Implement lists:reverse/1,2 as a nif

Also rewrite several lists module functions to not use this nif, either because
it was not necessary (lists:seq/2,3) or because a non-tail recursive loop
is more efficient memory-wise.

Signed-off-by: Paul Guyot <pguyot@kallisys.net>

Author: pguyot

libs/estdlib/src/lists.erl

@@ -33,6 +33,7 @@
     member/2,
     delete/2,
     reverse/1,
+    reverse/2,
     foreach/2,
     keydelete/3,
     keyfind/3,
@@ -111,28 +112,56 @@ delete(E, L) ->
 
 %% @private
 delete(_, [], Accum) ->
-    reverse(Accum);
+    ?MODULE:reverse(Accum);
 delete(E, [E | T], Accum) ->
-    reverse(Accum) ++ T;
+    ?MODULE:reverse(Accum) ++ T;
 delete(E, [H | T], Accum) ->
     delete(E, T, [H | Accum]).
 
 %%-----------------------------------------------------------------------------
 %% @param   L the list to reverse
 %% @returns the elements of L in reverse order
-%% @doc     Reverse the elements of L.
+%% @equiv lists:reverse(L, [])
 %% @end
 %%-----------------------------------------------------------------------------
--spec reverse(list()) -> list().
-reverse(L) ->
-    %% TODO this should be done in unit time in a BIF
-    reverse(L, []).
+-spec reverse(L :: list()) -> list().
+reverse(_L) ->
+    erlang:nif_error(undefined).
 
-%% @private
-reverse([], Accum) ->
-    Accum;
-reverse([H | T], Accum) ->
-    reverse(T, [H | Accum]).
+%%-----------------------------------------------------------------------------
+%% @param   L the list to reverse
+%% @param   T the tail to append to the reversed list
+%% @returns the elements of L in reverse order followed by T
+%% @doc     Reverse the elements of L, folled by T.
+%% If T is not a list or not a proper list, it is appended anyway and the result
+%% will be an improper list.
+%%
+%% Following Erlang/OTP tradition, `lists:reverse/1,2` is a nif. It computes
+%% the length and then allocates memory for the list at once (2 * n terms).
+%%
+%% While this is much faster with AtomVM as allocations are expensive with
+%% default heap growth strategy, it can consume more memory until the list
+%% passed is garbage collected, as opposed to a recursive implementation where
+%% the process garbage collect part of the input list during the reversal.
+%%
+%% Consequently, tail-recursive implementations calling `lists:reverse/2`
+%% can be as expensive or more expensive in memory than list comprehensions or
+%% non-tail recursive versions depending on the number of terms saved on the
+%% stack between calls.
+%%
+%% For example, a non-tail recursive join/2 implementation requires two terms
+%% on stack for each iteration, so when it returns it will use
+%% `n * 3' (stack) + `n * 4' (result list)
+%% a tail recursive version will use, on last iteration:
+%% `n * 4' (reversed list) + n * 4' (result list)
+%% @end
+%%-----------------------------------------------------------------------------
+-spec reverse
+    (L :: nonempty_list(E), T :: list(E)) -> nonempty_list(E);
+    (L :: nonempty_list(), T :: any()) -> maybe_improper_list();
+    (L :: [], T) -> T when T :: any().
+reverse(_L, _T) ->
+    erlang:nif_error(undefined).
 
 %%-----------------------------------------------------------------------------
 %% @param   Fun the predicate to evaluate
@@ -162,13 +191,13 @@ keydelete(K, I, L) ->
 
