Why does NaN - NaN == 0.0 with the Intel C++ Compiler?

📝 Why does NaN - NaN == 0.0 with the Intel C++ Compiler?

We've all heard of NaNs (Not a Number), but do you know how they propagate in arithmetic operations? 🤔 Well, if you're curious about this behavior, you're in the right place! In this blog post, we'll dive into the fascinating world of NaNs and understand why Intel C++ Compiler shows a peculiar behavior when it comes to subtracting NaNs.

Let's start with the context. A curious developer ran a small test to observe how NaNs behave in arithmetic operations using the Intel C++ compiler. Here's the code snippet they used:

#include <limits>
#include <cstdio>

int main(int argc, char* argv[]) {
    float qNaN = std::numeric_limits<float>::quiet_NaN();

    float neg = -qNaN;

    float sub1 = 6.0f - qNaN;
    float sub2 = qNaN - 6.0f;
    float sub3 = qNaN - qNaN;

    // ... other operations ...

    printf(
        "neg: %f\nsub: %f %f %f\nadd: %f %f\ndiv: %f %f %f\nmul: %f %f\n",
        neg, sub1, sub2, sub3, add1, add2, div1, div2, div3, mul1, mul2
    );

    return 0;
}

When they ran the code, they obtained the following output:

neg: -nan
sub: nan nan nan
add: nan nan
div: nan nan nan
mul: nan nan

So far, so good. The behavior observed aligns with what we would expect. However, things took an interesting turn when they tried it with the Intel C++ 15 Compiler. This time, the output was slightly different:

neg: -nan(ind)
sub: nan nan 0.000000
add: nan nan
div: nan nan nan
mul: nan nan

Woah! Did you notice that? 😮 Specifically, qNaN - qNaN evaluated to 0.0 with the Intel C++ Compiler.

Naturally, our developer was taken aback by this result and wondered if it could possibly be correct. So, let's dig deeper and find out what the relevant standards have to say about this behavior and why there's a difference between the compilers.

According to the ISO C, ISO C++, and IEEE 754 standards, the behavior of NaNs in arithmetic operations is defined, but there are some variations. In general, NaNs propagate through operations, meaning that if a NaN is involved in any operation, the result will also be NaN.

However, when it comes to subtracting NaNs, things get a bit more intriguing. In many cases, subtracting NaNs should indeed result in NaN. For example, on MSVC 2015, the output was consistent with our expectations. But with the Intel C++ Compiler, we observed the unexpected behavior where subtracting NaNs resulted in 0.0.

But why would this be the case? 🔍 Well, without diving too deep into the technical details, it might be due to some optimizations or non-conformance with the standard in the Intel C++ Compiler's implementation of floating-point arithmetic. This discrepancy between compilers may be attributed to their different interpretations or decisions made during the optimization process.

Now that we understand the context and the differing behavior between compilers, what can you do when faced with similar issues? Here are some tips:

1. Check for compiler-specific behavior: Whenever you encounter unexpected results in your code, it's worth investigating if the behavior is specific to a particular compiler. Testing your code with multiple compilers can help identify such discrepancies.

2. Consult the documentation and standards: If you come across unexpected behavior, consult the compiler documentation and the relevant standards (ISO C, ISO C++, IEEE 754) to understand how the behavior should ideally manifest.

3. Consider workarounds: If you're stuck with a particular compiler exhibiting non-standard behavior, you might need to find workarounds to achieve the desired outcome. This could involve alternative implementations or restructuring your code to avoid such scenarios.

In conclusion, NaNs are intriguing creatures that can behave unexpectedly in different scenarios, even leading to variations between compilers. Understanding the standards, consulting documentation, and exploring workarounds can help you navigate these peculiarities and achieve the desired results.

So, next time you encounter odd behavior when dealing with NaNs or any other aspect of floating-point arithmetic, remember to embrace curiosity, explore the standards, and adapt your code accordingly.

If you found this blog post intriguing and helpful, be sure to share it with other developers and leave your thoughts in the comments below. Let's continue the conversation! 💬🚀