add modes to ls010

[libreriscv.git] / openpower / sv / rfc / ls010.mdwn

# RFC ls009 SVP64 Zero-Overhead Loop Prefix Subsystem

Credits and acknowledgements:

* Luke Leighton
* Jacob Lifshay
* Hendrik Boom
* Richard Wilbur
* Alexandre Oliva
* Cesar Strauss
* NLnet Foundation, for funding
* OpenPOWER Foundation
* Paul Mackerras
* Toshaan Bharvani
* IBM for the Power ISA itself

Links:

* <https://bugs.libre-soc.org/show_bug.cgi?id=1045>

# Introduction

Simple-V is a type of Vectorisation best described as a "Prefix Loop Subsystem"
similar to the Z80 `LDIR` instruction and to the x86 `REP` Prefix instruction.
More advanced features are similar to the Z80 `CPIR` instruction. If viewed
as an actual Vector ISA it introduces over 1.5 million 64-bit Vector instructions.
SVP64, the instruction format, is therefore best viewed as an orthogonal
RISC-style "Prefixing" subsystem instead.

Except where explicitly stated all bit numbers remain as in the rest of the Power ISA:
in MSB0 form (the bits are numbered from 0 at the MSB on the left
and counting up as you move rightwards to the LSB end). All bit ranges are inclusive
(so `4:6` means bits 4, 5, and 6, in MSB0 order).  **All register numbering and
element numbering however is LSB0 ordering** which is a different convention from that used
elsewhere in the Power ISA.

The SVP64 prefix always comes before the suffix in PC order and must be considered
an independent "Defined word" that augments the behaviour of the following instruction,
but does **not** change the actual Decoding of that following instruction.
**All prefixed instructions retain their non-prefixed encoding and definition**.

*Architectural Resource Allocation note: it is prohibited to accept RFCs which
fundamentally violate this hard requirement.  Under no circumstances must the
Suffix space have an alternate instruction encoding allocated within SVP64 that is
entirely different from the non-prefixed Defined Word. Hardware Implementors
critically rely on this inviolate guarantee to implement High-Performance Multi-Issue
micro-architectures that can sustain 100% throughput*

| 0:5    | 6:31         | 32:63        |
|--------|--------------|--------------|
| EXT09  | v3.1  Prefix | v3.0/1  Suffix |

Subset implementations in hardware are permitted, as long as certain
rules are followed, allowing for full soft-emulation including future
revisions.  Compliancy Subsets exist to ensure minimum levels of binary
interoperability expectations within certain environments.

## Register files, elements, and Element-width Overrides

In the Upper Compliancy Levels the size of the GPR and FPR Register files are expanded
from 32 to 128 entries, and the number of CR Fields expanded from CR0-CR7 to CR0-CR127.

Memory access remains exactly the same: the effects of `MSR.LE` remain exactly the same,
affecting as they already do and remain **only** on the Load and Store memory-register
operation byte-order, and having nothing to do with the
ordering of the contents of register files or register-register operations.

Whilst the bits within the GPRs and FPRs are expected to be MSB0-ordered and for
numbering to be sequentially incremental the element offset numbering is naturally
**LSB0-sequentially-incrementing from zero not MSB0-incrementing.**  Expressed exclusively in
MSB0-numbering, SVP64 is unnecessarily complex to understand: the required
subtractions from 63, 31, 15 and 7 unfortunately become a hostile minefield.
Therefore for the purposes of this section the more natural
**LSB0 numbering is assumed** and it is up to the reader to translate to MSB0 numbering.

The Canonical specification for how element-sequential numbering and element-width
overrides is defined is expressed in the following c structure, assuming a Little-Endian
system, and naturally using LSB0 numbering everywhere because the ANSI c specification
is inherently LSB0:

```
    #pragma pack
    typedef union {
        uint8_t  b[]; // elwidth 8
        uint16_t s[]; // elwidth 16
        uint32_t i[]; // elwidth 32
        uint64_t l[]; // elwidth 64
        uint8_t actual_bytes[8];
    } el_reg_t;

    elreg_t int_regfile[128];

    void get_register_element(el_reg_t* el, int gpr, int element, int width) {
        switch (width) {
            case 64: el->l = int_regfile[gpr].l[element];
            case 32: el->i = int_regfile[gpr].i[element];
            case 16: el->s = int_regfile[gpr].s[element];
            case 8 : el->b = int_regfile[gpr].b[element];
        }
    }
    void set_register_element(el_reg_t* el, int gpr, int element, int width) {
        switch (width) {
            case 64: int_regfile[gpr].l[element] = el->l;
            case 32: int_regfile[gpr].i[element] = el->i;
            case 16: int_regfile[gpr].s[element] = el->s;
            case 8 : int_regfile[gpr].b[element] = el->b;
        }
    }
```

Example Vector-looped add operation implementation when elwidths are 64-bit:

```
 # add RT, RA,RB using the "uint64_t" union member, "l"
 for i in range(VL):
      int_regfile[RT].l[i] = int_regfile[RA].l[i] + int_regfile[RB].l[i]
```

However if elwidth overrides are set to 16 for both source and destination:

```
 # add RT, RA, RB using the "uint64_t" union member "s"
 for i in range(VL):
      int_regfile[RT].s[i] = int_regfile[RA].s[i] + int_regfile[RB].s[i]
```

Hardware Architectural note: to avoid a Read-Modify-Write at the register file it is
strongly recommended to implement byte-level write-enable lines exactly as has been
implemented in DRAM ICs for many decades. Additionally the predicate mask bit is advised
to be associated with the element operation and alongside the result ultimately
passed to the register file.
When element-width is set to 64-bit the relevant predicate mask bit may be repeated
eight times and pull all eight write-port byte-level lines HIGH. Clearly when element-width
is set to 8-bit the relevant predicate mask bit corresponds directly with one single
byte-level write-enable line.  It is up to the Hardware Architect to then amortise (merge)
elements together into both PredicatedSIMD Pipelines as well as simultaneous non-overlapping
Register File writes, to achieve High Performance designs.

## SVP64 encoding features

A number of features need to be compacted into a very small space of only 24 bits:

* Independent per-register Scalar/Vector tagging and range extension on every register
* Element width overrides on both source and destination
* Predication on both source and destination
* Two different sources of predication: INT and CR Fields
* SV Modes including saturation (for Audio, Video and DSP), mapreduce, fail-first and
  predicate-result mode.

Different classes of operations require different formats. The earlier sections cover
the c9mmon formats and the four separate modes follow: CR operations (crops),
Arithmetic/Logical (termed "normal"), Load/Store and Branch-Conditional.

## Definition of Reserved in this spec.

For the new fields added in SVP64, instructions that have any of their
fields set to a reserved value must cause an illegal instruction trap,
to allow emulation of future instruction sets, or for subsets of SVP64
to be implemented in hardware and the rest emulated.
This includes SVP64 SPRs: reading or writing values which are not
supported in hardware must also raise illegal instruction traps
in order to allow emulation.
Unless otherwise stated, reserved values are always all zeros.

This is unlike OpenPower ISA v3.1, which in many instances does not require a trap if reserved fields are nonzero.  Where the standard Power ISA definition
is intended the red keyword `RESERVED` is used.

##  Definition of "UnVectoriseable"

Any operation that inherently makes no sense if repeated is termed "UnVectoriseable"
or "UnVectorised".  Examples include `sc` or `sync` which have no registers. `mtmsr` is
also classed as UnVectoriseable because there is only one `MSR`.

## Scalar Identity Behaviour

SVP64 is designed so that when the prefix is all zeros, and
 VL=1, no effect or
influence occurs (no augmentation) such that all standard Power ISA
v3.0/v3 1 instructions covered by the prefix are "unaltered". This is termed `scalar identity behaviour` (based on the mathematical definition for "identity", as in, "identity matrix" or better "identity transformation").

Note that this is completely different from when VL=0.  VL=0 turns all operations under its influence into `nops` (regardless of the prefix)
 whereas when VL=1 and the SV prefix is all zeros, the operation simply acts as if SV had not been applied at all to the instruction  (an "identity transformation").

## Register Naming and size

As previously mentioned SV Registers are simply the INT, FP and CR register files extended
linearly to larger sizes; SV Vectorisation iterates sequentially through these registers
(LSB0 sequential ordering from 0 to VL-1).

Where the integer regfile in standard scalar
Power ISA v3.0B/v3.1B is r0 to r31, SV extends this as r0 to r127.
Likewise FP registers are extended to 128 (fp0 to fp127), and CR Fields
are
extended to 128 entries, CR0 thru CR127.

The names of the registers therefore reflects a simple linear extension
of the Power ISA v3.0B / v3.1B register naming, and in hardware this
would be reflected by a linear increase in the size of the underlying
SRAM used for the regfiles.

Note: when an EXTRA field (defined below) is zero, SV is deliberately designed
so that the register fields are identical to as if SV was not in effect
i.e. under these circumstances (EXTRA=0) the register field names RA,
RB etc. are interpreted and treated as v3.0B / v3.1B scalar registers.  This is part of
`scalar identity behaviour` described above.

## Future expansion.

With the way that EXTRA fields are defined and applied to register fields,
future versions of SV may involve 256 or greater registers. Backwards binary compatibility may be achieved with a PCR bit (Program Compatibility Register).  Further discussion is out of scope for this version of SVP64.

# Remapped Encoding (`RM[0:23]`)

To allow relatively easy remapping of which portions of the Prefix Opcode
Map are used for SVP64 without needing to rewrite a large portion of the
SVP64 spec, a mapping is defined from the OpenPower v3.1 prefix bits to
a new 24-bit Remapped Encoding denoted `RM[0]` at the MSB to `RM[23]`
at the LSB.

The mapping from the OpenPower v3.1 prefix bits to the Remapped Encoding
is defined in the Prefix Fields section.

## Prefix Fields

TODO incorporate EXT09

To "activate" svp64 (in a way that does not conflict with v3.1B 64 bit Prefix mode), fields within the v3.1B Prefix Opcode Map are set
(see Prefix Opcode Map, above), leaving 24 bits "free" for use by SV.
This is achieved by setting bits 7 and 9 to 1:

| Name       | Bits    | Value | Description                    |
|------------|---------|-------|--------------------------------|
| EXT01      | `0:5`   | `1`   | Indicates Prefixed 64-bit      |
| `RM[0]`    | `6`     |       | Bit 0 of Remapped Encoding     |
| SVP64_7    | `7`     | `1`   | Indicates this is SVP64        |
| `RM[1]`    | `8`     |       | Bit 1 of Remapped Encoding     |
| SVP64_9    | `9`     | `1`   | Indicates this is SVP64        |
| `RM[2:23]` | `10:31` |       | Bits 2-23 of Remapped Encoding |

Laid out bitwise, this is as follows, showing how the 32-bits of the prefix
are constructed:

| 0:5    | 6     | 7 | 8     | 9 | 10:31    |
|--------|-------|---|-------|---|----------|
| EXT01  | RM    | 1 | RM    | 1 | RM       |
| 000001 | RM[0] | 1 | RM[1] | 1 | RM[2:23] |

Following the prefix will be the suffix: this is simply a 32-bit v3.0B / v3.1
instruction.  That instruction becomes "prefixed" with the SVP context: the
Remapped Encoding field (RM).

It is important to note that unlike v3.1 64-bit prefixed instructions
there is insufficient space in `RM` to provide identification of
any SVP64 Fields without first partially decoding the
32-bit suffix.  Similar to the "Forms" (X-Form, D-Form) the
`RM` format is individually associated with every instruction.

Extreme caution and care must therefore be taken
when extending SVP64 in future, to not create unnecessary relationships
between prefix and suffix that could complicate decoding, adding latency.

# Common RM fields

The following fields are common to all Remapped Encodings:

| Field Name | Field bits | Description                            |
|------------|------------|----------------------------------------|
| MASKMODE   | `0`        | Execution (predication) Mask Kind      |
| MASK       | `1:3`      | Execution Mask                      |
| SUBVL      | `8:9`      | Sub-vector length                   |

The following fields are optional or encoded differently depending
on context after decoding of the Scalar suffix:

| Field Name | Field bits | Description                            |
|------------|------------|----------------------------------------|
| ELWIDTH       | `4:5`      | Element Width                       |
| ELWIDTH_SRC   | `6:7`      | Element Width for Source      |
| EXTRA         | `10:18`    | Register Extra encoding                |
| MODE          | `19:23`    | changes Vector behaviour               |

* MODE changes the behaviour of the SV operation (result saturation, mapreduce)
* SUBVL groups elements together into vec2, vec3, vec4 for use in 3D and Audio/Video DSP work
* ELWIDTH and ELWIDTH_SRC overrides the instruction's destination and source operand width
* MASK (and MASK_SRC) and MASKMODE provide predication (two types of sources: scalar INT and Vector CR).
* Bits 10 to 18 (EXTRA) are further decoded depending on the RM category for the instruction, which is determined only by decoding the Scalar 32 bit suffix.

Similar to Power ISA `X-Form` etc. EXTRA bits are given designations, such as `RM-1P-3S1D` which indicates for this example that the operation is to be single-predicated and that there are 3 source operand EXTRA tags and one destination operand tag.

Note that if ELWIDTH != ELWIDTH_SRC this may result in reduced performance or increased latency in some implementations due to lane-crossing.

# Mode

Mode is an augmentation of SV behaviour.  Different types of
instructions have different needs, similar to Power ISA
v3.1 64 bit prefix 8LS and MTRR formats apply to different
instruction types.  Modes include Reduction, Iteration, arithmetic
saturation, and Fail-First.  More specific details in each
section and in the [[svp64/appendix]]

* For condition register operations see [[sv/cr_ops]]
* For LD/ST Modes, see [[sv/ldst]].
* For Branch modes, see [[sv/branches]]
* For arithmetic and logical, see [[sv/normal]]

# ELWIDTH Encoding

Default behaviour is set to 0b00 so that zeros follow the convention of
`scalar identity behaviour`.  In this case it means that elwidth overrides
are not applicable.  Thus if a 32 bit instruction operates on 32 bit,
`elwidth=0b00` specifies that this behaviour is unmodified.  Likewise
when a processor is switched from 64 bit to 32 bit mode, `elwidth=0b00`
states that, again, the behaviour is not to be modified.

Only when elwidth is nonzero is the element width overridden to the
explicitly required value.

