Visual C++ 2008 bug?

Question

The very simple code (under MS Visual C++ 2008 Express):

#include <iostream>
using namespace std;

int main()
{
    for (int a=1; 2*a<=7; a++)
        cout << a << endl;
    return 0;
}

Debug mode gives me correct result:

1
2
3

But Release mode gives me wrong result:

1
2

Well, I understand possible answers "use 2*a<8"; "why not a<=3", "a<4".

I don't want to change the code, 'cause it is correct code (working well in Debug mode, all variables are initialized well, etc).

• Have you the same bug with Visual C++ 2008 Express?
• Is this bug present in younger versions (2010, 2012)?
• How to avoid this bug?
• Is there SP for fixing it?
• Maybe to change some compile options (not default options)?

---

Update:

when I write

cout << a+1 << endl;

or

cout << 2*a << endl;

it works/compiles correct (3 rows of output).

---

Note: I tried on different computers with VC++ 2008 Express. The same behavior.

Answer 1

I can repro that on VS2008 SP1. It is a code optimizer bug, as usual. You'd have to look at the disassembly to see the cause. It fumbles at 2*a <= 7 when it factors out the multiplication, it generates code for a <= 2. That's wrong of course, should have been a <= 3 or a < 4. Looks like it doesn't handle the <= operator correctly for divisions. Kinda tricky, it has to know the difference between odd and even numbers :)

The bug disappears when you don't force it to figure out how <= behaves with division, using 2*a < 8 works fine.

This bug has been fixed a while ago, I don't know exactly when since the bug reports for these old versions have been deleted from the public site. Best way to deal with optimizer bugs is to give them a chance to fix them, keeping your compiler updated is pretty important. You've got 3 newer versions of the Express edition to choose from, that's two dog lives in compiler development. Three with C++11 around :)