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

# SV Overview

[[discussion]] page feel free to add comments, questions.

[[!toc]]

This document provides an overview and introduction as to why SV (a
Cray-style Vector augmentation to OpenPOWER) exists, and how it works.

SIMD, the primary method for easy parallelism of the
past 30 years in Computer Architectures, is [known to be
harmful](https://www.sigarch.org/simd-instructions-considered-harmful/).
SIMD provides a seductive simplicity that is easy to implement in
hardware.  With each doubling in width it promises increases in raw performance without the complexity of either multi-issue or out-of-order execution.

Unfortunately, even with predication added, SIMD only becomes more and
more problematic with each power of two SIMD width increase introduced
through an ISA revision.  The opcode proliferation, at O(N^6), inexorably
spirals out of control in the ISA, detrimentally impacting the hardware,
the software, the compilers and the testing and compliance.

Cray-style variable-length Vectors on the other hand result in
stunningly elegant and small loops, exceptionally high data throughput
per instruction (by one *or greater* orders of magnitude than SIMD), with no alarmingly high setup and cleanup code, where
at the hardware level the microarchitecture may execute from one element
right the way through to tens of thousands at a time, yet the executable
remains exactly the same and the ISA remains clear, true to the RISC
paradigm, and clean.  Unlike in SIMD, powers of two limitations are not
involved in either the hardware or in the assembly code.

SimpleV takes the Cray style Vector principle and applies it in the
abstract to a Scalar ISA, in the process allowing register file size
increases using "tagging" (similar to how x86 originally extended
registers from 32 to 64 bit).

The fundamentals are:

* The Program Counter gains a "Sub Counter" context.
* Vectorisation pauses the PC and runs a loop from 0 to VL-1
 (where VL is Vector Length). This may be thought of as a
   "Sub-PC"
* Some registers may be "tagged" as Vectors
* During the loop, "Vector"-tagged register are incremented by
  one with each iteration, executing the *same instruction*
  but with *different registers*
* Once the loop is completed *only then* is the Program Counter
  allowed to move to the next instruction.

Hardware (and simulator) implementors are free and clear to implement this
as literally a for-loop, sitting in between instruction decode and issue.
Higher performance systems may deploy SIMD backends, multi-issue and
out-of-order execution, although it is strongly recommended to add
predication capability into all SIMD backend units.

In OpenPOWER ISA v3.0B pseudo-code form, an ADD operation, assuming both
source and destination have been "tagged" as Vectors, is simply:

    for i = 0 to VL-1:
         GPR(RT+i) = GPR(RA+i) + GPR(RB+i)

At its heart, SimpleV really is this simple.  On top of this fundamental
basis further refinements can be added which build up towards an extremely
powerful Vector augmentation system, with very little in the way of
additional opcodes required: simply external "context".

RISC-V RVV as of version 0.9 is over 180 instructions (more than the
rest of RV64G combined). Over 95% of that functionality is added to
OpenPOWER v3 0B, by SimpleV augmentation, with around 5 to 8 instructions.

Even in OpenPOWER v3.0B, the Scalar Integer ISA is around 150
instructions, with IEEE754 FP adding approximately 80 more. VSX, being
based on SIMD design principles, adds somewhere in the region of 600 more.
SimpleV again provides over 95% of VSX functionality, simply by augmenting
the *Scalar* OpenPOWER ISA, and in the process providing features such
as predication, which VSX is entirely missing.

AVX512, SVE2, VSX, RVV, all of these systems have to provide different
types of register files: Scalar and Vector is the minimum. AVX512
even provides a mini mask regfile, followed by explicit instructions
that handle operations on each of them *and map between all of them*.
SV simply not only uses the existing scalar regfiles (including CRs),
but because operations exist within OpenPOWER to cover interactions
between the scalar regfiles (`mfcr`, `fcvt`) there is very little that
needs to be added.

In fairness to both VSX and RVV, there are things that are not provided
by SimpleV:

* 128 bit or above arithmetic and other operations
  (VSX Rijndael and SHA primitives; VSX shuffle and bitpermute operations)
* register files above 128 entries
* Vector lengths over 64
* Unit-strided LD/ST and other comprehensive memory operations
  (struct-based LD/ST from RVV for example)
* 32-bit instruction lengths. [[svp64]] had to be added as 64 bit.

These are not insurmountable limitations, that, over time, may well be
added in future revisions of SV.