## Elwidth for Integers:

| Value | Mnemonic       | Description                        |
|-------|----------------|------------------------------------|
| 00    | DEFAULT        | default behaviour for operation    |
| 01    | `ELWIDTH=w`    | Word: 32-bit integer                 |
| 10    | `ELWIDTH=h`    | Halfword: 16-bit integer             |
| 11    | `ELWIDTH=b`    | Byte: 8-bit integer                  |

This encoding is chosen such that the byte width may be computed as
`8<<(3-ew)`

## Elwidth for FP Registers:

| Value | Mnemonic       | Description                        |
|-------|----------------|------------------------------------|
| 00    | DEFAULT        | default behaviour for FP operation     |
| 01    | `ELWIDTH=f32`  | 32-bit IEEE 754 Single floating-point  |
| 10    | `ELWIDTH=f16`  | 16-bit IEEE 754 Half floating-point   |
| 11    | `ELWIDTH=bf16` | Reserved for `bf16` |

Note:
[`bf16`](https://en.wikipedia.org/wiki/Bfloat16_floating-point_format)
is reserved for a future implementation of SV

Note that any IEEE754 FP operation in Power ISA ending in "s" (`fadds`) shall
perform its operation at **half** the ELWIDTH then padded back out
to ELWIDTH.  `sv.fadds/ew=f32` shall perform an IEEE754 FP16 operation that is then "padded" to fill out to an IEEE754 FP32. When ELWIDTH=DEFAULT
clearly the behaviour of `sv.fadds` is performed at 32-bit accuracy
then padded back out to fit in IEEE754 FP64, exactly as for Scalar
v3.0B "single" FP.  Any FP operation ending in "s" where ELWIDTH=f16
or ELWIDTH=bf16 is reserved and must raise an illegal instruction
(IEEE754 FP8 or BF8 are not defined).

## Elwidth for CRs:

Element-width overrides for CR Fields has no meaning. The bits
are therefore used for other purposes, or when Rc=1, the Elwidth
applies to the result being tested (a GPR or FPR), but not to the
Vector of CR Fields.

# SUBVL Encoding

the default for SUBVL is 1 and its encoding is 0b00 to indicate that
SUBVL is effectively disabled (a SUBVL for-loop of only one element). this
lines up in combination with all other "default is all zeros" behaviour.

| Value | Mnemonic  | Subvec  | Description            |
|-------|-----------|---------|------------------------|
| 00    | `SUBVL=1` | single  | Sub-vector length of 1 |
| 01    | `SUBVL=2` | vec2    | Sub-vector length of 2 |
| 10    | `SUBVL=3` | vec3    | Sub-vector length of 3 |
| 11    | `SUBVL=4` | vec4    | Sub-vector length of 4 |

The SUBVL encoding value may be thought of as an inclusive range of a
sub-vector.  SUBVL=2 represents a vec2, its encoding is 0b01, therefore
this may be considered to be elements 0b00 to 0b01 inclusive.

# MASK/MASK_SRC & MASKMODE Encoding

TODO: rename MASK_KIND to MASKMODE

One bit (`MASKMODE`) indicates the mode: CR or Int predication.   The two
types may not be mixed.

Special note: to disable predication this field must
be set to zero in combination with Integer Predication also being set
to 0b000. this has the effect of enabling "all 1s" in the predicate
mask, which is equivalent to "not having any predication at all"
and consequently, in combination with all other default zeros, fully
disables SV (`scalar identity behaviour`).

`MASKMODE` may be set to one of 2 values:

| Value | Description                                          |
|-----------|------------------------------------------------------|
| 0         | MASK/MASK_SRC are encoded using Integer Predication  |
| 1         | MASK/MASK_SRC are encoded using CR-based Predication |

Integer Twin predication has a second set of 3 bits that uses the same
encoding thus allowing either the same register (r3, r10 or r31) to be used
for both src and dest, or different regs (one for src, one for dest).

Likewise CR based twin predication has a second set of 3 bits, allowing
a different test to be applied.

Note that it is assumed that Predicate Masks (whether INT or CR)
are read *before* the operations proceed.  In practice (for CR Fields)
this creates an unnecessary block on parallelism.  Therefore,
it is up to the programmer to ensure that the CR fields used as
Predicate Masks are not being written to by any parallel Vector Loop.
Doing so results in **UNDEFINED** behaviour, according to the definition
outlined in the Power ISA v3.0B Specification.

Hardware Implementations are therefore free and clear to delay reading
of individual CR fields until the actual predicated element operation
needs to take place, safe in the knowledge that no programmer will
have issued a Vector Instruction where previous elements could have
overwritten (destroyed) not-yet-executed CR-Predicated element operations.

## Integer Predication (MASKMODE=0)

When the predicate mode bit is zero the 3 bits are interpreted as below.
Twin predication has an identical 3 bit field similarly encoded.

`MASK` and `MASK_SRC` may be set to one of 8 values, to provide the following meaning:

| Value | Mnemonic | Element `i` enabled if:      |
|-------|----------|------------------------------|
| 000   | ALWAYS   | predicate effectively all 1s |
| 001   | 1 << R3  | `i == R3`                    |
| 010   | R3       | `R3 & (1 << i)` is non-zero  |
| 011   | ~R3      | `R3 & (1 << i)` is zero      |
| 100   | R10      | `R10 & (1 << i)` is non-zero |
| 101   | ~R10     | `R10 & (1 << i)` is zero     |
| 110   | R30      | `R30 & (1 << i)` is non-zero |
| 111   | ~R30     | `R30 & (1 << i)` is zero     |

r10 and r30 are at the high end of temporary and unused registers, so as not to interfere with register allocation from ABIs.

## CR-based Predication (MASKMODE=1)

When the predicate mode bit is one the 3 bits are interpreted as below.
Twin predication has an identical 3 bit field similarly encoded.

`MASK` and `MASK_SRC` may be set to one of 8 values, to provide the following meaning:

| Value | Mnemonic | Element `i` is enabled if     |
|-------|----------|--------------------------|
| 000   | lt       | `CR[offs+i].LT` is set   |
| 001   | nl/ge    | `CR[offs+i].LT` is clear |
| 010   | gt       | `CR[offs+i].GT` is set   |
| 011   | ng/le    | `CR[offs+i].GT` is clear |
| 100   | eq       | `CR[offs+i].EQ` is set   |
| 101   | ne       | `CR[offs+i].EQ` is clear |
| 110   | so/un    | `CR[offs+i].FU` is set   |
| 111   | ns/nu    | `CR[offs+i].FU` is clear |

CR based predication.  TODO: select alternate CR for twin predication? see
[[discussion]]  Overlap of the two CR based predicates must be taken
into account, so the starting point for one of them must be suitably
high, or accept that for twin predication VL must not exceed the range
where overlap will occur, *or* that they use the same starting point
but select different *bits* of the same CRs

`offs` is defined as CR32 (4x8) so as to mesh cleanly with Vectorised Rc=1 operations (see below).  Rc=1 operations start from CR8 (TBD).

The CR Predicates chosen must start on a boundary that Vectorised
CR operations can access cleanly, in full.
With EXTRA2 restricting starting points
to multiples of 8 (CR0, CR8, CR16...) both Vectorised Rc=1 and CR Predicate
Masks have to be adapted to fit on these boundaries as well.

# Extra Remapped Encoding <a name="extra_remap"> </a>

Shows all instruction-specific fields in the Remapped Encoding `RM[10:18]` for all instruction variants.  Note that due to the very tight space, the encoding mode is *not* included in the prefix itself.  The mode is "applied", similar to Power ISA "Forms" (X-Form, D-Form) on a per-instruction basis, and, like "Forms" are given a designation (below) of the form `RM-nP-nSnD`. The full list of which instructions use which remaps is here [[opcode_regs_deduped]]. (*Machine-readable CSV files have been provided which will make the task of creating SV-aware ISA decoders easier*).

These mappings are part of the SVP64 Specification in exactly the same
way as X-Form, D-Form. New Scalar instructions added to the Power ISA
will need a corresponding SVP64 Mapping, which can be derived by-rote
from examining the Register "Profile" of the instruction.

There are two categories:  Single and Twin Predication.
Due to space considerations further subdivision of Single Predication
is based on whether the number of src operands is 2 or 3.  With only
9 bits available some compromises have to be made.

* `RM-1P-3S1D` Single Predication dest/src1/2/3, applies to 4-operand instructions (fmadd, isel, madd).
* `RM-1P-2S1D` Single Predication dest/src1/2 applies to 3-operand instructions (src1 src2 dest)
* `RM-2P-1S1D` Twin Predication (src=1, dest=1)
* `RM-2P-2S1D` Twin Predication (src=2, dest=1) primarily for LDST (Indexed)
* `RM-2P-1S2D` Twin Predication (src=1, dest=2) primarily for LDST Update

## RM-1P-3S1D

| Field Name | Field bits | Description                            |
|------------|------------|----------------------------------------|
| Rdest\_EXTRA2 | `10:11` | extends Rdest (R\*\_EXTRA2 Encoding)   |
| Rsrc1\_EXTRA2 | `12:13` | extends Rsrc1 (R\*\_EXTRA2 Encoding)   |
| Rsrc2\_EXTRA2 | `14:15` | extends Rsrc2 (R\*\_EXTRA2 Encoding)   |
| Rsrc3\_EXTRA2 | `16:17` | extends Rsrc3 (R\*\_EXTRA2 Encoding)   |
| EXTRA2_MODE   | `18`    | used by `divmod2du` and `maddedu` for RS   |

These are for 3 operand in and either 1 or 2 out instructions.
3-in 1-out includes `madd RT,RA,RB,RC`. (DRAFT) instructions
such as `maddedu` have an implicit second destination, RS, the
selection of which is determined by bit 18.

## RM-1P-2S1D

| Field Name | Field bits | Description                               |
|------------|------------|-------------------------------------------|
| Rdest\_EXTRA3 | `10:12` | extends Rdest  |
| Rsrc1\_EXTRA3 | `13:15` | extends Rsrc1  |
| Rsrc2\_EXTRA3 | `16:18` | extends Rsrc3  |

These are for 2 operand 1 dest instructions, such as `add RT, RA,
RB`. However also included are unusual instructions with an implicit dest
that is identical to its src reg, such as `rlwinmi`.

Normally, with instructions such as `rlwinmi`, the scalar v3.0B ISA would not have sufficient bit fields to allow
an alternative destination.  With SV however this becomes possible.
Therefore, the fact that the dest is implicitly also a src should not
mislead: due to the *prefix* they are different SV regs.

* `rlwimi RA, RS, ...`
* Rsrc1_EXTRA3 applies to RS as the first src
* Rsrc2_EXTRA3 applies to RA as the secomd src
* Rdest_EXTRA3 applies to RA to create an **independent** dest.

With the addition of the EXTRA bits, the three registers
each may be *independently* made vector or scalar, and be independently
augmented to 7 bits in length.

## RM-2P-1S1D/2S

| Field Name | Field bits | Description                 |
|------------|------------|----------------------------|
| Rdest_EXTRA3 | `10:12`    | extends Rdest             |
| Rsrc1_EXTRA3 | `13:15`    | extends Rsrc1             |
| MASK_SRC     | `16:18`    | Execution Mask for Source |

`RM-2P-2S` is for `stw` etc. and is Rsrc1 Rsrc2.

## RM-1P-2S1D

single-predicate, three registers (2 read, 1 write)

| Field Name | Field bits | Description                 |
|------------|------------|----------------------------|
| Rdest_EXTRA3 | `10:12`    | extends Rdest             |
| Rsrc1_EXTRA3 | `13:15`    | extends Rsrc1             |
| Rsrc2_EXTRA3 | `16:18`    | extends Rsrc2             |

## RM-2P-2S1D/1S2D/3S

The primary purpose for this encoding is for Twin Predication on LOAD
and STORE operations.  see [[sv/ldst]] for detailed anslysis.

RM-2P-2S1D:

| Field Name | Field bits | Description                     |
|------------|------------|----------------------------|
| Rdest_EXTRA2 | `10:11`  | extends Rdest (R\*\_EXTRA2 Encoding)   |
| Rsrc1_EXTRA2 | `12:13`  | extends Rsrc1 (R\*\_EXTRA2 Encoding)   |
| Rsrc2_EXTRA2 | `14:15`  | extends Rsrc2 (R\*\_EXTRA2 Encoding)   |
| MASK_SRC     | `16:18`  | Execution Mask for Source     |

Note that for 1S2P the EXTRA2 dest and src names are switched (Rsrc_EXTRA2
is in bits 10:11, Rdest1_EXTRA2 in 12:13)

Also that for 3S (to cover `stdx` etc.) the names are switched to 3 src: Rsrc1_EXTRA2, Rsrc2_EXTRA2, Rsrc3_EXTRA2.

Note also that LD with update indexed, which takes 2 src and 2 dest
(e.g. `lhaux RT,RA,RB`), does not have room for 4 registers and also
Twin Predication.  therefore these are treated as RM-2P-2S1D and the
src spec for RA is also used for the same RA as a dest.

Note that if ELWIDTH != ELWIDTH_SRC this may result in reduced performance or increased latency in some implementations due to lane-crossing.

# R\*\_EXTRA2/3

EXTRA is the means by which two things are achieved:

1. Registers are marked as either Vector *or Scalar*
2. Register field numbers (limited typically to 5 bit)
   are extended in range, both for Scalar and Vector.

The register files are therefore extended:

* INT is extended from r0-31 to r0-127
* FP is extended from fp0-32 to fp0-fp127
* CR Fields are extended from CR0-7 to CR0-127

However due to pressure in `RM.EXTRA` not all these registers
are accessible by all instructions, particularly those with
a large number of operands (`madd`, `isel`).

In the following tables register numbers are constructed from the
standard v3.0B / v3.1B 32 bit register field (RA, FRA) and the EXTRA2
or EXTRA3 field from the SV Prefix, determined by the specific
RM-xx-yyyy designation for a given instruction.
The prefixing is arranged so that
interoperability between prefixing and nonprefixing of scalar registers
is direct and convenient (when the EXTRA field is all zeros).

