Why does assigning a value to a complex variable cause an infinite loop in a C program, even though it appears to be a seemingly innocuous action?

C Compilation Enigma with Unpredictable Loop Behavior

The following code presents an intriguing compilation issue:

<code class="c++">#include <iostream>
#include <complex>
using namespace std;

int main() {
    complex<int> delta;
    complex<int> mc[4] = {0};

    for(int di = 0; di < 4; di++, delta = mc[di]) {
        cout << di << endl;
    }

    return 0;
}</code>

Contrary to the expected output of "0, 1, 2, 3" terminating, the code produces an endless repetition of "0, 1, 2, 3, 4, 5, ...". Upon investigation, it appears that the comparison di < 4 fails to work correctly, resulting in an infinite loop.

Disabling the seemingly innocuous assignment delta = mc[di] magically fixes the issue, generating the intended output. What is the problem that this simple action causes?

Delving into Undefined Behavior

The key to unraveling this mystery lies in understanding undefined behavior. The assignment delta = mc[di] triggers an access to an array out of bounds on the final iteration of the loop. While many compilers might assume no undefined behavior based on optimization strategies, this assumption is inherently flawed.

GCC, with optimizations turned on, can aggressively optimize the loop assuming the absence of undefined behavior. This optimization leads to an erroneous inference that di < 4 is always true, since an out-of-bounds array access would constitute undefined behavior.

As a result, the loop becomes an infinite one, as the condition di < 4 is always met. To prevent this erroneous optimization, the -fno-aggressive-loop-optimizations flag can be added to GCC.

Unveiling the Compiler's Inner Workings

A closer examination of the optimized code reveals that the di < 4 check is removed and replaced with an unconditional jmp instruction. This behavior aligns with the assumption of no undefined behavior, resulting in the infinite loop.

Contrary to this behavior, Clang with -fsanitize=undefined would catch this case. However, GCC with the same flag fails to raise a warning in this specific instance.

The Perils of Undefined Behavior

Undefined behavior, as defined by the C standard, allows for unpredictable results, including completely disregarding the situation. Undefined behavior should be avoided at all costs, as it can lead to both compiler inconsistencies and unexpected runtime behavior.

To avoid such pitfalls, programmers should strive for well-defined code that adheres to the intended semantics. Compilers should also implement appropriate defenses to alert developers to potential undefined behavior.

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