gccint.info: Interface

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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 (*note 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;

   Code compiled with GCC may call certain library routines.  Most of
them handle arithmetic for which there are no instructions.  This
includes multiply and divide on some machines, and floating point
operations on any machine for which floating point support is disabled
with `-msoft-float'.  Some standard parts of the C library, such as
`bcopy' or `memcpy', are also called automatically.  The usual function
call interface is used for calling the library routines.
   Some of these routines can be defined in mostly machine-independent
C; they appear in `libgcc2.c'.  Others must be hand-written in assembly
language for each processor.  Wherever they are defined, they are
compiled into the support library, `libgcc.a', which is automatically
searched when you link programs with GCC.