Why Does Increasing Optimization Level Halt the Swap_64 Function?

Optimization Pitfall: Why Function Swap_64 Halts When Optimization Level is Boosted

In a recent university lecture, a function known as Swap_64 was presented, which aimed to swap the 64-bit value by manipulating its 32-bit segments. However, when the optimization level was increased, the function was observed to behave unexpectedly.

Understanding the Optimization Issue

The Swap_64 function, as written, involves casting an unsigned 64-bit integer to an array of two unsigned 32-bit integers. This approach violates strict aliasing rules, which prohibit accessing an object through a pointer of a different type. In this case, accessing the 64-bit integer through a pointer to an array of 32-bit integers is considered unsafe.

According to strict aliasing, compilers assume that pointers of different types do not point to the same memory location. This allows for aggressive optimizations where aliased memory is assumed to be independent.

Consequences of Violation

In the Swap_64 function, the compiler is permitted to optimize out the assignments to the temporary variable tmp. This is because it assumes that the pointers used to access the 64-bit integer and its 32-bit segments do not alias each other.

By allowing this optimization, the compiler effectively removes the code responsible for swapping the bits. Consequently, when the optimization level is high, the Swap_64 function appears to do nothing as the bit manipulation assignments are optimized away.

Avoiding Undefined Behavior

To resolve this issue and ensure correct behavior even with high optimization levels, it's crucial to avoid violating strict aliasing rules. This can be achieved by using a union, which allows different types to occupy the same memory location.

Conclusion

Understanding strict aliasing rules is essential to avoid undefined behavior caused by compiler optimizations. By ensuring that objects are accessed only through compatible types, developers can prevent optimizations that may hinder program operation. The union approach showcased in the provided solution serves as an effective way to guarantee correctness even under aggressive optimization settings.

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