Register Type Tags

UNDER CONSIDERATION

A concept present in processors such as Texas Instruments DSPs and in the Mill Architecture to an advanced level, register "tags" allow the meaning and behaviour of instructions to change, polymorphically.

This for allows instructions originally designed to only be IEEE754 FP64 for example to become IEEE754 FP128 or even complex mumbers. With SVP64 supporting remap it is not conceptually that much of a leap to support complex numbers, given that the hardware to do so is already in place.

It is however extremely important to keep the tag context down to a bare minimum size, because, like SVSTATE, it has to be added to the interrupt context alongside SRR0 and SRR1.

Interestingly due to the size of complex numbers (and FP128) there is not much point having large tags. It is quite likely therefore that a 64 bit SPR of 2 bits per register, covering 128 registers, would be sufficient. Groups of 4 registers would be tagged:

• SVTAG[0:1] marks all of FPR[0..3] and GPR[0..3]

Links:

• http://lists.libre-soc.org/pipermail/libre-soc-dev/2021-June/003256.html

Complex numbers

Currently under discussion is which arithmetic operations should be supported: complex divide for example is known to be very tricky. Add and subtract are easy: multiply involves four scalar multiplies, one add and one subtract. This fits within the REMAP system capabilities.