Is the constant value of SLEN architecturally visible?

[aka-sps]

How can I get the value of the SLEN constant?

[aswaterman]

It's computable by putting a known pattern into a register group and examining the group with a different SEW/LMUL ratio. I believe the following will put SLEN into t0. (Note that vmv.x.s ignores LMUL, so is effectively accessing the register group with a different SEW/LMUL ratio than the vid.v instruction wrote it with.)

vsetvli t0, x0, e32,m2
vid.v v0
vmv.x.s t0, v1
slli t0, t0, 5

[aka-sps]

    /* vl=VLMAX for AVL=x0 */
    vsetvli x0, x0, e32,m2

    /* fill by 0..VLMAX-1, v1[0] is SLEN / 32  for e32 */
    vid.v v0
    
    /* vext.x.v t0, v1, x0  ignores LMUL and vector register groups, copy v1[0]=SLEN / 32 to t0 */
    vmv.x.s t0, v1

    /* t0 is SLEN */
    slli t0, t0, 5

It seems OK.

Very inconvenient model for programmers.
For a programmer, it would be more convenient to fix SLEN to min SEW, or to VLEN.

Why SLEN ≥ 32 but not SLEN ≥ 8 (min SEW) or, for example SLEN ≥ ELEN?

[aswaterman]

It shouldn't be inconvenient for programmers in practice. Following the code-generation rule that the SEW / LMUL ratio be held constant for a strip-mine loop, SLEN should be invisible to the programmer.

@kasanovic would have to comment on why SLEN ≥ 32.

[aka-sps]

It would be nice to give an example for

VLEN=256b, SLEN=64b, SEW=128b, LMUL=4

to illustrate

If SEW > SLEN, the striping pattern places one element in each vector register in the group before moving to the next vector register in the group. So, when LMUL=2, the even-numbered vector register contains the even-numbered elements of the vector and the odd-numbered vector register contains the odd-numbered elements of the vector.

in addition to the following examples.

4.2. Mapping with LMUL > 1
VLEN=256b, SLEN=128b, SEW=256b, LMUL=4
VLEN=256b, SLEN=128b, SEW=32b, LMUL=8

[aka-sps]

Do not disturb @kasanovic, please. He is probably very busy.
But such questions may arise not only from me.

I understand the case of SEW > SLEN, but an example would be useful to others.
Thank you.

[kasanovic]

@ubc-guy offered to improve SLEN text in issue #329. Closing here.