llvm dev - Aug 2019 - How to implement spills into a second bank of registers (?)

Hi All,

For my processor ABI, I have a second bank of registers. The instruction set has
‘mov’ instructions to copy to and from the two register banks. The registers in
the second bank have no regular use in instructions, however they are there for
temporary storage purposes and it would be desirable that they could be used as
spills when there’s shortage of primary registers.

The second bank of registers is defined as a belonging to a separate register
class. I have defined the required ‘mov' instructions to move between
register classes in tablegen. I also updated  copyPhysReg to account for these
moves.

However, LLVM does not seem to use the second register bank at all.

As a side note, the spills to the stack are implemented and they work fine. I
would have expected that LLVM would use the second register bank as backup
storage before creating actual stack spills, but this does not happen.

Why is that?. Should I implement this in some custom way?

any help would be appreciated 

Thanks.