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[libreriscv.git] / simple_v_extension / specification.mdwn

# Simple-V (Parallelism Extension Proposal) Specification

* Copyright (C) 2017, 2018, 2019 Luke Kenneth Casson Leighton
* Status: DRAFTv0.6
* Last edited: 21 jun 2019
* Ancillary resource: [[opcodes]]
* Ancillary resource: [[sv_prefix_proposal]]
* Ancillary resource: [[abridged_spec]]
* Ancillary resource: [[vblock_format]]
* Ancillary resource: [[appendix]]

With thanks to:

* Allen Baum
* Bruce Hoult
* comp.arch
* Jacob Bachmeyer
* Guy Lemurieux
* Jacob Lifshay
* Terje Mathisen
* 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, without adding any new opcodes, thus allowing
the implementor to focus on adding hardware where it is needed and necessary.
The primary target is for mobile-class 3D GPUs and VPUs, with secondary
goals being to reduce executable size (by extending the effectiveness of RV opcodes, RVC in particular) and reduce context-switch latency.

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
compliant 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**.

# Basic Operation

The principle of SV is as follows:

* Standard RV instructions are "prefixed" (extended) through a 48/64
  bit format (single instruction option) or a variable
 length VLIW-like prefix (multi or "grouped" option).
* The prefix(es) indicate which registers are "tagged" as
  "vectorised". Predicates can also be added, and element widths
  overridden on any src or dest register.
* A "Vector Length" CSR is set, indicating the span of any future
  "parallel" operations.
* If any operation (a **scalar** standard RV opcode) uses a register
  that has been so "marked" ("tagged"), a hardware "macro-unrolling loop"
  is activated, of length VL, that effectively issues **multiple**
  identical instructions using contiguous sequentially-incrementing
  register numbers, based on the "tags".
* **Whether they be executed sequentially or in parallel or a
  mixture of both or punted to software-emulation in a trap handler
  is entirely up to the implementor**.

In this way an entire scalar algorithm may be vectorised with
the minimum of modification to the hardware and to compiler toolchains.

To reiterate: **There are *no* new opcodes**. The scheme works *entirely*
on hidden context that augments *scalar* RISCV instructions.

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

* An optional "reshaping" CSR key-value table which remaps from a 1D
  linear shape to 2D or 3D, including full transposition.

There are five additional 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 User Mode there are the following CSRs:

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

There are also two additional CSRs for Supervisor-Mode:

* sePCVBLK
* seSTATE

And likewise for M-Mode:

* mePCVBLK
* meSTATE

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.

Thus for example, a User Mode trap will end up swapping STATE and ueSTATE
(on both entry and exit), allowing User Mode traps to have their own
Vectorisation Context set up, separated from and unaffected by normal
user applications.  If an M Mode trap occurs in the middle of the U Mode trap, STATE is swapped with meSTATE, and restored on exit: the U Mode trap continues unaware that the M Mode trap even occurred.

Likewise, Supervisor Mode may perform context-switches, safe in the
knowledge that its Vectorisation State is unaffected by User Mode.

The access pattern for these groups of CSRs in each mode follows the
same pattern for other CSRs that have M-Mode and S-Mode "mirrors":

* In M-Mode, the S-Mode and U-Mode CSRs are separate and distinct.
* In S-Mode, accessing and changing of the M-Mode CSRs is transparently
  identical
  to changing the S-Mode CSRs.  Accessing and changing the U-Mode
  CSRs is permitted.
* In U-Mode, accessing and changing of the S-Mode and U-Mode CSRs
  is prohibited.

An interesting side effect of SV STATE being
separate and distinct in S Mode
is that
Vectorised saving of an entire register file to the stack is a single
instruction (through accidental provision of LOAD-MULTI semantics).  If the
SVPrefix P64-LD-type format is used, LOAD-MULTI may even be done with a
single standalone 64 bit opcode (P64 may set up SUBVL, VL and MVL from an
immediate field, to cover the full regfile). It can even be predicated, which opens up some very
interesting possibilities.

(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).

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 hardware implementation a
little simpler.

The other important factor to note is that the actual MVL is internally
stored **offset by one**, so that it can fit into only 6 bits (for RV64)
and still cover a range up to XLEN bits.  Attempts to set MVL to zero will
return an exception.  This is expressed more clearly in the "pseudocode"
section, where there are subtle differences between CSRRW and CSRRWI.

## 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

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 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 third 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 (STATE).  This CSR by contrast 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: CSRWI's immediate
is limited to 0-31 (representing VL=1-32).

