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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [ValueTracking] try harder to find 'nuw'"
href="https://bugs.llvm.org/show_bug.cgi?id=38021">38021</a>
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<th>Summary</th>
<td>[ValueTracking] try harder to find 'nuw'
</td>
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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>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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</tr></table>
<p>
<div>
<pre>As discussed in:
<a href="https://reviews.llvm.org/D48662">https://reviews.llvm.org/D48662</a>
computeOverflowForUnsignedAdd() doesn't have the same level of analysis as
computeOverflowForSignedAdd(), so in this example:
define i8 @no_overflow(i8 %x) {
%clear_top_3_bits = lshr i8 %x, 3
%add = add i8 %clear_top_3_bits, 193 ; 0b11000001
ret i8 %add
}
...we determine that the add is 'nsw', but not 'nuw':
$ ./opt -instcombine addnuw.ll -S
define i8 @no_overflow(i8 %x) {
%clear_top_3_bits = lshr i8 %x, 3
%add = add nsw i8 %clear_top_3_bits, -63
ret i8 %add
}
<a href="https://rise4fun.com/Alive/lCJ">https://rise4fun.com/Alive/lCJ</a></pre>
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