After finding a register solution, it need to be executed as a sequence
of regmove instructions. This often requires a topological ordering of
the moves so they don't conflict.
When the solution contains cycles, try to grab an available scratch
register to implement the copies. Panic if that fails (later, we'll
implement emergency spilling in this case).
Make sure we handle odd aliasing in the arm32 floating point register
bank. Not everything is a simple cycle in that case, so make sure we
don't assume so.
Add an abi module with code that is probably useful to all ISAs when
implementing this function.
Add a unit() method to RegClassData which can be used to index the
register units in a class.
An SSA value is usually biased towards a specific register class or a
stack slot, depending on the constraints of the instructions using it.
Represent this bias as an Affinity enum, and implement a merging
algorithm for updating an affinity to satisfy a new constraint.
Affinities will be computed as part of the liveness analysis. This is
not implemented yet.
Ensure that the set of register classes is closed under intersection.
Provide a RegClass::intersect() method which finds the register class
representing the intersection of two classes.
Generate a bit-mask of subclasses for each register class to be used by
the intersect() method.
Ensure that register classes are sorted topologically. This is also used
by the intersect() method.
This set of available register units also manages register aliasing in
an efficient way.
Detect if the units in a register straddles mask words. The algorithm
for allocating multi-unit registers expect the whole register to be
inside a single mask word. We could handle this if necessary, but so far
no ISAs need it.
