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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - sizeof(struct) == 0 for struct with large, explicitly aligned non-static data member"
href="https://bugs.llvm.org/show_bug.cgi?id=49184">49184</a>
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<th>Summary</th>
<td>sizeof(struct) == 0 for struct with large, explicitly aligned non-static data member
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<th>Product</th>
<td>clang
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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
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>joejev@gmail.com
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<pre>Created <span class=""><a href="attachment.cgi?id=24525" name="attach_24525" title="A reproducer which shows how the structure size is incorrectly reported as 0.">attachment 24525</a> <a href="attachment.cgi?id=24525&action=edit" title="A reproducer which shows how the structure size is incorrectly reported as 0.">[details]</a></span>
A reproducer which shows how the structure size is incorrectly reported as 0.
For structs which contain very large non-static data members with an explicit
alignment, sizeof can report 0. I believe I have tracked this down to an
overflow in llvm::alignTo from clang/lib/AST/MathExtras.H (clarifying because I
found an alternative implementation in llvm/Support/Alignment.h).
The example struct I have tested is:
struct node {
alignas(2) char data[2305843009213693951u]; //
std::numeric_limits<std::ptrdiff_t>::max() / 4;
};
With this definition, sizeof(node) == 0 even though sizeof(node::data) ==
2305843009213693951u.
In ItaniumRecordLayoutBuilder::FinishLayout, when computing RoundedSize, which
appears to be the final size of the struct, we start with a proper Size and
DataSize of 18446744073709551608 (std::numeric_limits<std::ptrdiff_t>::max() /
4 * 8, where 8 is the number of bits in char on my system). The Alignment also
appears correct at 16 (2 * 8, where 8 is the number of bits in char on my
system).
llvm::alignTo is defined as: (Value + Align - 1 - Skew) / Align * Align + Skew
In this invocation, Skew is 0.
When we take Value + Align, 18446744073709551608 + 16, we overflow and get 8,
which is less than Align and the division truncates to 0.
I would be happy to contribute a patch to either fix this issue, but I am not
sure if this should be corrected in llvm::alignTo or if this case should be
detected earlier and error.</pre>
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