Inter register constraints

[Paulo Matos]

> -----Original Message-----
> From: Georg-Johann Lay [mailto:avr@gjlay.de]
> Sent: 05 July 2013 18:03
> To: Paulo Matos
> Cc: gcc@gcc.gnu.org
> Subject: Re: Inter register constraints
> 
> > 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.
> 
> What are you trying to achieve?  In order so synthesize MOVW
> instructions after reload, see respective RTL peepholes.
> 

Thanks Johann, that seems to be pretty much what I want to do.
However when, in avr, there's a HI move of a register pair only the first register shows up in the instruction.
So movw r0:r1, r2:r3

is a *movhi: (set (reg:HI r0) (reg:HI r2))

How does gcc know that r1 is going to be clobbered?

Is it because GET_MODE_SIZE (HImode) is twice the register size and so it assumed the following register is clobbered as well? (or is there any hook that needs to be set)

Paulo Matos