left and right
shift operators, particularly right-shift.  

For SV, where each operand's element bitwidth may be over-ridden, the
rule about determining the operation's bitwidth *still applies*, being
defined as the maximum bitwidth of RS1 and RS2.  *However*, this rule
**also applies to the truncation of RS2**.  In other words, *after*
determining the maximum bitwidth, RS2's range must **also be truncated**
to ensure a correct answer.  Example:

* RS1 is over-ridden to a 16-bit width
* RS2 is over-ridden to an 8-bit width
* RD is over-ridden to a 64-bit width
* the maximum bitwidth is thus determined to be 16-bit - max(8,16)
* RS2 is **truncated to a range of values from 0 to 15**: RS2 & (16-1)

Pseudocode (in spike) for this example would therefore be:

    WRITE_RD(sext_xlen(zext_16bit(RS1) << (RS2 & (16-1))));

## Polymorphic MULH/MULHU/MULHSU

MULH is designed to take the top half MSBs of a multiply that
does not fit within the range of the source operands, such that
smaller width operations may produce a full double-width multiply
in two cycles.  The issue is: SV allows the source operands to
have variable bitwidth.

Here again special attention has to be paid to the rules regarding
bitwidth, which, again, are that the operation is performed at
the maximum bitwidth of the **source** registers.  Therefore:

* An 8-bit x 8-bit multiply will create a 16-bit result that must
  be shifted down by 8 bits
* A 16-bit x 8-bit multiply will create a 24-bit result that must
  be shifted down by 16 bits (top 8 bits being zero)
* A 16-bit x 16-bit multiply will create a 32-bit result that must
  be shifted down by 16 bits
* A 32-bit x 16-bit multiply will create a 48-bit result that must
  be shifted down by 32 bits
* A 32-bit x 8-bit multiply will create a 40-bit result that must
  be shifted down by 32 bits

So again, just as with shift-left and shift-right, the result
is shifted down by the maximum of the two source register bitwidths.
And, exactly again, truncation or sign-extension is performed on the
result.  If sign-extension is to be carried out, it is performed
from the same maximum of the two source register bitwidths out
to the result element's bitwidth.

If truncation occurs, i.e. the top MSBs of the result are lost,
this is "Officially Not Our Problem", i.e. it is assumed that the
programmer actually desires the result to be truncated.  i.e. if the
programmer wanted all of the bits, they would have set the destination
elwidth to accommodate them.

## Polymorphic elwidth on LOAD/STORE <a name="elwidth_loadstore"></a>

Polymorphic element widths in vectorised form means that the data
being loaded (or stored) across multiple registers needs to be treated
(reinterpreted) as a contiguous stream of elwidth-wide items, where
the source register's element width is **independent** from the destination's.

This makes for a slightly more complex algorithm when using indirection
on the "addressed" register (source for LOAD and destination for STORE),
particularly given that the LOAD/STORE instruction provides important
information about the width of the data to be reinterpreted.

As LOAD/STORE may be twin-predicated, it is important to note that
the rules on twin predication still apply.  Where in previous
pseudo-code (elwidth=default for both source and target) it was
the *registers* that the predication was applied to, it is now the
**elements** that the predication is applied to.

The full pseudocode for all LD operations may be written out
as follows:

    function LBU(rd, rs):
        load_elwidthed(rd, rs, 8, true)
    function LB(rd, rs):
        load_elwidthed(rd, rs, 8, false)
    function LH(rd, rs):
        load_elwidthed(rd, rs, 16, false)
    ...
    ...
    function LQ(rd, rs):
        load_elwidthed(rd, rs, 128, false)

    # returns 1 byte of data when opwidth=8, 2 bytes when opwidth=16..
    function load_memory(rs, imm, i, opwidth):
        elwidth = int_csr[rs].elwidth
        bitwidth = bw(elwidth);
        elsperblock = min(1, opwidth / bitwidth)
        srcbase = ireg[rs+i/(elsperblock)];
        offs = i % elsperblock;
        return mem[srcbase + imm + offs]; # 1/2/4/8/16 bytes

    function load_elwidthed(rd, rs, opwidth, unsigned):
      destwid = int_csr[rd].elwidth # destination element width
      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++;
        val = load_memory(rs, imm, i, opwidth)
        if unsigned:
            val = zero_extend(val, min(opwidth, bitwidth))
        else:
            val = sign_extend(val, min(opwidth, bitwidth))
        set_polymorphed_reg(rd, bitwidth, j, val)
        if (int_csr[rs].isvec) i++;
        if (int_csr[rd].isvec) j++; else break;

# Predication Element Zeroing

The introduction of zeroing on traditional vector predication is usually
intended as an optimisation for lane-based microarchitectures with register
renaming to be able to save power by avoiding a register read on elements
that are passed through en-masse through the ALU.  Simpler microarchitectures
do not have this issue: they simply do not pass the element through to
the ALU at all, and therefore do not store it back in the destination.
More complex non-lane-based micro-architectures can, when zeroing is
not set, use the predication bits to simply avoid sending element-based
operations to the ALUs, entirely: thus, over the long term, potentially
keeping all ALUs 100% occupied even when elements are predicated out.

