Vector TG meeting minutes 2020/10/16


Krste Asanovic
 

Date: 2020/10/16
Task Group: Vector Extension
Chair: Krste Asanovic
Co-Chair: Roger Espasa
Number of Attendees: ~16
Current issues on github: https://github.com/riscv/riscv-v-spec

# Definition of Done checklist

We discussed what items on the "definitions of done" checklist are not
currently underway, including the architectural compliance tests and
the SAIL formal model.

# 550 Names/contents of initial vector subsets.

We discussed the proposals for initial vector subsets for
microcontroller/embedded use cases. The proposed five subsets are
Zve32x, Zve32f, Zve64x, Zve64f, Zve64d.

The consensus was to not include Zvamo in these, as some embedded
memory systems do not have caches, and many interconnects do not
support atomics.

vrgather instruction was discussed for omission, but the consensus was
that if indexed memory accesses were to be supported (which had
agreement), then vrgather was relatively easy to add and was very
useful, especially in low-ended embedded systems with limited memory
systems.

reductions were discussed for omission, but it was felt that they're
commonly used and they would fragment the software base too much if
left out.

There was some discussion about supporting only LMUL=1, but was noted
this would not allow use of the widening and narrowing instructions.
Another proposal was to support only LMUL<=1, as this would simplify
some implementations while still supporting the full instruction set.
This would form an even more minimal base.

There was no clear consensus on requiring a minimum VLEN>ELEN. This
would interact with a proposal to allow LMUL<=1, in that a longer VLEN
might be mandated if LMUL<=1 since larger LMULs could not be used to
hold longer vectors.

We did not reach agreement on whether 64b integer multiplies should be
included in Zve64f subset.

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