Register classes

[Daniel Berlin]

A whole lot of ports seem to have register classes that contain 
registers that can't possibly be used together.
They also have to seem weird groupings of registers that don't make 
sense to be used by any single pseudo.

What exactly is the goal of these register classes?
Is it to work around bad allocation decisions by the register 
allocator/regclass (IE get regclass to choose this new class as the 
preferred class so that the allocator makes a better decision later 
on)?

I recently had another discussion with Gregory Chaitan about register 
allocation, and two things he was adamant about (that I was also 
somewhat strong feeling, but not as adamant about during design of 
new-ra):

1. The interference graph should represent all interference constraints 
necessary to do allocation in some way.  IE for variables that need 
multiple registers, this needs to be represented in the interference 
graph in some way (through either extra or weighted edges).
IE you should need nothing but a single bit test to determine whether 
two variables can share a register or not.
New-ra didn't follow this, and it became quite messy to try to 
determine whether two variables actually interfered, and what the 
possible registers we could use for these variables were.  This took 
away most of the benefits of using an interference graph in the first 
place.

2. That a lot of parameterization be done properly, so that costs are 
done right for each thing, and the allocator can make the right 
decisions.


In our case, that means that register classes (if we keep them) should 
represent only one thing, whereas right now it seems they can represent 
one of three things:

1. The set of registers possible for some given mode/usage.
2. The set of registers good (but not all possibly usable registers for 
that mode/usage are included) for some given mode/usage.
3. The set of registers good (but including  unusable registers for 
that mode/usage) are included.

Have I missed anything (I haven't touched new-ra or old-ra in a year or 
so, so it's possibly i've lost it).
--Dan