Machine description question

[Hariharan]

Hello all,
Picochip has communication instructions that allow one array element to 
pass data to another. There are 3 such instructions PUT/GET/TSTPORT. 
Currently, all three of these use UNSPEC_VOLATILE side-effect 
expressions to make sure they don't get reordered. But, i wonder if it 
is an overkill to use UNSPEC_VOLATILE for this purpose and whether i 
should use UNSPEC instead. The only thing we care here is that they 
don't reordered with respect to each other. It is okay for other 
instructions to move around the communication instructions (as long as 
normal scheduler dependencies are taken care of). There are possibly one 
of two things i can do.

1. Introduce an implicit dependency between all communication 
instructions by adding a use/clobber of an imaginary register.
2. Introduce explicit dependency between them by using some target hook 
to add dependency links. I have not found any appropriate target hook to 
do this.

Can you tell me which one i should try? Has anyone tried doing anything 
similar? Any pointers/suggestions on this will be greatly appreciated.

Thanks
Hari