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

# TODO

<https://bugs.libre-soc.org/show_bug.cgi?id=213>

* idea 1: modify cmp (and other CR generators?) with qualifiers that
  create single bit prefix vector into int reg
* idea 2: override CR SO field in vector form to be predicate bit per element
* idea 3: reading of predicates is from bits of int reg
* idea 4: SO CR field no longer overflow, contains copy of int reg
  predicate element bit (passed through).  when OE set?


# Requirements

* must be easily implementable in any  microarchitecture including:
  - small and large out-of-order 
  - in-order
  - FSM (0.3 IPC or below)
* must not compromise or penalise any microarchitectural performance
* must cover up to 64 elements
* must still work for elwidth over-rides

## Additional Capabilities

* two modes, "zeroing" and "non-zeroing". zeroing mode places a zero in the masked-out element results, where non-zeroing leaves the destination (result) element unmodified.
* predicate must be invertable via an opcode bit (to avoid the need for an instruction which inverts all bits of the predicate mask)

Implementation note: even in in-order microarchitectures it is strongly adviseable to use byte-level write-enable lines on the register file.  This in combination with 8-bit SIMD element overrides allows, in "non-zeroing" mode, the predicate mask to be directly ANDed with the regfile write-enable lines to achieve the required functionality.  The alternative is to perform a READ-MODIFY-MASK-WRITE cycle which is costly and compromises performance.  Avoided very simply with byte-level write-enable.

# Proposals

## Adding new predicate register file type and associated opcodes

This idea, adding new predicate manipulation opcodes,
violates the fundamental design principles of SV to not add
new vector-related instructions unless essential or compelling.

All other proposals utilise existing scalar opcodes which already happen to have bitmanipulation, arithmetic, and inter-file transfer capability (mfcr, mfspr etc).
They also involve adding extra bitmanip opcodes, such that by utilising those scalar registers as predicate masks SV achieves "par" with other Cray-style Vector ISAs, all without actually having to add any actual Vector opcodes.

In addition those bitmanip operations, although some of them are obscure and unusual in the scalar world, do actually have practical applicatiobe outside of a vector context.

Adding a full set special vector opcodes just for manipulating predicate masks and being able to transfer them to other regfiles (a la mfcr) is however anomalous, costly, and unnecessary.

## CR-based predication proposal

this involves treating each CR as providing one bit of predicate. If
there is limited space in SVPrefix it will be a fixed bit (bit 0)
otherwise it may be selected (bit 0 to 3 of the CR) through a firld in the opcode.

the crucial advantage of this proposal is that the Function Units can
have one more register (a CR) added as their Read Dependency Hazards
just like all the other incoming source registers, and there is no need
for a special "Predicate Shadow Function Unit".

a big advantage of this is that unpredicated operations just set the
predicate to an immediate of all 1s and the actual ALUs require very
little modification.

a disadvantage is that to support the selection of 8 bit of predicate
from 8 CRs (via the "full" 8x CR port") would require allocating 32-bit
datapath to the relevant FUs.  This could be reduced by adding yet another
type of special virtual register port or datapath that masks out the
required predicate bits closer to the regfile.

another disadvantage is that the CR regfile needs to be expanded from 8x 4bit CRs to a minimum of 64x or preferably 128x 4-bit CRs.  Beyond that they can be transferred using vectorised mfcr and mtcrf into INT regs.  this is a huge number of CR regs, each of which will need a DM column in the FU-REGs Matrix.  however this cost can be mitigated through regfile cacheing, bringing FU-REGs column numbers back down to "sane".

### Predicated SIMD HI32-LO32 FUs

an analysis of changing the element widths (for SIMD) gives the following
potential arrangements, for which it is assumed that 2x 32-bit FUs
"pair up" for single 64 bit arithmetic, HI32 and LO32 style.

