py/runtime: Don't allocate iter buf for user-defined types.

A user-defined type that defines __iter__ doesn't need any memory to be
pre-allocated for its iterator (because it can't use such memory).  So
optimise for this case by not allocating the iter-buf.

py/objtype.c

@@ -882,7 +882,8 @@ mp_obj_t mp_obj_instance_call(mp_obj_t self_in, size_t n_args, size_t n_kw, cons
     return mp_call_method_self_n_kw(member[0], member[1], n_args, n_kw, args);
 }
 
-STATIC mp_obj_t instance_getiter(mp_obj_t self_in, mp_obj_iter_buf_t *iter_buf) {
+// Note that iter_buf may be NULL, and needs to be allocated if needed
+mp_obj_t mp_obj_instance_getiter(mp_obj_t self_in, mp_obj_iter_buf_t *iter_buf) {
     mp_obj_instance_t *self = MP_OBJ_TO_PTR(self_in);
     mp_obj_t member[2] = {MP_OBJ_NULL};
     struct class_lookup_data lookup = {
@@ -897,6 +898,9 @@ STATIC mp_obj_t instance_getiter(mp_obj_t self_in, mp_obj_iter_buf_t *iter_buf)
         return MP_OBJ_NULL;
     } else if (member[0] == MP_OBJ_SENTINEL) {
         mp_obj_type_t *type = mp_obj_get_type(self->subobj[0]);
+        if (iter_buf == NULL) {
+            iter_buf = m_new_obj(mp_obj_iter_buf_t);
+        }
         return type->getiter(self->subobj[0], iter_buf);
     } else {
         return mp_call_method_n_kw(0, 0, member);
@@ -1136,7 +1140,7 @@ mp_obj_t mp_obj_new_type(qstr name, mp_obj_t bases_tuple, mp_obj_t locals_dict)
     o->binary_op = instance_binary_op;
     o->attr = mp_obj_instance_attr;
     o->subscr = instance_subscr;
-    o->getiter = instance_getiter;
+    o->getiter = mp_obj_instance_getiter;
     //o->iternext = ; not implemented
     o->buffer_p.get_buffer = instance_get_buffer;
 

py/objtype.h

@@ -51,4 +51,7 @@ mp_obj_t mp_obj_instance_call(mp_obj_t self_in, size_t n_args, size_t n_kw, cons
 // this needs to be exposed for the above macros to work correctly
 mp_obj_t mp_obj_instance_make_new(const mp_obj_type_t *self_in, size_t n_args, size_t n_kw, const mp_obj_t *args);
 
+// this needs to be exposed for mp_getiter
+mp_obj_t mp_obj_instance_getiter(mp_obj_t self_in, mp_obj_iter_buf_t *iter_buf);
+
 #endif // MICROPY_INCLUDED_PY_OBJTYPE_H

py/runtime.c

@@ -34,6 +34,7 @@
 #include "py/objstr.h"
 #include "py/objtuple.h"
 #include "py/objlist.h"
+#include "py/objtype.h"
 #include "py/objmodule.h"
 #include "py/objgenerator.h"
 #include "py/smallint.h"
@@ -1165,13 +1166,13 @@ mp_obj_t mp_getiter(mp_obj_t o_in, mp_obj_iter_buf_t *iter_buf) {
         return o_in;
     }
 
-    // if caller did not provide a buffer then allocate one on the heap
-    if (iter_buf == NULL) {
-        iter_buf = m_new_obj(mp_obj_iter_buf_t);
-    }
-
     // check for native getiter (corresponds to __iter__)
     if (type->getiter != NULL) {
+        if (iter_buf == NULL && type->getiter != mp_obj_instance_getiter) {
+            // if caller did not provide a buffer then allocate one on the heap
+            // mp_obj_instance_getiter is special, it will allocate only if needed
+            iter_buf = m_new_obj(mp_obj_iter_buf_t);
+        }
         mp_obj_t iter = type->getiter(o_in, iter_buf);
         if (iter != MP_OBJ_NULL) {
             return iter;
@@ -1183,6 +1184,10 @@ mp_obj_t mp_getiter(mp_obj_t o_in, mp_obj_iter_buf_t *iter_buf) {
     mp_load_method_maybe(o_in, MP_QSTR___getitem__, dest);
     if (dest[0] != MP_OBJ_NULL) {
         // __getitem__ exists, create and return an iterator
+        if (iter_buf == NULL) {
+            // if caller did not provide a buffer then allocate one on the heap
+            iter_buf = m_new_obj(mp_obj_iter_buf_t);
+        }
         return mp_obj_new_getitem_iter(dest, iter_buf);
     }