Inter register constraints

[Paulo Matos]

Hi,

I am convinced that what I am about to ask is not possible but I would like someone to confirm this.

Can I define in an insn a register constraint that depends on another register value?
So, for 
(set (match_operand:SI 0 "register_operand" "r")
     (match_operand:SI 1 "register_operand" "r"))

What I would like to represent is the fact that 0 and 1 are register pairs, so op0 is even and op1 is op0 + 1.
I noticed that avr defines register pair constraints but only for specific registers. If you have 64 registers that will give you 22 pairs. I could, of course, create all of these by hand by defining 23 classes and define a single constraint that matches these classes but I would like to know if there's another way. 

Cheers,

Paulo Matos