[Bug c++/81028] GCC miscompiles placement new

[hstong at ca dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81028

Hubert Tong <hstong at ca dot ibm.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
            Version|7.0                         |8.0
            Summary|miscompiles placement new   |GCC miscompiles placement
                   |                            |new

--- Comment #1 from Hubert Tong <hstong at ca dot ibm.com> ---
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.

GCC miscompiles at -O2, at least with x86_64-pc-linux-gnu.

Online compiler: https://wandbox.org/permlink/ByNZmtd6oB6SgG0g

### SOURCE (<stdin>):
inline void* operator new(decltype(sizeof 0), void* __p) noexcept
{ return __p; }

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);
   }
}

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:
g++ -O2 -o ./a.out -x c++ -std=c++11 -
./a.out


### EXPECTED OUTPUT:
(rc=0)


### ACTUAL OUTPUT:
Aborted
(rc=134)


### COMPILER VERSION INFO (g++ -v):
Using built-in specs.
COLLECT_GCC=/opt/wandbox/gcc-head/bin/g++
COLLECT_LTO_WRAPPER=/opt/wandbox/gcc-head/libexec/gcc/x86_64-pc-linux-gnu/8.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../source/configure --prefix=/opt/wandbox/gcc-head
--enable-languages=c,c++ --disable-multilib --without-ppl --without-cloog-ppl
--enable-checking=release --disable-nls --enable-lto
LDFLAGS=-Wl,-rpath,/opt/wandbox/gcc-head/lib,-rpath,/opt/wandbox/gcc-head/lib64,-rpath,/opt/wandbox/gcc-head/lib32
Thread model: posix
gcc version 8.0.0 20170607 (experimental) (GCC)