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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 - Clang unnecessarily generates "movl $0, %edi" instead of "xorl %edi, %edi""
href="https://bugs.llvm.org/show_bug.cgi?id=46315">46315</a>
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<th>Summary</th>
<td>Clang unnecessarily generates "movl $0, %edi" instead of "xorl %edi, %edi"
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>10.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>josephcsible@gmail.com
</td>
</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Consider this code:
void h(int, int, int, int, int);
void f(void) {
h(0, 0, 2, 3, 1);
}
When compiled with Clang 10.0.0 on x86-64 Linux, it produces the following
assembly:
f:
movl $0, %edi
xorl %esi, %esi
movl $2, %edx
movl $3, %ecx
movl $1, %r8d
jmp h
<a href="https://godbolt.org/z/ovmjYV">https://godbolt.org/z/ovmjYV</a>
It missed the obvious optimization to use "xorl %edi, %edi" instead of "movl
$0, %edi".
Bizarrely, this is really sensitive to the other values passed to h. For
example, if I did "h(0, 0, 2, 3, 2)" instead of "h(0, 0, 2, 3, 1)", then it
would correctly use "xorl".</pre>
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