 %% @private
 keydelete(_K, _I, [], L) ->
-    reverse(L);
+    ?MODULE:reverse(L);
 keydelete(K, I, [H | T], L2) when is_tuple(H) ->
     case I =< tuple_size(H) of
         true ->
             case element(I, H) of
                 K ->
-                    reverse(L2) ++ T;
+                    ?MODULE:reverse(L2, T);
                 _ ->
                     keydelete(K, I, T, [H | L2])
             end;
@@ -256,7 +285,7 @@ keyreplace(K, I, [H | T], L, NewTuple, NewList) when is_tuple(H) andalso is_tupl
         true ->
             case element(I, H) of
                 K ->
-                    reverse(NewList) ++ [NewTuple | T];
+                    ?MODULE:reverse(NewList, [NewTuple | T]);
                 _ ->
                     keyreplace(K, I, T, L, NewTuple, [H | NewList])
             end;
@@ -298,7 +327,7 @@ foldl(Fun, Acc0, [H | T]) ->
     List :: list()
 ) -> Acc1 :: term().
 foldr(Fun, Acc0, List) ->
-    foldl(Fun, Acc0, reverse(List)).
+    foldl(Fun, Acc0, ?MODULE:reverse(List)).
 
 %%-----------------------------------------------------------------------------
 %% @param   Fun the predicate to evaluate
@@ -381,19 +410,8 @@ search(Pred, [H | T]) ->
 %% @end
 %%-----------------------------------------------------------------------------
 -spec filter(Pred :: fun((Elem :: term()) -> boolean()), List :: list()) -> list().
-filter(Pred, L) ->
-    filter(Pred, L, []).
-
-%% @private
-filter(_Pred, [], Accum) ->
-    reverse(Accum);
-filter(Pred, [H | T], Accum) ->
-    case Pred(H) of
-        true ->
-            filter(Pred, T, [H | Accum]);
-        _ ->
-            filter(Pred, T, Accum)
-    end.
+filter(Pred, L) when is_function(Pred, 1) ->
+    [X || X <- L, Pred(X)].
 
 %%-----------------------------------------------------------------------------
 %% @param   Sep the separator
@@ -403,16 +421,16 @@ filter(Pred, [H | T], Accum) ->
 %% @end
 %%-----------------------------------------------------------------------------
 -spec join(Sep :: any(), List :: list()) -> list().
-join(Sep, L) ->
-    join(L, Sep, []).
+join(_Sep, []) ->
+    [];
+join(Sep, [H | Tail]) ->
+    [H | join_1(Sep, Tail)].
 
 %% @private
-join([], _Sep, Accum) ->
-    lists:reverse(Accum);
-join([E | R], Sep, []) ->
-    join(R, Sep, [E]);
-join([E | R], Sep, Accum) ->
-    join(R, Sep, [E, Sep | Accum]).
+join_1(Sep, [H | Tail]) ->
+    [Sep, H | join_1(Sep, Tail)];
+join_1(_Sep, []) ->
+    [].
 
 %%-----------------------------------------------------------------------------
 %% @param   From    from integer
@@ -424,8 +442,8 @@ join([E | R], Sep, Accum) ->
 %% @end
 %%-----------------------------------------------------------------------------
 -spec seq(From :: integer(), To :: integer()) -> list().
-seq(From, To) ->
-    seq(From, To, 1).
+seq(From, To) when is_integer(From) andalso is_integer(To) andalso From =< To ->
+    seq_r(From, To, 1, []).
 
 %%-----------------------------------------------------------------------------
 %% @param   From    from integer
@@ -447,16 +465,12 @@ seq(From, To, Incr) when
 seq(To, To, 0) ->
     [To];
 seq(From, To, Incr) ->
-    seq(From, To, Incr, []).
+    Last = From + ((To - From) div Incr) * Incr,
+    seq_r(From, Last, Incr, []).
 
 %% @private
-seq(From, To, Incr, Accum) when
-    (Incr > 0 andalso From > To) orelse
-        (Incr < 0 andalso To > From)
-->
-    reverse(Accum);
-seq(From, To, Incr, Accum) ->
-    seq(From + Incr, To, Incr, [From | Accum]).
+seq_r(From, From, _Incr, Acc) -> [From | Acc];
+seq_r(From, To, Incr, Acc) -> seq_r(From, To - Incr, Incr, [To | Acc]).
 
 %%-----------------------------------------------------------------------------
 %% @param   List a list
@@ -523,20 +537,16 @@ unique(Sorted) ->
     unique(Sorted, fun(X, Y) -> X =< Y end).
 