* 64-bit operations.  2 FUs and their DM rows "collaborate" 
  - 2x 32-bit source registers gang together for 64 bit input
  - 2x 32-bit output registers likewise for output
  - 1x CR (from the LO32 FU DM side) for a predicate bit
* 32-bit operations.  2 FUs collaborate 2x32 SIMD style
  - 2x 32-bit source registers go into separate input halves of the
    SIMD ALU
  - 2x 32-bit outputs likewise for output
  - 2x CRs (one for HI32, one for LO32) for a predicate bit for each of
    the 2x32bit SIMD pair
* 16-bit operations. 2 FUs collaborate 4x16 SIMD style
  - 2x 2x16-bit source registers group together to provide 4x16 inputs
  - likewise for outputs
  - EITHER 2x 2xCRs (2 for HI32, 2 for LO32) provide 4 predicate bits
  - OR 1x 8xCR "full" port is utilised (on LO32 FU) followed by masking
    at the ALU behind the FU pair, extracting the required 4 predicate bits
* 8-bit operations. 2 FUs collaborate 8x8 SIMD style
  - 2x 4x8-bit source registers
  - likewise for outputs
  - 1x 8xCR "full" port is utilised (on LO32 FU) and all 8 bits are
    passed through to the underlying 64-bit ALU to perform 8x 8-bit
    predicated operations

### Predicated SIMD straight 64-bit FUs

* 64-bit operations. 1 FU, 1 64 bit operation
  - 1x 64-bit source register
  - 1x 64-bit output register
  - 1x CR for a predicate bit
* 32-bit operations.  1 FUs 2x32 SIMD style
  - 1x 64-bit source register dynamically splits to 2x 32-bit
  - 1x 64-bit output likewise
  - 2x CRs for a predicate bit for each of the 2x32bit SIMD pair
* 16-bit operations. 1 FUs 4x16 SIMD style
  - 1x 4x16-bit source registers 
  - likewise for outputs
  - 1x 8xCR "full" port is utilised followed by masking at the ALU behind
    the FU pair, extracting the required 4 predicate bits
* 8-bit operations. 1 FU 8x8 SIMD style
  - 1x 8x8-bit source registers
  - likewise for outputs
  - 1x 8xCR "full" port is utilised LO32 and all 8 bits used
    to perform 8x 8-bit predicated operations

Here again the underying 64-bit ALU requires the 8x predicate bits to
cover the 8x8-bit SIMD operations (7 of which are dormant/unused in 64-bit
predicated operations but still have to be there to cover 8x8-bit SIMD).

Given that the initial idea of using the "full" (virtual) 32-bit CR read
port (which reads all 8 CRs CR0-CR7 simultaneously) would require a
32-bit broadcast bus to every predication-capable Function Unit, the bus
bandwidth can again be reduced by performing the selection of the masks
(bit 0 thru bit 3 of each CR) closer to the regfile i.e. before hitting
the broadcast bus.

## One scalar int per predicate element.

Similar to RVV and similar to the one-CR-per-element concept above, the idea here is to use the LSB of any given element in a vector of predicates.  This idea has quite a lot of merit to it.

Implementation-wise just like in the CR-based case a special regfile port could be added that gets the LSB of each scalar integer register and routes them through to the broadcast bus.

The disadvantages appear on closer analysis:

* Unlike the "full" CR port (which reads 8x CRs CR0-7 in one hit) trying the same trick on the scalar integer regfile, to obtain just 8 predicate bits (each being an LSB of a given 64 bit scalar int), would require a whopping 8x64bit set of reads to the INT regfile instead of a scant 1x32bit read.  Resource-wise, then, this idea is expensive.
* With predicate bits being distributed out amongst 64 bit scalar registers, scalar bitmanipulation operations that can be performed after transferring Vectors of CMP operations from CRs to INTs (vectorised-mfcr) are more challenging and costly.  Rather than use vectorised mfcr, complex transfers of the LSBs into a single scalar int are required.

In a "normal" Vector ISA this would be solved by adding opcodes that perform the kinds of bitmanipulation operations normally needed for predicate masks, as specialist operations *on* those masks.  However for SV the rule has been set: "no unnecessary additional Vector Instructions" because it is possible to use existing PowerISA scalar bitmanip opcodes to cover the same job.

The problem is that vectors of LSBs need to be transferred *to* scalar int regs, bitmanip operations carried out, *and then transferred back*, which is exceptionally costly.

