Does this union break strict aliasing? What about floating point registers

Question

union
{
    Uint32 Integer;
    Float32 Real;
} Field;    

I have to use that union for a little IEEE trick, does that break strict aliasing? GCC is no

Answer 1

Aliasing via a union is defined in C but has undefined behaviour in C++; the undefined behaviour is equivalent to that which occurs when reading from an uninitialized variable (a lvalue-to-rvalue conversion).

Accordingly, the most likely way this would break is in the optimiser deciding to eliminate the read from the union, as it does not have a defined value. However, most C-and-C++ compilers are likely to give you the C behaviour as they need to support that anyway.

The safe way to alias the values is via bytewise copy e.g. std::memcpy or std::copy(reinterpret_cast<char *>(...), ...). Alternatively, if you can compile your project in both C and C++ you could move the union aliasing code to a C source file and compile just that code as C.

Answer 2

It is UB (but doesn't require strict aliasing). Also, uniond data is always stored in memory by implementations, AFAIK, else would require knowing which register the source data came from, which means knowing the source type.