[llvm-bugs] [Bug 38689] New: biasCriticalPath does not find the critical path consistently

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38689

            Bug ID: 38689
           Summary: biasCriticalPath does not find the critical path
                    consistently
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: gongyuwang at micron.com
                CC: llvm-bugs at lists.llvm.org

SUnit::biasCriticalPath() is a simple function that finds the deepest
Predecessor and swap it with the head of this->Preds vector.

Since the code is short, it is probably easier to explain with the code:
void SUnit::biasCriticalPath() {
  if (NumPreds < 2)
    return;

  SUnit::pred_iterator BestI = Preds.begin();
  unsigned MaxDepth = BestI->getSUnit()->getDepth();
  for (SUnit::pred_iterator I = std::next(BestI), E = Preds.end(); I != E;
       ++I) {
    if (I->getKind() == SDep::Data && I->getSUnit()->getDepth() > MaxDepth)
      BestI = I;  // Need to update MaxDepth

  }
  if (BestI != Preds.begin())
    std::swap(*Preds.begin(), *BestI);
}

As the comment in the above code suggest, after BestI is updated, MaxDepth (=
BestI->getSUnit()->getDepth()) should also be updated. But the current code
does not do this, which would fail to find the correct Predecessor
consistently. For example, if this SUnit has three Predecessors (the first
Predecessor has depth 1; the second and third Predecessors are both Data
dependences with depths of 3 and 2 respectively), the above code would fail to
find the second Predecessor as part of the critical path.

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