(no commit message)

[libreriscv.git] / openpower / sv / sprs.mdwn

# SPRs  <a name="sprs"></a>

The full list of SPRs for Simple-V is:

* **SVSTATE** 64-bit.
* **SVLR** 64-bit.
* **SVSHAPE0** 32-bit
* **SVSHAPE1** 32-bit
* **SVSHAPE2** 32-bit
* **SVSHAPE3** 32-bit

Future versions of Simple-V will have at least 7 more SVSTATE SPRs, in a small
"stack", as part of a full Zero-Overhead Loop Control subsystem.

## SVSTATE SPR 


The format of the SVSTATE SPR is as follows:

| Field | Name     | Description           |
| ----- | -------- | --------------------- |
| 0:6   | maxvl    | Max Vector Length     |
| 7:13  |    vl    | Vector Length         |
| 14:20 | srcstep  | for srcstep = 0..VL-1 |
| 21:27 | dststep  | for dststep = 0..VL-1 |
| 28:29 | dsubstep | for substep = 0..SUBVL-1  |
| 30:31 | ssubstep | for substep = 0..SUBVL-1  |
| 32:33 | mi0      | REMAP RA/FRA/BFA SVSHAPE0-3    |
| 34:35 | mi1      | REMAP RB/FRB/BFB SVSHAPE0-3    |
| 36:37 | mi2      | REMAP RC/FRT SVSHAPE0-3    |
| 38:39 | mo0      | REMAP RT/FRT/BF SVSHAPE0-3    |
| 40:41 | mo1      | REMAP EA/RS/FRS SVSHAPE0-3    |
| 42:46 | SVme     | REMAP enable (RA-RT)  |
| 47:52 | rsvd     | reserved              |
| 53    | pack     | PACK (srcstep reorder)  |
| 54    | unpack   | UNPACK (dststep order)  |
| 55:61 | hphint   | Horizontal Hint       |
| 62    | RMpst    | REMAP persistence     |
| 63    | vfirst   | Vertical First mode   |

Notes:

* The entries are truncated to be within range.  Attempts to set VL to
  greater than MAXVL will truncate VL.
* Setting srcstep, dststep to 64 or greater, or VL or MVL to greater
  than 64 is reserved and will cause an illegal instruction trap.

**SVSTATE Fields**

SVSTATE is a standard SPR that (if REMAP is not activated) contains sufficient
self-contaned information for a full context save/restore.
SVSTATE contains (and permits setting of):

* MVL (the Maximum Vector Length) - declares (statically) how
  much of a regfile is to be reserved for Vector elements
* VL - Vector Length
* dststep - the destination element offset of the current parallel
  instruction being executed
* srcstep - for twin-predication, the source element offset as well.
* ssubstep - the source subvector element offset of the current
  parallel instruction being executed
* dsubstep - the destination subvector element offset of the current
  parallel instruction being executed
* vfirst - Vertical First mode.  srcstep, dststep and substep
    **do not advance** unless explicitly requested to do so with svstep
* RMpst - REMAP persistence.  REMAP will apply only to the following
  instruction unless this bit is set, in which case REMAP "persists".
  Reset (cleared) on use of the `setvl` instruction if used to
  alter VL or MVL.
* Pack - if set then srcstep/ssubstep VL/SUBVL loop-ordering is inverted.
* UnPack - if set then dststep/dsubstep VL/SUBVL loop-ordering is inverted.
* hphint - Horizontal Parallelism Hint. Indicates that
  no Hazards exist between groups of elements in sequential multiples of this number
   (before REMAP).  By definition: elements for which `FLOOR(step/hphint)` is
   equal *before REMAP* are in the same parallelism "group", for both
   `srcstep` and `dststep`. In Vertical First Mode
   hardware **MUST** respect Strict Program Order but is permitted to
   merge multiple scalar loops into parallel batches, if Reservation Station resources
   are sufficient.  Set to zero to indicate "no hint".
* SVme - REMAP enable bits, indicating which register is to be
   REMAPed: RA, RB, RC, RT and EA are the canonical (typical) register names
   associated with each bit, with RA being the LSB and EA being the MSB.
   See table below for ordering. When `SVme` is zero (0b00000) REMAP
   is **fully disabled and inactive** regardless of the contents of
  `SVSTATE`, `mi0-mi2/mo0-mo1`, or the four `SVSHAPEn` SPRs
* mi0-mi2/mo0-mo1 - these
  indicate the SVSHAPE (0-3) that the corresponding register (RA etc)
  should use, as long as the register's corresponding SVme bit is set 

