<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 --- - InstCombine: '(x lshr C1) udiv C2 --> x udiv (C2 << C1)' incorrect if C2<<C1 overflows"
   href="http://llvm.org/bugs/show_bug.cgi?id=21255">21255</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>InstCombine: '(x lshr C1) udiv C2 --> x udiv (C2 << C1)'  incorrect if C2<<C1 overflows
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>libraries
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>trunk
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>All
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>All
          </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>Scalar Optimizations
          </td>
        </tr>

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

        <tr>
          <th>Reporter</th>
          <td>nunoplopes@sapo.pt
          </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>The following transformation in InstCombiner::visitUDiv() is incorrect if
C2<<C1 overflows (and btw correct otherwise):

; (x lshr C1) udiv C2 --> x udiv (C2 << C1)
%Op0 = lshr %X, C1
%r = udiv %Op0, C2
  =>
%r = udiv %X, (C2 << C1)

Done: 1
ERROR: Domain of definedness of Target is smaller than Source's for i2 %r

Example:
%X i2 = 0x0 (0)
C1 i2 = 0x1 (1)
C2 i2 = 0x2 (2, -2)
%Op0 i2 = 0x0 (0)
Source value: 0x0 (0)
Target value: undef



And bypassing the undef case:

Pre: ((C2 << C1) != 0)
%Op0 = lshr exact %X, C1
%r = udiv %Op0, C2
  =>
%r = udiv %X, (C2 << C1)


ERROR: Mismatch in values of i4 %r

Example:
%X i4 = 0x2 (2)
C1 i4 = 0x1 (1)
C2 i4 = 0x9 (9, -7)
%Op0 i4 = 0x1 (1)
Source value: 0x0 (0)
Target value: 0x1 (1)



And finally we have:

Pre: WillNotOverflowUnsignedShl(C2, C1)
%Op0 = lshr %X, C1
%r = udiv %Op0, C2
  =>
%r = udiv %X, (C2 << C1)

Done
Optimization is correct!</pre>
        </div>
      </p>
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