Generating the mapping from a register class to a register bank is
complex:
- there can be lots of register classes
- the mapping may be ambiguos
- a register class can span several register banks (e.g. a register
class containing all registers)
- the type information is not enough to decide which register bank to
map to (e.g. a register class containing floating point and vector
registers, and all register can represent a f64 value)
The approach taken here is to encode the register banks in an array
indexed by the ID of the register class. To save space, the entries are
packed into chunks of size 2^n.
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