How Can `memcpy` Solve Strict-Aliasing Issues in Type-Punning Float-to-Int Conversions?

Type-Punning Float-to-Int Conversions: Resolving Strict-Aliasing Issues with Memcpy

In programming, type-punning refers to accessing data of one type through a reference to a different type. While convenient for performance optimization, type-punning can introduce strict-aliasing violations in optimized compilers like GCC C .

Consider the code below, which performs an inverse square root operation using bit hacks:

float InverseSquareRoot(float x) {
    float xhalf = 0.5f*x;
    int32_t i = *(int32_t*)&x; // Dereferencing type-punned pointer, breaking strict-aliasing rules
    i = 0x5f3759df - (i >> 1);
    x = *(float*)&i;
    x = x*(1.5f - xhalf*x*x);
    return x;
}

This code generates a warning from GCC regarding strict-aliasing rules. To address this issue, the code should be modified to avoid accessing the float object through an int32_t lvalue. Instead, memcpy() can be used to safely copy the bytes between the float and int32_t objects.

float InverseSquareRoot(float x) {
    float xhalf = 0.5f*x;
    uint32_t i;
    assert(sizeof(x) == sizeof(i));
    std::memcpy(&i, &x, sizeof(i)); // Use memcpy to safely copy bytes
    i = 0x5f375a86 - (i >> 1);
    std::memcpy(&x, &i, sizeof(i));
    x = x*(1.5f - xhalf*x*x);
    return x;
}

By relying on memcpy, this code maintains data integrity while ensuring type safety and adherence to strict-aliasing rules.

The above is the detailed content of How Can `memcpy` Solve Strict-Aliasing Issues in Type-Punning Float-to-Int Conversions?. For more information, please follow other related articles on the PHP Chinese website!