Vector Task Group meeting minutes for 2021/2/19

Krste Asanovic

Date: 2021/02/19
Task Group: Vector Extension
Chair: Krste Asanovic
Vice-Chair: Roger Espasa
Number of Attendees: ~23
Current issues on github:

# Next Meeting/Freezing

The schedule is to meet again in two weeks (Friday March 5). The plan
is to have all pending updates and cleanups in spec by that date, to
be able to agree to freeze and move forward into public review (v1.0),
which should happen soon after this meeting. Please continue to send
PRs for any small typos and clarifications, and use mailing list for
larger issues.

Issues discussed

#640 Bound on VLMAX

The major issue raised was that software would otherwise have to cope
with indices that might not fit in 16b. The group agreed that
profiles and/or platform specs can set the upper bound, with a current
recommendation that all profiles limit VLMAX to 64K elements
(VLEN=64Kib, or 8K bytes per vector register). The current ISA spec can
support larger VLMAX already, but adding vrgatherei32 instruction
would be a useful addition (post-v1.0) if architectural vector
regfiles >256KiB become common.

It was also discussed that a privileged setting will be desired to
modulate visible VLEN to support thread migration, or just different
application vector profiles with different VLMAX in general.