[Bug c++/16590] New: Incorrect execution when compiling with -O2

["m dot galante at centrosistemi dot it" <gcc-bugzilla at gcc dot gnu dot org>]

I have reduced the testcase to a small file that only includes <cassert> in 
order to show where the execution fails. The assert in main() should never 
fail, but it fails if you compile with:

  g++ -O2 bug.cpp
  
The assert does not fail if you compile with one of the following:
  g++ -O0 bug.cpp
  g++ -O1 bug.cpp
  g++ -O2 -DWORKAROUND bug.cpp

This happens with gcc 3.4.1 (mingw special) on Windows XP and gcc 3.4.0 on SCO 
OPENSERVER 5.0.6, but NOT with gcc 3.4.0 on SUN Solaris; perhaps a bug in 
generating code for INTEL X86?


Output of "gcc -v" for gcc 3.4.1/MinGW on Windows XP (assert FAILS):
Reading specs from c:/mingw/bin/../lib/gcc/mingw32/3.4.1/specs
Configured with: ../gcc/configure --with-gcc --with-gnu-ld --with-gnu-as --
host=mingw32 --target=mingw32 --prefix=/mingw --enable-threads --disable-nls --
enable-languages=c,c++,f77,ada,objc,java --disable-win32-registry --disable-
shared --enable-sjlj-exceptions --enable-libgcj --disable-java-awt --without-
x --enable-java-gc=boehm --disable-libgcj-debug --enable-interpreter --enable-
hash-synchronization --enable-libstdcxx-debug
Thread model: win32
gcc version 3.4.1 (mingw special)


Output of "gcc -v" for gcc 3.4.0 on SCO OpenServer (assert FAILS):
Reading specs from /home/local/gcc-3.4.0/bin/../lib/gcc/i686-pc-
sco3.2v5.0.6/3.4.0/specs
Configured with: ../gcc-3.4.0/configure --with-gnu-as --prefix=/usr/local/gcc-
3.4.0
Thread model: single
gcc version 3.4.0


Output of "gcc -v" for gcc 3.4.0 on SUN Solaris (assert does NOT FAIL):
Reading specs from /usr/local/gcc-3.4.0/lib/gcc/sparc-sun-
solaris2.8/3.4.0/specs
Configured with: ../gcc-3.4.0/configure --disable-shared --
prefix=/usr/local/gcc-3.4.0
Thread model: posix
gcc version 3.4.0


// -------------------- bug.cpp ------------------------

#include <cassert>

typedef char* iterator_type;

struct queue;

struct iterator
{
#ifdef WORKAROUND
  iterator_type volatile pos_;
#else
  iterator_type pos_;
#endif
  queue const* queue_;

  iterator(queue const* queue, iterator_type pos) : pos_(pos), queue_(queue) {}
  iterator& operator--();
};

struct queue
{
  iterator_type const first_, last_;
  iterator_type put_, get_;

  queue(iterator_type first, iterator_type last)
  : first_(first), last_(last), put_(first), get_(first)
  {}

  iterator pbegin() const { return iterator(this, put_); }
  iterator pend() const { iterator end(this, get_); return --end; }
};

inline iterator& iterator::operator--()
{
  if(pos_ == queue_->first_) pos_ = queue_->last_;
  --pos_;
  return *this;
}

int main()
{
  char mem[4+1];
  queue buf(mem, mem+4+1);
  assert(buf.pend().pos_ == buf.pbegin().pos_+4);
  return 0;
}

-- 
           Summary: Incorrect execution when compiling with -O2
           Product: gcc
           Version: 3.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: m dot galante at centrosistemi dot it
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: mingw32 / i686-pc-sco3.2v5.0.6
GCC target triplet: mingw32 / i686-pc-sco3.2v5.0.6


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16590