A pseudocode algorithm explains the relationship, for INT/FP (see [[svp64/appendix]] for CRs)

```
    if extra3_mode:
        spec = EXTRA3
    else:
        spec = EXTRA2 << 1 # same as EXTRA3, shifted
    if spec[0]: # vector
         return (RA << 2) | spec[1:2]
    else:         # scalar
         return (spec[1:2] << 5) | RA
```

Future versions may extend to 256 by shifting Vector numbering up.
Scalar will not be altered.

Note that in some cases the range of starting points for Vectors
is limited.

## INT/FP EXTRA3

If EXTRA3 is zero, maps to
"scalar identity" (scalar Power ISA field naming).

Fields are as follows:

* Value: R_EXTRA3
* Mode: register is tagged as scalar or vector
* Range/Inc: the range of registers accessible from this EXTRA
  encoding, and the "increment" (accessibility). "/4" means
  that this EXTRA encoding may only give access (starting point)
  every 4th register.
* MSB..LSB: the bit field showing how the register opcode field
  combines with EXTRA to give (extend) the register number (GPR)

| Value | Mode | Range/Inc | 6..0 |
|-----------|-------|---------------|---------------------|
| 000       | Scalar | `r0-r31`/1 | `0b00 RA`      |
| 001       | Scalar | `r32-r63`/1 | `0b01 RA`      |
| 010       | Scalar | `r64-r95`/1 | `0b10 RA`      |
| 011       | Scalar | `r96-r127`/1 | `0b11 RA`      |
| 100       | Vector | `r0-r124`/4 | `RA 0b00`      |
| 101       | Vector | `r1-r125`/4 | `RA 0b01`      |
| 110       | Vector | `r2-r126`/4 | `RA 0b10`      |
| 111       | Vector | `r3-r127`/4 | `RA 0b11`      |

## INT/FP EXTRA2

If EXTRA2 is zero will map to
"scalar identity behaviour" i.e Scalar Power ISA register naming:

| Value | Mode | Range/inc | 6..0 |
|-----------|-------|---------------|-----------|
| 00       | Scalar | `r0-r31`/1 | `0b00 RA`     |
| 01       | Scalar | `r32-r63`/1 | `0b01 RA`      |
| 10       | Vector | `r0-r124`/4 | `RA 0b00`      |
| 11       | Vector | `r2-r126`/4 | `RA 0b10`   |

**Note that unlike in EXTRA3, in EXTRA2**:

* the GPR Vectors may only start from
  `r0, r2, r4, r6, r8` and likewise FPR Vectors.
* the GPR Scalars may only go from `r0, r1, r2.. r63` and likewise FPR Scalars.

as there is insufficient bits to cover the full range.

## CR Field EXTRA3

CR Field encoding is essentially the same but made more complex due to CRs being bit-based.  See [[svp64/appendix]] for explanation and pseudocode.
Note that Vectors may only start from `CR0, CR4, CR8, CR12, CR16, CR20`...
and Scalars may only go from `CR0, CR1, ... CR31`

Encoding shown MSB down to LSB

For a 5-bit operand (BA, BB, BT):

| Value | Mode | Range/Inc     | 8..5      | 4..2    | 1..0    |
|-------|------|---------------|-----------| --------|---------|
| 000   | Scalar | `CR0-CR7`/1   | 0b0000    | BA[4:2] | BA[1:0] |
| 001   | Scalar | `CR8-CR15`/1  | 0b0001    | BA[4:2] | BA[1:0] |
| 010   | Scalar | `CR16-CR23`/1 | 0b0010    | BA[4:2] | BA[1:0] |
| 011   | Scalar | `CR24-CR31`/1 | 0b0011    | BA[4:2] | BA[1:0] |
| 100   | Vector | `CR0-CR112`/16 | BA[4:2] 0 | 0b000   | BA[1:0] |
| 101   | Vector | `CR4-CR116`/16 | BA[4:2] 0 | 0b100   | BA[1:0] |
| 110   | Vector | `CR8-CR120`/16 | BA[4:2] 1 | 0b000   | BA[1:0] |
| 111   | Vector | `CR12-CR124`/16 | BA[4:2] 1 | 0b100   | BA[1:0] |

For a 3-bit operand (e.g. BFA):

| Value | Mode | Range/Inc     | 6..3      | 2..0    |
|-------|------|---------------|-----------| --------|
| 000   | Scalar | `CR0-CR7`/1   | 0b0000    | BFA   |
| 001   | Scalar | `CR8-CR15`/1  | 0b0001    | BFA      |
| 010   | Scalar | `CR16-CR23`/1 | 0b0010    | BFA      |
| 011   | Scalar | `CR24-CR31`/1 | 0b0011    | BFA      |
| 100   | Vector | `CR0-CR112`/16 | BFA 0 | 0b000   |
| 101   | Vector | `CR4-CR116`/16 | BFA 0 | 0b100   |
| 110   | Vector | `CR8-CR120`/16 | BFA 1 | 0b000   |
| 111   | Vector | `CR12-CR124`/16 | BFA 1 | 0b100   |

## CR EXTRA2

CR encoding is essentially the same but made more complex due to CRs being bit-based.  See separate section for explanation and pseudocode.
Note that Vectors may only start from CR0, CR8, CR16, CR24, CR32...


Encoding shown MSB down to LSB

For a 5-bit operand (BA, BB, BC):

| Value | Mode   | Range/Inc      | 8..5    | 4..2    | 1..0    |
|-------|--------|----------------|---------|---------|---------|
| 00    | Scalar | `CR0-CR7`/1    | 0b0000  | BA[4:2] | BA[1:0] |
| 01    | Scalar | `CR8-CR15`/1   | 0b0001  | BA[4:2] | BA[1:0] |
| 10    | Vector | `CR0-CR112`/16 | BA[4:2] 0 | 0b000   | BA[1:0] |
| 11    | Vector | `CR8-CR120`/16 | BA[4:2] 1 | 0b000   | BA[1:0] |

For a 3-bit operand (e.g. BFA):

| Value | Mode | Range/Inc     | 6..3      | 2..0    |
|-------|------|---------------|-----------| --------|
| 00    | Scalar | `CR0-CR7`/1   | 0b0000  | BFA   |
| 01    | Scalar | `CR8-CR15`/1  | 0b0001  | BFA     |
| 10    | Vector | `CR0-CR112`/16 | BFA 0 | 0b000   |
| 11    | Vector | `CR8-CR120`/16 | BFA 1 | 0b000   |


# Normal SVP64 Modes, for Arithmetic and Logical Operations

Normal SVP64 Mode covers Arithmetic and Logical operations
to provide suitable additional behaviour.  The Mode
field is bits 19-23 of the [[svp64]] RM Field.

## Mode

Mode is an augmentation of SV behaviour, providing additional
functionality.  Some of these alterations are element-based (saturation), others involve post-analysis (predicate result) and others are Vector-based (mapreduce, fail-on-first).

[[sv/ldst]],
[[sv/cr_ops]] and [[sv/branches]] are covered separately: the following
Modes apply to Arithmetic and Logical SVP64 operations:

* **simple** mode is straight vectorisation.  no augmentations: the vector comprises an array of independently created results.
* **ffirst** or data-dependent fail-on-first: see separate section.  the vector may be truncated depending on certain criteria.
  *VL is altered as a result*.
* **sat mode** or saturation: clamps each element result to a min/max rather than overflows / wraps.  allows signed and unsigned clamping for both INT
and FP.
* **reduce mode**. if used correctly, a mapreduce (or a prefix sum)
  is performed.    see [[svp64/appendix]].
  note that there are comprehensive caveats when using this mode.
* **pred-result** will test the result (CR testing selects a bit of CR and inverts it, just like branch conditional testing) and if the test fails it
is as if the
*destination* predicate bit was zero even before starting the operation.
When Rc=1 the CR element however is still stored in the CR regfile, even if the test failed.  See appendix for details.

Note that ffirst and reduce modes are not anticipated to be high-performance in some implementations.  ffirst due to interactions with VL, and reduce due to it requiring additional operations to produce a result.  simple, saturate and pred-result are however inter-element independent and may easily be parallelised to give high performance, regardless of the value of VL.

The Mode table for Arithmetic and Logical operations
 is laid out as follows:

| 0-1 |  2  |  3   4  |  description              |
| --- | --- |---------|-------------------------- |
| 00  |   0 |  dz  sz | simple mode                      |
| 00  |   1 | 0  RG   | scalar reduce mode (mapreduce) |
| 00  |   1 | 1  /    | reserved     |
| 01  | inv | CR-bit  | Rc=1: ffirst CR sel              |
| 01  | inv | VLi RC1 |  Rc=0: ffirst z/nonz |
| 10  |   N | dz   sz |  sat mode: N=0/1 u/s |
| 11  | inv | CR-bit  |  Rc=1: pred-result CR sel |
| 11  | inv | zz  RC1 |  Rc=0: pred-result z/nonz |

Fields:

* **sz / dz**  if predication is enabled will put zeros into the dest (or as src in the case of twin pred) when the predicate bit is zero.  otherwise the element is ignored or skipped, depending on context.
* **zz**: both sz and dz are set equal to this flag
* **inv CR bit** just as in branches (BO) these bits allow testing of a CR bit and whether it is set (inv=0) or unset (inv=1)
* **RG** inverts the Vector Loop order (VL-1 downto 0) rather
than the normal 0..VL-1
* **N** sets signed/unsigned saturation.
* **RC1** as if Rc=1, enables access to `VLi`.
* **VLi** VL inclusive: in fail-first mode, the truncation of
  VL *includes* the current element at the failure point rather
  than excludes it from the count.

For LD/ST Modes, see [[sv/ldst]].  For Condition Registers
see [[sv/cr_ops]].
For Branch modes, see [[sv/branches]].

# Rounding, clamp and saturate

See [[av_opcodes]] for relevant opcodes and use-cases.

To help ensure that audio quality is not compromised by overflow,
"saturation" is provided, as well as a way to detect when saturation
occurred if desired (Rc=1). When Rc=1 there will be a *vector* of CRs,
one CR per element in the result (Note: this is different from VSX which
has a single CR per block).

When N=0 the result is saturated to within the maximum range of an
unsigned value.  For integer ops this will be 0 to 2^elwidth-1. Similar
logic applies to FP operations, with the result being saturated to
maximum rather than returning INF, and the minimum to +0.0

When N=1 the same occurs except that the result is saturated to the min
or max of a signed result, and for FP to the min and max value rather
than returning +/- INF.

When Rc=1, the CR "overflow" bit is set on the CR associated with the
element, to indicate whether saturation occurred.  Note that due to
the hugely detrimental effect it has on parallel processing, XER.SO is
**ignored** completely and is **not** brought into play here.  The CR
overflow bit is therefore simply set to zero if saturation did not occur,
and to one if it did.

Note also that saturate on operations that set OE=1 must raise an
Illegal Instruction due to the conflicting use of the CR.so bit for
storing if
saturation occurred. Integer Operations that produce a Carry-Out (CA, CA32):
these two bits will be `UNDEFINED` if saturation is also requested.

Note that the operation takes place at the maximum bitwidth (max of
src and dest elwidth) and that truncation occurs to the range of the
dest elwidth.

*Programmer's Note: Post-analysis of the Vector of CRs to find out if any given element hit
saturation may be done using a mapreduced CR op (cror), or by using the
new crrweird instruction with Rc=1, which will transfer the required
CR bits to a scalar integer and update CR0, which will allow testing
the scalar integer for nonzero.  see [[sv/cr_int_predication]]*

## Reduce mode

Reduction in SVP64 is similar in essence to other Vector Processing
ISAs, but leverages the underlying scalar Base v3.0B operations.
Thus it is more a convention that the programmer may utilise to give
the appearance and effect of a Horizontal Vector Reduction. Due
to the unusual decoupling it is also possible to perform
prefix-sum (Fibonacci Series) in certain circumstances. Details are in the [[svp64/appendix]]

Reduce Mode should not be confused with Parallel Reduction [[sv/remap]].
As explained in the [[sv/appendix]] Reduce Mode switches off the check
which would normally stop looping if the result register is scalar.
Thus, the result scalar register, if also used as a source scalar,
may be used to perform sequential accumulation.  This *deliberately*
sets up a chain
of Register Hazard Dependencies, whereas Parallel Reduce [[sv/remap]]
deliberately issues a Tree-Schedule of operations that may be parallelised.

## Fail-on-first

Data-dependent fail-on-first has two distinct variants: one for LD/ST,
the other for arithmetic operations (actually, CR-driven).  Note in each
case the assumption is that vector elements are required to appear to be
executed in sequential Program Order. When REMAP is not active,
element 0 would be the first.

Data-driven (CR-driven) fail-on-first activates when Rc=1 or other
CR-creating operation produces a result (including cmp).  Similar to
branch, an analysis of the CR is performed and if the test fails, the
vector operation terminates and discards all element operations **at and
above the current one**, and VL is truncated to either
the *previous* element or the current one, depending on whether
VLi (VL "inclusive") is clear or set, respectively.

Thus the new VL comprises a contiguous vector of results,
all of which pass the testing criteria (equal to zero, less than zero etc
as defined by the CR-bit test).

*Note: when VLi is clear, the behaviour at first seems counter-intuitive.
A result is calculated but if the test fails it is prohibited from being
actually written.  This becomes intuitive again when it is remembered
that the length that VL is set to is the number of *written* elements,
and only when VLI is set will the current element be included in that
count.*

The CR-based data-driven fail-on-first is "new" and not found in ARM
SVE or RVV. At the same time it is "old" because it is almost
identical to a generalised form of Z80's `CPIR` instruction.
It is extremely useful for reducing instruction count,
however requires speculative execution involving modifications of VL
to get high performance implementations.  An additional mode (RC1=1)
effectively turns what would otherwise be an arithmetic operation
into a type of `cmp`.  The CR is stored (and the CR.eq bit tested
against the `inv` field).
If the CR.eq bit is equal to `inv` then the Vector is truncated and
the loop ends.

VLi is only available as an option when `Rc=0` (or for instructions
which do not have Rc). When set, the current element is always
also included in the count (the new length that VL will be set to).
This may be useful in combination with "inv" to truncate the Vector
to *exclude* elements that fail a test, or, in the case of implementations
of strncpy, to include the terminating zero.

In CR-based data-driven fail-on-first there is only the option to select
and test one bit of each CR (just as with branch BO).  For more complex
tests this may be insufficient.  If that is the case, a vectorised crop
such as crand, cror or [[sv/cr_int_predication]] crweirder may be used,
and ffirst applied to the crop instead of to
the arithmetic vector. Note that crops are covered by
the [[sv/cr_ops]] Mode format.

