Minutes from 2020/1/24 meeting

Krste Asanovic

Date: 2020/1/24
Task Group: Vector Extension
Chair: Krste Asanovic
Number of Attendees: ~20
github: https://github.com/riscv/riscv-v-spec

Outstanding items from prior meeting:
#362/#354, #341, #348, #347, #335, #326, #318, #235

Current items:
#362/#354, #341

New actionable items to be addressed in next meeting:

Is group tracking to roadmap? y

The main discussion in the group that occupied most of the time was
around #362/#354, whether to drop the fixed-size vector load/store
instructions, leaving only the SEW-sized load/store instructions.
Dropping these instructions would save considerable complexity in
memory pipelines. However, dropping support would also require
execution of additional instructions in some common cases. A remedy
would be to add more widening or quad-widening (quadening) compute
instructions to reduce this impact.

During the call a second issue was raised. The current design uses
constant SEW/LMUL ratios to align data types of different element
widths. If only SEW-sized load/stores were available, then a
computation using a mixture of element widths would have to use larger
LMUL for larger SEW values, which effectively reduces the number of
available registers and so increases register pressure. The fixed
width load/stores allow, e.g., a byte to be loaded into a vector
register with four-byte width with LMUL=1 so avoids this issue.

No resolution was reached, but this was to be studied further.

The second point that was quickly discussed at the end of the meeting
was the proposed separation of a vcsr from fcsr (#341). There was
little obvious enthusiasm in favor of one choice over the other, and
so the design is left as is for now.