On balance this is a less favourable option than vectorising CRs

## Scalar (single) integer as predicate, with one DM row

This idea has merit in that to perform predicate bitmanip operations the preficate is already in scalar INT reg form and consequently standard scalar INT bitmanip operations can be done straight away.  Vectorised mfcr can be used to get CMP results or Vectorised Rc=1 CRs into the scalar INT, easily.

This idea has several disadvantages.

* the single DM entry for the entire 64 bits creates a read hazard
  that has to be resolved through the addition of a special Shadowing
  Function Unit.  Only when the entire predicate is available can the
  die-cancel/ok be pulled on the FU elements each bit covers
* this situation is exacerbated if one vector creates a predicate
  mask that is then used to mask immediately following instructions.
  Ordinarily (i.e. without the predicate involved), Cray-style "chaining"
  would be possible.  The single DM entry for the entire predicate mask
  prohibits this because the subsequent operations can only proceed when
  the *entire* mask has been computed and placed in full
  into the scalar integer register.
* Allocation of bits to FUs gets particularly complex for SIMD (elwidth
  overrides) requiring shift and mask logic that is simply not needed
  compared to "one-for-one" schemes (above)

Overall there is very little in favour of this concept.

## Scalar (single) integer as predicate with one DM row per bit

The Dependency Matrix logic from the CR proposal favourably applies
equally to this proposal.  However there are additional caveats that
weigh against it:

* Like the single scalar DM entry proposal, the integer scalar register
  had to be covered also by a single DM entry (for when it is used *as*
  an integer register).
* Unlike the same, it must also be covered by a 64-wide suite of bitlevel
  Dependency Matrix Rows.  These numbers are so massive as to cause some
  concern.
* A solution is to introduce a virtual register naming scheme however
  this also introduces huge complexity as the register cache has to be
  capable of swapping reservations from 64 bitlevel to full 64bit scalar
  level *and* keep the Dependency Matrices synchronised

it is enormously complex and likely to result in debugging, verification
and ongoing maintenance difficulties.

## Schemes which split (a scalar) integer reg into mask "chunks"

These ideas are based on the principle that each chunk of 8 (or 16)
bits of a scalar integer register may be covered by its own DM column
  in FU-REGs.
8 chunks of a scalar 64-bit integer register for use as a bit-level
predicate mask onto 64 vector elements would for example require 8
DM entries.

This would, for vector sizes of 8, solve the "chaining" problem reasonably
well even when two FUs (or two clock cycles) were required to deal with
4 elements at a time.  The "compare" that generated the predicate would
be ready to go into the first "chunk" of predicate bits whilst the second
compare was still being issued.

It would also require a lot smaller DMs than the single-bit-per-element
ideas.

The problems start when trying to allocate bits of predicate to units.
Just like the single-DM-row per entire scalar reg case, a shadow-capable
Predicate Function Unit is now required (already determined to be costly)
except now if there are 8 chunks requiring 8 Predicate FUs *the problem
is now made 8x worse*.

Not only that but it is even more complex when trying to bring in virtual
register cacheing in order to bring down overall FU-REGs DM row count,
although the numbers are much lower: 8x 8-bit chunks of scalar int
only requires 8 DM Rows and 8 virtual subdivisions however *this is per
in-flight register*.

The additional complexity of the cross-over point between use as a chunked
predicate mask and when the same underlying register is used as an actual
scalar (or even vector) integer register is also carried over from the
bit-level DM subdivision case.

Out-of-order systems, to be effective, require several operations to
be "in-flight" (POWER10 has up to 1,000 in-flight instructions) and if
every predicated vector operation needed one 8-chunked scalar register
each it becomes exceedingly complex very quickly.

Even more than that, in a predicated chaining scenario, when computing
the mask from a vector "compare", the groupings are troublesome to
think through how to implement, which is itself a bad sign.  It is
suspected that chaining will be complex or adversely affected by certain
combinations of element width.

(see [[masked_vector_chaining]])

Overall this idea which initially seems to save resources brings together
all the least favourable implementation aspects of other proposals and
requires and combines all of them.