*Programmer's note: `VLi` is only accessible in normal operations
which in turn limits the CR field bit-testing to only `EQ/NE`.
[[sv/cr_ops]] are not so limited.  Thus it is possible to use for
example `sv.cror/ff=gt/vli *0,*0,*0`, which is not a `nop` because
it allows Fail-First Mode to perform a test and truncate VL.*

Two extremely important aspects of ffirst are:

* LDST ffirst may never set VL equal to zero.  This because on the first
  element an exception must be raised "as normal".
* CR-based data-dependent ffirst on the other hand **can** set VL equal
  to zero. This is the only means in the entirety of SV that VL may be set
  to zero (with the exception of via the SV.STATE SPR).  When VL is set
  zero due to the first element failing the CR bit-test, all subsequent
  vectorised operations are effectively `nops` which is
  *precisely the desired and intended behaviour*.

The second crucial aspect, compared to LDST Ffirst:

* LD/ST Failfirst may (beyond the initial first element
  conditions) truncate VL for any architecturally
  suitable reason. Beyond the first element LD/ST Failfirst is
  arbitrarily speculative and 100% non-deterministic.
* CR-based data-dependent first on the other hand MUST NOT truncate VL
  arbitrarily to a length decided by the hardware: VL MUST only be
  truncated based explicitly on whether a test fails.
  This because it is a precise Deterministic test on which algorithms
  can and will will rely.

**Floating-point Exceptions**

When Floating-point exceptions are enabled VL must be truncated at
the point where the Exception appears not to have occurred. If `VLi`
is set then VL must include the faulting element, and thus the
faulting element will always raise its exception.  If however `VLi`
is clear then VL **excludes** the faulting element and thus the
exception will **never** be raised.

Although very strongly
discouraged the Exception Mode that permits Floating Point Exception
notification to arrive too late to unwind is permitted
(under protest, due it violating
the otherwise 100% Deterministic nature of Data-dependent Fail-first).

**Use of lax FP Exception Notification Mode could result in parallel
computations proceeding with invalid results that have to be explicitly
detected, whereas with the strict FP Execption Mode enabled, FFirst
truncates VL, allows subsequent parallel computation to avoid
the exceptions entirely**

## Data-dependent fail-first on CR operations (crand etc)

Operations that actually produce or alter CR Field as a result
have their own SVP64 Mode, described
in [[sv/cr_ops]].

## pred-result mode

This mode merges common CR testing with predication, saving on instruction
count. Below is the pseudocode excluding predicate zeroing and elwidth
overrides. Note that the pseudocode for SVP64 CR-ops is slightly different.

```
    for i in range(VL):
        # predication test, skip all masked out elements.
        if predicate_masked_out(i):
             continue
        result = op(iregs[RA+i], iregs[RB+i])
        CRnew = analyse(result) # calculates eq/lt/gt
        # Rc=1 always stores the CR field
        if Rc=1 or RC1:
            CR.field[offs+i] = CRnew
        # now test CR, similar to branch
        if RC1 or CR.field[BO[0:1]] != BO[2]:
            continue # test failed: cancel store
        # result optionally stored but CR always is
        iregs[RT+i] = result
```

The reason for allowing the CR element to be stored is so that
post-analysis of the CR Vector may be carried out.  For example:
Saturation may have occurred (and been prevented from updating, by the
test) but it is desirable to know *which* elements fail saturation.

Note that RC1 Mode basically turns all operations into `cmp`.  The
calculation is performed but it is only the CR that is written. The
element result is *always* discarded, never written (just like `cmp`).

Note that predication is still respected: predicate zeroing is slightly
different: elements that fail the CR test *or* are masked out are zero'd.

# SV Load and Store

**Rationale**

All Vector ISAs dating back fifty years have extensive and comprehensive
Load and Store operations that go far beyond the capabilities of Scalar
RISC and most CISC processors, yet at their heart on an individual element
basis may be found to be no different from RISC Scalar equivalents.

The resource savings from Vector LD/ST are significant and stem from
the fact that one single instruction can trigger a dozen (or in some
microarchitectures such as Cray or NEC SX Aurora) hundreds of element-level Memory accesses.

Additionally, and simply: if the Arithmetic side of an ISA supports
Vector Operations, then in order to keep the ALUs 100% occupied the
Memory infrastructure (and the ISA itself) correspondingly needs Vector
Memory Operations as well.

Vectorised Load and Store also presents an extra dimension (literally)
which creates scenarios unique to Vector applications, that a Scalar
(and even a SIMD) ISA simply never encounters.  SVP64 endeavours to
add the modes typically found in *all* Scalable Vector ISAs,
without changing the behaviour of the underlying Base
(Scalar) v3.0B operations in any way.

## Modes overview

Vectorisation of Load and Store requires creation, from scalar operations,
a number of different modes:

* **fixed aka "unit" stride** - contiguous sequence with no gaps
* **element strided** - sequential but regularly offset, with gaps
* **vector indexed** - vector of base addresses and vector of offsets
* **Speculative fail-first** - where it makes sense to do so
* **Structure Packing** - covered in SV by [[sv/remap]] and Pack/Unpack Mode.

*Despite being constructed from Scalar LD/ST none of these Modes
exist or make sense in any Scalar ISA. They **only** exist in Vector ISAs*

Also included in SVP64 LD/ST is both signed and unsigned Saturation,
as well as Element-width overrides and Twin-Predication.

Note also that Indexed [[sv/remap]] mode may be applied to both
v3.0 LD/ST Immediate instructions *and* v3.0 LD/ST Indexed instructions.
LD/ST-Indexed should not be conflated with Indexed REMAP mode: clarification
is provided below.

**Determining the LD/ST Modes**

A minor complication (caused by the retro-fitting of modern Vector
features to a Scalar ISA) is that certain features do not exactly make
sense or are considered a security risk.  Fail-first on Vector Indexed
would allow attackers to probe large numbers of pages from userspace, where
strided fail-first (by creating contiguous sequential LDs) does not.

In addition, reduce mode makes no sense.
Realistically we need
an alternative table definition for [[sv/svp64]] `RM.MODE`.
The following modes make sense:

* saturation
* predicate-result (mostly for cache-inhibited LD/ST)
* simple (no augmentation)
* fail-first (where Vector Indexed is banned)
* Signed Effective Address computation (Vector Indexed only)
* Pack/Unpack (on LD/ST immediate operations only)

More than that however it is necessary to fit the usual Vector ISA
capabilities onto both Power ISA LD/ST with immediate and to
LD/ST Indexed. They present subtly different Mode tables, which, due
to lack of space, have the following quirks:

* LD/ST Immediate has no individual control over src/dest zeroing,
  whereas LD/ST Indexed does.
* LD/ST Immediate has no Saturated Pack/Unpack (Arithmetic Mode does)
* LD/ST Indexed has no Pack/Unpack (REMAP may be used instead)

# Format and fields

Fields used in tables below:

* **sz / dz**  if predication is enabled will put zeros into the dest (or as src in the case of twin pred) when the predicate bit is zero.  otherwise the element is ignored or skipped, depending on context.
* **zz**: both sz and dz are set equal to this flag.
* **inv CR bit** just as in branches (BO) these bits allow testing of a CR bit and whether it is set (inv=0) or unset (inv=1)
* **N** sets signed/unsigned saturation.
* **RC1** as if Rc=1, stores CRs *but not the result*
* **SEA** - Signed Effective Address, if enabled performs sign-extension on
  registers that have been reduced due to elwidth overrides

**LD/ST immediate**

The table for [[sv/svp64]] for `immed(RA)` which is `RM.MODE`
(bits 19:23 of `RM`) is:

| 0-1 |  2  |  3   4  |  description               |
| --- | --- |---------|--------------------------- |
| 00  | 0   |  zz els | simple mode                |
| 00  | 1   | PI  LF  | post-increment and Fault-First  |
| 01  | inv | CR-bit  | Rc=1: ffirst CR sel        |
| 01  | inv | els RC1 |  Rc=0: ffirst z/nonz       |
| 10  |   N | zz  els |  sat mode: N=0/1 u/s       |
| 11  | inv | CR-bit  |  Rc=1: pred-result CR sel  |
| 11  | inv | els RC1 |  Rc=0: pred-result z/nonz  |

The `els` bit is only relevant when `RA.isvec` is clear: this indicates
whether stride is unit or element:

```
    if RA.isvec:
        svctx.ldstmode = indexed
    elif els == 0:
        svctx.ldstmode = unitstride
    elif immediate != 0:
        svctx.ldstmode = elementstride
```

An immediate of zero is a safety-valve to allow `LD-VSPLAT`:
in effect the multiplication of the immediate-offset by zero results
in reading from the exact same memory location, *even with a Vector
register*. (Normally this type of behaviour is reserved for the
mapreduce modes)

For `LD-VSPLAT`, on non-cache-inhibited Loads, the read can occur
just the once and be copied, rather than hitting the Data Cache
multiple times with the same memory read at the same location.
The benefit of Cache-inhibited LD-splats is that it allows
for memory-mapped peripherals to have multiple
data values read in quick succession and stored in sequentially
numbered registers (but, see Note below).

For non-cache-inhibited ST from a vector source onto a scalar
destination: with the Vector
loop effectively creating multiple memory writes to the same location,
we can deduce that the last of these will be the "successful" one. Thus,
implementations are free and clear to optimise out the overwriting STs,
leaving just the last one as the "winner".  Bear in mind that predicate
masks will skip some elements (in source non-zeroing mode).
Cache-inhibited ST operations on the other hand **MUST** write out
a Vector source multiple successive times to the exact same Scalar
destination. Just like Cache-inhibited LDs, multiple values may be
written out in quick succession to a memory-mapped peripheral from
sequentially-numbered registers.

Note that any memory location may be Cache-inhibited
(Power ISA v3.1, Book III, 1.6.1, p1033)

*Programmer's Note: an immediate also with a Scalar source as
a "VSPLAT" mode is simply not possible: there are not enough
Mode bits. One single Scalar Load operation may be used instead, followed
by any arithmetic operation (including a simple mv) in "Splat"
mode.*

**LD/ST Indexed**

The modes for `RA+RB` indexed version are slightly different
but are the same `RM.MODE` bits (19:23 of `RM`):

| 0-1 |  2  |  3   4  |  description              |
| --- | --- |---------|-------------------------- |
| 00  | SEA |  dz  sz | simple mode        |
| 01  | SEA | dz sz   | Strided (scalar only source)   |
| 10  |   N | dz   sz |  sat mode: N=0/1 u/s |
| 11  | inv | CR-bit  |  Rc=1: pred-result CR sel |
| 11  | inv | zz  RC1 |  Rc=0: pred-result z/nonz |

Vector Indexed Strided Mode is qualified as follows:

    if mode = 0b01 and !RA.isvec and !RB.isvec:
        svctx.ldstmode = elementstride

A summary of the effect of Vectorisation of src or dest:

     imm(RA)  RT.v   RA.v   no stride allowed
     imm(RA)  RT.s   RA.v   no stride allowed
     imm(RA)  RT.v   RA.s   stride-select allowed
     imm(RA)  RT.s   RA.s   not vectorised
     RA,RB    RT.v  {RA|RB}.v Standard Indexed
     RA,RB    RT.s  {RA|RB}.v Indexed but single LD (no VSPLAT)
     RA,RB    RT.v  {RA&RB}.s VSPLAT possible. stride selectable
     RA,RB    RT.s  {RA&RB}.s not vectorised (scalar identity)

Signed Effective Address computation is only relevant for
Vector Indexed Mode, when elwidth overrides are applied.
The source override applies to RB, and before adding to
RA in order to calculate the Effective Address, if SEA is
set RB is sign-extended from elwidth bits to the full 64
bits.  For other Modes (ffirst, saturate),
all EA computation with elwidth overrides is unsigned.

Note that cache-inhibited LD/ST  when VSPLAT is activated will perform **multiple** LD/ST operations, sequentially.  Even with scalar src a
Cache-inhibited LD will read the same memory location *multiple times*, storing the result in successive Vector destination registers.  This because the cache-inhibit instructions are typically used to read and write memory-mapped peripherals.
If a genuine cache-inhibited LD-VSPLAT is required then a single *scalar*
cache-inhibited LD should be performed, followed by a VSPLAT-augmented mv,
copying the one *scalar* value into multiple register destinations.

Note also that cache-inhibited VSPLAT with Predicate-result is possible.
This allows for example to issue a massive batch of memory-mapped
peripheral reads, stopping at the first NULL-terminated character and
truncating VL to that point. No branch is needed to issue that large burst
of LDs, which may be valuable in Embedded scenarios.

## Vectorisation of Scalar Power ISA v3.0B

Scalar Power ISA Load/Store operations may be seen from their
pseudocode to be of the form:

    lbux RT, RA, RB
    EA <- (RA) + (RB)
    RT <- MEM(EA)

and for immediate variants:

    lb RT,D(RA)
    EA <- RA + EXTS(D)
    RT <- MEM(EA)

Thus in the first example, the source registers may each be independently
marked as scalar or vector, and likewise the destination; in the second
example only the one source and one dest may be marked as scalar or
vector.

