Hello LLVMers!
I recently added a generic machine rewriter/spiller that global register
allocator writers may find useful.
The API provided is the free function:
void eliminateVirtRegs(MachineFunction&, const VirtRegMap&)
declared in lib/CodeGen/VirtRegMap.h and implemented in
lib/CodeGen/VirtRegMap.cpp.
This function rewrites the machine code of MachineFunction to eliminate
virtual registers and adds spill code as specified by VirtRegMap. VirtRegMap
provides a mapping from virtual registers to physical registers and stack
slots. This object is what a global register allocator must populate in order
to use the generic spiller. Given a VirtRegMap, after the call to
eliminateVirtRegs(), all virtual register references are eliminated and spill
code is added to the instruction stream. Yes it is that simple 
Now, what should a VirtRegMap provide for this to work?
1) Each virtual register must (in general*) have a mapping to a physical
register. The spiller will replace all references to this virtual register
with the mapped physical register.
2) Each spilled virtual register must also have a mapping to a stack slot. The
spiller will use the physical register mapping for loads/stores from/to this
stack slot.
* I say in general because in architectures that support it, folding memory
operands into instructions may eliminate all references to a virtual register
(x86 for example). Obviously in that case a mapping to a physical register
need not be provided.
Hopefully most global register allocator writers will find this useful with
little or no modifications. Let me know if you have any questions, concerns
or suggestions!