On Wed, 12 Apr 2023, Kito Cheng wrote:
> Hi Richard:
>
> > > In order to model LMUL in backend, we have to the combination of
> > > scalar type and LMUL; possible LMUL is 1, 2, 4, 8, 1/2, 1/4, 1/8 - 8
> > > different types of LMUL, and we'll have QI, HI, SI, DI, HF, SF and DF,
> > > so basically we'll have 7 (LMUL type) * 7 (scalar type) here.
> >
> > Other archs have load/store-multiple instructions, IIRC those
> > are modeled with the appropriate set of operands. Do RVV LMUL
> > group inputs/outputs overlap with the non-LMUL grouped registers
> > and can they be used as aliases or is this supposed to be
> > implemented transparently on the register file level only?
>
> LMUL and non-LMUL (or LMUL=1) modes use the same vector register file.
>
> Reg for LMUL=1/2 : { {v0, v1, ...v31} }
> Reg for LMUL=1 : { {v0, v1, ...v31} }
> Reg for LMUL=2 : { {v0, v1}, {v2, v3}, ... {v30, v31} } // reg. must
> align to multiple of 2.
> Reg for LMUL=4 : { {v0, v1, v2, v3}, {v4, v5, v6, v7}, ... {v28, v29,
> v30, v31} } // reg. must align to multiple of 4.
> ..
> Reg for 2-tuples of LMUL=1 : { {v0, v1}, {v1, v2}, ... {v29, v30}, {v30, v31}
> }
> Reg for 2-tuples of LMUL=2 : { {v0, v1, v2, v3}, {v2, v3, v4, v5}, ...
> {v28, v29, v30, v31}, {v28, v29, v30, v31} } // reg. must align to
> multiple of 2.
> ...
>
> > But yes, implementing this as operations on multi-register
> > ops with large modes is probably the only sensible approach.
> >
> > I don't see how LMUL of 1/2, 1/4 or 1/8 is useful though? Can you
> > explain? Is that supposed to virtually increase the number of
> > registers? How do you represent r0:1/8:0 vs r0:1/8:3 (the first
> > and the third "virtual" register decomposed from r0) in GCC? To
> > me the natural way would be a subreg of r0?
> >
> > Somehow RVV seems to have more knobs than necessary for tuning
> > the actual vector register layout (aka N axes but only N-1 dimensions
> > thus the axes are
>
> The concept of fractional LMUL is the same as the concept of AArch64's
> partial SVE vectors,
> so they can only access the lowest part, like SVE's partial vector.
>
> We want to spill/restore the exact size of those modes (1/2, 1/4,
> 1/8), so adding dedicated modes for those partial vector modes should
> be unavoidable IMO.
>
> And even if we use sub-vector, we still need to define those partial
> vector types.
Could you use integer modes for the fractional vectors? For computation
you can always appropriately limit the LEN?
