Add bit rotate instructions (ROR, ROL)

[mbitsnbites]

Problem

Immediate mode rotation is currently possible to do with two instructions: EBFU + IBF.

    ; Rotate R1 right 7 positions (result in R2).
    ebfu    r2, r1, #<7:25>
    ibf     r2, r1, #<25:7>

Variable (register based) rotation on the other hand is much trickier and requires about five instructions, e.g:

    ; Rotate R1 right the number of positions specified by R2 (result in R1).
    and     r2, r2, #31
    lsr     r3, r1, r2
    sub     r2, #32, r2
    lsl     r1, r1, r2
    or      r1, r1, r3

Bit rotate is common in crypto, and it would be very helpful to have dedicated rotate instructions.

Suggestion

Consider adding:

• ROL ra, rb, rc
• ROR ra, rb, rc
• ROR ra, rb, #imm

Note: We only need one immediate variant since left rotate can easily be expressed as a right rotate when the constant shift amount is known. Possibly we can do without an immediate mode ROR (investigate).

Questions

Encoding of immediate mode ROR?

Using a format C slot wastes lots of encoding space, since we only need 5 of the 15 immediate bits. Can we live without the immediate mode rotate (require an extra LDI), or should we introduce a new encoding?

We could, for instance, use a variant of format A where the 5-bit register specifier is interpreted as an immediate value. This would, however, be a poor fit for MRISC64.

Hardware implications?

How well would the rotate functionality match the MKBF/EBF/IBF hardware? Would we introduce more gate delay?