SimpleV's design principle is not based on or influenced by
microarchitectural design factors: it is a hardware-level API.
Therefore, looking purely at whether zeroing is *useful* or not,
(whether less instructions are needed for certain scenarios),
given that a case can be made for zeroing *and* non-zeroing, the
decision was taken to add support for both.

## Single-predication (based on destination register)

Zeroing on predication for arithmetic operations is taken from
the destination register's predicate.  i.e. the predication *and*
zeroing settings to be applied to the whole operation come from the
CSR Predication table entry for the destination register.
Thus when zeroing is set on predication of a destination element,
if the predication bit is clear, then the destination element is *set*
to zero (twin-predication is slightly different, and will be covered
next).

Thus the pseudo-code loop for a predicated arithmetic operation
is modified to as follows:

      for (i = 0; i < VL; i++)
        if not zeroing: # an optimisation
           while (!(predval & 1<<i) && i < VL)
             if (int_vec[rd ].isvector)  { id += 1; }
             if (int_vec[rs1].isvector)  { irs1 += 1; }
             if (int_vec[rs2].isvector)  { irs2 += 1; }
           if i == VL:
             return
        if (predval & 1<<i)
           src1 = ....
           src2 = ...
           else:
               result = src1 + src2 # actual add (or other op) here
           set_polymorphed_reg(rd, destwid, ird, result)
           if int_vec[rd].ffirst and result == 0:
              VL = i # result was zero, end loop early, return VL
              return
           if (!int_vec[rd].isvector) return
        else if zeroing:
           result = 0
           set_polymorphed_reg(rd, destwid, ird, result)
        if (int_vec[rd ].isvector)  { id += 1; }
        else if (predval & 1<<i) return
        if (int_vec[rs1].isvector)  { irs1 += 1; }
        if (int_vec[rs2].isvector)  { irs2 += 1; }
        if (rd == VL or rs1 == VL or rs2 == VL): return

The optimisation to skip elements entirely is only possible for certain
micro-architectures when zeroing is not set.  However for lane-based
micro-architectures this optimisation may not be practical, as it
implies that elements end up in different "lanes".  Under these
circumstances it is perfectly fine to simply have the lanes
"inactive" for predicated elements, even though it results in
less than 100% ALU utilisation.

## Twin-predication (based on source and destination register)

Twin-predication is not that much different, except that that
the source is independently zero-predicated from the destination.
This means that the source may be zero-predicated *or* the
destination zero-predicated *or both*, or neither.

When with twin-predication, zeroing is set on the source and not
the destination, if a predicate bit is set it indicates that a zero
data element is passed through the operation (the exception being:
if the source data element is to be treated as an address - a LOAD -
then the data returned *from* the LOAD is zero, rather than looking up an
*address* of zero.

When zeroing is set on the destination and not the source, then just
as with single-predicated operations, a zero is stored into the destination
element (or target memory address for a STORE).

Zeroing on both source and destination effectively result in a bitwise
NOR operation of the source and destination predicate: the result is that
where either source predicate OR destination predicate is set to 0,
a zero element will ultimately end up in the destination register.

However: this may not necessarily be the case for all operations;
implementors, particularly of custom instructions, clearly need to
think through the implications in each and every case.

Here is pseudo-code for a twin zero-predicated operation:

    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, zerosrc = get_pred_val(FALSE, rs); # predication on src
      pd, zerodst = get_pred_val(FALSE, rd); # ... AND on dest
      for (int i = 0, int j = 0; i < VL && j < VL):
        if (int_csr[rs].isvec && !zerosrc) while (!(ps & 1<<i)) i++;
        if (int_csr[rd].isvec && !zerodst) while (!(pd & 1<<j)) j++;
        if ((pd & 1<<j))
            if ((pd & 1<<j))
                sourcedata = ireg[rs+i];
            else
                sourcedata = 0
            ireg[rd+j] <= sourcedata
        else if (zerodst)
            ireg[rd+j] <= 0
        if (int_csr[rs].isvec)
            i++;
        if (int_csr[rd].isvec)
            j++;
        else
            if ((pd & 1<<j))
                break;

Note that in the instance where the destination is a scalar, the hardware
loop is ended the moment a value *or a zero* is placed into the destination
register/element.  Also note that, for clarity, variable element widths
have been left out of the above.

# Exceptions

TODO: expand.  

# Hints

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

# Vector Block Format <a name="vliw-format"></a>

See ancillary resource: [[vblock_format]]

# Subsets of RV functionality

It is permitted to only implement SVprefix and not the VBLOCK instruction
format option, and vice-versa.  UNIX Platforms **MUST** raise illegal
instruction on seeing an unsupported VBLOCK or SVprefix opcode, so that
traps may emulate the format.

It is permitted in SVprefix to either not implement VL or not implement
SUBVL (see [[sv_prefix_proposal]] for full details. Again, UNIX Platforms
*MUST* raise illegal instruction on implementations that do not support
VL or SUBVL.

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.