Interfacing to GCC Output
*************************

GCC is normally configured to use the same function calling convention normally in use on the target system.  This is done with the machine-description macros described (see Target Macros).

However, returning of structure and union values is done differently on some target machines.  As a result, functions compiled with PCC returning such types cannot be called from code compiled with GCC, and vice versa.  This does not cause trouble often because few Unix library routines return structures or unions.

GCC code returns structures and unions that are 1, 2, 4 or 8 bytes long in the same registers used for int or double return values.  (GCC typically allocates variables of such types in registers also.)  Structures and unions of other sizes are returned by storing them into an address passed by the caller (usually in a register).  The machine-description macros STRUCT_VALUE and STRUCT_INCOMING_VALUE tell GCC where to pass this address.

By contrast, PCC on most target machines returns structures and unions of any size by copying the data into an area of static storage, and then returning the address of that storage as if it were a pointer value. The caller must copy the data from that memory area to the place where the value is wanted.  This is slower than the method used by GCC, and fails to be reentrant.

On some target machines, such as RISC machines and the 80386, the standard system convention is to pass to the subroutine the address of where to return the value.  On these machines, GCC has been configured to be compatible with the standard compiler, when this method is used.  It may not be compatible for structures of 1, 2, 4 or 8 bytes.

GCC uses the system's standard convention for passing arguments.  On some machines, the first few arguments are passed in registers; in others, all are passed on the stack.  It would be possible to use registers for argument passing on any machine, and this would probably result in a significant speedup.  But the result would be complete incompatibility with code that follows the standard convention.  So this change is practical only if you are switching to GCC as the sole C compiler for the system.  We may implement register argument passing on certain machines once we have a complete GNU system so that we can compile the libraries with GCC.

On some machines (particularly the SPARC), certain types of arguments are passed ``by invisible reference''.  This means that the value is stored in memory, and the address of the memory location is passed to the subroutine.

If you use longjmp, beware of automatic variables.  ISO C says that automatic variables that are not declared volatile have undefined values after a longjmp.  And this is all GCC promises to do, because it is very difficult to restore register variables correctly, and one of GCC's features is that it can put variables in registers without your asking it to.

If you want a variable to be unaltered by longjmp, and you don't want to write volatile because old C compilers don't accept it, just take the address of the variable.  If a variable's address is ever taken, even if just to compute it and ignore it, then the variable cannot go in a register:

     {
       int careful;
       &careful;
       ...
     }