Why Does This Integer Overflow Code Result in an Infinite Loop in GCC on x86?

Understanding Integer Overflow in GCC on x86

Problem:

The code provided enters an infinite loop on GCC when attempting integer overflow with an x86 architecture:

int i = 0x10000000;
while (i > 0) {
    i += i;
}

Reason:

Integer overflow in GCC on x86 is undefined behavior, and the behavior observed can vary. GCC usually implements integer arithmetic using x86 integer instructions, which typically wrap on overflow. However, in this case, the compiler assumes the overflow is not intended and performs optimizations that result in the infinite loop.

Solution:

To ensure wrap-around behavior, explicit overflow control is necessary. This can be achieved by:

• Enabling Overflow Checking: Passing the -fwrapv flag to GCC enforces twos-complement overflow semantics, ensuring predictable wrap-around. However, this can impact performance.

Alternative Implementations:

• Shift Operation: Left-shifting by 1 (i <<= 1;) does not exhibit the infinite loop behavior, as it doesn't cause overflow.

Code Explanation:

The assembly code of the problematic loop reveals that the compiler optimizes away the loop test based on the assumption that overflow will not occur. As a result, the loop continues indefinitely.

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