 %% @private
-unique(Sorted, Fun) ->
-    unique(Sorted, Fun, []).
-
-%% @private
-unique([], _Fun, []) ->
+unique([], _Fun) ->
     [];
-unique([X], _Fun, Acc) ->
-    lists:reverse([X | Acc]);
-unique([X, Y | Tail], Fun, Acc) ->
+unique([X], _Fun) ->
+    [X];
+unique([X, Y | Tail], Fun) ->
     case Fun(X, Y) andalso Fun(Y, X) of
         true ->
-            unique([Y | Tail], Fun, Acc);
+            unique([Y | Tail], Fun);
         false ->
-            unique([Y | Tail], Fun, [X | Acc])
+            [X | unique([Y | Tail], Fun)]
     end.
 
 %%-----------------------------------------------------------------------------
@@ -563,9 +573,9 @@ duplicate(Count, Elem, Acc) -> duplicate(Count - 1, Elem, [Elem | Acc]).
 %%-----------------------------------------------------------------------------
 -spec sublist([Elem], integer()) -> [Elem].
 sublist(List, Len) when is_integer(Len) andalso Len >= 0 ->
-    sublist0(List, Len, []).
+    sublist0(List, Len).
 
 %% @private
-sublist0(_List, 0, Acc) -> reverse(Acc);
-sublist0([], _, Acc) -> reverse(Acc);
-sublist0([H | Tail], Len, Acc) -> sublist0(Tail, Len - 1, [H | Acc]).
+sublist0([], _Len) -> [];
+sublist0(_, 0) -> [];
+sublist0([H | Tail], Len) -> [H | sublist0(Tail, Len - 1)].

src/libAtomVM/nifs.c

@@ -166,6 +166,7 @@ static term nif_base64_encode_to_string(Context *ctx, int argc, term argv[]);
 static term nif_base64_decode_to_string(Context *ctx, int argc, term argv[]);
 static term nif_code_load_abs(Context *ctx, int argc, term argv[]);
 static term nif_code_load_binary(Context *ctx, int argc, term argv[]);
+static term nif_lists_reverse(Context *ctx, int argc, term argv[]);
 static term nif_maps_next(Context *ctx, int argc, term argv[]);
 static term nif_unicode_characters_to_list(Context *ctx, int argc, term argv[]);
 static term nif_unicode_characters_to_binary(Context *ctx, int argc, term argv[]);
@@ -694,6 +695,11 @@ static const struct Nif code_load_binary_nif =
     .base.type = NIFFunctionType,
     .nif_ptr = nif_code_load_binary
 };
+static const struct Nif lists_reverse_nif =
+{
+    .base.type = NIFFunctionType,
+    .nif_ptr = nif_lists_reverse
+};
 static const struct Nif maps_next_nif =
 {
     .base.type = NIFFunctionType,
@@ -4236,6 +4242,36 @@ static term nif_code_load_binary(Context *ctx, int argc, term argv[])
     return result;
 }
 
+static term nif_lists_reverse(Context *ctx, int argc, term argv[])
+{
+    // Compared to erlang version, compute the length of the list and allocate
+    // at once the space for the reverse.
+    int proper;
+    size_t len = term_list_length(argv[0], &proper);
+    if (!proper) {
+        RAISE_ERROR(BADARG_ATOM);
+    }
+
+    if (UNLIKELY(memory_ensure_free_with_roots(ctx, len * CONS_SIZE, 2, argv, MEMORY_CAN_SHRINK) != MEMORY_GC_OK)) {
+        RAISE_ERROR(OUT_OF_MEMORY_ATOM);
+    }
+
+    term result = term_nil();
+    if (argc == 2) {
+        result = argv[1];
+    }
+    term list_crsr = argv[0];
+    while (!term_is_nil(list_crsr)) {
+        if (UNLIKELY(!term_is_nonempty_list(list_crsr))) {
+            RAISE_ERROR(BADARG_ATOM);
+        }
+        term *list_ptr = term_get_list_ptr(list_crsr);
+        result = term_list_prepend(list_ptr[LIST_HEAD_INDEX], result, &ctx->heap);
+        list_crsr = list_ptr[LIST_TAIL_INDEX];
+    }
+    return result;
+}
+
 static term nif_maps_next(Context *ctx, int argc, term argv[])
 {
     UNUSED(argc);

