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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 - Missed canonicalization for bit mask calculation"
href="https://bugs.llvm.org/show_bug.cgi?id=37603">37603</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed canonicalization for bit mask calculation
</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>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<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><a href="https://godbolt.org/g/VCMNpS">https://godbolt.org/g/VCMNpS</a>
<a href="https://rise4fun.com/Alive/idM">https://rise4fun.com/Alive/idM</a>
int mask_signed_add(int nbits) {
return (1 << nbits) - 1;
}
unsigned mask_unsigned_add(unsigned nbits) {
return (1 << nbits) - 1;
}
int mask_signed_xor(int nbits) {
return ~(-(1 << nbits));
}
unsigned mask_unsigned_xor(unsigned nbits) {
return ~(-(1 << nbits));
}
define dso_local i32 @mask_signed_add(int)(i32) local_unnamed_addr #0 {
%2 = shl i32 1, %0
%3 = add nsw i32 %2, -1
ret i32 %3
}
define dso_local i32 @mask_unsigned_add(unsigned int)(i32) local_unnamed_addr
#0 {
%2 = shl i32 1, %0
%3 = add nsw i32 %2, -1
ret i32 %3
}
define dso_local i32 @mask_signed_xor(int)(i32) local_unnamed_addr #0 {
%2 = shl i32 -1, %0
%3 = xor i32 %2, -1
ret i32 %3
}
define dso_local i32 @mask_unsigned_xor(unsigned int)(i32) local_unnamed_addr
#0 {
%2 = shl i32 -1, %0
%3 = xor i32 %2, -1
ret i32 %3
}
I think we really would prefer the second variant with `xor`,
because if we calculated mask, it is quite likely that
we will use it in `and` next, and then it can be fused into `andn`..
Thoughs?</pre>
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