[LLVMdev] Possible LiveInterval Bug

[David Greene]

I just ran into a problem here.  I'm in SimpleRegisterCoalescing at the point
where EXTRACT_SUBREG coalescing updates live ranges of aliased
registers (around line 473 of SimpleRegisterCoalescing.cpp).

There's a call to MergeValueInAsValue at line 50.  MergeValueInAsValue has
this code:

void LiveInterval::MergeValueInAsValue(const LiveInterval &RHS,
                                     const VNInfo *RHSValNo, VNInfo *LHSValNo) 
{
  SmallVector<VNInfo*, 4> ReplacedValNos;
  iterator IP = begin();
  for (const_iterator I = RHS.begin(), E = RHS.end(); I != E; ++I) {
    if (I->valno != RHSValNo)
      continue;
    unsigned Start = I->start, End = I->end;
    IP = std::upper_bound(IP, end(), Start);
    // If the start of this range overlaps with an existing liverange, trim 
it.
    if (IP != begin() && IP[-1].end > Start) {
      if (IP->valno != LHSValNo) {
        ReplacedValNos.push_back(IP->valno);

What happens if IP == end()?  We're pushing bogus information onto
ReplacedvalNos (IP->valno is not valid).

Is this a bug or should IP never be end() at this point?  If the latter is
true then I've got a bug somewhere else I need to hunt down.

                                                   -Dave