Date: 2020/12/04
Task Group: Vector Extension
Chair: Krste Asanovic
Co-Chair: Roger Espasa
Number of Attendees: ~12
Current issues on github: https://github.com/riscv/riscv-v-spec

Note: No meeting week Dec 11 due to Summit week.

Issues discussed:

# Memory ordering

Most of the meeting was spent discussing a strategy to handle vector
memory ordering wrt to the RISC-V MCM (RVWMO, RVTSO), i.e., ordering
as observed/influenced by other harts in the system.

A big concern is ordering of younger scalar loads after older vector
loads when both are the same address, as this complicates
high-performance in-order implementations (OoO implementations already
have to deal with ordering around unknown addresses in any case, so
not considered a significant additional burden there).  This load-load
ordering is required for the existing MCM, and the discussion was
around how it would be difficult to remove this ordering guarantee on
current vector load instructions while preserve existing software view
of memory, possibly either complicating mapping of standard languages
or requiring software to add fences that would hurt performance on a
large class of machines.

One possible approach that was discussed was to add separate vector
memory instructions with weaker memory ordering, either encoded as new
opcodes or with some CSR field that modifies behavior of existing
instruction encodings.  This might only be required for gather
operations, but some discussion was whether even greater weakening,
including intra-thread ordering should be considered.

It was felt defining and experimenting with these variants on
memory ordering would delay the vector spec even further, and so the
consensus was to enter public review with the current PoR that follows
standard RVWMO (or really, the standard MCM including TSO) at the
instruction level with the current instructions (intra-instruction
ordering was already relaxed per current draft spec), and consider
weaker instruction forms as a later extension.

# Mask handling

We further discussed the challenges of distributing mask register
values for machines with spatial wide datapaths using internal dynamic
data striping.  In particular, all common instructions used to produce
mask values are explicitly encoded in the ISA, except for loads from
memory.  Machines with internal dynamic data striping will therefore require
hiccups (additional microops) in the pipeline to rearrange load data
whenever used as a mask (heuristics/predictors might be possible to
reduce hiccups).

The most important case is that of mask register spill/refill, but
another important case is loading of packed bit vectors from memory
for use as masks.

Oblivious context save/restore would still likely require hiccups as
the save/restore code would not know data type assumption for next use
of a register, but these hiccups would be rare.

To help reduce these hiccups, we discussed the addition of new
unit-stride loads and stores that would use the lumop/sumop field to
encode EEW=1, and also use effective vl = ceil(vl/8) (implying
effectively EMUL<=1).  Proposed instructions would be:

       vle1.v vd, (rs1)    # Byte load with effective vl = ceil(vl/8)
       vse1.v vs2, (rs1)   # Byte store with effective vl = ceil(vl/8)

For context
switch, or where multiple vector lengths are present in a loop, whole
register versions would also be useful, and might be simpler to
provide and could be only alternative.

       vl1re1.v vd, (rs1) # Whole register load

These options, and whether any should be added to v1.0 for public
review to be discussed further on email.  How to treat the extra bits
in a byte loaded from memory is an open issue (1) 0s, or 2) 1s to match
tail-agnostic, or 3) use whole byte from memory - 3) is probably simplest for
implementations).