[Bug c++/36742] New: g++ -O2 produces wrong code

["michael at jarvis dot net" <gcc-bugzilla at gcc dot gnu dot org>]

The following code produces correct output with g++ -O, but wrong output with
g++ -O2:
The basic calculation should be just a product of two complex numbers, each
equal to (1,1), so the output should be (and is with g++ -O):

a = (0,2)

However, on my system, the output with the -O2 flag is:

a = (-1.22015e-313,1.22015e-313)

Furthermore, any one of the following changes "fixes" the problem:

Any one of the following changes "fix" the problem:

- Removing either Assert statement in foo
- Removing the imag(b.val) branch in foo
- Removing the trigraph in foo
- Removing s2 from Error
- Removing base class of A
- Removing junk from A

Here is the code:

#include <iostream>
#include <complex>
#include <string>

class B
{
  public:
    virtual size_t one() const = 0;
    virtual ~B() {}
};

class A : public B
{
  public:

    A(std::complex<double> _val) : itsval(_val), junk(1) {}
    ~A() {}

    std::complex<double>& val() { return itsval; }
    size_t one() const { return 1; }

  protected :

    std::complex<double> itsval;
    const size_t junk;
};

class Error
{
  public :
    std::string s1, s2;
    inline Error(std::string _s1, std::string _s2="") throw() :
      s1(_s1), s2(_s2) {}
    virtual inline ~Error() throw() {}
};

#define Assert(x) do { if(!(x)) { throw Error(#x); } } while(false)

void foo(A& a, std::complex<double> b)
{
  Assert(a.one() == 1);
  Assert(a.one() == 1);

  if (std::imag(b) == double(0))
    a.val() *= std::real(b);
  else
    a.val() *= (true ? b : std::conj(b));
}

int main()
{
  A a(std::complex<double>(1,1));
  std::complex<double> b(1,1);

  foo(a, b);
  std::cout<<"a = "<<a.val()<<std::endl;
  //std::cout<<"Should be (0,2)\n";

  return 0;
}


-- 
           Summary: g++ -O2 produces wrong code
           Product: gcc
           Version: 4.2.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: michael at jarvis dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36742