Note that when VL is set to 1, vector operations cease (but not subvector
operations: that requires setting SUBVL=1) the hardware loop is reduced
to a single element: scalar operations.  This is in effect the default,
normal operating mode. However it is important to appreciate that this
does **not** result in the Register table or SUBVL being disabled. Only
when the Register table is empty (P48/64 prefix fields notwithstanding)
would SV have no effect.

## SUBVL - Sub Vector Length

This is a "group by quantity" that effectively 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.

Another way to view SUBVL is that each element in the VL length vector is
now SUBVL times elwidth bits in length and now comprises SUBVL discrete
sub operations.  An inner SUBVL for-loop within a VL for-loop in effect,
with the sub-element increased every time in the innermost loop. This
is best illustrated in the (simplified) pseudocode example, in the
[[appendix]].

The primary use case for SUBVL is for 3D FP Vectors. A Vector of 3D
coordinates X,Y,Z for example may be loaded and multiplied then stored, per
VL element iteration, rather than having to set VL to three times larger.

Setting this CSR to 0 must raise an exception.  Setting it to a value
greater than 4 likewise.  To see the relationship with STATE, see below.

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

This saves a not insignificant number of instructions when handling 3D
vectors, as otherwise a much longer predicate mask would have to be set
up with regularly-repeated bit patterns.

See SUBVL Pseudocode illustration in the [[appendix]], for details.

## STATE

out of date, see http://lists.libre-riscv.org/pipermail/libre-riscv-dev/2019-June/001896.html

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.

Interestingly STATE may hypothetically also be modified to make the
immediately-following instruction to skip a certain number of elements,
by playing with destoffs and srcoffs (and the subvector offsets as well)

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
exactly 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, meSTATE
and seSTATE).

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   |

The relationship between SUBVL and the subvl field is:

| SUBVL | (25..24) |
| ----- | -------- |
| 1     | 0b00     |
| 2     | 0b01     |
| 3     | 0b10     |
| 4     | 0b11     |

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
* **SUBVL** which sets a SIMD-like quantity, has only 4 values so there
  are no changes needed
* **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
* **ssvoffs** will be truncated to be within the range 0 to SUBVL-1
* **dsvoffs** will be truncated to be within the range 0 to SUBVL-1

NOTE: if the following instruction is not a twin predicated instruction,
and destoffs or dsvoffs has been set to non-zero, subsequent execution
behaviour is undefined. **USE WITH CARE**.

### Hardware rules for when to increment STATE offsets

The offsets inside STATE are like the indices in a loop, except
in hardware. They are also partially (conceptually) similar to a
"sub-execution Program Counter". As such, and to allow proper context
switching and to define correct exception behaviour, the following rules
must be observed:

* When the VL CSR is set, srcoffs and destoffs are reset to zero.
* Each instruction that contains a "tagged" register shall start
  execution at the *current* value of srcoffs (and destoffs in the case
  of twin predication)
* Unpredicated bits (in nonzeroing mode) shall cause the element operation
  to skip, incrementing the srcoffs (or destoffs)
* On execution of an element operation, Exceptions shall **NOT** cause
  srcoffs or destoffs to increment.
* On completion of the full Vector Loop (srcoffs = VL-1 or destoffs =
  VL-1 after the last element is executed), both srcoffs and destoffs
  shall be reset to zero.

This latter is why srcoffs and destoffs may be stored as values from
0 to XLEN-1 in the STATE CSR, because as loop indices they refer to
elements. srcoffs and destoffs never need to be set to VL: their maximum
operating values are limited to 0 to VL-1.

The same corresponding rules apply to SUBVL, svsrcoffs and svdestoffs.

## MVL and VL Pseudocode

The pseudo-code for get and set of VL and MVL use the following internal
functions as follows:

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

    get_mvl_csr(rd):
        regs[rd] = STATE.VL

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

    get_vl_csr(rd):
        regs[rd] = STATE.VL
        return STATE.VL

Note that where setting MVL behaves as a normal CSR (returns the old
value), 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 pseudocode 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 storing that value.

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

    CSRRW_Set_VL(rs1, rd):
        value = regs[rs1]
        if value == 0 or value > XLEN:
            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)
        STATE.MVL = set_mvl_csr(value[11:6]+1)
        STATE.VL = set_vl_csr(value[5:0]+1)
        STATE.destoffs = value[23:18]>>18
        STATE.srcoffs = value[23:18]>>12

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

## 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" />

*NOTE: in prior versions of SV, this table used to be writable and
accessible via CSRs. It is now stored in the VBLOCK instruction format. Note
that this table does *not* get applied to the SVPrefix P48/64 format,
only to scalar opcodes*

The purpose of the Register table is three-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 bits (3 for some RVC ops) may
  access up to **128** 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.

