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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 known bits info for x*x"
href="https://bugs.llvm.org/show_bug.cgi?id=48683">48683</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missing known bits info for x*x
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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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>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>bool foo(unsigned x) {
return (x * x) & 2;
}
Square is always congruent to 0 or 1 mod 4 so we can fold it to return false
(For knownbits, second LSB is not set).
define i1 @src(i32 %0) {
%1:
%2 = mul i32 %0, %0
%3 = and i32 %2, 2
%4 = icmp ne i32 %3, 0
ret i1 %4
}
=>
define i1 @tgt(i32 %0) {
%1:
ret i1 0
}
Transformation seems to be correct!
<a href="https://en.wikipedia.org/wiki/Quadratic_reciprocity">https://en.wikipedia.org/wiki/Quadratic_reciprocity</a></pre>
</div>
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