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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Clang crashes when compiling __builtin_assume_aligned() with non-integer-constant alignment"
href="https://bugs.llvm.org/show_bug.cgi?id=47588">47588</a>
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<th>Summary</th>
<td>Clang crashes when compiling __builtin_assume_aligned() with non-integer-constant alignment
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<th>Product</th>
<td>clang
</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>
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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>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>bugsbugs@code.daltonmwoodard.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=23985" name="attach_23985" title="Clang stacktrace">attachment 23985</a> <a href="attachment.cgi?id=23985&action=edit" title="Clang stacktrace">[details]</a></span>
Clang stacktrace
The following reliably leads to a crash within Clang (current trunk, 12.0
series as provided on compiler explorer). This is the most reduced from of the
crash I could come up with, but it was originally discovered in a larger
program source I'm not able to attach here.
// main.c
constexpr unsigned long long a() { return 4; }
int main()
{
int i = 0;
(void)__builtin_assume_aligned(&i, a());
}
I've backtested this and it seems to first reproduce in Clang 10.0. Note, the
following snippets containing alternative formulations do _not_ lead to a
crash.
// main.c (crash negative)
constexpr unsigned long long a = 4;
int main()
{
int i = 0;
(void)__builtin_assume_aligned(&i, a);
}
// main.c (crash negative)
struct a { static constexpr unsigned long long value = 4; };
int main()
{
int i = 0;
(void)__builtin_assume_aligned(&i, a::value);
}
Also note that when the `constexpr` is removed from the declaration of a(),
Clang does not crash and correctly diagnoses it as an error with:
<source>:6:11: error: argument to '__builtin_assume_aligned' must be a constant
integer
(void)__builtin_assume_aligned(&i, a());
Noting this just for completeness, but it's what I expected since the stack
trace is pointing to LLVM IR generation.</pre>
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