+ /* the stack looks like this:
+
+ | | <- Sp + stk_offset
+ +-------------+
+ | |
+ | args |
+ | | <- Sp + ret_size + 1
+ +-------------+
+ | C fun | <- Sp + ret_size
+ +-------------+
+ | ret | <- Sp
+ +-------------+
+
+ ret is a placeholder for the return address, and may be
+ up to 2 words.
+
+ We need to copy the args out of the TSO, because when
+ we call suspendThread() we no longer own the TSO stack,
+ and it may move at any time - indeed suspendThread()
+ itself may do stack squeezing and move our args.
+ So we make a copy of the argument block.
+ */
+
+#define ROUND_UP_WDS(p) ((((StgWord)(p)) + sizeof(W_)-1)/sizeof(W_))
+
+ ffi_cif *cif = (ffi_cif *)marshall_fn;
+ nat nargs = cif->nargs;
+ nat ret_size;
+ nat i;
+ StgPtr p;
+ W_ ret[2]; // max needed
+ W_ *arguments[stk_offset]; // max needed
+ void *argptrs[nargs];
+ void (*fn)(void);
+
+ if (cif->rtype->type == FFI_TYPE_VOID) {
+ // necessary because cif->rtype->size == 1 for void,
+ // but the bytecode generator has not pushed a
+ // placeholder in this case.
+ ret_size = 0;
+ } else {
+ ret_size = ROUND_UP_WDS(cif->rtype->size);
+ }
+
+ memcpy(arguments, Sp+ret_size+1,
+ sizeof(W_) * (stk_offset-1-ret_size));
+
+ // libffi expects the args as an array of pointers to
+ // values, so we have to construct this array before making
+ // the call.
+ p = (StgPtr)arguments;
+ for (i = 0; i < nargs; i++) {
+ argptrs[i] = (void *)p;
+ // get the size from the cif
+ p += ROUND_UP_WDS(cif->arg_types[i]->size);
+ }