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Vector TG minutes 2020/11/13 meeting


Krste Asanovic
 

Date: 2020/11/13
Task Group: Vector Extension
Chair: Krste Asanovic
Co-Chair: Roger Espasa
Number of Attendees: ~22
Current issues on github: https://github.com/riscv/riscv-v-spec

Issues discussed:

# Removing half-precision FP as mandate in V extension.

The group discussed a proposal to remove half-precision floating-point
operations as part of mandated set for V extension, moving FP16
support to an additional standard extension. BF16 support could be
added in same way at later date. For both 16b FP formats, a standard
subextension supporting only conversion up/down to/from FP32 was
agreed to be defined (no integer conversions, and no .rod. variant).
This approach enables implementations to support either, both, or
neither of these 16-bit floating-point formats. The expected impact
on software stack is expected to be small given that half-precision FP
use is not yet standard.

Related, there is a discussion on what level of support should be
mandated in next application-processor profile (RVA21). There were
three options considered:
1) no FP16 supported mandate
2) RVA21 mandates FP16 convert operations only
3) V mandates FP16 convert operations only

This is to be discussed further, but group seemed to favor option 1 or
2.

# Mask handling

There was initial discussion of concerns over mask handling for wide
spatial implementations and/or implementations with vector register
renaming.

One proposal was to not allow "tail undisturbed" on mask results, as
this complicated renamed implementations and those with specialized
mask handling, while not being particularly useful to software.
Created new issue #602 for this proposal.