llvm dev - Apr 2012 - [LLVMdev] Should repetitive basic blocks be removed in the results of LoopBase::getExitBlocks()?

Hi, all. I'm using void LoopBase::getExitBlocks (SmallVectorImpl< BlockT
*
> &ExitBlocks) const to get all
exit blocks for a loop. The problem I find with this API is that it returns
repetitive basic blocks in certain
situations. Should repetitive basic blocks be removed?

I have an example to show the problem. Following is the source code and the
CFG is enclosed.
int main()
{
  int i = rand()%10;

  while ( i < 20 )
    {
      i++;
      if ( i == 5)
return 21;
      if ( i == 9)
continue;
      else if ( i == 10)
break;
    }
  return i;
}

If you use getExitBlocks() to get the exit basic blocks for the loop, the
block "10" will show up twice in the
results.

Xiaoming
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On Apr 19, 2012, at 10:43 PM, xiaoming gu <xiaoming.gu at gmail.com>
wrote:
> Hi, all. I'm using void LoopBase::getExitBlocks (SmallVectorImpl<
BlockT * > &ExitBlocks) const to get all
> exit blocks for a loop. The problem I find with this API is that it returns
repetitive basic blocks in certain
> situations. Should repetitive basic blocks be removed?
Users generally expect a unique set of exit blocks, but don't make any
strong assumption. The worst that can happen is missed optimization or redundant
analysis. In most cases, the loop is in LoopSimplifyForm, so it's probably
not a problem in practice.

-Andy

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On Apr 23, 2012, at 10:31 PM, Andrew Trick <atrick at apple.com> wrote:
> 
> On Apr 19, 2012, at 10:43 PM, xiaoming gu <xiaoming.gu at gmail.com>
wrote:
> 
>> Hi, all. I'm using void LoopBase::getExitBlocks
(SmallVectorImpl< BlockT * > &ExitBlocks) const to get all
>> exit blocks for a loop. The problem I find with this API is that it
returns repetitive basic blocks in certain
>> situations. Should repetitive basic blocks be removed?
> 
> Users generally expect a unique set of exit blocks, but don't make any
strong assumption. The worst that can happen is missed optimization or redundant
analysis. In most cases, the loop is in LoopSimplifyForm, so it's probably
not a problem in practice.
Another thing I should mention. The iteration order of ExitBlocks is important.
In llvm, generating unique sets is a pain because values are not numbered and
iteration order needs to be reproducible. We would need to keep a SmallPtrSet
for membership checking while populating the result vector.

-Andy
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