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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - False positive alignment error with -fsanitize=undefined with virtual base"
href="https://bugs.llvm.org/show_bug.cgi?id=35902">35902</a>
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<th>Summary</th>
<td>False positive alignment error with -fsanitize=undefined with virtual base
</td>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>5.0
</td>
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<tr>
<th>Hardware</th>
<td>PC
</td>
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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>-New Bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>securesneakers@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=19654" name="attach_19654" title="Minimal example that reproduces the issue">attachment 19654</a> <a href="attachment.cgi?id=19654&action=edit" title="Minimal example that reproduces the issue">[details]</a></span>
Minimal example that reproduces the issue
Attached program generates false misalignment errors when compiled with
-fsanitize=undefined
$ clang++ --version
clang version 5.0.0 (tags/RELEASE_500/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
$ uname -s -m
Linux x86_64
$ clang++ -std=c++11 -O2 -fsanitize=undefined minimal.cpp && ./a.out
minimal.cpp:9:8: runtime error: constructor call on misaligned address
0x7ffdd1e1e658 for type 'struct Base2', which requires 16 byte alignment
The program crashes afterwards, I have reported a separate bug:
<a class="bz_bug_link
bz_status_NEW "
title="NEW - Unaligned movaps in virtual base constructor"
href="show_bug.cgi?id=35901">https://bugs.llvm.org/show_bug.cgi?id=35901</a>. This bug is about false positive
error.
Attached example contains following hierarchy:
struct alignas(16) Base1 { };
struct Base2 : virtual Base1 { };
struct Base3 : virtual Base2 { };
alignof(Base2) is set to 16 due to alignment of its base class. But when Base3
is instantiated, Base2 is placed with alignment of 8 as it should be according
to Itanium C++ ABI (due to its non-virtual alignment being equal 8):
<a href="https://refspecs.linuxfoundation.org/cxxabi-1.75.html#class-types">https://refspecs.linuxfoundation.org/cxxabi-1.75.html#class-types</a>. Yet
sanitizer complains about alignment not being 16.
Seems that sanitizer checks address using "normal" alignment when "non-virtual
alignment" should be used.
I have reported the same issue for GCC:
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83780">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83780</a></pre>
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