Thus we can see that Vector Indexed may be covered, and, as demonstrated
with the pseudocode below, the immediate can be used to give unit
stride or element stride.  With there being no way to tell which from
the Power v3.0B Scalar opcode alone, the choice is provided instead by
the SV Context.

```
    # LD not VLD!  format - ldop RT, immed(RA)
    # op_width: lb=1, lh=2, lw=4, ld=8
    op_load(RT, RA, op_width, immed, svctx, RAupdate):
      ps = get_pred_val(FALSE, RA); # predication on src
      pd = get_pred_val(FALSE, RT); # ... AND on dest
      for (i=0, j=0, u=0; i < VL && j < VL;):
        # skip nonpredicates elements
        if (RA.isvec) while (!(ps & 1<<i)) i++;
        if (RAupdate.isvec) while (!(ps & 1<<u)) u++;
        if (RT.isvec) while (!(pd & 1<<j)) j++;
        if postinc:
            offs = 0; # added afterwards
            if RA.isvec: srcbase = ireg[RA+i]
            else         srcbase = ireg[RA]
        elif svctx.ldstmode == elementstride:
          # element stride mode
          srcbase = ireg[RA]
          offs = i * immed              # j*immed for a ST
        elif svctx.ldstmode == unitstride:
          # unit stride mode
          srcbase = ireg[RA]
          offs = immed + (i * op_width) # j*op_width for ST
        elif RA.isvec:
          # quirky Vector indexed mode but with an immediate
          srcbase = ireg[RA+i]
          offs = immed;
        else
          # standard scalar mode (but predicated)
          # no stride multiplier means VSPLAT mode
          srcbase = ireg[RA]
          offs = immed

        # compute EA
        EA = srcbase + offs
        # load from memory
        ireg[RT+j] <= MEM[EA];
        # check post-increment of EA
        if postinc: EA = srcbase + immed;
        # update RA?
        if RAupdate: ireg[RAupdate+u] = EA;
        if (!RT.isvec)
            break # destination scalar, end now
        if (RA.isvec) i++;
        if (RAupdate.isvec) u++;
        if (RT.isvec) j++;
```

Indexed LD is:

```
    # format: ldop RT, RA, RB
    function op_ldx(RT, RA, RB, RAupdate=False) # LD not VLD!
      ps = get_pred_val(FALSE, RA); # predication on src
      pd = get_pred_val(FALSE, RT); # ... AND on dest
      for (i=0, j=0, k=0, u=0; i < VL && j < VL && k < VL):
        # skip nonpredicated RA, RB and RT
        if (RA.isvec) while (!(ps & 1<<i)) i++;
        if (RAupdate.isvec) while (!(ps & 1<<u)) u++;
        if (RB.isvec) while (!(ps & 1<<k)) k++;
        if (RT.isvec) while (!(pd & 1<<j)) j++;
        if svctx.ldstmode == elementstride:
            EA = ireg[RA] + ireg[RB]*j   # register-strided
        else
            EA = ireg[RA+i] + ireg[RB+k] # indexed address
        if RAupdate: ireg[RAupdate+u] = EA
        ireg[RT+j] <= MEM[EA];
        if (!RT.isvec)
            break # destination scalar, end immediately
        if (RA.isvec) i++;
        if (RAupdate.isvec) u++;
        if (RB.isvec) k++;
        if (RT.isvec) j++;
```

Note that Element-Strided uses the Destination Step because with both
sources being Scalar as a prerequisite condition of activation of
Element-Stride Mode, the source step (being Scalar) would never advance.

Note in both cases that [[sv/svp64]] allows RA-as-a-dest in "update" mode (`ldux`) to be effectively a *completely different* register from RA-as-a-source.  This because there is room in svp64 to extend RA-as-src as well as RA-as-dest, both independently as scalar or vector *and* independently extending their range.

*Programmer's note: being able to set RA-as-a-source
 as separate from RA-as-a-destination as Scalar is **extremely valuable**
 once it is remembered that Simple-V element operations must
 be in Program Order, especially in loops, for saving on
 multiple address computations. Care does have
 to be taken however that RA-as-src is not overwritten by
 RA-as-dest unless intentionally desired, especially in element-strided Mode.*

## LD/ST Indexed vs Indexed REMAP

Unfortunately the word "Indexed" is used twice in completely different
contexts, potentially causing confusion.

* There has existed instructions in the Power ISA `ld RT,RA,RB` since
  its creation: these are called "LD/ST Indexed" instructions and their
  name and meaning is well-established.
* There now exists, in Simple-V, a REMAP mode called "Indexed"
  Mode that can be applied to *any* instruction **including those
  named LD/ST Indexed**.

Whilst it may be costly in terms of register reads to allow REMAP
Indexed Mode to be applied to any Vectorised LD/ST Indexed operation such as
`sv.ld *RT,RA,*RB`, or even misleadingly
labelled  as redundant, firstly the strict
application of the RISC Paradigm that Simple-V follows makes it awkward
to consider *preventing* the application of Indexed REMAP to such
operations, and secondly they are not actually the same at all.

Indexed REMAP, as applied to RB in the instruction `sv.ld *RT,RA,*RB`
effectively performs an *in-place* re-ordering of the offsets, RB.
To achieve the same effect without Indexed REMAP would require taking
a *copy* of the Vector of offsets starting at RB, manually explicitly
reordering them, and finally using the copy of re-ordered offsets in
a non-REMAP'ed `sv.ld`.  Using non-strided LD as an example,
pseudocode showing what actually occurs,
where the pseudocode for `indexed_remap` may be found in [[sv/remap]]:

```
    # sv.ld *RT,RA,*RB with Index REMAP applied to RB
    for i in 0..VL-1:
        if remap.indexed:
            rb_idx = indexed_remap(i) # remap
        else:
            rb_idx = i # use the index as-is
        EA = GPR(RA) + GPR(RB+rb_idx)
        GPR(RT+i) = MEM(EA, 8)
```

Thus it can be seen that the use of Indexed REMAP saves copying
and manual reordering of the Vector of RB offsets.

## LD/ST ffirst

LD/ST ffirst treats the first LD/ST in a vector (element 0 if REMAP
is not active) as an ordinary one, with all behaviour with respect to
Interrupts Exceptions Page Faults Memory Management being identical
in every regard to Scalar v3.0 Power ISA LD/ST. However for elements
1 and above, if an exception would occur, then VL is **truncated**
to the previous element: the exception is **not** then raised because
the LD/ST that would otherwise have caused an exception is *required*
to be cancelled. Additionally an implementor may choose to truncate VL
for any arbitrary reason *except for the very first*.

ffirst LD/ST to multiple pages via a Vectorised Index base is
considered a security risk due to the abuse of probing multiple
pages in rapid succession and getting speculative feedback on which
pages would fail.  Therefore Vector Indexed LD/ST is prohibited
entirely, and the Mode bit instead used for element-strided LD/ST.
See <https://bugs.libre-soc.org/show_bug.cgi?id=561>

```
    for(i = 0; i < VL; i++)
        reg[rt + i] = mem[reg[ra] + i * reg[rb]];
```

High security implementations where any kind of speculative probing
of memory pages is considered a risk should take advantage of the fact that
implementations may truncate VL at any point, without requiring software
to be rewritten and made non-portable. Such implementations may choose
to *always* set VL=1 which will have the effect of terminating any
speculative probing (and also adversely affect performance), but will
at least not require applications to be rewritten.

Low-performance simpler hardware implementations may also
choose (always) to also set VL=1 as the bare minimum compliant implementation of
LD/ST Fail-First. It is however critically important to remember that
the first element LD/ST **MUST** be treated as an ordinary LD/ST, i.e.
**MUST** raise exceptions exactly like an ordinary LD/ST.

For ffirst LD/STs, VL may be truncated arbitrarily to a nonzero value for any implementation-specific reason. For example: it is perfectly reasonable for implementations to alter VL when ffirst LD or ST operations are initiated on a nonaligned boundary, such that within a loop the subsequent iteration of that loop begins the following ffirst LD/ST operations on an aligned boundary
such as the beginning of a cache line, or beginning of a Virtual Memory
page. Likewise, to reduce workloads or balance resources.

Vertical-First Mode is slightly strange in that only one element
at a time is ever executed anyway.  Given that programmers may
legitimately choose to alter srcstep and dststep in non-sequential
order as part of explicit loops, it is neither possible nor
safe to make speculative assumptions about future LD/STs.
Therefore, Fail-First LD/ST in Vertical-First is `UNDEFINED`.
This is very different from Arithmetic (Data-dependent) FFirst
where Vertical-First Mode is fully deterministic, not speculative.

## LOAD/STORE Elwidths <a name="elwidth"></a>

Loads and Stores are almost unique in that the Power Scalar ISA
provides a width for the operation (lb, lh, lw, ld).  Only `extsb` and
others like it provide an explicit operation width.  There are therefore
*three* widths involved:

* operation width (lb=8, lh=16, lw=32, ld=64)
* src element width override (8/16/32/default)
* destination element width override (8/16/32/default)

Some care is therefore needed to express and make clear the transformations,
which are expressly in this order:

* Calculate the Effective Address from RA at full width
  but (on Indexed Load) allow srcwidth overrides on RB
* Load at the operation width (lb/lh/lw/ld) as usual
* byte-reversal as usual
* Non-saturated mode:
   - zero-extension or truncation from operation width to dest elwidth
   - place result in destination at dest elwidth
* Saturated mode:
   - Sign-extension or truncation from operation width to dest width
   - signed/unsigned saturation down to dest elwidth

In order to respect Power v3.0B Scalar behaviour the memory side
is treated effectively as completely separate and distinct from SV
augmentation.  This is primarily down to quirks surrounding LE/BE and
byte-reversal.

It is rather unfortunately possible to request an elwidth override
on the memory side which
does not mesh with the overridden operation width: these result in
`UNDEFINED`
behaviour.  The reason is that the effect of attempting a 64-bit `sv.ld`
operation with a source elwidth override of 8/16/32 would result in
overlapping memory requests, particularly on unit and element strided
operations.  Thus it is `UNDEFINED` when the elwidth is smaller than
the memory operation width. Examples include `sv.lw/sw=16/els` which
requests (overlapping) 4-byte memory reads offset from
each other at 2-byte intervals.  Store likewise is also `UNDEFINED`
where the dest elwidth override is less than the operation width.

Note the following regarding the pseudocode to follow:

* `scalar identity behaviour` SV Context parameter conditions turn this
  into a straight absolute fully-compliant Scalar v3.0B LD operation
* `brev` selects whether the operation is the byte-reversed variant (`ldbrx`
  rather than `ld`)
* `op_width` specifies the operation width (`lb`, `lh`, `lw`, `ld`) as
  a "normal" part of Scalar v3.0B LD
* `imm_offs` specifies the immediate offset `ld r3, imm_offs(r5)`, again
  as a "normal" part of Scalar v3.0B LD
* `svctx` specifies the SV Context and includes VL as well as
  source and destination elwidth overrides.

Below is the pseudocode for Unit-Strided LD (which includes Vector capability). Observe in particular that RA, as the base address in
both Immediate and Indexed LD/ST,
does not have element-width overriding applied to it.

Note that predication, predication-zeroing,
and other modes except saturation have all been removed,
for clarity and simplicity:

```
    # LD not VLD!
    # this covers unit stride mode and a type of vector offset
    function op_ld(RT, RA, op_width, imm_offs, svctx)
      for (int i = 0, int j = 0; i < svctx.VL && j < svctx.VL):
        if not svctx.unit/el-strided:
            # strange vector mode, compute 64 bit address which is
            # not polymorphic! elwidth hardcoded to 64 here
            srcbase = get_polymorphed_reg(RA, 64, i)
        else:
            # unit / element stride mode, compute 64 bit address
            srcbase = get_polymorphed_reg(RA, 64, 0)
            # adjust for unit/el-stride
            srcbase += ....

        # read the underlying memory
        memread <= MEM(srcbase + imm_offs, op_width)

        # check saturation.
        if svpctx.saturation_mode:
            # ... saturation adjustment...
            memread = clamp(memread, op_width, svctx.dest_elwidth)
        else:
            # truncate/extend to over-ridden dest width.
            memread = adjust_wid(memread, op_width, svctx.dest_elwidth)

        # takes care of inserting memory-read (now correctly byteswapped)
        # into regfile underlying LE-defined order, into the right place
        # within the NEON-like register, respecting destination element
        # bitwidth, and the element index (j)
        set_polymorphed_reg(RT, svctx.dest_elwidth, j, memread)

        # increments both src and dest element indices (no predication here)
        i++;
        j++;
```

Note above that the source elwidth is *not used at all* in LD-immediate.

For LD/Indexed, the key is that in the calculation of the Effective Address,
RA has no elwidth override but RB does.  Pseudocode below is simplified
for clarity: predication and all modes except saturation are removed:

```
    # LD not VLD! ld*rx if brev else ld*
    function op_ld(RT, RA, RB, op_width, svctx, brev)
      for (int i = 0, int j = 0; i < svctx.VL && j < svctx.VL):
        if not svctx.el-strided:
            # RA not polymorphic! elwidth hardcoded to 64 here
            srcbase = get_polymorphed_reg(RA, 64, i)
        else:
            # element stride mode, again RA not polymorphic
            srcbase = get_polymorphed_reg(RA, 64, 0)
        # RB *is* polymorphic
        offs = get_polymorphed_reg(RB, svctx.src_elwidth, i)
        # sign-extend
        if svctx.SEA: offs = sext(offs, svctx.src_elwidth, 64)

        # takes care of (merges) processor LE/BE and ld/ldbrx
        bytereverse = brev XNOR MSR.LE

        # read the underlying memory
        memread <= MEM(srcbase + offs, op_width)

        # optionally performs byteswap at op width
        if (bytereverse):
            memread = byteswap(memread, op_width)

        if svpctx.saturation_mode:
            # ... saturation adjustment...
            memread = clamp(memread, op_width, svctx.dest_elwidth)
        else:
            # truncate/extend to over-ridden dest width.
            memread = adjust_wid(memread, op_width, svctx.dest_elwidth)

        # takes care of inserting memory-read (now correctly byteswapped)
        # into regfile underlying LE-defined order, into the right place
        # within the NEON-like register, respecting destination element
        # bitwidth, and the element index (j)
        set_polymorphed_reg(RT, svctx.dest_elwidth, j, memread)

        # increments both src and dest element indices (no predication here)
        i++;
        j++;
```

# Remapped LD/ST

In the [[sv/remap]] page the concept of "Remapping" is described.
Whilst it is expensive to set up (2 64-bit opcodes minimum) it provides
a way to arbitrarily perform 1D, 2D and 3D "remapping" of up to 64
elements worth of LDs or STs.  The usual interest in such re-mapping
is for example in separating out 24-bit RGB channel data into separate
contiguous registers.

REMAP easily covers this capability, and with dest
elwidth overrides and saturation may do so with built-in conversion that
would normally require additional width-extension, sign-extension and
min/max Vectorised instructions as post-processing stages.

Thus we do not need to provide specialist LD/ST "Structure Packed" opcodes
because the generic abstracted concept of "Remapping", when applied to
LD/ST, will give that same capability, with far more flexibility.

It is worth noting that Pack/Unpack Modes of SVSTATE, which may be
established through `svstep`, are also an easy way to perform regular
Structure Packing, at the vec2/vec3/vec4 granularity level.  Beyond
that, REMAP will need to be used.

