Re: Vector TG meeting minutes 2021/07/02
Task Group: Vector Extension
Chair: Krste Asanovic
Vice-Chair: Roger Espasa
Number of Attendees: ~12
Current issues on github: https://urldefense.com/v3/__https://github.com/riscv/riscv-v-spec__;!!EHscmS1ygiU1lA!RMBBi1gNhOHzOU2abQmClNEMWkaI2-aaN2062y7AMPrW99eX1JSWR8QC9lFNZlI$
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.
I think having the exact instruction sequence will both help clarify support for wide, re-organizing datapaths and ensure that the envisioned sequence isn't only 99% correct.
#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.
Not just the rational but what cases need particular care to avoid unexpected behavior.
#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.