Merge pull request #2091 from bjorng/bjorn/compiler/beam_ssa_type

Improve type optimizations

2 files changed, 286 insertions, 43 deletions

diff --git a/lib/compiler/src/beam_ssa_type.erl b/lib/compiler/src/beam_ssa_type.erl
index 752533ace7..ab5ea4b1a4 100644
--- a/lib/compiler/src/beam_ssa_type.erl
+++ b/lib/compiler/src/beam_ssa_type.erl
@@ -23,7 +23,7 @@
 
 -include("beam_ssa.hrl").
 -import(lists, [all/2,any/2,droplast/1,foldl/3,last/1,member/2,
-                reverse/1,sort/1]).
+                partition/2,reverse/1,sort/1]).
 
 -define(UNICODE_INT, #t_integer{elements={0,16#10FFFF}}).
 
@@ -443,12 +443,12 @@ update_successors(#b_br{bool=#b_var{}=Bool,succ=Succ,fail=Fail}, Ts0, D0) ->
             %% no need to include the type database passed on to the
             %% successors of this block.
             Ts = maps:remove(Bool, Ts0),
-            D = update_successor(Fail, Ts, D0),
-            SuccTs = infer_types(Bool, Ts, D0),
+            {SuccTs,FailTs} = infer_types(Bool, Ts, D0),
+            D = update_successor(Fail, FailTs, D0),
             update_successor(Succ, SuccTs, D);
         false ->
-            D = update_successor_bool(Bool, false, Fail, Ts0, D0),
-            SuccTs = infer_types(Bool, Ts0, D0),
+            {SuccTs,FailTs} = infer_types(Bool, Ts0, D0),
+            D = update_successor_bool(Bool, false, Fail, FailTs, D0),
             update_successor_bool(Bool, true, Succ, SuccTs, D)
     end;
 update_successors(#b_switch{arg=#b_var{}=V,fail=Fail,list=List}, Ts0, D0) ->
@@ -465,7 +465,8 @@ update_successors(#b_switch{arg=#b_var{}=V,fail=Fail,list=List}, Ts0, D0) ->
                 end,
             foldl(F, D, List);
         false ->
-            D = update_successor(Fail, Ts0, D0),
+            FailTs = subtract_types([{V,join_sw_list(List, Ts0, none)}], Ts0),
+            D = update_successor(Fail, FailTs, D0),
             F = fun({Val,S}, A) ->
                         T = get_type(Val, Ts0),
                         update_successor(S, Ts0#{V=>T}, A)
@@ -474,6 +475,10 @@ update_successors(#b_switch{arg=#b_var{}=V,fail=Fail,list=List}, Ts0, D0) ->
         end;
 update_successors(#b_ret{}, _Ts, D) -> D.
 
+join_sw_list([{Val,_}|T], Ts, Type) ->
+    join_sw_list(T, Ts, join(Type, get_type(Val, Ts)));
+join_sw_list([], _, Type) -> Type.
+
 update_successor_bool(#b_var{}=Var, BoolValue, S, Ts, D) ->
     case t_is_boolean(get_type(Var, Ts)) of
         true ->
@@ -549,6 +554,14 @@ type(call, [#b_remote{mod=#b_literal{val=Mod},
                 {_,_} ->
                     #t_tuple{}
             end;
+        {erlang,'++',[List1,List2]} ->
+            case get_type(List1, Ts) =:= cons orelse
+                get_type(List2, Ts) =:= cons of
+                true -> cons;
+                false -> list
+            end;
+        {erlang,'--',[_,_]} ->
+            list;
         {math,_,_} ->
             case is_math_bif(Name, length(Args)) of
                 false -> any;
@@ -880,10 +893,84 @@ get_type(#b_literal{val=Val}, _Ts) ->
             any
     end.
 