# Condition Register SVP64 Operations

Condition Register Fields are only 4 bits wide: this presents some
interesting conceptual challenges for SVP64, which was designed
primarily for vectors of arithmetic and logical operations. However
if predicates may be bits of CR Fields it makes sense to extend
Simple-V to cover CR Operations, especially given that Vectorised Rc=1
may be processed by Vectorised CR Operations tbat usefully in turn
may become Predicate Masks to yet more Vector operations, like so:

```
    sv.cmpi/ew=8 *B,*ra,0    # compare bytes against zero
    sv.cmpi/ew=8 *B2,*ra,13. # and against newline
    sv.cror PM.EQ,B.EQ,B2.EQ # OR compares to create mask
    sv.stb/sm=EQ    ...      # store only nonzero/newline
```

Element width however is clearly meaningless for a 4-bit collation of
Conditions, EQ LT GE SO. Likewise, arithmetic saturation (an important
part of Arithmetic SVP64) has no meaning. An alternative Mode Format is
required, and given that elwidths are meaningless for CR Fields the bits
in SVP64 `RM` may be used for other purposes.

This alternative mapping **only** applies to instructions that **only**
reference a CR Field or CR bit as the sole exclusive result. This section
**does not** apply to instructions which primarily produce arithmetic
results that also, as an aside, produce a corresponding
CR Field (such as when Rc=1).
Instructions that involve Rc=1 are definitively arithmetic in nature,
where the corresponding Condition Register Field can be considered to
be a "co-result". Such CR Field "co-result" arithmeric operations
are firmly out of scope for
this section, being covered fully by [[sv/normal]].

* Examples of v3.0B instructions to which this section does
  apply is
  - `mfcr` and `cmpi` (3 bit operands) and
  - `crnor` and `crand` (5 bit operands).
* Examples to which this section does **not** apply include
  `fadds.` and `subf.` which both produce arithmetic results
  (and a CR Field co-result).

The CR Mode Format still applies to `sv.cmpi` because despite
taking a GPR as input, the output from the Base Scalar v3.0B `cmpi`
instruction is purely to a Condition Register Field.

Other modes are still applicable and include:

* **Data-dependent fail-first**.
  useful to truncate VL based on
  analysis of a Condition Register result bit.
* **Reduction**.
  Reduction is useful
for analysing a Vector of Condition Register Fields
and reducing it to one
single Condition Register Field.

Predicate-result does not make any sense because
when Rc=1 a co-result is created (a CR Field). Testing the co-result
allows the decision to be made to store or not store the main
result, and for CR Ops the CR Field result *is*
the main result.

## Format

SVP64 RM `MODE` (includes `ELWIDTH_SRC` bits) for CR-based operations:

|6 | 7 |19-20|  21 | 22   23 |  description     |
|--|---|-----| --- |---------|----------------- |
|/ | / |0 RG |   0 | dz  sz  | simple mode                      |
|/ | / |0 RG |   1 | dz  sz  | scalar reduce mode (mapreduce) |
|zz|SNZ|1 VLI| inv |  CR-bit | Ffirst 3-bit mode      |
|/ |SNZ|1 VLI| inv |  dz sz  | Ffirst 5-bit mode (implies CR-bit from result) |

Fields:

* **sz / dz**  if predication is enabled will put zeros into the dest (or as src in the case of twin pred) when the predicate bit is zero.  otherwise the element is ignored or skipped, depending on context.
* **zz** set both sz and dz equal to this flag
* **SNZ** In fail-first mode, on the bit being tested, when sz=1 and SNZ=1 a value "1" is put in place of "0".
* **inv CR-bit** just as in branches (BO) these bits allow testing of a CR bit and whether it is set (inv=0) or unset (inv=1)
* **RG** inverts the Vector Loop order (VL-1 downto 0) rather
than the normal 0..VL-1
* **SVM** sets "subvector" reduce mode
* **VLi** VL inclusive: in fail-first mode, the truncation of
  VL *includes* the current element at the failure point rather
  than excludes it from the count.

## Data-dependent fail-first on CR operations

The principle of data-dependent fail-first is that if, during
the course of sequentially evaluating an element's Condition Test,
one such test is encountered which fails,
then VL (Vector Length) is truncated (set) at that point. In the case
of Arithmetic SVP64 Operations the Condition Register Field generated from
Rc=1 is used as the basis for the truncation decision.
However with CR-based operations that CR Field result to be
tested is provided
*by the operation itself*.

Data-dependent SVP64 Vectorised Operations involving the creation or
modification of a CR can require an extra two bits, which are not available
in the compact space of the SVP64 RM `MODE` Field. With the concept of element
width overrides being meaningless for CR Fields it is possible to use the
`ELWIDTH` field for alternative purposes.

Condition Register based operations such as `sv.mfcr` and `sv.crand` can thus
be made more flexible.  However the rules that apply in this section
also apply to future CR-based instructions.

There are two primary different types of CR operations:

* Those which have a 3-bit operand field (referring to a CR Field)
* Those which have a 5-bit operand (referring to a bit within the
   whole 32-bit CR)

Examining these two types it is observed that the
difference may be considered to be that the 5-bit variant
*already* provides the
prerequisite information about which CR Field bit (EQ, GE, LT, SO) is to
be operated on by the instruction.
Thus, logically, we may set the following rule:

* When a 5-bit CR Result field is used in an instruction, the
  5-bit variant of Data-Dependent Fail-First
  must be used. i.e. the bit of the CR field to be tested is
  the one that has just been modified (created) by the operation.
* When a 3-bit CR Result field is used the 3-bit variant
  must be used, providing as it does the missing `CRbit` field
  in order to select which CR Field bit of the result shall
  be tested (EQ, LE, GE, SO)

The reason why the 3-bit CR variant needs the additional CR-bit
field should be obvious from the fact that the 3-bit CR Field
from the base Power ISA v3.0B operation clearly does not contain
and is missing the two CR Field Selector bits. Thus, these two
bits (to select EQ, LE, GE or SO) must be provided in another
way.

Examples of the former type:

* crand, cror, crnor. These all are 5-bit (BA, BB, BT). The bit
  to be tested against `inv` is the one selected by `BT`
* mcrf. This has only 3-bit (BF, BFA). In order to select the
  bit to be tested, the alternative encoding must be used.
  With `CRbit` coming from the SVP64 RM bits 22-23 the bit
  of BF to be tested is identified.

Just as with SVP64 [[sv/branches]] there is the option to truncate
VL to include the element being tested (`VLi=1`) and to exclude it
(`VLi=0`).

Also exactly as with [[sv/normal]] fail-first, VL cannot, unlike
[[sv/ldst]], be set to an arbitrary value.  Deterministic behaviour
is *required*.

## Reduction and Iteration

Bearing in mind as described in the svp64 Appendix, SVP64 Horizontal
Reduction is a deterministic schedule on top of base Scalar v3.0 operations,
the same rules apply to CR Operations, i.e. that programmers must
follow certain conventions in order for an *end result* of a
reduction to be achieved.  Unlike
other Vector ISAs *there are no explicit reduction opcodes*
in SVP64: Schedules however achieve the same effect.

Due to these conventions only reduction on operations such as `crand`
and `cror` are meaningful because these have Condition Register Fields
as both input and output.
Meaningless operations are not prohibited because the cost in hardware
of doing so is prohibitive, but neither are they `UNDEFINED`. Implementations
are still required to execute them but are at liberty to optimise out
any operations that would ultimately be overwritten, as long as Strict
Program Order is still obvservable by the programmer.

Also bear in mind that 'Reverse Gear' may be enabled, which can be
used in combination with overlapping CR operations to iteratively accumulate
results.  Issuing a `sv.crand` operation for example with `BA`
differing from `BB` by one Condition Register Field would
result in a cascade effect, where the first-encountered CR Field
would set the result to zero, and also all subsequent CR Field
elements thereafter:

```
    # sv.crand/mr/rg CR4.ge.v, CR5.ge.v, CR4.ge.v
    for i in VL-1 downto 0 # reverse gear
         CR.field[4+i].ge &= CR.field[5+i].ge
```

`sv.crxor` with reduction would be particularly useful for parity calculation
for example, although there are many ways in which the same calculation
could be carried out after transferring a vector of CR Fields to a GPR
using crweird operations.

Implementations are free and clear to optimise these reductions in any
way they see fit, as long as the end-result is compatible with Strict Program
Order being observed, and Interrupt latency is not adversely impacted.

## Unusual and quirky CR operations

**cmp and other compare ops**

`cmp` and `cmpi` etc take GPRs as sources and create a CR Field as a result.

    cmpli BF,L,RA,UI
    cmpeqb BF,RA,RB

With `ELWIDTH` applying to the source GPR operands this is perfectly fine.

**crweird operations**

There are 4 weird CR-GPR operations and one reasonable one in
the [[cr_int_predication]] set:

* crrweird
* mtcrweird
* crweirder
* crweird
* mcrfm - reasonably normal and referring to CR Fields for src and dest.

The "weird" operations have a non-standard behaviour, being able to
treat *individual bits* of a GPR effectively as elements.  They are
expected to be Micro-coded by most Hardware implementations.


## SVP64 Branch Conditional behaviour

Please note: although similar, SVP64 Branch instructions should be
considered completely separate and distinct from
standard scalar OpenPOWER-approved v3.0B branches.
**v3.0B branches are in no way impacted, altered,
changed or modified in any way, shape or form by
the SVP64 Vectorised Variants**.

It is also
extremely important to note that Branches are the
sole pseudo-exception in SVP64 to `Scalar Identity Behaviour`.
SVP64 Branches contain additional modes that are useful
for scalar operations (i.e. even when VL=1 or when
using single-bit predication).

**Rationale**

Scalar 3.0B Branch Conditional operations, `bc`, `bctar` etc. test a
Condition Register.  However for parallel processing it is simply impossible
to perform multiple independent branches: the Program Counter simply
cannot branch to multiple destinations based on multiple conditions.
The best that can be done is
to test multiple Conditions and make a decision of a *single* branch,
based on analysis of a *Vector* of CR Fields
which have just been calculated from a *Vector* of results.

In 3D Shader
binaries, which are inherently parallelised and predicated, testing all or
some results and branching based on multiple tests is extremely common,
and a fundamental part of Shader Compilers.  Example:
without such multi-condition
test-and-branch, if a predicate mask is all zeros a large batch of
instructions may be masked out to `nop`, and it would waste
CPU cycles to run them.  3D GPU ISAs can test for this scenario
and, with the appropriate predicate-analysis instruction,
jump over fully-masked-out operations, by spotting that
*all* Conditions are false.

Unless Branches are aware and capable of such analysis, additional
instructions would be required which perform Horizontal Cumulative
analysis of Vectorised Condition Register Fields, in order to
reduce the Vector of CR Fields down to one single yes or no
decision that a Scalar-only v3.0B Branch-Conditional could cope with.
Such instructions would be unavoidable, required, and costly
by comparison to a single Vector-aware Branch.
Therefore, in order to be commercially competitive, `sv.bc` and
other Vector-aware Branch Conditional instructions are a high priority
for 3D GPU (and OpenCL-style) workloads.

Given that Power ISA v3.0B is already quite powerful, particularly
the Condition Registers and their interaction with Branches, there
are opportunities to create extremely flexible and compact
Vectorised Branch behaviour.  In addition, the side-effects (updating
of CTR, truncation of VL, described below) make it a useful instruction
even if the branch points to the next instruction (no actual branch).

## Overview

When considering an "array" of branch-tests, there are four
primarily-useful modes:
AND, OR, NAND and NOR of all Conditions.
NAND and NOR may be synthesised from AND and OR by
inverting `BO[1]` which just leaves two modes:

* Branch takes place on the **first** CR Field test to succeed
  (a Great Big OR of all condition tests). Exit occurs
  on the first **successful** test.
* Branch takes place only if **all** CR field tests succeed:
  a Great Big AND of all condition tests.  Exit occurs
  on the first **failed** test.

Early-exit is enacted such that the Vectorised Branch does not
perform needless extra tests, which will help reduce reads on
the Condition Register file.

*Note: Early-exit is **MANDATORY** (required) behaviour.
Branches **MUST** exit at the first sequentially-encountered
failure point, for
exactly the same reasons for which it is mandatory in
programming languages doing early-exit: to avoid
damaging side-effects and to provide deterministic
behaviour. Speculative testing of Condition
Register Fields is permitted, as is speculative calculation
of CTR, as long as, as usual in any Out-of-Order microarchitecture,
that speculative testing is cancelled should an early-exit occur.
i.e. the speculation must be "precise": Program Order must be preserved*

Also note that when early-exit occurs in Horizontal-first Mode,
srcstep, dststep etc. are all reset, ready to begin looping from the
beginning for the next instruction. However for Vertical-first
Mode srcstep etc. are incremented "as usual" i.e. an early-exit
has no special impact, regardless of whether the branch
occurred or not. This can leave srcstep etc. in what may be
considered an unusual
state on exit from a loop and it is up to the programmer to
reset srcstep, dststep etc. to known-good values
*(easily achieved with `setvl`)*.

Additional useful behaviour involves two primary Modes (both of
which may be enabled and combined):

* **VLSET Mode**: identical to Data-Dependent Fail-First Mode
  for Arithmetic SVP64 operations, with more
  flexibility and a close interaction and integration into the
  underlying base Scalar v3.0B Branch instruction.
  Truncation of VL takes place around the early-exit point.
* **CTR-test Mode**: gives much more flexibility over when and why
  CTR is decremented, including options to decrement if a Condition
  test succeeds *or if it fails*.

With these side-effects, basic Boolean Logic Analysis advises that
it is important to provide a means
to enact them each based on whether testing succeeds *or fails*. This
results in a not-insignificant number of additional Mode Augmentation bits,
accompanying VLSET and CTR-test Modes respectively.

Predicate skipping or zeroing may, as usual with SVP64, be controlled
by `sz`.
Where the predicate is masked out and
zeroing is enabled, then in such circumstances
the same Boolean Logic Analysis dictates that
rather than testing only against zero, the option to test
against one is also prudent. This introduces a new
immediate field, `SNZ`, which works in conjunction with
`sz`.


Vectorised Branches can be used
in either SVP64 Horizontal-First or Vertical-First Mode. Essentially,
at an element level, the behaviour is identical in both Modes,
although the `ALL` bit is meaningless in Vertical-First Mode.

