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

[[!tag standards]]

# OpenPOWER SV setvl/setvli

See links:

* <http://lists.libre-soc.org/pipermail/libre-soc-dev/2020-November/001366.html>
* <https://bugs.libre-soc.org/show_bug.cgi?id=535>
* <https://bugs.libre-soc.org/show_bug.cgi?id=587>
* <https://bugs.libre-soc.org/show_bug.cgi?id=568> TODO
* <https://bugs.libre-soc.org/show_bug.cgi?id=862> VF Predication
* <https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#vsetvlivsetvl-instructions>
* [[sv/svstep]]
* pseudocode [[openpower/isa/simplev]]

Use of setvl results in changes to the MVL, VL and STATE SPRs. see [[sv/sprs]]

# Behaviour and Rationale

SV's Vector Engine is based on Cray-style Variable-length Vectorisation,
just like RVV.  However unlike RVV, SV sits on top of the standard Scalar
regfiles: there is no separate Vector register numbering.  Therefore, also
unlike RVV, SV does not have hard-coded "Lanes": microarchitects
may use *ordinary* in-order, out-of-order, or superscalar designs
as the basis for SV. By contrast, the relevant parameter
in RVV is "MAXVL" and this is architecturally hard-coded into RVV systems,
anywhere from 1 to tens of thousands of Lanes in supercomputers.

SV is more like how MMX used to sit on top of the x86 FP regfile.
Therefore when Vector operations are performed, the question has to
be asked, "well, how much of the regfile do you want to allocate to
this operation?" because if it is too small an amount performance may
be affected, and if too large then other registers would overlap and
cause data  corruption, or even if allocated correctly would require
spill to memory.

The answer effectively needs to be parameterised.  Hence: MAXVL (MVL)
is set from an immediate, so that the compiler may decide, statically, a
guaranteed resource allocation according to the needs of the application.

While RVV's MAXVL was a hw limit, SV's MVL is simply a loop
optimization. It does not carry side-effects for the arch, though for
a specific cpu it may affect hw unit usage.

Other than being able to set MVL, SV's VL (Vector Length) works just like
RVV's VL, with one minor twist.  RVV permits the `setvl` instruction to
set VL to an arbitrary explicit value.  Within the limit of MVL, VL
**MUST** be set to the requested value. Given that RVV only works on Vector Loops,
this is fine and part of its value and design.  However, SV sits on top
of the standard register files.  When MVL=VL=2, a Vector Add on `r3`
will perform two Scalar Adds: one on `r3` and one on `r4`.

Thus there is the opportunity to set VL to an explicit value (within the
limits of MVL) with the reasonable expectation that if two operations
are requested (by setting VL=2) then two operations are guaranteed.
This avoids the need for a loop (with not-insignificant use of the
regfiles for counters), simply two instructions:

    setvli r0, MVL=64, VL=64
    ld r0.v, 0(r30) # load exactly 64 registers from memory

Page Faults etc. aside this is *guaranteed* 100% without fail to perform
64 unit-strided LDs starting from the address pointed to by r30 and put
the contents into r0 through r63.  Thus it becomes a "LOAD-MULTI". Twin
Predication could even be used to only load relevant registers from
the stack.  This *only works if VL is set to the requested value* rather
than, as in RVV, allowing the hardware to set VL to an arbitrary value
(caveat being, limited to not exceed MVL)

Also available is the option to set VL from CTR (`VL = MIN(CTR, MVL)`.
In combination with SVP64 [[sv/branches]] this can save one instruction
inside critical inner loops. Note: to avoid having an extra bit in `setvl`,
to select CTR is slightly convoluted.

# Format

*(Allocation of opcode TBD pending OPF ISA WG approval)*,
using EXT22 temporarily and fitting into the
[[sv/bitmanip]] space

Form: SVL-Form (see [[isatables/fields.text]])

| 0.5|6.10|11.15|16..22| 23...25    | 26.30 |31|  name   |
| -- | -- | --- | ---- |----------- | ----- |--| ------- |
|OPCD| RT | RA  | SVi  |   ms vs vf | 11011 |Rc| setvl   |

Instruction format:

    setvl RT,RA,SVi,vf,vs,ms
    setvl. RT,RA,SVi,vf,vs,ms

Note that the immediate (`SVi`) spans 7 bits (16 to 22)

* `ms` - bit 23 - allows for setting of MVL
* `vs` - bit 24 - allows for setting of VL
* `vf` - bit 25 - sets "Vertical First Mode".

Note that in immediate setting mode VL and MVL start from **one**
i.e. that an immediate value of zero will result in VL/MVL being set to 1.
0b111111 results in VL/MVL being set to 64. This is because setting
VL/MVL to 1 results in "scalar identity" behaviour, where setting VL/MVL
to 0 would result in all Vector operations becoming `nop`.  If this is
truly desired (nop behaviour) then setting VL and MVL to zero is to be
done via the [[SVSTATE SPR|sv/sprs]]

Note that setmvli is a pseudo-op, based on RA/RT=0, and setvli likewise

    setvli VL=8    : setvl r5, r0, VL=8
    setmvli MVL=8  : setvl r0, r0, MVL=8

Additional pseudo-op for obtaining VL without modifying it:

    getvl r5       : setvl r5, r0, vf=0, vs=0, ms=0

For Vertical-First mode, a pseudo-op for explicit incrementing
of srcstep and dststep:

    svstep.        : setvl. 0, 0, vf=1, vs=0, ms=0

Note that whilst it is possible to set both MVL and VL from the same
immediate, it is not possible to set them to different immediates in
the same instruction.  That would require two instructions.

