[LLVMdev] An unexpected behavior in RegionInfo's block_iterator

[Tobias Grosser]

On 02/05/2014 23:26, Paul Vario wrote:
> Hi Fellows,
>
>     I notice an unexpected behavior in RegionInfo's block_iterator. Consider
> the following situation: a user creates her own region containing just a
> single basic block TheBB, then tries to have the block_iterator to print a
> DFS traversal of the region. The expected behavior should be that only the
> single basic block TheBB will be printed, but the real behavior is that
> block_iterator prints out all the basic blocks of the CFG starting from
> TheBB to the end.
>
>      It looks like the issue originates from setting the end iterator to
> (BasicBlock *) 0. I understand that the "region" detected by RegionInfo
> should never contain a single basic block or even a sequence of basic
> blocks. So maybe the above degenerated case is considered "will never
> happen"?
>
>      ... ...
>
>      BasicBlock *BB = Func.getARandomBasicBlock;
>
>      Region *R = new Region(BB, BB, RI, DT);
>      for (Region::block_iterator i = R->block_begin(),
>             e = R->block_end(); i != e; ++i) {
>           errs() << (*i)->getName() << "\n";
>        }


Hi Paul,

the exit basic block is the block after the region.

  BB0 < Entering Block
   |
   v
  BB1 < Entry and Exiting block
   |
   v
  BB2 < Exit block

So your code should probably be:

new Region(BB, *pred_begin(BB), RI, DT)

Let me know if it works for you. In case it does, we could add an assert 
to the constructor to catch these cases.

Cheers,
Tobias