[gas] new port advice

[Peter Breuer <peter dot t dot breuer at gmail dot com>]

Is there a pointer for me out there on how to do a little modification
of gas for a certain architecture?

I've read the documentation, googled, perused the source, and made
changes that result in objdump understanding and printing a new
instruction I've added. Now ...  

  I want to change gas's handling of addi (and other instructions with
  immediate data) so that it is treated like a macro that causes three
  new machine code instructions to be emitted instead of just the one
  old one. Thus

  addi r1,r2,0x345 -> newins1 0x3;newins2 0x4;addi r1,r2,0x5

  That should be turned on and off by a directive in the assembler.

Which file(s) do I twiddle?

Yes, I could do this by changing the c compiler to output the intended
assembler in the first place.  I'll try this first.

Advice gratefully received

PTB