**Selecting CTR to set VL**

There is considerable opcode pressure, consequently to set MVL and VL
from different sources is as follows:

| condition           | effect         |
| - | - |
| `vs=1, RA=0, RT!=0` | VL,RT set to MIN(MVL, CTR)  |
| `vs=1, RA=0, RT=0`  | VL set to MIN(MVL, SVi+1)  |
| `vs=1, RA!=0, RT=0` | VL set to MIN(MVL, RA)  |
| `vs=1, RA!=0, RT!=0` | VL,RT set to MIN(MVL, RA)  |

The reasoning here is that the opportunity to set RT equal to the
immediate `SVi+1` is sacrificed in favour of setting from CTR.

# Vertical First Mode

Vertical First is effectively like an implicit single bit predicate
applied to every SVP64 instruction.  **ONLY** one element in each
SVP64 Vector instruction is executed; srcstep and dststep do **not**
increment, and the Program Counter progresses **immediately** to
the next instruction just as it would for any standard scalar v3.0B
instruction.

An explicit mode of setvl is called which can move srcstep and
dststep on to the next element, still respecting predicate
masks.  

In other words, where normal SVP64 Vectorisation acts "horizontally"
by looping first through 0 to VL-1 and only then moving the PC
to the next instruction, Vertical-First moves the PC onwards
(vertically) through multiple instructions **with the same
srcstep and dststep**, then an explict instruction used to
advance srcstep/dststep, and an outer loop is expected to be
used (branch instruction) which completes a series of
Vector operations.

```svstep``` mode is enabled when vf=1, vs=0 and ms=0. 
When Rc=1 it is possible to determine when any level of
loops reach an end condition, or if VL has been reached. The immediate can
be reinterpreted as indicating which SVSTATE (0-3)
should be tested and placed into CR0.

* setvl immediate = 1: only VL testing is enabled. CR0.SO is set
  to 1 when either srcstep or dststep reach VL
* setvl immediate = 2: also include inner middle and outer
  loop end conditions from SVSTATE0 into CR.EQ CR.LE CR.GT
* setvl immediate = 3: test SVSTATE1
* setvl immediate = 4: test SVSTATE2
* setvl immediate = 5: test SVSTATE3

Testing any end condition of any loop of any REMAP state allows branches to be used to create loops.

*Programmers should be aware that VL, srcstep and dststep are global in nature.
Nested looping with different schedules is perfectly possible, as is
calling of functions, however SVSTATE (and any associated SVSTATE) should be stored on the stack.*

**SUBVL**

Sub-vector elements are not be considered "Vertical". The vec2/3/4
is to be considered as if the "single element".  Caveats exist for
[[sv/mv.swizzle]] and [[sv/mv.vec]] when Pack/Unpack is enabled.

# Pseudocode

    // instruction fields:
    rd = get_rt_field();         // bits 6..10
    ra = get_ra_field();         // bits 11..15
    vf = get_vf_field();         // bit 23
    vs = get_vs_field();         // bit 24
    ms = get_ms_field();         // bit 25
    Rc = get_Rc_field();         // bit 31

    if vf and not vs and not ms {
        // increment src/dest step mode
        // NOTE! this is in no way complete! predication is not included
        // and neither is SUBVL mode
        srcstep = SPR[SV].srcstep
        dststep = SPR[SV].dststep
        VL = SPR[SV].VL
        srcstep++
        dststep++
        rollover = (srcstep == VL or dststep == VL)
        if rollover:
            // Reset srcstep, dststep, and also exit "Vertical First" mode
            srcstep = 0
            dststep = 0
            MSR[6] = 0
        SPR[SV].srcstep = srcstep
        SPR[SV].dststep = dststep

        // write CR? helps for doing Vertical loops, detects end
        // of Vector Elements
        if Rc = 1 {
            // update CR to indicate that srcstep/dststep "rolled over"
            CR0.eq = rollover
        }
    } else {
        // add one. MVL/VL=1..64 not 0..63
        vlimmed = get_immed_field()+1; //  16..22

        // set VL (or not).
        // 4 options: from SPR, from immed, from ra, from CTR
        if vs {
           // VL to be sourced from fields/regs
           if ra != 0 {
               VL = GPR[ra]
           } else {
               VL = vlimmed
           }
        } else {
           // VL not to change (except if MVL is reduced)
           // read from SPRs
           VL = SPR[SV_VL]
        }

        // set MVL (or not).
        // 2 options: from SPR, from immed
        if ms {
           MVL = vlimmed
        } else {
           // MVL not to change, read from SPRs
           MVL = SPR[SV_MVL]
        }

        // calculate (limit) VL
        VL = min(VL, MVL)

        // store VL, MVL
        SVSTATE.VL = VL
        SVSTATE.MVL = MVL

        // write rd
        if rt != 0 {
            // rt is not zero
            regs[rt] = VL;
        }
        // write CR?
        if Rc = 1 {
            // update CR from VL (not rt)
            CR0.eq = (VL == 0)
            ...
            ...
        }
        // write Vertical-First mode
        SVSTATE.vf = vf
    }

# Examples

## Core concept loop

```
loop:
    setvl a3, a0, MVL=8    #  update a3 with vl
                           # (# of elements this iteration)
                           # set MVL to 8
    # do vector operations at up to 8 length (MVL=8)
    # ...
    sub a0, a0, a3   # Decrement count by vl
    bnez a0, loop    # Any more?
```

## Loop using Rc=1

    my_fn:
      li r3, 1000
      b test
    loop:
      sub r3, r3, r4
      ...
    test:
      setvli. r4, r3, MVL=64
      bne cr0, loop
    end:
      blr