The documentation says that this is only used for heuristics, but it
is never actually called. This should be removed for now and perhaps
added back later if we find an actual use for it.
- Support preferred and non-preferred subsets of a register class. This
allows allocating, e.g., caller-saved registers before callee-saved
registers.
- Allow branch blockparam args to start an a certain offset in branch
operands; this allows branches to have other operands too (e.g.,
conditional-branch inputs).
- Allow `OperandOrAllocation` to be constructed from an `Allocation` and
`OperandKind` as well (i.e., an allocation with an use/def bit).
The main enhancement in this commit is support for reference types and
stackmaps. This requires tracking whether each VReg is a "reference" or
"pointer". At certain instructions designated as "safepoints", the
regalloc will (i) ensure that all references are in spillslots rather
than in registers, and (ii) provide a list of exactly which spillslots
have live references at that program point. This can be used by, e.g., a
GC to trace and possibly modify pointers. The stackmap of spillslots is
precise: it includes all live references, and *only* live references.
This commit also brings in some API tweaks as part of the in-progress
Cranelift glue. In particular, it makes Allocations and Operands
mutually disjoint by using the same bitfield for the type-tag in both
and choosing non-overlapping tags. This will allow instructions to carry
an Operand for each register slot and then overwrite these in place with
Allocations. The `OperandOrAllocation` type does the necessary magic to
make this look like an enum, but staying in 32 bits.
