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, 1 deletions

diff --git a/py/objfun.c b/py/objfun.c
index b03d4194f..ce6fd22a5 100644
--- a/py/objfun.c
+++ b/py/objfun.c
@@ -318,7 +318,7 @@ STATIC mp_obj_t fun_bc_call(mp_obj_t self_in, size_t n_args, size_t n_kw, const
         // must be an exception because normal functions can't yield
         assert(vm_return_kind == MP_VM_RETURN_EXCEPTION);
         // return value is in fastn[0]==state[n_state - 1]
-        result = code_state->state[n_state - 1];
+        result = code_state->state[0];
     }
 
     #if MICROPY_ENABLE_PYSTACK