Vector Task Group minutes, 2020/10/23


Krste Asanovic
 

Reminder: No meeting next Friday October 30.

Date: 2020/10/23
Task Group: Vector Extension
Chair: Krste Asanovic
Co-Chair: Roger Espasa
Number of Attendees: ~12
Current issues on github: https://github.com/riscv/riscv-v-spec

There is no task group meeting next week (October 30).

We have identified potential resource for building SAIL model for V
extension.

We will investigate various paths to building compliance suite,
including Imperas released suite.

Issues discussed:

#575 Are the reserved bits of vtype WARL?

No. Implementation can set vill if not a supported configuration.
While implementations are allowed to trap on reserved values, we don't
want to mandate data-dependent traps. vsetvli instructions have a 11b
value written to vtype (the sign bit of the 12b imm field selects
immediate/register forms of vsetvl). For future extension, we could
view vtype as an 11b CSR, with additional CSRs added for more bits
later.

#519 Reserve wider EEW

Proposal to reserve EEW of 128-1024 for v1.0 ratification, with
anticipation to use these encodings later as proposed. Agreed.

#522 Forbid register overlap in reductions?

This issue was closed, as agreed that there were no additional
constraints above those needed for regular renaming or restart.

#544 Proposal to add a "addvl" instruction

This would include a multiply immediate to scale vlenb by number of
vector registers to be saved/restored to reduce code size.

It was noted that Zba will reduce the cost in some cases as
shift-and-add instructions can perform some small immediate
multiplies.

The consensus was that more experience was needed with software use
cases before adding any new instruction.

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