py/objfun: Factor out macro for decoding codestate size.

fun_bc_call() starts with almost the same code as
mp_obj_fun_bc_prepare_codestate(), the only difference is a way to
allocate the codestate object (heap vs stack with heap fallback).
Still, would be nice to avoid code duplication to make further
refactoring easier.

So, this commit factors out the common code before the allocation -
decoding and calculating codestate size. It produces two values,
so structured as a macro which writes to 2 variables passed as
arguments.

py/objfun.c

@@ -195,20 +195,30 @@ STATIC void dump_args(const mp_obj_t *a, size_t sz) {
 // than this will try to use the heap, with fallback to stack allocation.
 #define VM_MAX_STATE_ON_STACK (11 * sizeof(mp_uint_t))
 
-// Set this to enable a simple stack overflow check.
+// Set this to 1 to enable a simple stack overflow check.
 #define VM_DETECT_STACK_OVERFLOW (0)
 
+#define DECODE_CODESTATE_SIZE(bytecode, n_state_out_var, state_size_out_var) \
+    { \
+        /* bytecode prelude: state size and exception stack size */               \
+        n_state_out_var = mp_decode_uint_value(bytecode);                         \
+        size_t n_exc_stack = mp_decode_uint_value(mp_decode_uint_skip(bytecode)); \
+                                                                                  \
+        n_state += VM_DETECT_STACK_OVERFLOW;                                      \
+                                                                                  \
+        /* state size in bytes */                                                 \
+        state_size_out_var = n_state * sizeof(mp_obj_t) + n_exc_stack * sizeof(mp_exc_stack_t); \
+    }
+
 #if MICROPY_STACKLESS
 mp_code_state_t *mp_obj_fun_bc_prepare_codestate(mp_obj_t self_in, size_t n_args, size_t n_kw, const mp_obj_t *args) {
     MP_STACK_CHECK();
     mp_obj_fun_bc_t *self = MP_OBJ_TO_PTR(self_in);
 
-    // bytecode prelude: state size and exception stack size
-    size_t n_state = mp_decode_uint_value(self->bytecode);
-    size_t n_exc_stack = mp_decode_uint_value(mp_decode_uint_skip(self->bytecode));
+    size_t n_state, state_size;
+    DECODE_CODESTATE_SIZE(self->bytecode, n_state, state_size);
 
     // allocate state for locals and stack
-    size_t state_size = n_state * sizeof(mp_obj_t) + n_exc_stack * sizeof(mp_exc_stack_t);
     mp_code_state_t *code_state;
     code_state = m_new_obj_var_maybe(mp_code_state_t, byte, state_size);
     if (!code_state) {
@@ -238,16 +248,10 @@ STATIC mp_obj_t fun_bc_call(mp_obj_t self_in, size_t n_args, size_t n_kw, const
     mp_obj_fun_bc_t *self = MP_OBJ_TO_PTR(self_in);
     DEBUG_printf("Func n_def_args: %d\n", self->n_def_args);
 
-    // bytecode prelude: state size and exception stack size
-    size_t n_state = mp_decode_uint_value(self->bytecode);
-    size_t n_exc_stack = mp_decode_uint_value(mp_decode_uint_skip(self->bytecode));
-
-#if VM_DETECT_STACK_OVERFLOW
-    n_state += 1;
-#endif
+    size_t n_state, state_size;
+    DECODE_CODESTATE_SIZE(self->bytecode, n_state, state_size);
 
     // allocate state for locals and stack
-    size_t state_size = n_state * sizeof(mp_obj_t) + n_exc_stack * sizeof(mp_exc_stack_t);
     mp_code_state_t *code_state = NULL;
     if (state_size > VM_MAX_STATE_ON_STACK) {
         code_state = m_new_obj_var_maybe(mp_code_state_t, byte, state_size);