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| <a name="Interfacing-to-GCC-Output"></a> |
| <h2 class="chapter">3 Interfacing to GCC Output</h2> |
| <a name="index-interfacing-to-GCC-output"></a> |
| <a name="index-run_002dtime-conventions"></a> |
| <a name="index-function-call-conventions"></a> |
| <a name="index-conventions_002c-run_002dtime"></a> |
| |
| <p>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 <a href="Target-Macros.html#Target-Macros">Target Macros</a>). |
| </p> |
| <a name="index-unions_002c-returning"></a> |
| <a name="index-structures_002c-returning"></a> |
| <a name="index-returning-structures-and-unions"></a> |
| <p>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. |
| </p> |
| <p>GCC code returns structures and unions that are 1, 2, 4 or 8 bytes |
| long in the same registers used for <code>int</code> or <code>double</code> 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 target hook <code>TARGET_STRUCT_VALUE_RTX</code> |
| tells GCC where to pass this address. |
| </p> |
| <p>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. |
| </p> |
| <p>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. |
| </p> |
| <a name="index-argument-passing"></a> |
| <a name="index-passing-arguments"></a> |
| <p>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. |
| </p> |
| <p>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. |
| </p> |
| <a name="index-longjmp-and-automatic-variables"></a> |
| <p>If you use <code>longjmp</code>, beware of automatic variables. ISO C says that |
| automatic variables that are not declared <code>volatile</code> have undefined |
| values after a <code>longjmp</code>. 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. |
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