To get a trial key
fill out the form below
Team License (a basic version)
Enterprise License (an extended version)
* By clicking this button you agree to our Privacy Policy statement

Request our prices
New License
License Renewal
--Select currency--
USD
EUR
RUB
* By clicking this button you agree to our Privacy Policy statement

Free PVS-Studio license for Microsoft MVP specialists
* By clicking this button you agree to our Privacy Policy statement

To get the licence for your open-source project, please fill out this form
* By clicking this button you agree to our Privacy Policy statement

I am interested to try it on the platforms:
* By clicking this button you agree to our Privacy Policy statement

Message submitted.

Your message has been sent. We will email you at


If you haven't received our response, please do the following:
check your Spam/Junk folder and click the "Not Spam" button for our message.
This way, you won't miss messages from our team in the future.

>
>
>
Optimization in the world of 64-bit err…

Optimization in the world of 64-bit errors

Jan 11 2010
Author:

In the previous blog-post I promised to tell you why it is difficult to demonstrate 64-bit errors by simple examples. We spoke about operator[] and I told that in simple cases even incorrect code might work.

Here is such an example:

class MyArray
{
public:
  char *m_p;
  size_t m_n;
  MyArray(const size_t n)
  {
    m_n = n;
    m_p = new char[n];
  }
  ~MyArray() { delete [] m_p; }
  char &operator[](int index)
    { return m_p[index]; }
  char &operator()(ptrdiff_t index)
    { return m_p[index]; }
  ptrdiff_t CalcSum()
  {
    ptrdiff_t sum = 0;
    for (size_t i = 0; i != m_n; ++i)
      sum += m_p[i];
    return sum;
  }
};
void Test()
{
  ptrdiff_t a = 2560;
  ptrdiff_t b = 1024;
  ptrdiff_t c = 1024;
  MyArray array(a * b * c);
  for (ptrdiff_t i = 0; i != a * b * c; ++i)
    array(i) = 1;
  ptrdiff_t sum1 = array.CalcSum();
  for (int i = 0; i != a * b * c; ++i)
    array[i] = 2;
  ptrdiff_t sum2 = array.CalcSum();
  if (sum1 != sum2 / 2)
    MessageBox(NULL, _T("Normal error"),
        _T("Test"), MB_OK);
  else
    MessageBox(NULL, _T("Fantastic"),
        _T("Test"), MB_OK);
}

Briefly, this code does the following:

  • Creates an array of 2.5 Gbytes (more than INT_MAX items).
  • Fills the array with ones by using the correct operator() with ptrdiff_t parameter.
  • Calculates the sum of all the items and writes it into the variable sum1.
  • Fills the array with twos by using the incorrect operator[] with int parameter. Theoretically, int does not allow us to address the items whose numbers are more than INT_MAX. There is one more error in the loop "for (int i = 0; i != a * b * c; ++i)". Here, we also use int as the index. This double error is made for the compiler not to generate warnings about a 64-bit value converting to a 32-bit one. Actually, an overflow and addressing an item with a negative number must take place what will result in a crash. By the way, this is what happens in the debug-version.
  • Calculates the sum of all the items and writes it into the variable sum2.
  • If (sum1 == sum2 / 2), it means that the impossible became true and you see the message "Fantastic".

Despite the two errors in this code, it successfully works in the 64-bit release-version and prints the message "Fantastic"!

Now let us make out why. The point is that the compiler guessed our wish to fill the array with the values 1 and 2. And in the both cases it optimized our code by calling memset function:

0047_Optimization_in_the_world_of_64-bit_errors/image1-48dd9e204e8554b3e7ec9b89d0549af5.png

The first conclusion is: the compiler is a clever guy in the questions of optimization. The second conclusion - stay watchful.

This error might be easily detected in the debug-version where there is no optimization and the code writing twos into the array leads to a crash. What is dangerous, this code behaves incorrectly only when dealing with large arrays. Most likely, processing of more than two milliards of items will not be present in the unit-tests run for the debug-version. And the release-version might keep this error a secret for a long time. The error can occur quite unexpectedly at a slightest change of the code. Look what can happen if we introduce one more variable, n:

void Test()
{
  ptrdiff_t a = 2560;
  ptrdiff_t b = 1024;
  ptrdiff_t c = 1024;
  ptrdiff_t n = a * b * c;
  MyArray array(n);
  for (ptrdiff_t i = 0; i != n; ++i)
    array(i) = 1;
  ptrdiff_t sum1 = array.CalcSum();
  for (int i = 0; i != n; ++i)
    array[i] = 2;
  ptrdiff_t sum2 = array.CalcSum();
  ...
}

The release-version crashed this time. Look at the assembler code.

0047_Optimization_in_the_world_of_64-bit_errors/image2-af06ffcb112b6db07ade304d90a9f461.png

The compiler again built the code with a memset call for the correct operator(). This part still works well as before. But in the code where operator[] is used, an overflow occurs because "i != n" condition does not hold. It is not quite the code I wished to create but it is difficult to implement what I wanted in a small code while a large code is difficult to examine. Anyways, the fact remains. The code now crashes as it should be.

Why have I devoted so much time to this topic? Perhaps I am tormented with the problem that I cannot demonstrate 64-bit errors by simple examples. I write something simple for the purpose of demonstration and what a pity it is when one tries it and it works well in the release-version. And therefore it seems that there is no error. But there are errors and they are very insidious and difficult to detect. So, I will repeat once again. You might easily miss such errors during debugging and while running unit-tests for the debug-version. Hardly has anyone so much patience to debug a program or wait for the tests to complete when they process gigabytes. The release-version might pass a large serious testing. But if there is a slight change in the code or a new version of the compiler is used, the next build will fail to work at a large data amount.

To learn about diagnosis of this error, see the previous post where the new warning V302 is described.

Comments (0)

Next comments
This website uses cookies and other technology to provide you a more personalized experience. By continuing the view of our web-pages you accept the terms of using these files. If you don't want your personal data to be processed, please, leave this site.
Learn More →
Accept