[libre-riscv-dev] microwatt decoder tables: M-Form and X-Form switched RS and RB

[Luke Kenneth Casson Leighton]

anton, paul, and team, hi,

we noticed that in microwatt decode1.vhdl tables, M-Form has:

* op1 is RA
* op2 is a constant or RB, which is in the *3rd* field (M-Form) position
* op3 is RS, which is in the *second* Form position

where in X-Form it is:

* op1 is RA
* op2 is a constant or RB, where RB is from the *2nd* field
* op3 is RS (or RC), which is in the 3rd position.

so decode1.vhdl has this, for rldcl:
0b1000,SHIFT_ROT,OP_RLCL,NONE,RB,RS,RA

we're going to flip that to:
0b1000,SHIFT_ROT,OP_RLCL,NONE,RS,RB,RA

this will put the shift-rot pipeline "in line" with the arithmetic
(and logical) ones.

it *might* be the case that if you did the same thing, you would be
able to cut at least one pair of 64-bit MUXes from microwatt, because
RS and RB (2nd and 3rd operands) would no longer be
special-case-swapped for any Shift-Rot operations.  i mention it
because microwatt is intended to be tiny.

best,

l.

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