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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Clang miscompiles placement-new"
href="https://llvm.org/bugs/show_bug.cgi?id=28767">28767</a>
</td>
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<tr>
<th>Summary</th>
<td>Clang miscompiles placement-new
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Keywords</th>
<td>miscompilation
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hstong@ca.ibm.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>In the following program, the only iteration of the loop increments the int
object pointed-to by p, and then replaces said object via q using
placement-new.
Clang miscompiles at -O1 at least on x86_64-unknown-linux-gnu and
powerpc64le-unknown-linux-gnu.
Observation: passes with -fno-strict-aliasing.
Observation: -mllvm -print-before-all is indistinguishable between the
placement-new and the commented-out reinterpret_cast.
Online compiler: <a href="http://melpon.org/wandbox/permlink/h0aEUwT90gAnbGtv">http://melpon.org/wandbox/permlink/h0aEUwT90gAnbGtv</a>
### SOURCE (<stdin>):
void *operator new(decltype(sizeof 0), void *) noexcept;
extern "C" void abort();
typedef int A;
typedef float B;
B *qq;
void foo(A *p, A *q, long unk) {
for (long i = 0; i < unk; ++i) {
++*p;
qq = new (static_cast<void *>(&q[i])) B(42);
// Note: the following is a TBAA violation.
//qq = &(*reinterpret_cast<B *>(static_cast<void *>(&q[i])) = B(42));
}
}
void (*volatile fp)(A *, A *, long);
int main(void) {
union { A x; B f; } u = { 0 };
fp = foo;
fp(&u.x, &u.x, 1);
if (*qq != 42) abort();
}
### COMMANDS TO REPRODUCE:
clang -O1 -o ./a.out -x c++ -std=c++11 -
./a.out
### EXPECTED OUTPUT:
(rc=0)
### ACTUAL OUTPUT:
Aborted (core dumped)
Return: 0x86:134
### COMPILER VERSION INFO (clang -v):
clang version 4.0.0 (trunk 276983) (llvm/trunk 276982)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/llvm-head/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6.3
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64</pre>
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