The rest of this document builds on the above simple loop to add:

* Vector-Scalar, Scalar-Vector and Scalar-Scalar operation
 (of all register files: Integer, FP *and CRs*)
* Traditional Vector operations (VSPLAT, VINSERT, VCOMPRESS etc)
* Predication masks (essential for parallel if/else constructs)
* 8, 16 and 32 bit integer operations, and both FP16 and BF16.
* Compacted operations into registers (normally only provided by SIMD)
* Fail-on-first (introduced in ARM SVE2)
* A new concept: Data-dependent fail-first
* Condition-Register based *post-result* predication (also new)
* A completely new concept: "Twin Predication"
* vec2/3/4 "Subvectors" and Swizzling (standard fare for 3D)

All of this is *without modifying the OpenPOWER v3.0B ISA*, except to add
"wrapping context", similar to how v3.1B 64 Prefixes work.

# Adding Scalar / Vector

The first augmentation to the simple loop is to add the option for all
source and destinations to all be either scalar or vector.  As a FSM
this is where our "simple" loop gets its first complexity.

    function op_add(RT, RA, RB) # add not VADD!
      int id=0, irs1=0, irs2=0;
      for i = 0 to VL-1:
        ireg[RT+id] <= ireg[RA+irs1] + ireg[RB+irs2];
        if (!RT.isvec) break;
        if (RT.isvec)  { id += 1; }
        if (RA.isvec)  { irs1 += 1; }
        if (RB.isvec)  { irs2 += 1; }

With some walkthroughs it is clear that the loop exits immediately
after the first scalar destination result is written, and that when the
destination is a Vector the loop proceeds to fill up the register file,
sequentially, starting at `rd` and ending at `rd+VL-1`. The two source
registers will, independently, either remain pointing at `RB` or `RA`
respectively, or, if marked as Vectors, will march incrementally in
lockstep, producing element results along the way, as the destination
also progresses through elements.

In this way all the eight permutations of Scalar and Vector behaviour
are covered, although without predication the scalar-destination ones are
reduced in usefulness.  It does however clearly illustrate the principle.

Note in particular: there is no separate Scalar add instruction and
separate Vector instruction and separate Scalar-Vector instruction, *and
there is no separate Vector register file*: it's all the same instruction,
on the standard register file, just with a loop.  Scalar happens to set
that loop size to one.

# Adding single predication

The next step is to add a single predicate mask.  This is where it gets
interesting.  Predicate masks are a bitvector, each bit specifying, in
order, whether the element operation is to be skipped ("masked out")
or allowed. If there is no predicate, it is set to all 1s, which is
effectively the same as "no predicate".

    function op_add(RT, RA, RB) # add not VADD!
      int id=0, irs1=0, irs2=0;
      predval = get_pred_val(FALSE, rd);
      for i = 0 to VL-1:
        if (predval & 1<<i) # predication bit test
           ireg[RT+id] <= ireg[RA+irs1] + ireg[RB+irs2];
           if (!RT.isvec) break;
        if (RT.isvec)  { id += 1; }
        if (RA.isvec)  { irs1 += 1; }
        if (RB.isvec)  { irs2 += 1; }

The key modification is to skip the creation and storage of the result
if the relevant predicate mask bit is clear, but *not the progression
through the registers*.

A particularly interesting case is if the destination is scalar, and the
first few bits of the predicate are zero.  The loop proceeds to increment
the Scalar *source* registers until the first nonzero predicate bit is
found, whereupon a single result is computed, and *then* the loop exits.
This therefore uses the predicate to perform Vector source indexing.
This case was not possible without the predicate mask.

If all three registers are marked as Vector then the "traditional"
predicated Vector behaviour is provided.  Yet, just as before, all other
options are still provided, right the way back to the pure-scalar case,
as if this were a straight OpenPOWER v3.0B non-augmented instruction.

Single Predication therefore provides several modes traditionally seen
in Vector ISAs:

* VINSERT: the predicate may be set as a single bit, the sources are scalar and the destination a vector.
* VSPLAT (result broadcasting) is provided by making the sources scalar and the destination a vector, and having no predicate set or having multiple bits set.
* VSELECT is provided by setting up (at least one of) the sources as a vector, using a single bit in olthe predicate, and the destination as a scalar.

