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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - llvm-mca does not understand x86 zeroing idioms"
href="https://bugs.llvm.org/show_bug.cgi?id=41982">41982</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>llvm-mca does not understand x86 zeroing idioms
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</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>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>llvm-mca
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>brennan.vincent@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>andrea.dibiagio@gmail.com, llvm-bugs@lists.llvm.org, matthew.davis@sony.com
</td>
</tr></table>
<p>
<div>
<pre>This seems similar: <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED FIXED - [llvm-mca][x86] Teach how to identify optimizable register-register moves and zero-idioms."
href="show_bug.cgi?id=36671">https://bugs.llvm.org/show_bug.cgi?id=36671</a>
It was marked fixed, but llvm-mca is still not recognizing xor as a zeroing
idiom in this simple test case. Note that the second `divl` has to wait for the
first to finish, despite the outputs being zeroed.
```
~ ❯❯❯ cat asm.s
xorl %eax, %eax
xorl %edx, %edx
divl %edi
~ ❯❯❯ ~/clangmr/bin/llvm-mca -timeline -timeline-max-cycles=200 ~/asm.s
✘ 130
<snip>
Timeline view:
0123456789 0123456789 0123456789
0123456789 0123456789 0123456789 0123456789
0123456789
Index 0123456789 0123456789 0123456789
0123456789 0123456789 0123456789 0123456789
0123456789 01
[0,0] DeER . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
. . .. xorl %eax, %eax
[0,1] DeER . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
. . .. xorl %edx, %edx
[0,2]
.DeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeER.
. . . . . . . . . . . . . . .
.. divl %edi
[1,0] .
.D=======================================================================eER
. . . . . . . . . . . . . . . ..
xorl %eax, %eax
[1,1] .
.D=======================================================================eER
. . . . . . . . . . . . . . . ..
xorl %edx, %edx
[1,2] . .
D=======================================================================eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeER
.. divl %edi
[2,0] . . .
D==============================================================================================================================================eER
.. xorl %eax, %eax
[2,1] . . .
D==============================================================================================================================================eER
.. xorl %edx, %edx
```</pre>
</div>
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