Simple C Program - Failing in a particular scenario

Question

Hi guys,

Looks at this simple C Code.

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
    int k;
    int ctr = 0;
    for (k = 0; k < 5; k++)
    {
        if ( k == 0 )
        {
            printf("\n----------");
        }
        ctr++;
    }
    printf("\n Counter is supposed to be 5. But the Value is  = %d ",ctr);
    puts("\n End of K Loop");
    return 0;
}

I believe that this program should print the counter value as 5. But it prints the value as 1. Any Idea why is this so?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

But this works,

int main(void)
{
    int k;
    int ctr = 0;
    for (k = 0; k < 5; k++)
    {
        if ( k <1 )
        {
            printf("\n----------");
        }
        ctr++;
    }
    printf("\n Counter is supposed to be 5. But the Value is  = %d ",ctr);
    puts("\n End of K Loop");
    return 0;
}

~~~~~~~~~~~~~~~~~~~

This also works

int main(void)
{
    int k;
    int ctr = 0;

    int dum=0;

    for (k = 0; k < 5; k++)
    {
        if ( k == 0 )
        {
            dum=0;
        }
        ctr++;
    }
    printf("\n Counter is supposed to be 5. But the Value is  = %d ",ctr);
    puts("\n End of K Loop");
    return 0;
}

Any clues? This happens in Visual Studio 2008 SP1.

Thanks,

Vasanth

Answer 1

On 3/8/2012 4:00 PM, Vasanth Sivanatarajan wrote:

#include<stdio.h>
#include<stdlib.h>
int main(void)
{
     int k;
     int ctr = 0;
     for (k = 0; k<  5; k++)
     {
         if ( k == 0 )
         {
             printf("\n----------");
         }
         ctr++;
     }
     printf("\n Counter is supposed to be 5. But the Value is  = %d ",ctr);
     puts("\n End of K Loop");
     return 0;
}

I believe that this program should print the counter value as 5. But it prints the value as 1. Any Idea why is this so?

Prints 5 for me. Can't reproduce - working as expected.

---

Igor Tandetnik

Answer 2

It's an optimization issue which will manifest when a
Release Build is done, but not in a Debug build.

- Wayne

Answer 3

On 3/8/2012 4:21 PM, WayneAKing wrote:

It's an optimization issue which will manifest when a
Release Build is done, but not in a Debug build.

Works as expected in Release, too. Any particular optimization options to trigger the problem?

---

Igor Tandetnik

Answer 4

Works as expected in Release, too. Any particular optimization options to trigger the problem?

The OP is using Visual Studio 2008 SP1. I'm using
Visual C++ 2008 Express without SP1.

When a Release Build is done with Optimizations "Disabled"
or set to "Minimize Size", the correct output of 5 is
displayed. If built with "Maximize Speed" or "Full
Optimization" then "1" is shown.

- Wayne

Answer 5

On 3/8/2012 4:31 PM, WayneAKing wrote:

Works as expected in Release, too. Any particular optimization options to trigger the problem?

The OP is using Visual Studio 2008 SP1. I'm using
Visual C++ 2008 Express without SP1.

When a Release Build is done with Optimizations "Disabled"
or set to "Minimize Size", the correct output of 5 is
displayed. If built with "Maximize Speed" or "Full
Optimization" then "1" is shown.

Yes, can reproduce with /O2. Looks very much like a bug. One can file it, though I suspect there isn't much point unless the problem also reproduces with VC10 or, better still, VC11 beta.

Looking at disassembly, the compiler unrolled the loop entirely, and effectively transformed the code into

printf("\n----------");
printf("\n Counter is supposed to be 5. But the Value is  = %d ", 1);

The constant is wrong, of course.

---

Igor Tandetnik

Answer 6

Yes, can reproduce with /O2. Looks very much like a bug. One can file it, though I suspect there isn't much point unless the problem also reproduces with VC10 or, better still, VC11 beta.

On VC10 it seems even worse. I seem to get the problem in Release build no matter what the optimization settings.
 
Debug works fine.
 

---

David Wilkinson | Visual C++ MVP

Answer 7

Hi Guys,

Thanks for the observations.

This segment of code was in a production program and it was working for years, which was initially compiled in VC 2005. As per the end application requirement, we had to move to VC 2008 and something weird happened. Finally, we found this issue. You are correct, this happens in Release Mode and not in Debug Mode.

I understand that it's related to Optimization. But we were thinking that this might be happening whenever we use == in a IF condition. Is that true? If yes, we are scared that lot many other programs that are in production, may fail too now, as all were recompiled in VS 2008. In a year, we would be moving to VS 2010, as per the end application requirement.

What action can we take at this point?

Thanks,

Vasanth Sivanatarajan

Answer 8

On 3/8/2012 7:05 PM, Vasanth Sivanatarajan wrote:

This segment of code was in a production program

I wonder why you've written your code this way in the first place. What's the purpose of the loop? It's equivalent to

int ctr = 5;
printf("\n----------");

I understand that it's related to Optimization. But we were thinking that this might be happening whenever we use == in a IF condition. Is that true?

That's highly unlikely. It would have broken millions of lines of code, and couldn't possibly have gone unnoticed for so long.

The problem appears to be triggered by a trivial loop in your code, that the optimizer decided it could eliminate, but didn't quite do that correctly. One way to avoid this is to not write trivial loops.

---

Igor Tandetnik

Answer 9

Hi,

This code was written by my friend few years ago. He reads multiple data files, each one of which will have thousands of lines(Each line contains several double values).

In one particular function, he has three level nested loops. In the inner most loop (say K loop), he has condition that certain things has to be done only for the first item of K loop by saying IF (K == 0). In the actual production code, instead of executing this segment only one time, it got executed for all the K values. Inside the If condition, he has assigned values for a double ** (previously allocated memory with dynamic allocation). We did check the memory, leaks etc.,

He pulled out a simple code segment, where he could kind of reproduce the problem. I understand, the simple code that I had pasted first, might look weird but that was written just to show one of the problem.

For now, we could disable the Optimization and also change the condition to IF (K < 1).

Thanks,

Vasanth

Answer 10

On 3/8/2012 8:26 PM, Vasanth Sivanatarajan wrote:

In one particular function, he has three level nested loops. In the
inner most loop (say K loop), he has condition that certain things
has to be done only for the first item of K loop by saying IF (K ==
0). In the actual production code, instead of executing this segment
only one time, it got executed for all the K values. Inside the If
condition, he has assigned values for a double ** (previously
allocated memory with dynamic allocation). We did check the memory,
leaks etc.,

For now, we could disable the Optimization and also change the condition to IF (K<  1).

Why not remove this inner loop entirely? If you only need to exectute a piece of code one time, why do you put it in a loop at all? The very fact that you have this issue demonstrates that the loop isn't actually needed (since the optimizer attempted to eliminate it, albeit unsuccessfully).

---

Igor Tandetnik

Answer 11

Yes. I need to check with him. If that's the only thing he does inside the third loop, then obviously it makes sense to remove that loop. If not, we will see if there's a way to reorganize this.

Thanks,

Vasanth

Answer 12

he has condition that certain things has to be done only for the first item of K loop by saying IF (K == 0). 

Why he need a loop here. He can check the condition for first value directly.

This is the infamous for-if anti-pattern
see http://blogs.msdn.com/b/oldnewthing/archive/2011/12/27/10251210.aspx

Answer 13

This is fixed in VS11. Here is the message,

Greetings from Microsoft Connect!

This notification was generated for feedback item: <a href="http://connect.microsoft.com/VisualStudio/feedback/details/729818/for-loop-error">For loop Error</a> which you submitted at the <a href="http://connect.microsoft.com">Microsoft Connect</a> site.

Hi, thanks for this report. This is a bug in the optimizer, and will be fixed in the release of VS11. If you are curious, here are the details.

There is an optimization called if-unswitching that turns:

for (i=L; i<U; i++) {
  if (i == 3)
     // do something
}

Into this (removing the for-loop entirely):

if (L <= 3 < U) {
  i=3;
  // do something
}

The bug is that we were not considering uses of 'i' beyond the loop body when performing if-unswitching. This will be fixed in VS11.

Thanks!
Eric
Microsoft VC++ Team

You may receive a general "Feedback Item Updated" notification as well, if any other changes were made by Microsoft.

Thank you for using Microsoft Connect!

Regards,

the Microsoft Connect Team

Answer 14

Vasanth -

Thanks for submitting it to MS and for posting the
result here.

To avoid possible confusion for new readers of this thread,
I suggest that you UNmark as "Answer" the reply from Igor
which stated: "Can't reproduce - working as expected."
and mark your last post with the official reply from
Microsoft Connect confirming an optimizer bug as
the "Answer".

- Wayne