# Predicate "zeroing" mode

Sometimes with predication it is ok to leave the masked-out element
alone (not modify the result) however sometimes it is better to zero the
masked-out elements.  Zeroing can be combined with bit-wise ORing to build
up vectors from multiple predicate patterns: the same combining with
nonzeroing involves more mv operations and predicate mask operations.
Our pseudocode therefore ends up as follows, to take the enhancement
into account:

    function op_add(RT, RA, RB) # add not VADD!
      int id=0, irs1=0, irs2=0;
      predval = get_pred_val(FALSE, rd);
      for i = 0 to VL-1:
        if (predval & 1<<i) # predication bit test
           ireg[RT+id] <= ireg[RA+irs1] + ireg[RB+irs2];
           if (!RT.isvec) break;
        else if zeroing:   # predicate failed
           ireg[RT+id] = 0 # set element  to zero
        if (RT.isvec)  { id += 1; }
        if (RA.isvec)  { irs1 += 1; }
        if (RB.isvec)  { irs2 += 1; }

Many Vector systems either have zeroing or they have nonzeroing, they
do not have both.  This is because they usually have separate Vector
register files. However SV sits on top of standard register files and
consequently there are advantages to both, so both are provided.

# Element Width overrides

All good Vector ISAs have the usual bitwidths for operations: 8/16/32/64
bit integer operations, and IEEE754 FP32 and 64.  Often also included
is FP16 and more recently BF16.  The *really* good Vector ISAs have
variable-width vectors right down to bitlevel, and as high as 1024 bit
arithmetic per element, as well as IEEE754 FP128.

SV has an "override" system that *changes* the bitwidth of operations
that were intended by the original scalar ISA designers to have (for
example) 64 bit operations (only).  The override widths are 8, 16 and
32 for integer, and FP16 and FP32 for IEEE754 (with BF16 to be added in
the future).

This presents a particularly intriguing conundrum given that the OpenPOWER
Scalar ISA was never designed with for example 8 bit operations in mind,
let alone Vectors of 8 bit.

The solution comes in terms of rethinking the definition of a Register
File.  The typical regfile may be considered to be a multi-ported SRAM
block, 64 bits wide and usually 32 entries deep, to give 32 64 bit
registers.  Conceptually, to get our variable element width vectors,
we may think of the regfile as insead being the following c-based data
structure:

    typedef union {
        uint8_t  actual_bytes[8];
        uint8_t  b[0]; // array of type uint8_t
        uint16_t s[0];
        uint32_t i[0];
        uint64_t l[0]; // default OpenPOWER ISA uses this
    } reg_t;

    reg_t int_regfile[128]; // SV extends to 128 regs

Then, our simple loop, instead of accessing the array of regfile entries
with a computed index, would access the appropriate element of the
appropriate type.  Thus we have a series of overlapping conceptual arrays
that each start at what is traditionally thought of as "a register".
It then helps if we have a couple of routines:

    get_polymorphed_reg(reg, bitwidth, offset):
        reg_t res = 0;
        if (!reg.isvec): # scalar
            offset = 0
        if bitwidth == 8:
            reg.b = int_regfile[reg].b[offset]
        elif bitwidth == 16:
            reg.s = int_regfile[reg].s[offset]
        elif bitwidth == 32:
            reg.i = int_regfile[reg].i[offset]
        elif bitwidth == default: # 64
            reg.l = int_regfile[reg].l[offset]
        return res

    set_polymorphed_reg(reg, bitwidth, offset, val):
        if (!reg.isvec): # scalar
            offset = 0
        if bitwidth == 8:
            int_regfile[reg].b[offset] = val
        elif bitwidth == 16:
            int_regfile[reg].s[offset] = val
        elif bitwidth == 32:
            int_regfile[reg].i[offset] = val
        elif bitwidth == default: # 64
            int_regfile[reg].l[offset] = val

These basically provide a convenient parameterised way to access the
register file, at an arbitrary vector element offset and an arbitrary
element width.  Our first simple loop thus becomes:

    for i = 0 to VL-1:
       src1 = get_polymorphed_reg(RA, srcwid, i)
       src2 = get_polymorphed_reg(RB, srcwid, i)
       result = src1 + src2 # actual add here
       set_polymorphed_reg(rd, destwid, i, result)

With this loop, if elwidth=16 and VL=3 the first 48 bits of the target
register will contain three 16 bit addition results, and the upper 16
bits will be *unaltered*.

Note that things such as zero/sign-extension (and predication) have
been left out to illustrate the elwidth concept. Also note that it turns
out to be important to perform the operation at the maximum bitwidth -
`max(srcwid, destwid)` - such that any truncation, rounding errors or
other artefacts may all be ironed out.  This turns out to be important
when applying Saturation for Audio DSP workloads.

