Re: Vector extension TG meeting minutes 2020/5/1

David Horner

On 2020-05-01 10:27 p.m., Krste Asanovic wrote:
Date: 2020/5/1
Task Group: Vector Extension
Chair: Krste Asanovic
Co-Chair: Roger Espasa
Number of Attendees: ~20
Current issues on github:

Issues discussed:

#440 LMUL contiguous?

Discussed the recent change to add extra bit for fractional LMUL.
Explained that the reason to revert to backwards-compatible layout was
to reduce implementation churn. Not intended as a precedent that
pre-freeze specs have to ensure backwards compatibility. Can be
changed in later or 1.0 version.

#418 Drop LMUL

Agreed to retain LMUL given discussion on github, but to bring up some
of the detailed suggestions as separate issues.

#424/425 New explicit EEW scheme in load/stores

Group was generally favorable on the new scheme that encodes effective
element width in loads/stores.

The spec still has not been updated to add 4* widening and 2* widening
operations that would sign/zero extend from 1/4*SEW and 1/2*SEW to

Some concern expressed that dropping fixed-width sign/zero-extending
byte/halfword load/stores will hurt performance of some key loops.
Also noted that bringing in smaller data items as fractional LMUL will
avoid tying up as many architectural registers as bringing as full
width elements. Will keep studying application kernels.

#434 SLEN=VLEN as optional extension

General sentiment was that it will be bad to fragment and we should
find way to retain SLEN<=VLEN as single standard. Also, that
SLEN=VLEN will be too expensive even on large cores, with only a
subset of workload needing in-memory format in registers.

Proposals suggested including adding another bit to vsetvli to change
to in-memory format in vector registers (potentially reducing VLEN),
or adding "cast" instructions to change in/out of memory byte order.
The latter could be in-place overwrites that turn into NOPs in
SLEN=VLEN implementations.

Group to provide proposals.

#423 transient vtype modifier

Briefly discussed adding a transient vtype modifier, but no decisions.
Note: To help understand the #423 as the meeting introduction only mentioned transient SEW and LMUL overrides.
The proposal is generalized and includes extending modifiers beyond what are mapped by vsetvli.
It also includes a persistent variant (which is discribed first).

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