https://llvm.org/bugs/show_bug.cgi?id=28767
Bug ID: 28767
Summary: Clang miscompiles placement-new
Product: clang
Version: trunk
Hardware: All
OS: All
Status: NEW
Keywords: miscompilation
Severity: normal
Priority: P
Component: C++
Assignee: unassignedclangb...@nondot.org
Reporter: hst...@ca.ibm.com
CC: dgre...@apple.com, llvm-bugs@lists.llvm.org
Classification: Unclassified
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: http://melpon.org/wandbox/permlink/h0aEUwT90gAnbGtv
### 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
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