Why Does Signed Integer Overflow Lead to Infinite Loops in GCC x86 Optimizations?

Why Does Integer Overflow Cause Infinite Looping in GCC on x86?

The provided code enters an infinite loop on GCC due to undefined behavior caused by signed integer overflow. While x86 integer instructions typically wrap on overflow, GCC optimizes away the loop test in this case.

Details:

• Because integer overflow is undefined behavior, anything can occur, including unusual behavior such as not wrapping around.
• GCC typically performs optimizations assuming well-defined behavior, including loop test optimization.
• However, the undefined behavior caused by overflow disrupts this assumption.
• The optimized code skips the loop test, resulting in an infinite loop.

Resolution:

• To obtain wraparound behavior, use the -fwrapv flag when compiling with GCC, which enables well-defined wrapping semantics for signed integers.
• Be aware that this can impact performance compared to unprotected optimized behavior.

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