py: Fix stack underflow with optimised for loop.

py/compile.c

@@ -1745,7 +1745,7 @@ void compile_while_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
 // And, if the loop never runs, the loop variable should never be assigned
 void compile_for_stmt_optimised_range(compiler_t *comp, mp_parse_node_t pn_var, mp_parse_node_t pn_start, mp_parse_node_t pn_end, mp_parse_node_t pn_step, mp_parse_node_t pn_body, mp_parse_node_t pn_else) {
     START_BREAK_CONTINUE_BLOCK
-    comp->break_label |= MP_EMIT_BREAK_FROM_FOR;
+    // note that we don't need to pop anything when breaking from an optimise for loop
 
     uint top_label = comp_next_label(comp);
     uint entry_label = comp_next_label(comp);

py/vm.c

@@ -159,8 +159,8 @@ mp_vm_return_kind_t mp_execute_bytecode(const byte *code, const mp_obj_t *args,
 
 #if DETECT_VM_STACK_OVERFLOW
     if (vm_return_kind == MP_VM_RETURN_NORMAL) {
-        if (sp != state) {
-            printf("Stack misalign: %d\n", sp - state);
+        if (sp < state) {
+            printf("VM stack underflow: " INT_FMT "\n", sp - state);
             assert(0);
         }
     }
@@ -178,7 +178,7 @@ mp_vm_return_kind_t mp_execute_bytecode(const byte *code, const mp_obj_t *args,
             }
         }
         if (overflow) {
-            printf("VM stack overflow state=%p n_state+1=%u\n", state, n_state);
+            printf("VM stack overflow state=%p n_state+1=" UINT_FMT "\n", state, n_state);
             assert(0);
         }
     }

tests/basics/for_return.py

+# test returning from within a for loop
+
+def f():
+    for i in [1, 2, 3]:
+        return i
+
+print(f())