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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - llvm misses a chance to eliminate a scratch alloca used to swap two bignums"
href="https://bugs.llvm.org/show_bug.cgi?id=38509">38509</a>
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<th>Summary</th>
<td>llvm misses a chance to eliminate a scratch alloca used to swap two bignums
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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
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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
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<th>Component</th>
<td>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>vsk@apple.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>Example IR: <a href="https://godbolt.org/g/Uri5QR">https://godbolt.org/g/Uri5QR</a>
The optimizer misses a chance to eliminate %t1.i.i.i, a scratch alloca used to
swap the contents of two bignums within a loop in a typical `tmp = a; a = b; b
= tmp` pattern. It should be possible to swap these bignums by simply swapping
their pointers.
Here is the original Rust example: <a href="https://pastebin.com/0GGiX9yi">https://pastebin.com/0GGiX9yi</a>
```
sdrodge: It seems to me that the output assembly has an unnecessary write to
memory at line 27
sdrodge: And if that got optimized away, then 32 bytes of stack would be
unnecessary.
sdrodge: As it is, there's a whole 16 bytes that are never used, and 16 bytes
that are written to but never read from.
sdrodge: In other words, I think llvm should be able to figure out that the
%t1.i.i.i allocation is unnecessary and the swap can be done using registers.
sdrodge: and it _almost_ does?
sdrodge: just keeps one useless write and fails to optimized the allocation
away
```
Reported by sdrodge on IRC.</pre>
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