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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 - Missing optimization with (var&const)==const"
href="https://bugs.llvm.org/show_bug.cgi?id=47797">47797</a>
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<th>Summary</th>
<td>Missing optimization with (var&const)==const
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>blubban@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>$ clang -Oz
void t();
void x(int a) { if ((a&1234) == 1234) t(); }
void y(int a) { if (!(1234&~a)) t(); }
Output (x86_64):
x(int): # @x(int)
mov eax, 1234
and edi, eax
cmp edi, eax
je t() # TAILCALL
ret
y(int): # @y(int)
mov eax, 1234
and edi, eax
cmp edi, eax
je t() # TAILCALL
ret
Expected output: `not edi; test edi,1234; je t(); ret` for both, it's one byte
smaller.
It intuitively seems faster too, since and+cmp calculates a less-than flag that
not+test doesn't, but modern x86 performance is quite unintuitive, so who
knows.
Compiler Explorer: <a href="https://godbolt.org/z/Yeofdo">https://godbolt.org/z/Yeofdo</a></pre>
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</p>
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