What's a proper way of type-punning a float to an int and vice-versa?

Question

The code below performs a fast inverse square root operation by some bit hacks. The algorithm was probably developed by Silicon Graphics in early 1990's and it's appeared in Quake 3 too. more info

However I get the following warning from GCC C++ compiler: dereferencing type-punned pointer will break strict-aliasing rules

Should I use static_cast, reinterpret_cast or dynamic_cast instead in such situations?

float InverseSquareRoot(float x)
{
    float xhalf = 0.5f*x;
    int32_t i = *(int32_t*)&x;
    i = 0x5f3759df - (i>>1);
    x = *(float*)&i;
    x = x*(1.5f - xhalf*x*x);
    return x;
}

Answer 1

Forget casts. Use memcpy.

float xhalf = 0.5f*x;
uint32_t i;
assert(sizeof(x) == sizeof(i));
std::memcpy(&i, &x, sizeof(i));
i = 0x5f375a86 - (i>>1);
std::memcpy(&x, &i, sizeof(i));
x = x*(1.5f - xhalf*x*x);
return x;

The original code tries to initialize the int32_t by first accessing the float object through an int32_t pointer, which is where the rules are broken. The C-style cast is equivalent to a reinterpret_cast, so changing it to reinterpret_cast would not make much difference.

The important difference when using memcpy is that the bytes are copied from the float into the int32_t, but the float object is never accessed through an int32_t lvalue, because memcpy takes pointers to void and its insides are "magical" and don't break the aliasing rules.