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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - umulll_overflow with constant powers of two are not well optimized"
href="https://llvm.org/bugs/show_bug.cgi?id=28426">28426</a>
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<th>Summary</th>
<td>umulll_overflow with constant powers of two are not well optimized
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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>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>jmuizelaar@mozilla.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>bool k(unsigned long long i) {
unsigned long long result;
return __builtin_umulll_overflow(i, 4, &result);
}
compiles to:
movl $4, %ecx
movq %rdi, %rax
mulq %rcx
seto %al
retq
A better alternative would be this code:
bool k(unsigned long long i)
{
return i >> (64-2);
}
which compiles to:
shrq $62, %rdi
setne %al
retq
This idiom comes from compiled rust code which does an overflow check when
allocating Vecs so it is quite prevalent.</pre>
</div>
</p>
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