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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - InstCombine: shift amount reassociation when doing bit test"
href="https://bugs.llvm.org/show_bug.cgi?id=42399">42399</a>
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<th>Summary</th>
<td>InstCombine: shift amount reassociation when doing bit test
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<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>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
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<pre><a href="https://godbolt.org/z/Kq82aC">https://godbolt.org/z/Kq82aC</a>
<a href="https://rise4fun.com/Alive/kBb">https://rise4fun.com/Alive/kBb</a>
bool bad(unsigned x, unsigned y, unsigned skip0, unsigned skip1) {
x >>= skip0;
return x & (y << skip1);
}
bool good(unsigned x, unsigned y, unsigned skip0, unsigned skip1) {
return x & (y << (skip1+skip0));
}
Name:
Pre: C1+C2 u< 32
%o4 = lshr i32 %x, C1
%o5 = shl i32 %y, C2
%o6 = and i32 %o5, %o4
%r = icmp ne i32 %o6, 0
=>
%n4 = add i32 C1, C2
%n5 = shl i32 %y, %n4
%n6 = and i32 %n5, %x
%r = icmp ne i32 %n6, 0
Still from real code..</pre>
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</p>
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