Re: V extension groups analogue to the standard groups

Colin Schmidt

We had a long discussion about how to name different portions of the vector ISA at one of the recent meetings.

You can see the issue here:
And the meeting minutes here:


On Fri, Aug 21, 2020 at 5:49 PM Andrew Waterman <andrew@...> wrote:

On Fri, Aug 21, 2020 at 4:51 PM Guy Lemieux <glemieux@...> wrote:
I think a common embedded and FPGA scenarios  will be F on the scalar side but no F on the vector side. Adding F to V is nontrivial in area, particularly for FPGAs that lack FPUs, yet an integer-only V makes a lot of sense for pixel processing etc. F in the scalar is a nice-to-have for code size and to calculate scalar parameters, eg in OpenCV and OpenVX.

The current nomenclature assumptions don’t allow this, but I think that they should do so.

We definitely want to sanction configurations that have different datatype support on scalar and vector.  The current thinking is that the letter V means "whatever apps-profile processors want", just like what "G" means on the scalar side.  Perhaps the "vector-with-fewer-datatypes-than-scalar" case can be described as Zvbase instead of V?


On Fri, Aug 21, 2020 at 4:30 PM Andrew Waterman <andrew@...> wrote:

On Fri, Aug 21, 2020 at 2:43 PM Simon Davidmann <simond@...> wrote:
A question to clarify. You state:
      RV32IV doesn’t mandate any FP hardware in the vector unit, whereas RV32IFV means both scalar and vector support single-precision, etc.  

This means if I understand you that we need to add F to get F hardware in the vector unit - so RV32IV means V with no F hardware, and RV32IFV includes F hardware.

So for consistency...

What does RV32IV means for M hardware multiply - do I need to RV32IMV to get scalar and vector hardware multiply?

I don’t believe the spec explicitly addresses this question, but I agree it makes sense. Alternatively, V could require M, since it doesn’t make much sense to pay for a vector unit but be too stingy to pay for a multiplier. But that might be less consistent. (My recommendation is that RV32IV continue to mean “no multiplier”, even though it’s a silly configuration.)

RV32IV means no F and no M hardware? - so I need to explicitly include the extensions I need as V assumes nothing but I?

My recommendation is to clarify in the spec that RV32IV is a valid config with no FPU in the vector unit, and RV32IFV is also a valid config with an FPU in both scalar and vector.

Or is something assumed for M?

If we choose to define that V implies M, RV32IV and RV32IMV would be synonyms.


On Thu, Aug 20, 2020 at 8:48 PM Andrew Waterman <andrew@...> wrote:
Quad-widening ops have been moved to a separate extension, Zvqmac.

I believe the intent is that the capital-V V extension supports the same FP datatypes as the scalar ISA, so e.g., RV32IV doesn’t mandate any FP hardware in the vector unit, whereas RV32IFV means both scalar and vector support single-precision, etc.

I’m surprised all those hashtags made it past the spam filter!

On Thu, Aug 20, 2020 at 11:42 AM Strauch, Tobias (HENSOLDT Cyber GmbH) <tobias.strauch@...> wrote:

Apologies if this is old stuff already dismissed. But I give it a try anyway.

Wouldn't it make sense to separate more complex vector instructions from more trivial ones? Already with the very first base release ? Vector instructions can also be helpful in small devices #IOT #Edge #GAP8

#RISCY without the need to fully support floating point instructions or without the need for a quad multiply.

The suggestion would be to basically group vector extensions analogue to the standard instructions (I, M, F, D, Q, …), instead of having an already complex base and then subtract or re-define subsets of instructions

again ?

Wouldn't that be in-line with the RISC-V philosophy of modularity and simplicity ? The beauty would be that you have a non-vector and a vector group version.

Possible nomenclature based on order:

M: Standard Multiply Divide Instructions (MUL, ...)

V: Very Basic Vector Instructions (VSETVL, ...)

MV: Standard Multiply Divide Instructions and Very Basic Vector Instructions (MUL, VSETVL, ...

VM: Standard Multiply Divide Instructions, Very Basic Vector Instructions and Vector Integer Multiply\Divide Instructions (MUL, VSETVL, VMUL, ...)

F, D, Q analogue to M as suggested.

The V version will not be a 1:1 match with the standard version and will cover additional aspects. But it can be argued, that when you implement the V version (of M, F, D, Q, ...), then you most likely will have

the relevant standard counterparts implemented as well anyway.

Kind Regards, Tobias

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