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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [InstCombine] Preferred shift direction in bit test"
href="https://bugs.llvm.org/show_bug.cgi?id=42466">42466</a>
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<th>Summary</th>
<td>[InstCombine] Preferred shift direction in bit test
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Scalar Optimizations
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>As discussed in <a href="https://reviews.llvm.org/D63829">https://reviews.llvm.org/D63829</a>
These two patterns are identical:
define i1 @v0(i32 %x, i32 %y, i32 %z) {
%t0 = lshr i32 %x, %y
%t2 = and i32 %t0, %z
%t3 = icmp ne i32 %t2, 0
ret i1 %t3
}
define i1 @v1(i32 %x, i32 %y, i32 %z) {
%t0 = shl i32 %y, %y
%t2 = and i32 %t0, %x
%t3 = icmp ne i32 %t2, 0
ret i1 %t3
}
We should canonicalize to one of them.
Probably to @v0.
With one obvious exception of `(1 << y) & x`.
<a href="https://godbolt.org/z/KPgPPQ">https://godbolt.org/z/KPgPPQ</a>
<a href="https://rise4fun.com/Alive/0avd">https://rise4fun.com/Alive/0avd</a></pre>
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