<html>

    <head>

      <base href="http://llvm.org/bugs/" />

    </head>

    <body><table border="1" cellspacing="0" cellpadding="8">

        <tr>

          <th>Bug ID</th>

          <td><a class="bz_bug_link 

          bz_status_NEW "

   title="NEW --- - A code flaw found in function CodeGenRegBank::computeRegUnitSets"

   href="http://llvm.org/bugs/show_bug.cgi?id=20122">20122</a>

          </td>

        </tr>

        <tr>

          <th>Summary</th>

          <td>A code flaw found in function CodeGenRegBank::computeRegUnitSets

          </td>

        </tr>

        <tr>

          <th>Product</th>

          <td>new-bugs

          </td>

        </tr>

        <tr>

          <th>Version</th>

          <td>3.3

          </td>

        </tr>

        <tr>

          <th>Hardware</th>

          <td>PC

          </td>

        </tr>

        <tr>

          <th>OS</th>

          <td>Linux

          </td>

        </tr>

        <tr>

          <th>Status</th>

          <td>NEW

          </td>

        </tr>

        <tr>

          <th>Severity</th>

          <td>normal

          </td>

        </tr>

        <tr>

          <th>Priority</th>

          <td>P

          </td>

        </tr>

        <tr>

          <th>Component</th>

          <td>new bugs

          </td>

        </tr>

        <tr>

          <th>Assignee</th>

          <td>unassignedbugs@nondot.org

          </td>

        </tr>

        <tr>

          <th>Reporter</th>

          <td>wuhui1973@163.com

          </td>

        </tr>

        <tr>

          <th>CC</th>

          <td>llvmbugs@cs.uiuc.edu

          </td>

        </tr>

        <tr>

          <th>Classification</th>

          <td>Unclassified

          </td>

        </tr></table>

      <p>

        <div>

        <pre>In function CodeGenRegBank::computeRegUnitSets, below code fragment is used to

calculate the transitive closure of set union of intersecting sets.

  // Iterate over all unit sets, including new ones added by this loop.

  unsigned NumRegUnitSubSets = RegUnitSets.size();

  for (unsigned Idx = 0, EndIdx = RegUnitSets.size(); Idx != EndIdx; ++Idx) {

    // In theory, this is combinatorial. In practice, it needs to be bounded

    // by a small number of sets for regpressure to be efficient.

    // If the assert is hit, we need to implement pruning.

    assert(Idx < (2*NumRegUnitSubSets) && "runaway unit set inference");

    // Compare new sets with all original classes.

    for (unsigned SearchIdx = (Idx >= NumRegUnitSubSets) ? 0 : Idx+1;

         SearchIdx != EndIdx; ++SearchIdx) {

      std::set<unsigned> Intersection;

      std::set_intersection(RegUnitSets[Idx].Units.begin(),

                            RegUnitSets[Idx].Units.end(),

                            RegUnitSets[SearchIdx].Units.begin(),

                            RegUnitSets[SearchIdx].Units.end(),

                            std::inserter(Intersection, Intersection.begin()));

      if (Intersection.empty())

        continue;

      // Speculatively grow the RegUnitSets to hold the new set.

      RegUnitSets.resize(RegUnitSets.size() + 1);

      RegUnitSets.back().Name =

        RegUnitSets[Idx].Name + "+" + RegUnitSets[SearchIdx].Name;

      std::set_union(RegUnitSets[Idx].Units.begin(),

                     RegUnitSets[Idx].Units.end(),

                     RegUnitSets[SearchIdx].Units.begin(),

                     RegUnitSets[SearchIdx].Units.end(),

                     std::inserter(RegUnitSets.back().Units,

                                   RegUnitSets.back().Units.begin()));

      // Find an existing RegUnitSet, or add the union to the unique sets.

      std::vector<RegUnitSet>::const_iterator SetI =

        findRegUnitSet(RegUnitSets, RegUnitSets.back());

      if (SetI != llvm::prior(RegUnitSets.end()))

        RegUnitSets.pop_back();

    }

  }

However, the use of EndIdx like that is not correct, it makes the for loop

exiting too early without handling the new added sets. The correct way should

be: