Re: g++ Bug? Why w/ and w/o "O3" options gives different result(g++3.3.3) for this program?

[Peng Yu]

Thank you for all the people replied my previous post.

The following minimum program shows that if you uncomment the
commented line, with and w/o -O3 option will give different results.

However, if you commented that line, with and w/o -O3 option will give
the same results.

If you make g_range_down an integer and uncomment the first cout
statement, the results will also be the same.

I wonder why the first cout statement and the declaration of
g_range_down as an array will affect the compile result.

g++3.3.3 is used.

Thanks,
Peng

#include <iostream>

using namespace std;

int main(int argc, char *argv[])
{
  double g_range_down[3];
  g_range_down[2] = - 0.5;
//  cout << g_range_down[2] << endl;
  cout << ceil(g_range_down[2] / 0.02) << endl;
}



On 01 Dec 2004 17:45:37 -0800, Paul Pluzhnikov
<ppluzhnikov-nsp@charter.net> wrote:

>Peng Yu <pengyu.ut@gmail.com> writes:
>
>> Does it indicate g++ has a bug? 
>
>We can't tell: your test case is too long, and nobody so far has
>shown inclination to spend hours analyzing it.
>
>Try to produce a *minimal* test case that demonstrates the problem.
>
>> Or I used some C++ language feature
>> that -O3 option doesn't support?
>
>It is quite a bit more likely that you have a bug in your program,
>but finding it without a minimal test case is difficult.
>
>FWIW, your program is "valgrind-clean", but shows reading
>uninitialized memory from stack under Chaperon (with and without -O3):
>
>[PC: 0x402c0835 std::ostream::operator<<()+0x000000a5] (Thread 0) 
>**READ_UNINIT_MEM(read)**
>
>  Reading uninitialized memory.
>
>  Pointer : 0xbfffda25
>  Stack trace where the error occurred:
>        std::ostream::operator<<()  pc: 0x402c0835
>std::basic_ostream<char, std::char_traits<char> >& std::operator<< <double, 
>char, std::char_traits<char> >()  pc: 0x080498a6  
>/usr/include/c++/3.2.2/complex, 393
>                            main()  pc: 0x0804931d  junk.cc, 106
>
>temp = (0.0140056,0)
>temp = (0.0140056,0)
>
>Cheers,