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Vector Task Group minutes 2020/11/20 meeting


Krste Asanovic
 

Next meeting today in usual time slot as on calendar,
Krste


Date: 2020/11/20
Task Group: Vector Extension
Chair: Krste Asanovic
Co-Chair: Roger Espasa
Number of Attendees: ~17
Current issues on github: https://github.com/riscv/riscv-v-spec

No meeting Nov 27 due to US Thanksgiving.

Issues discussed:

# Mask handling

We discussed the challenges of distributing mask register values for
machines with spatial wide datapaths. Machines that perform dynamic
microarchitectural striping to reduce datapath wiring can do the same
for masks when it is known that a mask value is being written.
Currently mask writes are explicit in the ISA, except for loads from
memory (e.g., in a mask register spill/refill sequence).

One suggestion was that a heuristic could be used for loads, where it
would be assumed to not be a mask if LMUL>1 or not SEW=8, but this
would not work for many cases.

Another suggestion was to add new instructions to support load/store
of masks. This could be encoded as unit-stride instructions using
reserved space in lumop[4:0]. These would look like EEW=8 load/stores
except with vector length of ceil(vl/8) (implying effectively
EMUL<=1). The detailed behavior was not discussed.