-infer_types(#b_var{}=V, Ts, #d{ds=Ds}) ->
+%% infer_types(Var, Types, #d{}) -> {SuccTypes,FailTypes}
+%%  Looking at the expression that defines the variable Var, infer
+%%  the types for the variables in the arguments. Return the updated
+%%  type database for the case that the expression evaluates to
+%%  true, and and for the case that it evaluates to false.
+%%
+%%  Here is an example. The variable being asked about is
+%%  the variable Bool, which is defined like this:
+%%
+%%     Bool = is_nonempty_list L
+%%
+%%  If 'is_nonempty_list L' evaluates to 'true', L must
+%%  must be cons. The meet of the previously known type of L and 'cons'
+%%  will be added to SuccTypes.
+%%
+%%  On the other hand, if 'is_nonempty_list L' evaluates to false, L
+%%  is not cons and cons can be subtracted from the previously known
+%%  type for L. For example, if L was known to be 'list', subtracting
+%%  'cons' would give 'nil' as the only possible type. The result of the
+%%  subtraction for L will be added to FailTypes.
+%%
+%%  Here is another example, asking about the variable Bool:
+%%
+%%     Head = bif:hd L
+%%     Bool = succeeded Head
+%%
+%%  'succeeded Head' will evaluate to 'true' if the instrution that
+%%  defined Head succeeded. In this case, it is the 'bif:hd L'
+%%  instruction, which will succeed if L is 'cons'. Thus, the meet of
+%%  the previous type for L and 'cons' will be added to SuccTypes.
+%%
+%%  If 'succeeded Head' evaluates to 'false', it means that 'bif:hd L'
+%%  failed and that L is not 'cons'. 'cons' can be subtracted from the
+%%  previously known type for L and the result put in FailTypes.
+
+infer_types(#b_var{}=V, Ts, #d{ds=Ds,once=Once}) ->
     #{V:=#b_set{op=Op,args=Args}} = Ds,
-    Types = infer_type(Op, Args, Ds),
-    meet_types(Types, Ts).
+    Types0 = infer_type(Op, Args, Ds),
+
+    %% We must be careful with types inferred from '=:='.
+    %%
+    %% If we have seen L =:= [a], we know that L is 'cons' if the
+    %% comparison succeeds. However, if the comparison fails, L could
+    %% still be 'cons'. Therefore, we must not subtract 'cons' from the
+    %% previous type of L.
+    %%
+    %% However, it is safe to subtract a type inferred from '=:=' if
+    %% it is single-valued, e.g. if it is [] or the atom 'true'.
+    EqTypes0 = infer_eq_type(Op, Args, Ts, Ds),
+    {Types1,EqTypes} = partition(fun({_,T}) ->
+                                         is_singleton_type(T)
+                                 end, EqTypes0),
+
+    %% Don't bother updating the types for variables that
+    %% are never used again.
+    Types2 = Types1 ++ Types0,
+    Types = [P || {InfV,_}=P <- Types2, not cerl_sets:is_element(InfV, Once)],
+
+    {meet_types(EqTypes++Types, Ts),subtract_types(Types, Ts)}.
+
+infer_eq_type({bif,'=:='}, [#b_var{}=Src,#b_literal{}=Lit], Ts, Ds) ->
+    Def = maps:get(Src, Ds),
+    Type = get_type(Lit, Ts),
+    [{Src,Type}|infer_tuple_size(Def, Lit) ++
+         infer_first_element(Def, Lit)];
+infer_eq_type({bif,'=:='}, [#b_var{}=Arg0,#b_var{}=Arg1], Ts, _Ds) ->
+    %% As an example, assume that L1 is known to be 'list', and L2 is
+    %% known to be 'cons'. Then if 'L1 =:= L2' evaluates to 'true', it can
+    %% be inferred that L1 is 'cons' (the meet of 'cons' and 'list').
+    Type0 = get_type(Arg0, Ts),
+    Type1 = get_type(Arg1, Ts),
+    Type = meet(Type0, Type1),
+    [{V,MeetType} ||
+        {V,OrigType,MeetType} <-
+            [{Arg0,Type0,Type},{Arg1,Type1,Type}],
+        OrigType =/= MeetType];
+infer_eq_type(_Op, _Args, _Ts, _Ds) ->
+    [].
 
