Why Does This C Code Produce an Infinite Loop on GCC with Optimizations Enabled?

Integer Overflow on x86: Unmasking the Infinite Loop Riddle

Consider the following code snippet that mysteriously plunges into an infinite loop when compiled using GCC:

int i = 0x10000000;
do {
    i += i;
    cout << i << endl;
} while (i > 0);

The Mystery Unraveled

While integer overflow is typically undefined behavior, the x86 architecture's integer arithmetic instructions normally wrap around if overflow occurs. However, in this case, GCC's optimizations introduce an anomaly.

Optimization's Misadventures

With optimizations enabled, GCC assumes integer overflow is impossible and eliminates the loop exit condition check. As a result, when the integer i wraps around to a negative value, the loop continues interminably.

Visual Studio's Correct Handling

Visual Studio, on the other hand, correctly handles the integer overflow and exits the loop as intended.

Determining Undefined Behavior

This bizarre behavior highlights the unpredictable nature of undefined behavior. Even though integer wraparound is expected on x86, undefined behavior can manifest in unexpected ways, frustrating the compiler's predictions.

Workarounds

• Disable optimizations (-O0) to enforce explicit overflow checks.
• Use the -fwrapv flag to enable well-defined overflow semantics. However, be aware of potential performance penalties.

Conclusion

GCC's choice to optimize away the loop exit condition in the presence of undefined behavior leads to the perplexing infinite loop. This serves as a reminder of the unpredictable consequences of violating undefined behavior rules, demanding caution when working with integer arithmetic on x86 platforms.

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