Why doesn't GCC optimize structs?

Question

Systems demand that certain primitives be aligned to certain points within the memory (ints to bytes that are multiples of 4, shorts to bytes that are multiples of 2, etc.).

Answer 1

Not saying it's a good idea, but you can certainly write code that relies on the order of the members of a struct. For example, as a hack, often people cast a pointer to a struct as the type of a certain field inside that they want access to, then use pointer arithmetic to get there. To me this is a pretty dangerous idea, but I've seen it used, especially in C++ to force a variable that's been declared private to be publicly accessible when it's in a class from a 3rd party library and isn't publicly encapsulated. Reordering the members would totally break that.