How can modern C mechanisms like `std::bit_cast` help you safely perform type punning?

Modern Type Punning in C

In C , type punning, the practice of interpreting a bit pattern of one type as a different type, can be a useful technique in specific situations like floating-point optimizations. However, it comes with its pitfalls, including unspecified behavior, strict aliasing, and lifetime issues.

To address these challenges, C has introduced several mechanisms for safe and efficient type punning, including:

• std::reinterpret_cast: The traditional reinterpret_cast still has issues with undefined behavior and aliasing violations.
• std::static_cast: Combined with void*, static_cast can provide a more explicit and safer cast than reinterpret_cast.
• std::bit_cast: Introduced in C 20, std::bit_cast provides a type-safe and efficient way to perform bit-wise conversion without violating strict aliasing rules.
• std::memcpy: While not explicitly designed for type punning, memcpy can be used to copy the bit pattern of one object to another, allowing for safe reinterpretation.
• std::launder: Can be used to convert a pointer to a pointer that doesn't violate strict aliasing rules.

For the concrete problem of rewriting the fast inverse square root function, using std::bit_cast would be the safest and most performant approach:

float fast_inv_sqrt(float number) {
  std::uint32_t i;
  float x2, y;
  const float threehalfs = 1.5F;

  x2 = number * 0.5F;
  y = number;
  i = std::bit_cast<std::uint32_t>(y);                       // type-safe bit-wise conversion
  i = 0x5f3759df - (i >> 1);               // what the
  y = std::bit_cast<float>(i);
  y = y * (threehalfs - (x2 * y * y));   // 1st iteration
//  y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed

  return y;
}

This solution avoids undefined behavior and aliasing violations while preserving performance. std::bit_cast is well-optimized by compilers, producing efficient code that adheres to the C type system.

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