Programmer's Note: the fact that REMAP is entirely dormant when `SVme` is zero
allows establishment of REMAP context well in advance, followed by utilising `svremap`
at a precise (or the very last) moment.  Some implementations may exploit this
to cache (or take some time to prepare caches) in the background whilst other
(unrelated) instructions are being executed. This is particularly important to
bear in mind when using `svindex` which will require hardware to perform (and
cache) additional GPR reads.

Programmer's Note: when REMAP is activated it becomes necessary on any
context-switch (Interrupt or Function call) to detect (or know in advance)
that REMAP is enabled and to additionally explicitly save/restore the four SVSHAPE
SPRs, SVHAPE0-3.  Given that this is expected to be a rare occurrence it was
deemed unreasonable to burden every context-switch or function call with
mandatory save/restore of SVSHAPEs, and consequently it is a *callee*
(and Trap Handler) responsibility.  Callees (and Trap Handlers) **MUST**
avoid using all and any SVP64 instructions during the period where state
could be adversely affected.  SVP64 purely relies on Scalar instructions,
so Scalar instructions (except the SVP64 Management ones and mtspr and
mfspr) are 100% guaranteed to have zero impact on SVP64 state.

**Max Vector Length (maxvl)** <a name="mvl" />

MAXVECTORLENGTH is a static (immediate-operand only) compile-time declaration
of the maximum number of elements in a Vector. MVL is limited to 7 bits
(in the first version of SVP64) and consequently the maximum number of
elements is limited to between 0 and 127.

MAXVL is normally (in other True-Scalable Vector ISAs) an Architecturally-defined
quantity related indirectly to the total available number of bits in the Vector
Register File.  Cray Vectors had a Hardware-Architectural set limit of MAXVL=64.
RISC-V RVV has MAXVL defined in terms of a Silicon-Partner-selectable fixed number
of bits.  MAXVL in Simple-V is set in terms of the number of *elements* and
may change at runtime.

Programmer's Note: Except by directly using `mtspr` on SVSTATE, which may
result in performance penalties on some hardware implementations, SVSTATE's `maxvl`
field may only be set **statically** as an immediate, by the `setvl` instruction.
It may **NOT** be set dynamically from a register.  Compiler writers and assembly
programmers are expected to perform static register file analysis, subdivision,
and allocation and only utilise `setvl`. Direct writing to SVSTATE in order to
"bypass" this Note could, in less-advanced implementations, potentially cause stalling,
particularly if SVP64 instructions are issued directly after the `mtspr` to SVSTATE.

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

The actual Vector length, the number of elements in a "Vector", `SVSTATE.vl` may be set
entirely dynamically at runtime from a number of sources. `setvl` is the primary
instruction for setting Vector Length.
`setvl` is conceptually similar but different from the Cray, SX Aurora, and RISC-V RVV
equivalent. Similar to RVV, VL is set to be within
the range 0 <= VL <= MVL. Unlike RVV, VL is set **exactly** according to the following:

```
    VL = (RT|0) = MIN(vlen, MVL)
```

where `0 <= MVL <= 127`, and vlen may come from an immediate, `RA`, or from the `CTR` SPR,
depending on options selected with the `setvl` instruction.

Programmer's Note: conceptual understanding of Cray-style Vectors is far beyond the scope
of the Power ISA Technical Reference.  Guidance on the 50-year-old Cray Vector paradigm is
best sought elsewhere: good studies include Academic Courses given on the 1970s
Cray Supercomputers over at least the past three decades.

**Horizontal Parallelism**

A problem exists for hardware where it may not be able to detect
that a programmer (or compiler) knows of opportunities for parallelism
and lack of overlap between loops, despite these being easy for a compiler
to statically detect and potentially express.
`hphint` is such an expression, declaring that elements within a batch are
independent of each other (no Register *or Memory* Hazards).

