<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 --- - Sparse switches lowered to an else-if chain"
   href="http://llvm.org/bugs/show_bug.cgi?id=18347">18347</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Sparse switches lowered to an else-if chain
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>libraries
          </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>Common Code Generator Code
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedbugs@nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>jn@sirrida.de
          </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 SelectionDAGBuilder::handleBTSplitSwitchCase
(lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp) seems to be a quirk which
can effectively create an else-if chain from a very sparse switch statement
instead of a balanced binary tree.

The reason is that a marginal single switch label is weighted too much causing
it to be split off; a possible solution is to weight it with 0. To do this one
can replace the assignments to LDensity und RDensity by:
    volatile double LDensity =
       (double)(LSize-1).roundToDouble() /
                           (LEnd - First + 1ULL).roundToDouble();
    volatile double RDensity =
      (double)(RSize-1).roundToDouble() /
                           (Last - RBegin + 1ULL).roundToDouble();

I replaced ?Size by (?Size-1).

Here is a more or less stupid test program (translate with "clang -O3 -S
test.cpp" and then look at test.s):

int main(int argc, char **argv) {
  switch (argc) {
    case 100: return 1;
    case 200: return 2;
    case 300: return 3;
    case 400: return 4;
    case 500: return 5;
    case 600: return 6;
    case 700: return 7;
    case 800: return 8;
    case 900: return 9;
    default: return 0;
    }
  }

As far as I can see this quirk is also present in trunk.</pre>
        </div>
      </p>
      <hr>
      <span>You are receiving this mail because:</span>
      
      <ul>
          <li>You are on the CC list for the bug.</li>
      </ul>
    </body>
</html>