[otbn] double increment and dmem byte addressing

[felixmiller]

With byte addressing of dmem we need to increment the dmem pointer by 32 (full WLEN word) when auto increment is used. In OTBN we only support a single increment of either the reg pointer or the dmem pointer directly in the load/store instruction. If both pointers are supposed to be incremented we add an additional ADDI. In the modexp code snippet we need to adjust this (e.g. here) in order to work with byte addressing.

There are two options:

1. Increment the dmem pointer not the reg pointer in the load/store instruction and increment the reg pointer by 1 with the ADDI
2. Keep incrementing the reg pointer but increment the dmem pointer with ADDI by 32

To me, #1 seems to be the cleaner solution and should also be the default when porting code.

[imphil]

Both options work and don't make much of a difference, the addi immediate can deal with the value 32 without trouble. But I agree with you, option 1 seems slightly nicer.