Vector TG meeting minutes 2021/07/02


Krste Asanovic
 

Date: 2021/07/02
Task Group: Vector Extension
Chair: Krste Asanovic
Vice-Chair: Roger Espasa
Number of Attendees: ~12
Current issues on github: https://github.com/riscv/riscv-v-spec

We discussed several issues that will result in clarifications to the
current spec, but no substantive changes. The plan is to release a
v1.0-rc2 next week, then have a (hopefully) final meeting next week
before releasing for public review.

#701 Whole register mask loads

The group revisited the decision to exclude these from the v1.0 spec.
The discussion concluded that the use was narrow (code that knew the
target register would be used as a mask, but where vector length was
different than current vector length), and that the effect could be
synthesized with a four instruction sequence to save vl, set max
vector length, then use mask load, then restore vl (in some cases, a
portion of the vl manipulation could possibly be amortized with other
code). The decision was to continue to exclude these from v1.0 and
the spec commentary will be updated to describe the alternative code
sequence.

#702 Reductions when vl=0

The group discussed the behavior of reductions when vl=0, and
concluded we'd retain the current definition. This only affects the
case where the source and destination scalar are in different vector
registers, which was determined to be a rare case that would often be
known statically in code. Commentary will be added to text to provide
rationale.

#703 Clarify that vl=0 does not affect moves from vector registers

After reviewing #702, it was realized the current text could be
confusing for the effect of vl=0 on vector->scalar move instructions,
so the text will be clarified.

#704 Add vector memory operations to RISC-V MCM

As already noted in spec, a more formal definition of vector memory
operations interaction with MCM is needed.

#705 Vector memory operations under TSO.

While not necessary for v1.0 (as Ztso is still not ratified, though
frozen), the group realised that the behavior under RVTSO needs to be
clarified.

#664 drop tail undisturbed

This closed issue was raised again. The group agreed that tail
undisturbed caused implementation complexity for renamed/OoO
implementations, but also acknowledged there were some important use
cases for tail undisturbed. The observation was made that the
complexity for renamed/OoO machines mostly arises from optimizing for
performance, so implementations that did not care about
tail-undisturbed use cases could simplify microarchitecture. A note
will be added explaining this tradeoff.

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