[Bug c++/58417] New: Incorrect optimization

[mirzayanovmr at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58417

            Bug ID: 58417
           Summary: Incorrect optimization
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mirzayanovmr at gmail dot com

Created attachment 30820
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30820&action=edit
Compile it with "g++ -O2 a.cpp" and run. It will output 0 instead of 10.

The following code writes 0 (but 10 is expected) with -O1 or -O2. Checked on
4.7.2 and 4.8.1 (mingw), on Fedora with g++ (GCC) 4.8.1 20130603 (Red Hat
4.8.1-1).

Command line to compile: g++ -O2 a.cpp

Code:

#include<iostream>

using namespace std;

long long arr[6] = {0, 1, 2, 3, 4, 5};

int main()
{
    int n = 5;
    long long sum = 0, prevsum = 0;

    for(int i = 1; i <= n; i++)
    {
        cout << "sum = " << sum << endl;
        sum = (i - 1) * arr[i] - prevsum;
        // cout<<"sum : "<<sum<<endl;
        prevsum += arr[i];
    }

    cout << "Final Result: " << sum << endl;
}