Other than that, element width overrides, which can be applied to *either*
source or destination or both, are pretty straightforward, conceptually.
The details, for hardware engineers, involve byte-level write-enable
lines, which is exactly what is used on SRAMs anyway.  Compiler writers
have to alter Register Allocation Tables to byte-level granularity.

One critical thing to note: upper parts of the underlying 64 bit
register are *not zero'd out* by a write involving a non-aligned Vector
Length. An 8 bit operation with VL=7 will *not* overwrite the 8th byte
of the destination.  The only situation where a full overwrite occurs
is on "default" behaviour.  This is extremely important to consider the
register file as a byte-level store, not a 64-bit-level store.

# Quick recap so far

The above functionality pretty much covers around 85% of Vector ISA needs.
Predication is provided so that parallel if/then/else constructs can
be performed: critical given that sequential if/then statements and
branches simply do not translate successfully to Vector workloads.
VSPLAT capability is provided which is approximately 20% of all GPU
workload operations.  Also covered, with elwidth overriding, is the
smaller arithmetic operations that caused ISAs developed from the
late 80s onwards to get themselves into a tiz when adding "Multimedia"
acceleration aka "SIMD" instructions.

Experienced Vector ISA readers will however have noted that VCOMPRESS
and VEXPAND are missing, as is Vector "reduce" (mapreduce) capability
and VGATHER and VSCATTER.  Compress and Expand are covered by Twin
Predication, and yet to also be covered is fail-on-first, CR-based result
predication, and Subvectors and Swizzle.

## SUBVL <a name="subvl"></a>

Adding in support for SUBVL is a matter of adding in an extra inner
for-loop, where register src and dest are still incremented inside the
inner part.  Predication is still taken from the VL index, however it
is applied to the whole subvector:

    function op_add(RT, RA, RB) # add not VADD!
      int id=0, irs1=0, irs2=0;
      predval = get_pred_val(FALSE, rd);
      for i = 0 to VL-1:
        if (predval & 1<<i) # predication uses intregs
          for (s = 0; s < SUBVL; s++)
            sd = id*SUBVL + s
            srs1 = irs1*SUBVL + s
            srs2 = irs2*SUBVL + s
            ireg[RT+sd] <= ireg[RA+srs1] + ireg[RB+srs2];
          if (!RT.isvec) break;
        if (RT.isvec)  { id += 1; }
        if (RA.isvec)  { irs1 += 1; }
        if (RB.isvec)  { irs2 += 1; }

# Swizzle <a name="subvl"></a>

Swizzle is particularly important for 3D work.  It allows in-place
reordering of XYZW, ARGB etc. and access of sub-portions of the same in
arbitrary order *without* requiring timeconsuming scalar mv instructions
(scalar due to the convoluted offsets).  With somewhere around 10% of
operations in 3D Shaders involving swizzle this is a huge saving and
reduces pressure on register files.

In SV given the percentage of operations that also involve initialisation
to 0.0 or 1.0 into subvector elements the decision was made to include
those:

    swizzle = get_swizzle_immed() # 12 bits
    for (s = 0; s < SUBVL; s++)
        remap = (swizzle >> 3*s) & 0b111
        if remap < 4:
           sm = id*SUBVL + remap
           ireg[rd+s] <= ireg[RA+sm]
        elif remap == 4:
              ireg[rd+s] <= 0.0
        elif remap == 5:
              ireg[rd+s] <= 1.0

Note that a value of 6 (and 7) will leave the target subvector element
untouched. This is equivalent to a predicate mask which is built-in,
in immediate form, into the [[sv/mv.swizzle]] operation.  mv.swizzle is
rare in that it is one of the few instructions needed to be added that
are never going to be part of a Scalar ISA.  Even in High Performance
Compute workloads it is unusual: it is only because SV is targetted at
3D and Video that it is being considered.

Some 3D GPU ISAs also allow for two-operand subvector swizzles.  These are
sufficiently unusual, and the immediate opcode space required so large,
that the tradeoff balance was decided in SV to only add mv.swizzle.

