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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [InstCombine] Failure to recognize sign bit test if the high bit extract is followed by trunc"
href="https://bugs.llvm.org/show_bug.cgi?id=43564">43564</a>
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<th>Summary</th>
<td>[InstCombine] Failure to recognize sign bit test if the high bit extract is followed by trunc
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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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
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre><a href="https://godbolt.org/z/hzDvD_">https://godbolt.org/z/hzDvD_</a>
Given
int bad(unsigned input, unsigned len) {
unsigned diff = input & ((1 << len)-1);
// If the first bit is 1 we need to turn this into a negative number
if (diff >> (len - 1))
diff -= (1 << len);
return diff;
}
we get
%5 = icmp slt i32 %0, 0
But for
int bad2(unsigned long input, unsigned len) {
unsigned diff = input >> (64 - len);
// If the first bit is 1 we need to turn this into a negative number
if (diff >> (len - 1))
diff -= (1 << len);
return diff;
}
we no longer recognize it, and remain with
%7 = add i32 %1, -1
%8 = lshr i32 %6, %7
%9 = icmp eq i32 %8, 0</pre>
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