From 5b5562c1d13fa0b78fd13731cebdaa911eb47726 Mon Sep 17 00:00:00 2001
From: Damien George <damien.p.george@gmail.com>
Date: Sat, 31 May 2014 17:59:11 +0100
Subject: [PATCH] py: Fix stack underflow with optimised for loop.

---
 py/compile.c               | 2 +-
 py/vm.c                    | 6 +++---
 tests/basics/for_return.py | 7 +++++++
 3 files changed, 11 insertions(+), 4 deletions(-)
 create mode 100644 tests/basics/for_return.py

diff --git a/py/compile.c b/py/compile.c
index c90772a7e3..1f0d90570e 100644
--- a/py/compile.c
+++ b/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);
diff --git a/py/vm.c b/py/vm.c
index aa7e0e2cfc..75093d2401 100644
--- a/py/vm.c
+++ b/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);
         }
     }
diff --git a/tests/basics/for_return.py b/tests/basics/for_return.py
new file mode 100644
index 0000000000..0441352ad9
--- /dev/null
+++ b/tests/basics/for_return.py
@@ -0,0 +1,7 @@
+# test returning from within a for loop
+
+def f():
+    for i in [1, 2, 3]:
+        return i
+
+print(f())