py: Make all LOAD_FAST ops check for unbound local.

This is necessary to catch all cases where locals are referenced before
assignment.  We still keep the _0, _1, _2 versions of LOAD_FAST to help
reduced the byte code size in RAM.

Addresses issue #457.

py/bc0.h

@@ -17,7 +17,6 @@
 #define MP_BC_LOAD_FAST_1        (0x21)
 #define MP_BC_LOAD_FAST_2        (0x22)
 #define MP_BC_LOAD_FAST_N        (0x23) // uint
-#define MP_BC_LOAD_FAST_CHECKED  (0x24) // uint
 #define MP_BC_LOAD_DEREF         (0x25) // uint
 #define MP_BC_LOAD_NAME          (0x26) // qstr
 #define MP_BC_LOAD_GLOBAL        (0x27) // qstr

py/emitbc.c

@@ -413,17 +413,11 @@ STATIC void emit_bc_load_null(emit_t *emit) {
 STATIC void emit_bc_load_fast(emit_t *emit, qstr qstr, uint id_flags, int local_num) {
     assert(local_num >= 0);
     emit_bc_pre(emit, 1);
-    if (id_flags & ID_FLAG_IS_DELETED) {
+    switch (local_num) {
-        // This local may be deleted, so need to do a checked load.
+        case 0: emit_write_byte_code_byte(emit, MP_BC_LOAD_FAST_0); break;
-        emit_write_byte_code_byte_uint(emit, MP_BC_LOAD_FAST_CHECKED, local_num);
+        case 1: emit_write_byte_code_byte(emit, MP_BC_LOAD_FAST_1); break;
-    } else {
+        case 2: emit_write_byte_code_byte(emit, MP_BC_LOAD_FAST_2); break;
-        // This local is never deleted, so can do a fast, uncheched load.
+        default: emit_write_byte_code_byte_uint(emit, MP_BC_LOAD_FAST_N, local_num); break;
-        switch (local_num) {
-            case 0: emit_write_byte_code_byte(emit, MP_BC_LOAD_FAST_0); break;
-            case 1: emit_write_byte_code_byte(emit, MP_BC_LOAD_FAST_1); break;
-            case 2: emit_write_byte_code_byte(emit, MP_BC_LOAD_FAST_2); break;
-            default: emit_write_byte_code_byte_uint(emit, MP_BC_LOAD_FAST_N, local_num); break;
-        }
     }
 }
 

py/emitpass1.c

@@ -96,7 +96,9 @@ STATIC void emit_pass1_store_id(emit_t *emit, qstr qstr) {
 
 STATIC void emit_pass1_delete_id(emit_t *emit, qstr qstr) {
     id_info_t *id = get_id_for_modification(emit->scope, qstr);
-    id->flags |= ID_FLAG_IS_DELETED;
+    // this flag is unused
+    //id->flags |= ID_FLAG_IS_DELETED;
+    (void)id; // suppress compiler warning
 }
 
 const emit_method_table_t emit_pass1_method_table = {

py/showbc.c

@@ -132,11 +132,6 @@ void mp_byte_code_print(const byte *ip, int len) {
                 printf("LOAD_FAST_N " UINT_FMT, unum);
                 break;
 
-            case MP_BC_LOAD_FAST_CHECKED:
-                DECODE_UINT;
-                printf("LOAD_FAST_CHECKED " UINT_FMT, unum);
-                break;
-
             case MP_BC_LOAD_DEREF:
                 DECODE_UINT;
                 printf("LOAD_DEREF " UINT_FMT, unum);

py/vm.c

@@ -252,25 +252,21 @@ dispatch_loop:
                         break;
 
                     case MP_BC_LOAD_FAST_0:
-                        PUSH(fastn[0]);
+                        obj1 = fastn[0];
-                        break;
+                        goto load_check;
 
                     case MP_BC_LOAD_FAST_1:
-                        PUSH(fastn[-1]);
+                        obj1 = fastn[-1];
-                        break;
+                        goto load_check;
 
                     case MP_BC_LOAD_FAST_2:
-                        PUSH(fastn[-2]);
+                        obj1 = fastn[-2];
-                        break;
+                        goto load_check;
 
                     case MP_BC_LOAD_FAST_N:
-                        DECODE_UINT;
-                        PUSH(fastn[-unum]);
-                        break;
-
-                    case MP_BC_LOAD_FAST_CHECKED:
                         DECODE_UINT;
                         obj1 = fastn[-unum];
+                        load_check:
                         if (obj1 == MP_OBJ_NULL) {
                             local_name_error:
                             nlr_raise(mp_obj_new_exception_msg(&mp_type_NameError, "local variable referenced before assignment"));
@@ -281,11 +277,7 @@ dispatch_loop:
                     case MP_BC_LOAD_DEREF:
                         DECODE_UINT;
                         obj1 = mp_obj_cell_get(fastn[-unum]);
-                        if (obj1 == MP_OBJ_NULL) {
+                        goto load_check;
-                            goto local_name_error;
-                        }
-                        PUSH(obj1);
-                        break;
 
                     case MP_BC_LOAD_NAME:
                         DECODE_QSTR;

tests/basics/unboundlocal.py

@@ -0,0 +1,19 @@
+# locals referenced before assignment
+
+def f1():
+    print(x)
+    x = 1
+
+def f2():
+    for i in range(0):
+        print(i)
+    print(i)
+
+def check(f):
+    try:
+        f()
+    except NameError:
+        print("NameError")
+
+check(f1)
+check(f2)