[LLVMdev] Possible bug in LiveIntervalAnalysis?

[Roman Levenstein]

Hi,

In the LiveIntervalAnalysis::runOnMachineFunction, there is a code to
compute the MBB2IdxMap, by remembering for each MBB its start and end
instruction numbers:

  unsigned MIIndex = 0;
  for (MachineFunction::iterator MBB = mf_->begin(), E = mf_->end();
       MBB != E; ++MBB) {
    unsigned StartIdx = MIIndex;

    for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
         I != E; ++I) {
      bool inserted = mi2iMap_.insert(std::make_pair(I,
MIIndex)).second;
      assert(inserted && "multiple MachineInstr -> index mappings");
      i2miMap_.push_back(I);
      MIIndex += InstrSlots::NUM;
    }

    // Set the MBB2IdxMap entry for this MBB.
    MBB2IdxMap[MBB->getNumber()] = std::make_pair(StartIdx, MIIndex -
1);

 For empty MBBs, the last line would create a pair, where the end
element is smaller than the start element, which is a bit
counter-intuitive. This may lead to some assertion failures in other
parts of the code generator, if they use MBB2IdxMap for checking is an
instruction belongs to a given MBB.

 Question: Is this way of handling empty MBBs in the
LiveIntervalAnalysis a bug or a feature? Any special reason, why the
pair for an empty block does not look e.g. like (start, start)?

 -Roman


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