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    <head>
      <base href="https://llvm.org/bugs/" />
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    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW --- - AVX512: We don't always need EVEX for VEX-encodable instructions."
   href="https://llvm.org/bugs/show_bug.cgi?id=23376">23376</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>AVX512: We don't always need EVEX for VEX-encodable instructions.
          </td>
        </tr>

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

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

        <tr>
          <th>Hardware</th>
          <td>PC
          </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>Backend: X86
          </td>
        </tr>

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

        <tr>
          <th>Reporter</th>
          <td>ahmed.bougacha@gmail.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>This:

define float @t(float %a, float %b) {
  %res = fadd float %a, %b
  ret float %res
}

Generates the EVEX-prefixed:

    62f17e0858c1        vaddss    %xmm1, %xmm0, %xmm0

instead of using VEX:

    c5fa58c1            vaddss    %xmm1, %xmm0, %xmm0


AFAIK, there is no transition penalty between AVX512 and AVX, so I believe the
only reason to use EVEX in this case is to access more registers (xmm0-31).

That's not always useful, so we should recognize that, and relax EVEX->VEX when
possible.</pre>
        </div>
      </p>
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