vm: Don't unconditionally allocate state on stack, do that only if needed.

This makes sure that only as much stack allocated as actually used, reducing
stack usage for each Python function call.

py/vm.c

@@ -28,6 +28,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <assert.h>
+#include <alloca.h>
 
 #include "mpconfig.h"
 #include "nlr.h"
@@ -117,21 +118,23 @@ mp_vm_return_kind_t mp_execute_bytecode(const byte *code, const mp_obj_t *args,
     ip += 4;
 
     // allocate state for locals and stack
-    mp_obj_t temp_state[VM_MAX_STATE_ON_STACK];
-    mp_obj_t *state = &temp_state[0];
 #if DETECT_VM_STACK_OVERFLOW
     n_state += 1;
 #endif
+    mp_obj_t *state;
     if (n_state > VM_MAX_STATE_ON_STACK) {
         state = m_new(mp_obj_t, n_state);
+    } else {
+        state = alloca(sizeof(mp_obj_t) * n_state);
     }
     mp_obj_t *sp = &state[0] - 1;
 
     // allocate state for exceptions
-    mp_exc_stack_t exc_state[VM_MAX_EXC_STATE_ON_STACK];
-    mp_exc_stack_t *exc_stack = &exc_state[0];
+    mp_exc_stack_t *exc_stack;
     if (n_exc_stack > VM_MAX_EXC_STATE_ON_STACK) {
         exc_stack = m_new(mp_exc_stack_t, n_exc_stack);
+    } else {
+        exc_stack = alloca(sizeof(mp_exc_stack_t) * n_exc_stack);
     }
     mp_exc_stack_t *exc_sp = &exc_stack[0] - 1;