[LLVMdev] Problem using a label to a basic block

[Jeroen Dobbelaere]

Hi everybody,

I am creating an llvm backend for a custom architecture, and I am struggling
with the generation of a 'call to register', used when doing a call
through a function pointer.

The problem is the following:
- the architecture uses a link register R15 for function calls.
- But, it has no special 'branch and link' instruction that accepts a register destination.

I try to implement this by replacing a call to register by:
- splitting the basic block around the call. 
- replacing the call (which is at the end of the first basic block) with :
-- Loading the link register with the address of the new basic block
-- and generating a 'CALL_R' which in the end results in a 'branch to register'
   (but, for llvm, it is treated as a call)

So:

 __BB_2:
  ...
  call_to_reg Rx
  ...

results in:

 __BB_2:
  ...
  load_immediate R15, __BB_3
  branch_to_reg Rx
 __BB_3:
  ...

where __BB_2 falls through to __BB__3

This works when the optimization level is low.
At higher optimization levels I am observing the following :

  load_immediate R15, __BB_3
  branch_to_reg Rx
 __BB_3:
  branch_to __BB_22

is converted into 

  load_immediate R15, "__BB_-1" // basic block 3 was removed
  branch_to_reg Rx
 __BB_22:
  ...

How do I learn my backend that, when the successor of __BB_2 changes,
that it also needs to update the 'load_immediate' with the new successor ?
(Or maybe, there is a different way to achieve this ?)

Thanks,

Jeroen Dobbelaere