Re: Non-contiguous opcodes

["Frank Ch. Eigler" <fche at redhat dot com>]

Hi -

On Thu, Jun 21, 2018 at 09:29:39AM +0200, Christian Eggers wrote:
> [...]
> I would like to port GNU binutils for the NXP i.MX SDMA coprocessor
> (see https://www.nxp.com/docs/en/reference-manual/IMX6ULRM.pdf, 
> page 2682 for instruction set).

Neat.

> In contrast to other ISAs found in the cpu files, the SDMA ISA has it's
> opcode splitted into several variable length field within a fixed size 
> 16 bit instruction code:
> 
> 00000jjj00000000  <- opcode @ 15 5, 7 8
> 00000jjj00000001  <- opcode @ 15 5, 7 8
> 000000ff00000111  <- opcode @ 15 6, 7 8
> 10aaaaaaaaaaaaaa  <- opcode @ 15 2
> 11aaaaaaaaaaaaaa  <- opcode @ 15 2
> ...
> 
> I've tried to solve this with multi-ifields:
> [...]

Without digging into the ISA deeply, nor the two-decade-old memories,
have you considered not doing it that way?  Consider instead treating
the opcode-like subfields separately inside the define-normal-insn.
So ditch the single insn-enum as it is, and instead of:

(define-normal-insn
  revblo "Reverse Low Order Bytes" ()
  "revblo $r"
  (+ OP_REVBLO r)
  [...]
)

try:

(define-normal-insn
  revblo "Reverse Low Order Bytes" ()
  "revblo $r"
  (+ (f-op15x5 17) (f-op7x8 0) r)
  [...]
)


- FChE