How Can We Safely Convert Floats to Integers in C While Avoiding Strict-Aliasing Violations?

Type-Punning: A Discussion on Proper Float-to-Int Conversion

The fast inverse square root operation presented in the code employs bit hacks to achieve efficiency. However, it raises concerns regarding type-punning and potential rule violations.

The Issue: Strict-Aliasing Violation

The compiler warns of dereferencing a type-punned pointer, thereby violating strict-aliasing rules. Strict-aliasing refers to the assumption that accessing memory through pointers of different types can lead to unintended consequences.

Alternative Casting Options

The question posed explores the suitability of using static_cast, reinterpret_cast, or dynamic_cast as potential solutions.

static_cast

Static_cast performs implicit conversion between compatible types. However, in this case, float and int32_t are not compatible types, making static_cast unsuitable.

reinterpret_cast

Reinterpret_cast allows for conversion between unrelated types. However, it merely changes the interpretation of the bits and does not guarantee type safety. Using reinterpret_cast would not resolve the aliasing violation.

dynamic_cast

Dynamic_cast is not applicable in this context since it is used for object-oriented programming and verifying type relationships at runtime.

The Correct Approach

The suggested solution involves using memcpy to achieve the type conversion. Memcpy copies bytes between memory locations without type interpretation, effectively bypassing the strict-aliasing issue.

Code Implementation

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;

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