[LLVMdev] An unexpected behavior in RegionInfo's block_iterator

[Paul Vario]

Hi Tobias,

     Thanks so much for the quick response. Your approach fixes the issue.
     On a bigger context, would it make more sense to make the region exit
part of the region? For example, a while loop gets lowered down to LLVM IR
contains while.cond, while.body and while.end. If one tries to use
RegionInfo as a substitute for structural analysis, she might think
while.end should belong to the region/structure ... is it necessary to
exclude the exit from being part of the region or both ways are equally
correct in terms of uniquely representing regions?

Best,
Paul


[image: Inline image 1]


On Fri, May 2, 2014 at 4:42 PM, Tobias Grosser <tobias at grosser.es> wrote:

> 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
>
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