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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 opt with ctlz and shifts of power of 2 constants"
href="https://bugs.llvm.org/show_bug.cgi?id=41988">41988</a>
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<th>Summary</th>
<td>Missing opt with ctlz and shifts of power of 2 constants
</td>
</tr>
<tr>
<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>All
</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>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>Matthew.Arsenault@amd.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>It should be possible to eliminate a ctlz of a shift of a power of 2 by turning
it into an add:
declare i32 @llvm.ctlz.i32(i32, i1)
define i32 @foo(i32 %log2) {
%div = lshr i32 8387584, %log2
%ctlz = call i32 @llvm.ctlz.i32(i32 %div, i1 false)
ret i32 %ctlz
}
define i32 @bar(i32 %log2) {
%ctlz.const = call i32 @llvm.ctlz.i32(i32 8387584, i1 false)
%add = add i32 %log2, %ctlz.const
ret i32 %add
}
define void @test() {
%foo = call i32 @foo(i32 6)
store volatile i32 %foo, i32* undef
%bar = call i32 @bar(i32 6)
store volatile i32 %bar, i32* undef
ret void
}</pre>
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</p>
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