[Bug c++/91215] New: Compiled program loops endlessly because of -O2 with g++ 8.3.0

[sboisvert at gydle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91215

            Bug ID: 91215
           Summary: Compiled program loops endlessly because of -O2 with
                    g++ 8.3.0
           Product: gcc
           Version: 8.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sboisvert at gydle dot com
  Target Milestone: ---

Created attachment 46614
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46614&action=edit
Minimum reproducer C++ source code

Hello,

I compile my program with -O2 with g++ 8.3.0.
When I run my program, there is an infinite loop.

With -O0, this is not the case.


Steps to reproduce
==================

Step 1: Acquire g++-8 (8.3.0)
I used g++ (Ubuntu 8.3.0-6ubuntu1~18.04.1) 8.3.0.

Step 2: build with O2

Command:

/usr/bin/g++-8 -O2 g++-8.3.0-infinite-loop-bug.cpp -o
g++-8.3.0-infinite-loop-bug

Step 3: execute the program

./g++-8.3.0-infinite-loop-bug


Expected result
===============

The body of the for loop is done 50 times (i = 0, ..., i = 49).
Then the program returns 0.


Actual result
=============

The body of the for loop is performed beyond i = 49.
There is an infinite loop.


Minimum reproducer
==================

/*
 * Buggy for loop with g++ (Ubuntu 8.3.0-6ubuntu1~18.04.1) 8.3.0
 *
 *
 /usr/bin/g++-8 -O2 g++-8.3.0-infinite-loop-bug.cpp -o
g++-8.3.0-infinite-loop-bug
 ./g++-8.3.0-infinite-loop-bug

 */
#include <iostream>

int doY()
{
    for (int i = 0; i < 50; ++i) {
        std::cout << "DEBUG []";
        std::cout << "  i: " << i;
        std::cout << std::endl;
    }

    // return 0; // This fixes the bug.
}

int main(int argc, char* argv[]){
    doY();
    return 0;
}