[LLVMdev] request for help writing a register allocator

[Susan Horwitz]

I'm having no luck getting my register allocator to work.  I'm trying to 
do it using the "indirect" approach; i.e., using a VirtRegMap, with calls 
to assignVirt2Phys, assignVirt2StackSlot, etc. and a call to a "spiller" 
at the end.

As a warm-up exercise (before implementing register allocation via graph 
coloring) I'm trying to implement a very simple scheme in which NO 
pseudo-registers are allocated to physical registers across instructions. 
So for each virtual register I call assignVirt2StackSlot.  I thought that 
I should call assignVirt2Phys as well for each virtual register in an 
instruction, mapping that vreg to an unused preg.  For example, for an 
instruction like this:
 	%reg1024<def> = MOV32rr %ESP
I assign virtual register 1024 to some available physical register in the 
appropriate class so that the generated code will assign to that physical 
register.  (I also assign virtual register 1024 to a stack slot so that 
the value assigned to the physical register is spilled to that slot.)

If a virtual register is used in an instruction, rather than defined, I 
again assign it to a physical register preg (but not to a stack slot 
because it will already have been assigned a slot) so that the generated 
code will load from the stack into that preg.

However, if the same virtual register is used in two different 
instructions, I get an abort with an error message saying that I'm trying 
to assign a physical register to a virtual register that's already been 
mapped.  If I don't call assignVirt2Phys then I get an abort with an error 
message saying that some virtual register was not assigned a physical 
register.

I've tried calling addSpillPoint and addRestorePoint for every instruction 
that involves a virtual register, and also not calling those functions, 
and I still get the same aborts.

I would very much appreciate any help!

Susan Horwitz