Elements are considered to be in the same source batch if they have
the same value of `FLOOR(srcstep/hphint)`. Likewise in the same destination batch
for the same value `FLOOR(dststep/hphint)`.
Four key observations here:

1. predication is **not** involved here.  the number of actual elements
involved is considered *before* predicate masks are applied.
2. twin predication can result in srcstep and dststep being in different
batches
3. batch evaluation is done *before* REMAP, making Hazard elimination easier
   for Multi-Issue systems.
4. `hphint` is *not* limited to power-of-two. Hardware implementors may choose
   a lower parallelism hint up to `hphint` and may find power-of-two more
   convenient.

Regarding (4): if a smaller hint is chosen by hardware, actual parallelism
(Dependency Hazard relaxation) must **never**
exceed `hphint` and must still respect the batch boundaries, even if this results
in just one element being considered Hazard-independent.  Even under these
circumstances Multi-Issue Register-renaming is possible, to introduce parallelism
by a different route.

*Hardware Architect note: each element within the same group may be treated as
100% independent from any other element within that group, and therefore
neither Register Hazards nor Memory Hazards inter-element exist,
but crucially inter-group definitely remains.  This makes
implementation far easier on resources because the Hazard Dependencies are
effectively at a much coarser granularity than a single register.
With element-width overrides extending down to the byte level reducing Dependency
Hazard hardware complexity becomes even more important.*

`hphint` may legitimately be set greater than `MAXVL`. This indicates to Multi-Issue
hardware that even though MAXVL is relatively small the batches are *still independent*
and therefore if Multi-Issue hardware chooses to allocate several batches up to
`MAXVL` in size they are still independent, even if Register-renaming is deployed.
This helps greatly simplify Multi-Issue systems by significantly reducing Hazards.

**Considerable care** must be taken when setting `hphint`. Matrix Outer Product
could produce corrupted results if `hphint` is set to greater than the innermost
loop depth. Parallel Reduction, DCT and FFT REMAP all are similarly critically affected
by `hphint` in ways that if used correctly greatly increases ease of parallelism but
if done incorrectly will also result in data corruption.  Reduction/Iteration
also requires care to correctly declare in `hphint` how many elements are
independent. In the case of most Reduction use-cases the answer is almost certainly
"none".

`hphint` must never be set on Atomic Memory operations, Cache-Inhibited
Memory operations, or Load-Reservation Store-Conditional. Also if Load-with-Update
Data-Dependent Fail-First is ever used for linked-list pointer-chasing, `hphint`
should again definitely be disabled. Failure to do so results in `UNDEFINED`
behaviour.

`hphint` may only be ignored by Hardware Implementors as long as full element-level
Register and Memory Hazards are implemented *in full* (including right down to individual
bytes of each register for when elwidth=8/16/32). In other words if `hphint` is to
be ignored then implementations must consider the situation as if `hphint=0`.

**Horizontal Parallelism in Vertical-First Mode**

Setting `hphint` with Vertical-First is perfectly legitimate.  Under these circumstances
single-element strict Program Execution Order must be preserved at all times, but
should there be a small enough program loop, than Out-of-Order Hardware may
take the opportunity to *merge*
consecutive element-based instructions into the *same Reservation Stations*, for
multiple operations to be passed to massive-wide back-end SIMD ALUs or Vector-Chaining ALUs.
**Only** elements within the same `hphint` group (across multiple such looped instructions)
may be treated as mergeable in this fashion.

Note that if the loop of Vertical-First instructions cannot fit entirely into Reservation
Stations then Hardware clearly cannot exploit the above optimisation opportunity, but at
least there is no harm done: the loop is still correctly executed as Scalar instructions.
Programmers do need to be aware though that short loops on some Hardware Implementations
can be made considerably faster than on other Implementations.

## SVLR

SV Link Register, exactly analogous to LR (Link Register) may
be used for temporary storage of SVSTATE, and, in particular,
Vectorised Branch-Conditional instructions may interchange
SVLR and SVSTATE whenever LR and NIA are.

Note that there is no equivalent Link variant of SVREMAP or
SVSHAPE0-3 (it would be too costly), so SVLR has limited applicability:
REMAP SPRs must be saved and restored explicitly.

-----------

[[!tag standards]]