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/vm.c b/py/vm.c
index 8da40c9b6..828ea79e5 100644
--- a/py/vm.c
+++ b/py/vm.c
@@ -1468,7 +1468,7 @@ unwind_loop:
             } else {
                 // propagate exception to higher level
                 // Note: ip and sp don't have usable values at this point
-                fastn[0] = MP_OBJ_FROM_PTR(nlr.ret_val); // must put exception here because sp is invalid
+                code_state->state[0] = MP_OBJ_FROM_PTR(nlr.ret_val); // put exception here because sp is invalid
                 return MP_VM_RETURN_EXCEPTION;
             }
         }