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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 optimization for ashr with zero or all ones as LHS"
href="https://bugs.llvm.org/show_bug.cgi?id=52190">52190</a>
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<th>Summary</th>
<td>Missed optimization for ashr with zero or all ones as LHS
</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>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>LLVM does optimize these cases - remove ashr:
int ok(__int128 f, int g)
{
return (f >> 127) >> g;
}
int ok2(int f, int g)
{
return (f >> 31) >> g;
}
int ok3(int f, int g)
{
if (f == 0 || f == 1)
return (-f) >> g;
__builtin_unreachable();
}
But not these:
int bad1(int f, int g)
{
if (f == 0 || f == -1)
return f >> g;
__builtin_unreachable();
}
int bad2(int f, int g)
{
if (f == 0 || f == 1)
return (f-1) >> g;
return 0;
}
int bad3(int f, int g)
{
if (f == 6 || f == 7)
return (f-7) >> g;
__builtin_unreachable();
}
<a href="https://godbolt.org/z/4P8vWcnWj">https://godbolt.org/z/4P8vWcnWj</a></pre>
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</p>
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