py/vm: When VM raises exception put exc obj at beginning of func state.

Instead of at end of state, n_state - 1.  It was originally (way back in
v1.0) put at the end of the state because the VM didn't have a pointer to
the start.  But now that the VM takes a mp_code_state_t pointer it does
have a pointer to the start of the state so can put the exception object
there.

This commit saves about 30 bytes of code on all architectures, and, more
importantly, reduces C-stack usage by a couple of words (8 bytes on Thumb2
and 16 bytes on x86-64) for every (non-generator) call of a bytecode
function because fun_bc_call no longer needs to remember the n_state
variable.

1 files changed, 1 insertions, 2 deletions

diff --git a/py/objgenerator.c b/py/objgenerator.c
index 038c15fc3..58a33d40b 100644
--- a/py/objgenerator.c
+++ b/py/objgenerator.c
@@ -140,9 +140,8 @@ mp_vm_return_kind_t mp_obj_gen_resume(mp_obj_t self_in, mp_obj_t send_value, mp_
             break;
 
         case MP_VM_RETURN_EXCEPTION: {
-            size_t n_state = mp_decode_uint_value(self->code_state.fun_bc->bytecode);
             self->code_state.ip = 0;
-            *ret_val = self->code_state.state[n_state - 1];
+            *ret_val = self->code_state.state[0];
             // PEP479: if StopIteration is raised inside a generator it is replaced with RuntimeError
             if (mp_obj_is_subclass_fast(MP_OBJ_FROM_PTR(mp_obj_get_type(*ret_val)), MP_OBJ_FROM_PTR(&mp_type_StopIteration))) {
                 *ret_val = mp_obj_new_exception_msg(&mp_type_RuntimeError, "generator raised StopIteration");