From ae712b58c557909a24bd049468f8304a2b5797c8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20H=C3=B6gberg?= <john@erlang.org>
Date: Tue, 9 Apr 2019 07:18:34 +0200
Subject: [PATCH] Skip redundant enif_is_empty_list checks during encode

enif_get_list_cell fails when the list is empty.
---
 c_src/encoder.c | 25 +++++--------------------
 1 file changed, 5 insertions(+), 20 deletions(-)

diff --git a/c_src/encoder.c b/c_src/encoder.c
index dc848ae..9d7a734 100644
--- a/c_src/encoder.c
+++ b/c_src/encoder.c
@@ -656,17 +656,13 @@ encode_iter(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
         if(enif_is_identical(curr, e->atoms->ref_object)) {
             curr = termstack_pop(&stack);
 
-            if(enif_is_empty_list(env, curr)) {
+            if(!enif_get_list_cell(env, curr, &item, &curr)) {
                 if(!enc_end_object(e)) {
                     ret = enc_error(e, "internal_error");
                     goto done;
                 }
                 continue;
             }
-            if(!enif_get_list_cell(env, curr, &item, &curr)) {
-                ret = enc_error(e, "internal_error");
-                goto done;
-            }
             if(!enif_get_tuple(env, item, &arity, &tuple)) {
                 ret = enc_obj_error(e, "invalid_object_member", item);
                 goto done;
@@ -694,7 +690,7 @@ encode_iter(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
         } else if(enif_is_identical(curr, e->atoms->ref_array)) {
             curr = termstack_pop(&stack);
 
-            if(enif_is_empty_list(env, curr)) {
+            if(!enif_get_list_cell(env, curr, &item, &curr)) {
                 if(!enc_end_array(e)) {
                     ret = enc_error(e, "internal_error");
                     goto done;
@@ -705,10 +701,6 @@ encode_iter(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
                 ret = enc_error(e, "internal_error");
                 goto done;
             }
-            if(!enif_get_list_cell(env, curr, &item, &curr)) {
-                ret = enc_error(e, "internal_error");
-                goto done;
-            }
 
             termstack_push(&stack, curr);
             termstack_push(&stack, e->atoms->ref_array);
@@ -766,17 +758,13 @@ encode_iter(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
                 ret = enc_error(e, "internal_error");
                 goto done;
             }
-            if(enif_is_empty_list(env, tuple[0])) {
+            if(!enif_get_list_cell(env, tuple[0], &item, &curr)) {
                 if(!enc_end_object(e)) {
                     ret = enc_error(e, "internal_error");
                     goto done;
                 }
                 continue;
             }
-            if(!enif_get_list_cell(env, tuple[0], &item, &curr)) {
-                ret = enc_error(e, "internal_error");
-                goto done;
-            }
             if(!enif_get_tuple(env, item, &arity, &tuple)) {
                 ret = enc_obj_error(e, "invalid_object_member", item);
                 goto done;
@@ -811,17 +799,14 @@ encode_iter(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
                 ret = enc_error(e, "internal_error");
                 goto done;
             }
-            if(enif_is_empty_list(env, curr)) {
+
+            if(!enif_get_list_cell(env, curr, &item, &curr)) {
                 if(!enc_end_array(e)) {
                     ret = enc_error(e, "internal_error");
                     goto done;
                 }
                 continue;
             }
-            if(!enif_get_list_cell(env, curr, &item, &curr)) {
-                ret = enc_error(e, "internal_error");
-                goto done;
-            }
 
             termstack_push(&stack, curr);
             termstack_push(&stack, e->atoms->ref_array);