How to type-pun and why

[Gabriel Dos Reis]

 Mattias Engdegård <mattias@virtutech.se> writes:

| Using a union intermediate instead,
| 
|   union { double d; uint64_t x; } u;
|   u.d = something;
|   uint64_t x = u.x;
| 
| works as expected, but I would like to know why - if it is a matter of luck
| with the current implementation it might cease working when the compiler
| changes or becomes more aggressive in the future.

that union trick is invalid as well, from the standard point of view.
However, GCC makes a guarantee that it will be supported.