src/libAtomVM/nifs.gperf

@@ -140,6 +140,8 @@ base64:encode/1, &base64_encode_nif
 base64:decode/1, &base64_decode_nif
 base64:encode_to_string/1, &base64_encode_to_string_nif
 base64:decode_to_string/1, &base64_decode_to_string_nif
+lists:reverse/1, &lists_reverse_nif
+lists:reverse/2, &lists_reverse_nif
 maps:next/1, &maps_next_nif
 unicode:characters_to_list/1, &unicode_characters_to_list_nif
 unicode:characters_to_list/2, &unicode_characters_to_list_nif

src/libAtomVM/opcodesswitch.h

@@ -3011,11 +3011,10 @@ HOT_FUNC int scheduler_entry_point(GlobalContext *glb)
                 #ifdef IMPL_EXECUTE_LOOP
                     TRACE("get_list/3 %lx, %c%i, %c%i\n", src_value, T_DEST_REG(head_dreg), T_DEST_REG(tail_dreg));
 
-                    term head = term_get_list_head(src_value);
-                    term tail = term_get_list_tail(src_value);
+                    term *list_ptr = term_get_list_ptr(src_value);
 
-                    WRITE_REGISTER(head_dreg, head);
-                    WRITE_REGISTER(tail_dreg, tail);
+                    WRITE_REGISTER(head_dreg, list_ptr[LIST_HEAD_INDEX]);
+                    WRITE_REGISTER(tail_dreg, list_ptr[LIST_TAIL_INDEX]);
                 #endif
 
                 #ifdef IMPL_CODE_LOADER

src/libAtomVM/term.h

@@ -88,6 +88,9 @@ extern "C" {
 #define CONS_SIZE 2
 #define REFC_BINARY_CONS_OFFSET 4
 
+#define LIST_HEAD_INDEX 1
+#define LIST_TAIL_INDEX 0
+
 #define TERM_BINARY_SIZE_IS_HEAP(size) ((size) < REFC_BINARY_MIN)
 
 #if TERM_BYTES == 4
@@ -1288,7 +1291,7 @@ static inline term term_list_from_list_ptr(term *list_elem)
 static inline term term_get_list_head(term t)
 {
     term *list_ptr = term_get_list_ptr(t);
-    return list_ptr[1];
+    return list_ptr[LIST_HEAD_INDEX];
 }
 
 /**
@@ -1300,7 +1303,7 @@ static inline term term_get_list_head(term t)
 static inline term term_get_list_tail(term t)
 {
     term *list_ptr = term_get_list_ptr(t);
-    return *list_ptr;
+    return list_ptr[LIST_TAIL_INDEX];
 }
 
 /**
@@ -1312,7 +1315,7 @@ static inline term term_get_list_tail(term t)
  */
 MALLOC_LIKE static inline term *term_list_alloc(Heap *heap)
 {
-    return memory_heap_alloc(heap, 2);
+    return memory_heap_alloc(heap, CONS_SIZE);
 }
 
 /**

tests/erlang_tests/CMakeLists.txt

@@ -132,6 +132,7 @@ compile_erlang(test_list_processes)
 compile_erlang(test_tl)
 compile_erlang(test_list_to_atom)
 compile_erlang(test_list_to_existing_atom)
+compile_erlang(test_lists_reverse)
 compile_erlang(test_binary_to_atom)
 compile_erlang(test_binary_to_existing_atom)
 compile_erlang(test_atom_to_list)
@@ -575,6 +576,7 @@ add_custom_target(erlang_test_modules DEPENDS
     test_tl.beam
     test_list_to_atom.beam
     test_list_to_existing_atom.beam
+    test_lists_reverse.beam
     test_binary_to_atom.beam
     test_binary_to_existing_atom.beam
     test_atom_to_list.beam