llvm error: Found more than one landing-pad successor

Question

llvm has a MachineVerifier pass which checks a MachineFunction for correctness based on different parameters like: liveness, dead Basic Block, correct register class etc. Recently I came across this error from the MachineVerifier pass. I'm wondering why is it not okay to have multiple landing pad successors? In the example I am getting this error the layout (IR) is like this;

BB0: 
...
succ: BB1


BB1: EH_LANDING_PAD
pred: BB0, BB1
...
succ: BB1, BB2


BB2: EH_LANDING_PAD
...

 Bad machine code: MBB has more than one landing pad successor ***
   - function:    _Z7ex_testv
   - basic block: BB#1 lpad (0x1bf7c90)
   LLVM ERROR: Found 1 machine code errors.

Does it mean that if an exception occurs, exception handling will be ambiguous. Can someone please explain. Thanks.

Answer 1

Yes. Multiple landing pads are prohibited. landing pad is where the exception is landed by the unwinder. Strictly speaking, there is no problem of handling the landing pads with non-intersecting typeinfo's, but when you have e.g. cleanup or filter you might end in the trouble...