How Can Compiler Optimizations Lead to Undefined Function Behavior, and How Can This Be Avoided?

Optimization and Function Behavior: Addressing Undefined Function Behavior

In the realm of programming, achieving optimal performance often comes with a trade-off. One such compromise is the potential for unforeseen function behavior due to compiler optimizations. To illustrate this phenomenon, let's delve into a specific scenario involving the following function:

inline u64 Swap_64(u64 x) {
    u64 tmp;
    (*(u32*)&tmp) = Swap_32(*(((u32*)&x)+1));
    (*(((u32*)&tmp)+1)) = Swap_32(*(u32*)&x);

    return tmp;
}

Initially, this function operated effortlessly in production code. However, upon enabling high optimization levels, it inexplicably ceased to function. The compiler's aggressive optimizations inadvertently eliminated all assignments to the temporary variable tmp, rendering the function essentially useless.

Delving into the cause behind this behavior, the culprit lies in the violation of strict aliasing rules. These rules forbid accessing an object through a pointer of a different type. In this instance, the code manipulates x through both u64 and u32 pointers, a violation that the compiler assumes is safe to optimize away.

The resulting code invokes undefined behavior, meaning the compiler is free to behave in any unpredictable manner. As a result, the expected function behavior is compromised, leading to the observed failure.

To mitigate this issue and ensure consistent function performance across optimization levels, the strict aliasing rules must be respected. One effective solution is type-punning through a union, a technique that allows accessing an object through multiple types while maintaining compiler compliance.

In the context of the given function, employing a union to achieve this type-punning would involve the following code:

typedef union {
    uint32_t u32;
    uint16_t u16[2];
} U32;

uint32_t Swap_64(uint32_t arg) {
    U32 in;
    uint16_t lo;
    uint16_t hi;

    in.u32 = arg;
    hi = in.u16[0];
    lo = in.u16[1];
    in.u16[0] = lo;
    in.u16[1] = hi;

    return in.u32;
}

By adhering to strict aliasing rules, this revised code ensures the expected function behavior is preserved even under aggressive compiler optimizations.

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