Re: Vector TG minutes, 2021/1/29

Bill Huffman


I think the round float to integer as float sequence needs to use vmfle.vf with the usage of the mask inverted. Otherwise NaN values will use the integer instead of the original NaN.


-----Original Message-----
From: tech-vector-ext@... <tech-vector-ext@...> On Behalf Of Krste Asanovic
Sent: Friday, January 29, 2021 8:52 PM
To: tech-vector-ext@...
Subject: [RISC-V] [tech-vector-ext] Vector TG minutes, 2021/1/29


Date: 2021/01/29
Task Group: Vector Extension
Chair: Krste Asanovic
Co-Chair: Roger Espasa
Number of Attendees: ~16
Current issues on github:;!!EHscmS1ygiU1lA!S65qKyz6MSIPofhHPvb7N7aMd1B9iyvNSfZqcbXAgxL4BM6BEBnR3Y3RZOmaCz8$

# Meetings

Meetings will continue in the regular Friday time slot as given on the task groups' Google calendar.

# v0.10 release

Version v0.10 (zero-point-ten) is tagged in repo, incorporating all specification changes agreed to date. A numbered version was requested by toolchain folks to help with their release process. An archived pdf build of v0.10 is available at:;!!EHscmS1ygiU1lA!S65qKyz6MSIPofhHPvb7N7aMd1B9iyvNSfZqcbXAgxL4BM6BEBnR3Y3RUN2S-AA$

This version is intended to provide a stable milestone for internal development, but is not ready for public review or ratification.
However, the intent is that there are no substantial changes in instruction specifications before reaching frozen v1.0 status for public review, though of course there cannot be any guarantees. The repo version name has been updated for the next stage of editing.

Issues discussed:

#623 Round Float to integer as float

The current ISA spec does not have an instruction that rounds a float to an integer but leaving result as a float (IEEE RoundToIntegral*).
The following five vector instruction sequence was quickly proposed, but not checked for correctness or completeness:
vfcvt.rtz.x.f.v v8, v4 # Round to integer of same width (could use frm)
vfcvt.f.x.v v8, v8 # Round back to float of same width
vfabs.f.f.v v12, v4 # Get magnitude of original value.
vmfgt.vf v0, v4, f0 # Is large integer already? f0 has threshold.
vmerge.vvm v4, v8, v4, v0 # Leave alone if already an integer

Group was to evaluate correct equivalent sequence and to determine relative importance of supporting the operation directly.

#550 Zve* subsets

There was discussion on the exact composition of the Zve* subsets, though mostly there was agreement with decisions in earlier meetings.
One question is whether multiplies that produce the upper word of a ELEN*ELEN-bit product (vmulh* and vsmul*) should be mandated on all embedded subsets when ELEN>=64.

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