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<body><span class="vcard"><a class="email" href="mailto:ahmed.bougacha@gmail.com" title="Ahmed Bougacha <ahmed.bougacha@gmail.com>"> <span class="fn">Ahmed Bougacha</span></a>
</span> changed
<a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - UBSan reports downcast/upcast of misaligned address in std::set<long long>"
href="https://llvm.org/bugs/show_bug.cgi?id=23413">bug 23413</a>
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<th>What</th>
<th>Removed</th>
<th>Added</th>
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<td style="text-align:right;">Status</td>
<td>NEW
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<td>RESOLVED
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<td style="text-align:right;">CC</td>
<td>
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<td>ahmed.bougacha@gmail.com
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<td style="text-align:right;">Resolution</td>
<td>---
</td>
<td>INVALID
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<b><a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - UBSan reports downcast/upcast of misaligned address in std::set<long long>"
href="https://llvm.org/bugs/show_bug.cgi?id=23413#c2">Comment # 2</a>
on <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - UBSan reports downcast/upcast of misaligned address in std::set<long long>"
href="https://llvm.org/bugs/show_bug.cgi?id=23413">bug 23413</a>
from <span class="vcard"><a class="email" href="mailto:ahmed.bougacha@gmail.com" title="Ahmed Bougacha <ahmed.bougacha@gmail.com>"> <span class="fn">Ahmed Bougacha</span></a>
</span></b>
<pre>Aah, I remember this! I think this is actually a legitimate libstdc++ bug;
gcc's implementation of UBSan doesn't catch it, but there's undefined behavior
nonetheless.
The RB tree iterator (used for std::set/map) is defined as a pointer to a node
struct. This struct subclasses the "base" node type (with the color and
children and whatnot), adding the value type as a (properly aligned) field.
std::set/map::end(), however, takes a pointer to the base node type, and casts
it to the iterator type, i.e., a pointer to the actual node type. The latter
has alignment >= that of the value type. The former can be (and is) less
aligned, with an alignment == that of the children/color types.
The alignment sanitizer complains, because the cast by itself is undefined
behavior: quoting C11 6.3.2.3, paragraph 7:
<span class="quote">> A pointer to an object type may be converted to a pointer to a different
> object type. If the resulting pointer is not correctly aligned 68) for the
> referenced type, the behavior is undefined.</span >
Let me know if I missed something!</pre>
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