It is also important
to bear in mind that, fundamentally, Vectorised Branch-Conditional
is still extremely close to the Scalar v3.0B Branch-Conditional
instructions, and that the same v3.0B Scalar Branch-Conditional
instructions are still
*completely separate and independent*, being unaltered and
unaffected by their SVP64 variants in every conceivable way.

*Programming note: One important point is that SVP64 instructions are 64 bit.
(8 bytes not 4). This needs to be taken into consideration when computing
branch offsets: the offset is relative to the start of the instruction,
which **includes** the SVP64 Prefix*

## Format and fields

With element-width overrides being meaningless for Condition
Register Fields, bits 4 thru 7 of SVP64 RM may be used for additional
Mode bits.

SVP64 RM `MODE` (includes repurposing `ELWIDTH` bits 4:5,
and `ELWIDTH_SRC` bits 6-7 for *alternate* uses) for Branch
Conditional:

| 4 | 5 | 6 | 7 | 17 | 18 | 19 | 20 |  21 | 22  23 |  description     |
| - | - | - | - | -- | -- | -- | -- | --- |--------|----------------- |
|ALL|SNZ| / | / | SL |SLu | 0  | 0  | /   | LRu sz | simple mode      |
|ALL|SNZ| / |VSb| SL |SLu | 0  | 1  | VLI | LRu sz | VLSET mode       |
|ALL|SNZ|CTi| / | SL |SLu | 1  | 0  | /   | LRu sz | CTR-test mode         |
|ALL|SNZ|CTi|VSb| SL |SLu | 1  | 1  | VLI | LRu sz | CTR-test+VLSET mode   |

Brief description of fields:

* **sz=1** if predication is enabled and `sz=1` and a predicate
  element bit is zero, `SNZ` will
  be substituted in place of the CR bit selected by `BI`,
  as the Condition tested.
  Contrast this with
  normal SVP64 `sz=1` behaviour, where *only* a zero is put in
  place of masked-out predicate bits.
* **sz=0** When `sz=0` skipping occurs as usual on
  masked-out elements, but unlike all
  other SVP64 behaviour which entirely skips an element with
  no related side-effects at all, there are certain
  special circumstances where CTR
  may be decremented.  See CTR-test Mode, below.
* **ALL** when set, all branch conditional tests must pass in order for
  the branch to succeed. When clear, it is the first sequentially
  encountered successful test that causes the branch to succeed.
  This is identical behaviour to how programming languages perform
  early-exit on Boolean Logic chains.
* **VLI** VLSET is identical to Data-dependent Fail-First mode.
  In VLSET mode, VL *may* (depending on `VSb`) be truncated.
  If VLI (Vector Length Inclusive) is clear,
  VL is truncated to *exclude* the current element, otherwise it is
  included. SVSTATE.MVL is not altered: only VL.
* **SL** identical to `LR` except applicable to SVSTATE. If `SL`
  is set, SVSTATE is transferred to SVLR (conditionally on
  whether `SLu` is set).
* **SLu**: SVSTATE Link Update, like `LRu` except applies to SVSTATE.
* **LRu**: Link Register Update, used in conjunction with LK=1
  to make LR update conditional
* **VSb** In VLSET Mode, after testing,
  if VSb is set, VL is truncated if the test succeeds.  If VSb is clear,
  VL is truncated if a test *fails*. Masked-out (skipped)
  bits are not considered
  part of testing when `sz=0`
* **CTi** CTR inversion. CTR-test Mode normally decrements per element
  tested. CTR inversion decrements if a test *fails*. Only relevant
  in CTR-test Mode.

LRu and CTR-test modes are where SVP64 Branches subtly differ from
Scalar v3.0B Branches. `sv.bcl` for example will always update LR, whereas
`sv.bcl/lru` will only update LR if the branch succeeds.

Of special interest is that when using ALL Mode (Great Big AND
of all Condition Tests), if `VL=0`,
which is rare but can occur in Data-Dependent Modes, the Branch
will always take place because there will be no failing Condition
Tests to prevent it. Likewise when not using ALL Mode (Great Big OR
of all Condition Tests) and `VL=0` the Branch is guaranteed not
to occur because there will be no *successful* Condition Tests
to make it happen.

## Vectorised CR Field numbering, and Scalar behaviour

It is important to keep in mind that just like all SVP64 instructions,
the `BI` field of the base v3.0B Branch Conditional instruction
may be extended by SVP64 EXTRA augmentation, as well as be marked
as either Scalar or Vector. It is also crucially important to keep in mind
that for CRs, SVP64 sequentially increments the CR *Field* numbers.
CR *Fields* are treated as elements, not bit-numbers of the CR *register*.

The `BI` operand of Branch Conditional operations is five bits, in scalar
v3.0B this would select one bit of the 32 bit CR,
comprising eight CR Fields of 4 bits each.  In SVP64 there are
16 32 bit CRs, containing 128 4-bit CR Fields.  Therefore, the 2 LSBs of
`BI` select the bit from the CR Field (EQ LT GT SO), and the top 3 bits
are extended to either scalar or vector and to select CR Fields 0..127
as specified in SVP64 [[sv/svp64/appendix]].

When the CR Fields selected by SVP64-Augmented `BI` is marked as scalar,
then as the usual SVP64 rules apply:
the Vector loop ends at the first element tested
(the first CR *Field*), after taking
predication into consideration. Thus, also as usual, when a predicate mask is
given, and `BI` marked as scalar, and `sz` is zero, srcstep
skips forward to the first non-zero predicated element, and only that
one element is tested.

In other words, the fact that this is a Branch
Operation (instead of an arithmetic one) does not result, ultimately,
in significant changes as to
how SVP64 is fundamentally applied, except with respect to:

* the unique properties associated with conditionally
 changing the Program
Counter (aka "a Branch"), resulting in early-out
opportunities
* CTR-testing

Both are outlined below, in later sections.

## Horizontal-First and Vertical-First Modes

In SVP64 Horizontal-First Mode, the first failure in ALL mode (Great Big
AND) results in early exit: no more updates to CTR occur (if requested);
no branch occurs, and LR is not updated (if requested). Likewise for
non-ALL mode (Great Big Or) on first success early exit also occurs,
however this time with the Branch proceeding.  In both cases the testing
of the Vector of CRs should be done in linear sequential order (or in
REMAP re-sequenced order): such that tests that are sequentially beyond
the exit point are *not* carried out. (*Note: it is standard practice in
Programming languages to exit early from conditional tests, however
a little unusual to consider in an ISA that is designed for Parallel
Vector Processing. The reason is to have strictly-defined guaranteed
behaviour*)

In Vertical-First Mode, setting the `ALL` bit results in `UNDEFINED`
behaviour. Given that only one element is being tested at a time
in Vertical-First Mode, a test designed to be done on multiple
bits is meaningless.

## Description and Modes

Predication in both INT and CR modes may be applied to `sv.bc` and other
SVP64 Branch Conditional operations, exactly as they may be applied to
other SVP64 operations.  When `sz` is zero, any masked-out Branch-element
operations are not included in condition testing, exactly like all other
SVP64 operations, *including* side-effects such as potentially updating
LR or CTR, which will also be skipped. There is *one* exception here,
which is when
`BO[2]=0, sz=0, CTR-test=0, CTi=1` and the relevant element
predicate mask bit is also zero:
under these special circumstances CTR will also decrement.

When `sz` is non-zero, this normally requests insertion of a zero
in place of the input data, when the relevant predicate mask bit is zero.
This would mean that a zero is inserted in place of `CR[BI+32]` for
testing against `BO`, which may not be desirable in all circumstances.
Therefore, an extra field is provided `SNZ`, which, if set, will insert
a **one** in place of a masked-out element, instead of a zero.

(*Note: Both options are provided because it is useful to deliberately
cause the Branch-Conditional Vector testing to fail at a specific point,
controlled by the Predicate mask. This is particularly useful in `VLSET`
mode, which will truncate SVSTATE.VL at the point of the first failed
test.*)

Normally, CTR mode will decrement once per Condition Test, resulting
under normal circumstances that CTR reduces by up to VL in Horizontal-First
Mode. Just as when v3.0B Branch-Conditional saves at
least one instruction on tight inner loops through auto-decrementation
of CTR, likewise it is also possible to save instruction count for
SVP64 loops in both Vertical-First and Horizontal-First Mode, particularly
in circumstances where there is conditional interaction between the
element computation and testing, and the continuation (or otherwise)
of a given loop. The potential combinations of interactions is why CTR
testing options have been added.

Also, the unconditional bit `BO[0]` is still relevant when Predication
is applied to the Branch because in `ALL` mode all nonmasked bits have
to be tested, and when `sz=0` skipping occurs.
Even when VLSET mode is not used, CTR
may still be decremented by the total number of nonmasked elements,
acting in effect as either a popcount or cntlz depending on which
mode bits are set.
In short, Vectorised Branch becomes an extremely powerful tool.

**Micro-Architectural Implementation Note**: *when implemented on
top of a Multi-Issue Out-of-Order Engine it is possible to pass
a copy of the predicate and the prerequisite CR Fields to all
Branch Units, as well as the current value of CTR at the time of
multi-issue, and for each Branch Unit to compute how many times
CTR would be subtracted, in a fully-deterministic and parallel
fashion. A SIMD-based Branch Unit, receiving and processing
multiple CR Fields covered by multiple predicate bits, would
do the exact same thing. Obviously, however, if CTR is modified
within any given loop (mtctr) the behaviour of CTR is no longer
deterministic.*

### Link Register Update

For a Scalar Branch, unconditional updating of the Link Register
LR is useful and practical. However, if a loop of CR Fields is
tested, unconditional updating of LR becomes problematic.

For example when using `bclr` with `LRu=1,LK=0` in Horizontal-First Mode,
LR's value will be unconditionally overwritten after the first element,
such that for execution (testing) of the second element, LR
has the value `CIA+8`. This is covered in the `bclrl` example, in
a later section.

The addition of a LRu bit modifies behaviour in conjunction
with LK, as follows:

* `sv.bc` When LRu=0,LK=0, Link Register is not updated
* `sv.bcl` When LRu=0,LK=1, Link Register is updated unconditionally
* `sv.bcl/lru` When LRu=1,LK=1, Link Register will
  only be updated if the Branch Condition fails.
* `sv.bc/lru` When LRu=1,LK=0, Link Register will only be updated if
  the Branch Condition succeeds.

This avoids
destruction of LR during loops (particularly Vertical-First
ones).

**SVLR and SVSTATE**

For precisely the reasons why `LK=1` was added originally to the Power
ISA, with SVSTATE being a peer of the Program Counter it becomes
necessary to also add an SVLR (SVSTATE Link Register)
and corresponding control bits `SL` and `SLu`.

### CTR-test

Where a standard Scalar v3.0B branch unconditionally decrements
CTR when `BO[2]` is clear, CTR-test Mode introduces more flexibility
which allows CTR to be used for many more types of Vector loops
constructs.

CTR-test mode and CTi interaction is as follows: note that
`BO[2]` is still required to be clear for CTR decrements to be
considered, exactly as is the case in Scalar Power ISA v3.0B

* **CTR-test=0, CTi=0**: CTR decrements on a per-element basis
  if `BO[2]` is zero. Masked-out elements when `sz=0` are
  skipped (i.e. CTR is *not* decremented when the predicate
  bit is zero and `sz=0`).
* **CTR-test=0, CTi=1**: CTR decrements on a per-element basis
  if `BO[2]` is zero and a masked-out element is skipped
  (`sz=0` and predicate bit is zero). This one special case is the
  **opposite** of other combinations, as well as being
  completely different from normal SVP64 `sz=0` behaviour)
* **CTR-test=1, CTi=0**: CTR decrements on a per-element basis
  if `BO[2]` is zero and the Condition Test succeeds.
  Masked-out elements when `sz=0` are skipped (including
  not decrementing CTR)
* **CTR-test=1, CTi=1**: CTR decrements on a per-element basis
  if `BO[2]` is zero and the Condition Test *fails*.
  Masked-out elements when `sz=0` are skipped (including
  not decrementing CTR)

`CTR-test=0, CTi=1, sz=0` requires special emphasis because it is the
only time in the entirety of SVP64 that has side-effects when
a predicate mask bit is clear.  **All** other SVP64 operations
entirely skip an element when sz=0 and a predicate mask bit is zero.
It is also critical to emphasise that in this unusual mode,
no other side-effects occur: **only** CTR is decremented, i.e. the
rest of the Branch operation is skipped.

### VLSET Mode

VLSET Mode truncates the Vector Length so that subsequent instructions
operate on a reduced Vector Length. This is similar to
Data-dependent Fail-First and LD/ST Fail-First, where for VLSET the
truncation occurs at the Branch decision-point.

Interestingly, due to the side-effects of `VLSET` mode
it is actually useful to use Branch Conditional even
to perform no actual branch operation, i.e to point to the instruction
after the branch. Truncation of VL would thus conditionally occur yet control
flow alteration would not.

`VLSET` mode with Vertical-First is particularly unusual. Vertical-First
is designed to be used for explicit looping, where an explicit call to
`svstep` is required to move both srcstep and dststep on to
the next element, until VL (or other condition) is reached.
Vertical-First Looping is expected (required) to terminate if the end
of the Vector, VL, is reached. If however that loop is terminated early
because VL is truncated, VLSET with Vertical-First becomes meaningless.
Resolving this would require two branches: one Conditional, the other
branching unconditionally to create the loop, where the Conditional
one jumps over it.

Therefore, with `VSb`, the option to decide whether truncation should occur if the
branch succeeds *or* if the branch condition fails allows for the flexibility
required.  This allows a Vertical-First Branch to *either* be used as
a branch-back (loop) *or* as part of a conditional exit or function
call from *inside* a loop, and for VLSET to be integrated into both
types of decision-making.

In the case of a Vertical-First branch-back (loop), with `VSb=0` the branch takes
place if success conditions are met, but on exit from that loop
(branch condition fails), VL will be truncated. This is extremely
useful.

`VLSET` mode with Horizontal-First when `VSb=0` is still
useful, because it can be used to truncate VL to the first predicated
(non-masked-out) element.

