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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - InstCombine: lshr + "sext" => ashr canonicalization"
href="https://bugs.llvm.org/show_bug.cgi?id=42389">42389</a>
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<th>Summary</th>
<td>InstCombine: lshr + "sext" => ashr canonicalization
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<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>
</tr>
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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
</td>
</tr>
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I have not triaged yet what folds are missing, but this gigantic fold is
missing:
<a href="https://godbolt.org/z/E9-txU">https://godbolt.org/z/E9-txU</a>
int bad(unsigned input, unsigned len) {
unsigned diff = input >> (32 - 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;
}
int good(unsigned input, unsigned len) {
return int(input) >> (32 - len);
}
<a href="https://rise4fun.com/Alive/l3k">https://rise4fun.com/Alive/l3k</a>
Name: ashr
%o3 = sub i32 32, %nbits
%o4 = lshr i32 %x, %o3
%o5 = add i32 %nbits, -1
%o6 = lshr i32 %o4, %o5
%o7 = icmp eq i32 %o6, 0
%o8 = shl i32 1, %nbits
%o9 = select i1 %o7, i32 0, i32 %o8
%r = sub i32 %o4, %o9
=>
%n0 = sub i32 32, %nbits
%r = ashr i32 %x, %n0
I'm seeing this in real code.</pre>
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</p>
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