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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Failure to optimize integer conversion pattern"
href="https://bugs.llvm.org/show_bug.cgi?id=48692">48692</a>
</td>
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<tr>
<th>Summary</th>
<td>Failure to optimize integer conversion pattern
</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>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>gabravier@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>int64_t f(int64_t x)
{
return ((x << 48 >> 48) & -65536) | (x & 65535);
}
This can be optimized to `return (x << 48 >> 48);` (i.e. `(int16_t)x`), see
also Alive2 test here: <a href="https://alive2.llvm.org/ce/z/L7z-JQ">https://alive2.llvm.org/ce/z/L7z-JQ</a>
Should be generalizable to shifts/and patterns of any size.
Note: I originally found this while investigating how GCC and LLVM optimize
this piece of code:
union reg {
uint64_t r;
uint16_t x;
};
reg cbw(reg dest)
{
dest.r = (int16_t)dest.x;
return dest;
}
Which (on little endian targets) GCC optimizes to `return (int16_t)x` and LLVM
optimizes to the sample code I gave above.</pre>
</div>
</p>
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