IRA conflict graph & alternative selection

[Jeff Law <law at redhat dot com>]

I've been thinking further about instruction alternative selection prior 
to allocation and one of the questions in my mind is how this interacts 
with IRA.

We select an alternative for each insn based on some "best guess" 
heuristic -- the selection of an alternative will often restrict the 
register classes available for each of the operands.  Presumably we'd 
want to encode that information it the conflict graph so that IRA would 
allocate registers so as to fit the constraints of the early insn 
alternative selection.  Right?   In the case where the graph is 
uncolorable, do we allow IRA to override the alternative selection, or 
do we insert copies to simplify the conflict graph or some mixture of both?

Thoughts?

jeff