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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - llvm missing opportunities to overlap non-interfering local variables"
href="https://llvm.org/bugs/show_bug.cgi?id=25776">25776</a>
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<th>Summary</th>
<td>llvm missing opportunities to overlap non-interfering local variables
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</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>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>thanm@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=15416" name="attach_15416" title="stack-ifthen.cc">attachment 15416</a> <a href="attachment.cgi?id=15416&action=edit" title="stack-ifthen.cc">[details]</a></span>
stack-ifthen.cc
When I compile the attached testcase "stack-ifthen.cc" with "clang++ -g -O"
using LLVM trunk (linux x86-64), I observe that the size of the stack frame is
larger than it needs to be (~1024 bytes).
Compiling with "g++-4.8 -g -O" yields a stack frame size of ~512 bytes; gcc is
able to recognize that the two local arrays can share the same stack space.
Clang/llvm creates a separate stack allocation for each array, resulting in a
larger stack frame size.
Interestingly, gcc/g++ appears to overlap the two arrays even when compiling
with -O0, which leads me to assume the layout is being done in the front end as
opposed to in the back end.</pre>
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