Fix Arm calling convention by storing arguments to stack

Author: DrXiao

src/globals.c

@@ -947,7 +947,31 @@ func_t *add_func(char *func_name, bool synthesize)
     hashmap_put(FUNC_MAP, func_name, func);
     /* Use interned string for function name */
     strcpy(func->return_def.var_name, intern_string(func_name));
-    func->stack_size = 4;
+    /* Prepare space for function arguments and global pointer.
+     *
+     * For Arm architecture, the first four arguments are passed to r0 ~ r3,
+     * and any additional arguments are passed to the stack.
+     *
+     * +-------------+ <-- sp + 20
+     * | (val of ip) |
+     * +-------------+ <-- sp + 16
+     * |    arg 8    |
+     * +-------------+ <-- sp + 12
+     * |    arg 7    |
+     * +-------------+ <-- sp + 8
+     * |    arg 6    |
+     * +-------------+ <-- sp + 4
+     * |    arg 5    |
+     * +-------------+ <-- sp
+     *
+     * However, register r12 (ip) holds the global pointer that points to
+     * global stack, but this register may be changed after an external call.
+     *
+     * Therefore, the current strategy is preparing additional space for ip,
+     * saves its orignal value to stack before an external call, and restore
+     * it from the stack after the external function returns.
+     * */
+    func->stack_size = 20;
 
     if (synthesize)
         return func;

src/reg-alloc.c

@@ -768,12 +768,40 @@ void reg_alloc(void)
                     if (!callee_func->num_params)
                         spill_alive(bb, insn);
 
-                    if (dynlink)
+                    if (dynlink) {
                         callee_func->is_used = true;
+                        if (!callee_func->bbs) {
+                            int ofs = 0;
+                            if (args > 4) {
+                                /* Store args to stack for Arm output */
+                                for (int i = 4; i < args; i++) {
+                                    ir = bb_add_ph2_ir(bb, OP_store);
+                                    ir->src0 = i;
+                                    ir->src1 = ofs;
+                                    ofs += 4;
+                                }
+                            }
+                            /* FIXME:
+                             * use a better way to preserve global pointer */
+                            ir = bb_add_ph2_ir(bb, OP_store);
+                            ir->src0 = 12;
+                            ir->src1 = 16;
+                        }
+                    }
 
                     ir = bb_add_ph2_ir(bb, OP_call);
                     strcpy(ir->func_name, insn->str);
 
+                    if (dynlink) {
+                        if (!callee_func->bbs) {
+                            /* FIXME:
+                             * use a better way to restore global pointer */
+                            ir = bb_add_ph2_ir(bb, OP_load);
+                            ir->dest = 12;
+                            ir->src0 = 16;
+                        }
+                    }
+
                     is_pushing_args = false;
                     args = 0;