# Twin Predication

Twin Predication is cool.  Essentially it is a back-to-back
VCOMPRESS-VEXPAND (a multiple sequentially ordered VINSERT).  The compress
part is covered by the source predicate and the expand part by the
destination predicate.  Of course, if either of those is all 1s then
the operation degenerates *to* VCOMPRESS or VEXPAND, respectively.

    function op(RT, RS):
      ps = get_pred_val(FALSE, RS); # predication on src
      pd = get_pred_val(FALSE, RT); # ... AND on dest
      for (int i = 0, int j = 0; i < VL && j < VL;):
        if (RS.isvec) while (!(ps & 1<<i)) i++;
        if (RT.isvec) while (!(pd & 1<<j)) j++;
        reg[RT+j] = SCALAR_OPERATION_ON(reg[RS+i])
        if (int_csr[RS].isvec) i++;
        if (int_csr[RT].isvec) j++; else break

Here's the interesting part: given the fact that SV is a "context"
extension, the above pattern can be applied to a lot more than just MV,
which is normally only what VCOMPRESS and VEXPAND do in traditional
Vector ISAs: move registers.  Twin Predication can be applied to `extsw`
or `fcvt`, LD/ST operations and even `rlwinmi` and other operations
taking a single source and immediate(s) such as `addi`.  All of these
are termed single-source, single-destination (LDST Address-generation,
or AGEN, is a single source).

It also turns out that by using a single bit set in the source or
destination, *all* the sequential ordered standard patterns of Vector
ISAs are provided: VSPLAT, VSELECT, VINSERT, VCOMPRESS, VEXPAND.

The only one missing from the list here, because it is non-sequential,
is VGATHER: moving registers by specifying a vector of register indices
(`regs[rd] = regs[regs[rs]]` in a loop).  This one is tricky because it
typically does not exist in standard scalar ISAs.  If it did it would
be called [[sv/mv.x]]

# CR predicate result analysis

OpenPOWER has Condition Registers.  These store an analysis of the result
of an operation to test it for being greater, less than or equal to zero.
What if a test could be done, similar to branch BO testing, which hooked
into the predication system?

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

Note that whilst the Vector of CRs is always written to the CR regfile,
only those result elements that pass the BO test get written to the
integer regfile.

Here for example if FP overflow occurred, and the CR testing was carried
out for that, all valid results would be stored but invalid ones would
not, but in addition the Vector of CRs would contain the indicators of
which ones failed.  With the invalid results being simply not written
this could save resources (save on register file writes).

Also expected is, due to the fact that the predicate mask is effectively
ANDed with the post-result analysis as a secondary type of predication,
that there would be savings to be had in some types of operations where
the post-result analysis, if not included in SV, would need a second
predicate calculation followed by a predicate mask AND operation.

Note, hilariously, that Vectorised Condition Register Operations (crand, cror) may
also have post-result analysis applied to them.  With Vectors of CRs being
utilised *for* predication, possibilities for compact and elegant code
begin to emerge from this innocuous-looking addition to SV.

# Exception-based Fail-on-first

One of the major issues with Vectorised LD/ST operations is when a batch of LDs cross a page-fault boundary.  With considerable resources being taken up with in-flight data, a large Vector LD being cancelled or unable to roll back is either a detriment to performance or can cause data corruption.

What if, then, rather than cancel an entire Vector LD because the last operation would cause a page fault, instead truncate the Vector to the last successful element?

This is called "fail-on-first".  Here is strncpy, illustrated from RVV:

    strncpy:
        c.mv a3, a0               # Copy dst
    loop:
        setvli x0, a2, vint8    # Vectors of bytes.
        vlbff.v v1, (a1)        # Get src bytes
        vseq.vi v0, v1, 0       # Flag zero bytes
        vmfirst a4, v0          # Zero found?
        vmsif.v v0, v0          # Set mask up to and including zero byte.
        vsb.v v1, (a3), v0.t    # Write out bytes
        c.bgez a4, exit           # Done
        csrr t1, vl             # Get number of bytes fetched
        c.add a1, a1, t1          # Bump src pointer
        c.sub a2, a2, t1          # Decrement count.
        c.add a3, a3, t1          # Bump dst pointer
        c.bnez a2, loop           # Anymore?

    exit:
        c.ret

Vector Length VL is truncated inherently at the first page faulting byte-level LD.  Otherwise, with more powerful hardware the number of elements LOADed from memory could be dozens to hundreds or greater (memory bandwidth permitting).

With VL truncated the analysis looking for the zero byte and the subsequent STORE (a straight ST, not a ffirst ST) can proceed, safe in the knowledge that every byte loaded in the Vector is valid.  Implementors are even permitted to "adapt" VL, truncating it early so that, for example, subsequent iterations of loops will have LD/STs on aligned boundaries.

SIMD strncpy hand-written assembly routines are, to be blunt about it, a total nightmare.  240 instructions is not uncommon, and the worst thing about them is that they are unable to cope with detection of a page fault condition.