Passing Classes Directly on Various Processors

Classes (and unions) passed directly by the C++ compiler are passed exactly as the C compiler would pass a struct (or union). However, C++ structs and unions are passed differently on different architectures.

On SPARC V7/V8, structs and unions are passed and returned by allocating storage within the caller and passing a pointer to that storage. (That is, all structs and unions are passed by reference.)

On SPARC V9, structs with a size no greater than 16 bytes (32 bytes) are passed (returned) in registers. Unions and all other structs are passed and returned by allocating storage within the caller and passing a pointer to that storage. (That is, small structs are passed in registers; unions and large structs are passed by reference.) As a consequence, small value classes are passed as efficiently as primitive types.

On x86, structs and unions are passed by allocating space on the stack and copying the argument onto the stack. Structs and unions are returned by allocating a temporary object in the caller's frame and passing the address of the temporary object as an implicit first parameter.