The truncation point for VL, when VLi is clear, must not include skipped
elements that preceded the current element being tested.
Example: `sz=0, VLi=0, predicate mask = 0b110010` and the Condition
Register failure point is at CR Field element 4.

* Testing at element 0 is skipped because its predicate bit is zero
* Testing at element 1 passed
* Testing elements 2 and 3 are skipped because their
  respective predicate mask bits are zero
* Testing element 4 fails therefore VL is truncated to **2**
  not 4 due to elements 2 and 3 being skipped.

If `sz=1` in the above example *then* VL would have been set to 4 because
in non-zeroing mode the zero'd elements are still effectively part of the
Vector (with their respective elements set to `SNZ`)

If `VLI=1` then VL would be set to 5 regardless of sz, due to being inclusive
of the element actually being tested.

### VLSET and CTR-test combined

If both CTR-test and VLSET Modes are requested, it's important to
observe the correct order. What occurs depends on whether VLi
is enabled, because VLi affects the length, VL.

If VLi (VL truncate inclusive) is set:

1. compute the test including whether CTR triggers
2. (optionally) decrement CTR
3. (optionally) truncate VL (VSb inverts the decision)
4. decide (based on step 1) whether to terminate looping
   (including not executing step 5)
5. decide whether to branch.

If VLi is clear, then when a test fails that element
and any following it
should **not** be considered part of the Vector. Consequently:

1. compute the branch test including whether CTR triggers
2. if the test fails against VSb, truncate VL to the *previous*
   element, and terminate looping. No further steps executed.
3. (optionally) decrement CTR
4. decide whether to branch.

## Boolean Logic combinations

In a Scalar ISA, Branch-Conditional testing even of vector
results may be performed through inversion of tests. NOR of
all tests may be performed by inversion of the scalar condition
and branching *out* from the scalar loop around elements,
using scalar operations.

In a parallel (Vector) ISA it is the ISA itself which must perform
the prerequisite logic manipulation.
Thus for SVP64 there are an extraordinary number of nesessary combinations
which provide completely different and useful behaviour.
Available options to combine:

* `BO[0]` to make an unconditional branch would seem irrelevant if
  it were not for predication and for side-effects (CTR Mode
  for example)
* Enabling CTR-test Mode and setting `BO[2]` can still result in the
  Branch
  taking place, not because the Condition Test itself failed, but
  because CTR reached zero **because**, as required by CTR-test mode,
  CTR was decremented as a  **result** of Condition Tests failing.
* `BO[1]` to select whether the CR bit being tested is zero or nonzero
* `R30` and `~R30` and other predicate mask options including CR and
  inverted CR bit testing
* `sz` and `SNZ` to insert either zeros or ones in place of masked-out
  predicate bits
* `ALL` or `ANY` behaviour corresponding to `AND` of all tests and
  `OR` of all tests, respectively.
* Predicate Mask bits, which combine in effect with the CR being
  tested.
* Inversion of Predicate Masks (`~r3` instead of `r3`, or using
  `NE` rather than `EQ`) which results in an additional
  level of possible ANDing, ORing etc. that would otherwise
  need explicit instructions.

The most obviously useful combinations here are to set `BO[1]` to zero
in order to turn `ALL` into Great-Big-NAND and `ANY` into
Great-Big-NOR.  Other Mode bits which perform behavioural inversion then
have to work round the fact that the Condition Testing is NOR or NAND.
The alternative to not having additional behavioural inversion
(`SNZ`, `VSb`, `CTi`) would be to have a second (unconditional)
branch directly after the first, which the first branch jumps over.
This contrivance is avoided by the behavioural inversion bits.

## Pseudocode and examples

Please see the SVP64 appendix regarding CR bit ordering and for
the definition of `CR{n}`

For comparative purposes this is a copy of the v3.0B `bc` pseudocode

```
    if (mode_is_64bit) then M <- 0
    else M <- 32
    if ¬BO[2] then CTR <- CTR - 1
    ctr_ok <- BO[2] | ((CTR[M:63] != 0) ^ BO[3])
    cond_ok <- BO[0] | ¬(CR[BI+32] ^ BO[1])
    if ctr_ok & cond_ok then
      if AA then NIA <-iea EXTS(BD || 0b00)
      else       NIA <-iea CIA + EXTS(BD || 0b00)
    if LK then LR  <-iea  CIA + 4
```

Simplified pseudocode including LRu and CTR skipping, which illustrates
clearly that SVP64 Scalar Branches (VL=1) are **not** identical to
v3.0B Scalar Branches.  The key areas where differences occur are
the inclusion of predication (which can still be used when VL=1), in
when and why CTR is decremented (CTRtest Mode) and whether LR is
updated (which is unconditional in v3.0B when LK=1, and conditional
in SVP64 when LRu=1).

Inline comments highlight the fact that the Scalar Branch behaviour
and pseudocode is still clearly visible and embedded within the
Vectorised variant:

```
    if (mode_is_64bit) then M <- 0
    else M <- 32
    # the bit of CR to test, if the predicate bit is zero,
    # is overridden
    testbit = CR[BI+32]
    if ¬predicate_bit then testbit = SVRMmode.SNZ
    # otherwise apart from the override ctr_ok and cond_ok
    # are exactly the same
    ctr_ok <- BO[2] | ((CTR[M:63] != 0) ^ BO[3])
    cond_ok <- BO[0] | ¬(testbit ^ BO[1])
    if ¬predicate_bit & ¬SVRMmode.sz then
      # this is entirely new: CTR-test mode still decrements CTR
      # even when predicate-bits are zero
      if ¬BO[2] & CTRtest & ¬CTi then
        CTR = CTR - 1
      # instruction finishes here
    else
      # usual BO[2] CTR-mode now under CTR-test mode as well
      if ¬BO[2] & ¬(CTRtest & (cond_ok ^ CTi)) then CTR <- CTR - 1
      # new VLset mode, conditional test truncates VL
      if VLSET and VSb = (cond_ok & ctr_ok) then
        if SVRMmode.VLI then SVSTATE.VL = srcstep+1
        else                 SVSTATE.VL = srcstep
      # usual LR is now conditional, but also joined by SVLR
      lr_ok <- LK
      svlr_ok <- SVRMmode.SL
      if ctr_ok & cond_ok then
        if AA then NIA <-iea EXTS(BD || 0b00)
        else       NIA <-iea CIA + EXTS(BD || 0b00)
        if SVRMmode.LRu then lr_ok <- ¬lr_ok
        if SVRMmode.SLu then svlr_ok <- ¬svlr_ok
      if lr_ok   then LR   <-iea CIA + 4
      if svlr_ok then SVLR <- SVSTATE
```

Below is the pseudocode for SVP64 Branches, which is a little less
obvious but identical to the above. The lack of obviousness is down
to the early-exit opportunities.

Effective pseudocode for Horizontal-First Mode:

```
    if (mode_is_64bit) then M <- 0
    else M <- 32
    cond_ok = not SVRMmode.ALL
    for srcstep in range(VL):
        # select predicate bit or zero/one
        if predicate[srcstep]:
            # get SVP64 extended CR field 0..127
            SVCRf = SVP64EXTRA(BI>>2)
            CRbits = CR{SVCRf}
            testbit = CRbits[BI & 0b11]
            # testbit = CR[BI+32+srcstep*4]
        else if not SVRMmode.sz:
            # inverted CTR test skip mode
            if ¬BO[2] & CTRtest & ¬CTI then
              CTR = CTR - 1
            continue # skip to next element
        else
            testbit = SVRMmode.SNZ
        # actual element test here
        ctr_ok <- BO[2] | ((CTR[M:63] != 0) ^ BO[3])
        el_cond_ok <- BO[0] | ¬(testbit ^ BO[1])
        # check if CTR dec should occur
        ctrdec = ¬BO[2]
        if CTRtest & (el_cond_ok ^ CTi) then
           ctrdec = 0b0
        if ctrdec then CTR <- CTR - 1
        # merge in the test
        if SVRMmode.ALL:
            cond_ok &= (el_cond_ok & ctr_ok)
        else
            cond_ok |= (el_cond_ok & ctr_ok)
        # test for VL to be set (and exit)
        if VLSET and VSb = (el_cond_ok & ctr_ok) then
            if SVRMmode.VLI then SVSTATE.VL = srcstep+1
            else                 SVSTATE.VL = srcstep
            break
        # early exit?
        if SVRMmode.ALL != (el_cond_ok & ctr_ok):
             break
        # SVP64 rules about Scalar registers still apply!
        if SVCRf.scalar:
           break
    # loop finally done, now test if branch (and update LR)
    lr_ok <- LK
    svlr_ok <- SVRMmode.SL
    if cond_ok then
        if AA then NIA <-iea EXTS(BD || 0b00)
        else       NIA <-iea CIA + EXTS(BD || 0b00)
        if SVRMmode.LRu then lr_ok <- ¬lr_ok
        if SVRMmode.SLu then svlr_ok <- ¬svlr_ok
    if lr_ok then LR <-iea CIA + 4
    if svlr_ok then SVLR <- SVSTATE
```

Pseudocode for Vertical-First Mode:

```
    # get SVP64 extended CR field 0..127
    SVCRf = SVP64EXTRA(BI>>2)
    CRbits = CR{SVCRf}
    # select predicate bit or zero/one
    if predicate[srcstep]:
        if BRc = 1 then # CR0 vectorised
            CR{SVCRf+srcstep} = CRbits
        testbit = CRbits[BI & 0b11]
    else if not SVRMmode.sz:
        # inverted CTR test skip mode
        if ¬BO[2] & CTRtest & ¬CTI then
           CTR = CTR - 1
        SVSTATE.srcstep = new_srcstep
        exit # no branch testing
    else
        testbit = SVRMmode.SNZ
    # actual element test here
    cond_ok <- BO[0] | ¬(testbit ^ BO[1])
    # test for VL to be set (and exit)
    if VLSET and cond_ok = VSb then
        if SVRMmode.VLI
            SVSTATE.VL = new_srcstep+1
        else
            SVSTATE.VL = new_srcstep
```

### Example Shader code

```
    // assume f() g() or h() modify a and/or b
    while(a > 2) {
        if(b < 5)
            f();
        else
            g();
        h();
    }
```

which compiles to something like:

```
    vec<i32> a, b;
    // ...
    pred loop_pred = a > 2;
    // loop continues while any of a elements greater than 2
    while(loop_pred.any()) {
        // vector of predicate bits
        pred if_pred = loop_pred & (b < 5);
        // only call f() if at least 1 bit set
        if(if_pred.any()) {
            f(if_pred);
        }
    label1:
        // loop mask ANDs with inverted if-test
        pred else_pred = loop_pred & ~if_pred;
        // only call g() if at least 1 bit set
        if(else_pred.any()) {
            g(else_pred);
        }
        h(loop_pred);
    }
```

which will end up as:

```
       # start from while loop test point
       b looptest
    while_loop:
       sv.cmpi CR80.v, b.v, 5     # vector compare b into CR64 Vector
       sv.bc/m=r30/~ALL/sz CR80.v.LT skip_f # skip when none
       # only calculate loop_pred & pred_b because needed in f()
       sv.crand CR80.v.SO, CR60.v.GT, CR80.V.LT # if = loop & pred_b
       f(CR80.v.SO)
    skip_f:
       # illustrate inversion of pred_b. invert r30, test ALL
       # rather than SOME, but masked-out zero test would FAIL,
       # therefore masked-out instead is tested against 1 not 0
       sv.bc/m=~r30/ALL/SNZ CR80.v.LT skip_g
       # else = loop & ~pred_b, need this because used in g()
       sv.crternari(A&~B) CR80.v.SO, CR60.v.GT, CR80.V.LT
       g(CR80.v.SO)
    skip_g:
       # conditionally call h(r30) if any loop pred set
       sv.bclr/m=r30/~ALL/sz BO[1]=1 h()
    looptest:
       sv.cmpi CR60.v a.v, 2      # vector compare a into CR60 vector
       sv.crweird r30, CR60.GT # transfer GT vector to r30
       sv.bc/m=r30/~ALL/sz BO[1]=1 while_loop
    end:
```

### LRu example

show why LRu would be useful in a loop.  Imagine the following
c code:

```
    for (int i = 0; i < 8; i++) {
        if (x < y) break;
    }
```

Under these circumstances exiting from the loop is not only
based on CTR it has become conditional on a CR result.
Thus it is desirable that NIA *and* LR only be modified
if the conditions are met


v3.0 pseudocode for `bclrl`:

```
    if (mode_is_64bit) then M <- 0
    else M <- 32
    if ¬BO[2]  then CTR <- CTR - 1
    ctr_ok <- BO[2] | ((CTR[M:63] != 0) ^ BO[3])
    cond_ok <- BO[0] | ¬(CR[BI+32] ^  BO[1])
    if ctr_ok & cond_ok then NIA <-iea LR[0:61] || 0b00
    if LK then LR <-iea CIA + 4
```

the latter part for SVP64 `bclrl` becomes:

```
    for i in 0 to VL-1:
        ...
        ...
        cond_ok <- BO[0] | ¬(CR[BI+32] ^  BO[1])
        lr_ok <- LK
        if ctr_ok & cond_ok then
           NIA <-iea LR[0:61] || 0b00
           if SVRMmode.LRu then lr_ok <- ¬lr_ok
        if lr_ok then LR <-iea CIA + 4
        # if NIA modified exit loop
```

The reason why should be clear from this being a Vector loop:
unconditional destruction of LR when LK=1 makes `sv.bclrl`
ineffective, because the intention going into the loop is
that the branch should be to the copy of LR set at the *start*
of the loop, not half way through it.
However if the change to LR only occurs if
the branch is taken then it becomes a useful instruction.

The following pseudocode should **not** be implemented because
it violates the fundamental principle of SVP64 which is that
SVP64 looping is a thin wrapper around Scalar Instructions.
The pseducode below is more an actual Vector ISA Branch and
as such is not at all appropriate:

```
    for i in 0 to VL-1:
        ...
        ...
        cond_ok <- BO[0] | ¬(CR[BI+32] ^  BO[1])
        if ctr_ok & cond_ok then NIA <-iea LR[0:61] || 0b00
    # only at the end of looping is LK checked.
    # this completely violates the design principle of SVP64
    # and would actually need to be a separate (scalar)
    # instruction "set LR to CIA+4 but retrospectively"
    # which is clearly impossible
    if LK then LR <-iea CIA + 4
```