intro to #421 Fractional vtype field vfill and #418 vlmt...
Previous issues I opened on fractional LMUL were exploratory, suggesting various ways to encode and enable the feature.
The latest 4 issues opened on github are specific proposals based on the strawman recommendation Krste made on March 24th.
These are complimentary proposals which can be implemented individually or collectively.
#411 CLSTR and clstr: width specifiers for data cluster in each SLEN chunk. (when LMUL<=1/2)
Defines the user visible machine specific fractional LMUL characteristics.
#421 Fractional vtype field vfill – Fractional Fill order and Fractional Instruction eLement Location
This proposal enables more modes of operation over the strawman model.
It reduces to LMUL>=1 when vfill=00 and to the strawman model when vfill=01.
The additional modes (vfill=10 and vfil=11) allow for
1) doubling the capacity of fractional registers (and register groups if #418 is also enacted)
2) processing pairs of clustered elements in tandem.
3) allow a single physical register to source the data for widening operations in the common case LMUL=1/2.
I realize everyone is busy with various aspects of the standard and that not everyone subscribes to github riscv-v-spec, so I wanted to send out his overview prior to tomorrows meeting.