Basically, what I want is to "store" a signed 32-bit int inside (in the 32 rightmost bits) an unsigned 64-bit int - since I want to use the leftmost 32 bits for other purposes.
What I'm doing right now is a simple cast and mask:
#define packInt32(X) ((uint64_t)X | INT_MASK)
But this approach has an obvious issue: If X is a positive int (the first bit is not set), everything goes fine. If it's negative, it becomes messy.
The question is:
How to achieve the above, also supporting negative numbers, in the fastest and most-efficient way?
The "mess" you mention happens because you cast a small signed type to a large unsigned type. During this conversion the size is adjusted first with applying sign extension. This is what causes your trouble.
You can simply cast the (signed) integer to an unsigned type of same size first. Then casting to 64 bit will not trigger sign extension:
#define packInt32(X) ((uint64_t)(uint32_t)(X) | INT_MASK)