Note: clearly, if an RVC operation uses a 3 bit spec'd register (x8-x15)
and the Register table contains entried that only refer to registerd
x1-x14 or x16-x31, such operations will *never* activate the VL hardware
loop!

If however the (16 bit) Register table does contain such an entry (x8-x15
or x2 in the case of LWSP), that src or dest reg may be redirected
anywhere to the *full* 128 register range. Thus, RVC becomes far more
powerful and has many more opportunities to reduce code size that in
Standard RV32/RV64 executables.

[[!inline raw="yes" pages="simple_v_extension/reg_table_format" ]]

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.

The 8 bit format is used for a much more compact expression. "isvec"
is implicit and, similar to [[sv_prefix_proposal]], the target vector
is "regnum<<2", implicitly. Contrast this with the 16-bit format where
the target vector is *explicitly* named in bits 8 to 14, and bit 15 may
optionally set "scalar" mode.

Note that whilst SVPrefix adds one extra bit to each of rd, rs1 etc.,
and thus the "vector" mode need only shift the (6 bit) regnum by 1 to
get the actual (7 bit) register number to use, there is not enough space
in the 8 bit format (only 5 bits for regnum) so "regnum<<2" is required.

vew has the following meanings, indicating that the instruction's
operand size is "over-ridden" in a polymorphic fashion:

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

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

[[!inline raw="yes" pages="simple_v_extension/reg_table" ]]

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

*NOTE: in prior versions of SV, this table used to be writable and
accessible via CSRs. It is now stored in the VBLOCK instruction format. 
The table does **not** apply to SVPrefix opcodes*

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.

It 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 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
  register from 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

[[!inline raw="yes" pages="simple_v_extension/pred_table_format" ]]

The 8 bit format is a compact and less expressive variant of the full
16 bit format.  Using the 8 bit format is very different: the predicate
register to use is implicit, and numbering begins inplicitly from x9. The
regnum is still used to "activate" predication, in the same fashion as
described above.

The 16 bit Predication CSR Table is a key-value store, so
implementation-wise it will be faster to turn the table around (maintain
topologically equivalent state).  Opportunities then exist to access
registers in unary form instead of binary, saving gates and power by
only activating "redirection" with a single AND gate, instead of
multiple multi-bit XORs (a CAM):

[[!inline raw="yes" pages="simple_v_extension/pred_table" ]]

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 is 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))
           result = iop(s1 ? regfile[rs1+i] : regfile[rs1],
                        s2 ? regfile[rs2+i] : regfile[rs2]);
           (d ? regfile[rd+i] : regfile[rd]) = result
           if preg.ffirst and result == 0:
              VL = i # result was zero, end loop early, return VL
              return
        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 table) if they are
  vectors.
* fail-on-first mode stops execution early whenever an operation
  returns a zero value.  floating-point results count both
  positive-zero as well as negative-zero as "fail".

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

[[!inline raw="yes" pages="simple_v_extension/get_pred_value" ]]

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

## 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.

See [[appendix]] for more details on fail-on-first modes, as well as
pseudo-code, below.

## REMAP and SHAPE CSRs <a name="remap" />

See optional [[remap]] section.

# 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 <a name="instructions" />

See [[appendix]] for specific cases where instruction behaviour is
augmented.  A greatly simplified example is below.  Note that this
is the ADD implementation, not a separate VADD instruction:

[[!inline raw="yes" pages="simple_v_extension/simple_add_example" ]]

Note that several things have been left out of this example.
See [[appendix]] for additional examples that show how to add
support for additional features (twin predication, elwidth,
zeroing, SUBVL etc.)

# Exceptions

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**
and **PCVBLK** CSRs are 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 will be necessary for the microarchitecture to **discard** or terminate
operations with higher indices.  Optimisated microarchitectures could
hypothetically store (cache) results, for subsequent replay if appropriate.

In short: exception handling **MUST** be precise, in-order, and exactly
like Standard RISC-V as far as the instruction execution order is
concerned, regardless of whether it is PC, PCVBLK, VL or SUBVL that
is currently being incremented.

# 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

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

The VBLOCK Format allows Register, Predication and Vector Length to be contextually associated with a group of RISC-V scalar opcodes.  The format is as follows:

[[!inline raw="yes" pages="simple_v_extension/vblock_format_table" ]]

For more details, including the CSRs, see ancillary resource: [[vblock_format]]

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

See [[discussion]]