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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 - [InstCombine] failed to set/use popcount range metadata"
href="https://bugs.llvm.org/show_bug.cgi?id=52094">52094</a>
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</tr>
<tr>
<th>Summary</th>
<td>[InstCombine] failed to set/use popcount range metadata
</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>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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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>
<tr>
<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre><a href="https://alive2.llvm.org/ce/z/c2tUxf">https://alive2.llvm.org/ce/z/c2tUxf</a>
define i32 @src(i32 %0) {
%2 = and i32 %0, 15
%3 = tail call i32 @llvm.ctpop.i32(i32 %2)
%4 = and i32 %3, 5
%5 = icmp eq i32 %4, 1
%6 = zext i1 %5 to i32
ret i32 %6
}
define i32 @tgt(i32 %0) {
%2 = and i32 %0, 15
%3 = tail call i32 @llvm.ctpop.i32(i32 %2)
%4 = and i32 %3, 1
ret i32 %4
}
declare i32 @llvm.ctpop.i32(i32) #2
---------------------------------------------------------------------------
We fail to recognize this as a parity check which could lead to far worse
codegen (especially for a target that does not have a popcount instruction):
IR corresponds to source like this:
int xor4_popcount(int num) {
int masknum = num & 0xf;
int pop = __builtin_popcount(masknum);
return pop == 1 || pop == 3;
}
This example is derived from the post-commit discussion in:
<a href="https://reviews.llvm.org/D110170">https://reviews.llvm.org/D110170</a></pre>
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