 infer_type({bif,element}, [#b_literal{val=Pos},#b_var{}=Tuple], _Ds) ->
     if
@@ -892,20 +979,27 @@ infer_type({bif,element}, [#b_literal{val=Pos},#b_var{}=Tuple], _Ds) ->
         true ->
             []
     end;
-infer_type({bif,'=:='}, [#b_var{}=Src,#b_literal{}=Lit], Ds) ->
-    Def = maps:get(Src, Ds),
-    Type = get_type(Lit, #{}),
-    [{Src,Type}|infer_tuple_size(Def, Lit) ++
-         infer_first_element(Def, Lit)];
+infer_type({bif,element}, [#b_var{}=Position,#b_var{}=Tuple], _Ds) ->
+    [{Position,t_integer()},{Tuple,#t_tuple{}}];
 infer_type({bif,Bif}, [#b_var{}=Src]=Args, _Ds) ->
     case inferred_bif_type(Bif, Args) of
         any -> [];
         T -> [{Src,T}]
     end;
+infer_type({bif,binary_part}, [#b_var{}=Src,_], _Ds) ->
+    [{Src,{binary,8}}];
 infer_type({bif,is_map_key}, [_,#b_var{}=Src], _Ds) ->
     [{Src,map}];
 infer_type({bif,map_get}, [_,#b_var{}=Src], _Ds) ->
     [{Src,map}];
+infer_type({bif,Bif}, [_,_]=Args, _Ds) ->
+    case inferred_bif_type(Bif, Args) of
+        any -> [];
+        T -> [{A,T} || #b_var{}=A <- Args]
+    end;
+infer_type({bif,binary_part}, [#b_var{}=Src,Pos,Len], _Ds) ->
+    [{Src,{binary,8}}|
+     [{V,t_integer()} || #b_var{}=V <- [Pos,Len]]];
 infer_type(bs_start_match, [#b_var{}=Bin], _Ds) ->
     [{Bin,{binary,1}}];
 infer_type(is_nonempty_list, [#b_var{}=Src], _Ds) ->
@@ -976,6 +1070,7 @@ inferred_bif_type(is_number, [_]) -> number;
 inferred_bif_type(is_tuple, [_]) -> #t_tuple{};
 inferred_bif_type(abs, [_]) -> number;
 inferred_bif_type(bit_size, [_]) -> {binary,1};
+inferred_bif_type('bnot', [_]) -> t_integer();
 inferred_bif_type(byte_size, [_]) -> {binary,1};
 inferred_bif_type(ceil, [_]) -> number;
 inferred_bif_type(float, [_]) -> number;
@@ -983,10 +1078,25 @@ inferred_bif_type(floor, [_]) -> number;
 inferred_bif_type(hd, [_]) -> cons;
 inferred_bif_type(length, [_]) -> list;
 inferred_bif_type(map_size, [_]) -> map;
+inferred_bif_type('not', [_]) -> t_boolean();
 inferred_bif_type(round, [_]) -> number;
 inferred_bif_type(trunc, [_]) -> number;
 inferred_bif_type(tl, [_]) -> cons;
 inferred_bif_type(tuple_size, [_]) -> #t_tuple{};
+inferred_bif_type('and', [_,_]) -> t_boolean();
+inferred_bif_type('or', [_,_]) -> t_boolean();
+inferred_bif_type('xor', [_,_]) -> t_boolean();
+inferred_bif_type('band', [_,_]) -> t_integer();
+inferred_bif_type('bor', [_,_]) -> t_integer();
+inferred_bif_type('bsl', [_,_]) -> t_integer();
+inferred_bif_type('bsr', [_,_]) -> t_integer();
+inferred_bif_type('bxor', [_,_]) -> t_integer();
+inferred_bif_type('div', [_,_]) -> t_integer();
+inferred_bif_type('rem', [_,_]) -> t_integer();
+inferred_bif_type('+', [_,_]) -> number;
+inferred_bif_type('-', [_,_]) -> number;
+inferred_bif_type('*', [_,_]) -> number;
+inferred_bif_type('/', [_,_]) -> number;
 inferred_bif_type(_, _) -> any.
 
 infer_tuple_size(#b_set{op={bif,tuple_size},args=[#b_var{}=Tuple]},
@@ -1095,6 +1205,11 @@ t_tuple_size(#t_tuple{size=Size,exact=true}) ->
 t_tuple_size(_) ->
     none.
 
+is_singleton_type(#t_atom{elements=[_]}) -> true;
+is_singleton_type(#t_integer{elements={V,V}}) -> true;
+is_singleton_type(nil) -> true;
+is_singleton_type(_) -> false.
+
 %% join(Type1, Type2) -> Type
 %%  Return the "join" of Type1 and Type2. The join is a more general
 %%  type than Type1 and Type2. For example:
@@ -1159,14 +1274,40 @@ gcd(A, B) ->
 
 meet_types([{V,T0}|Vs], Ts) ->
     #{V:=T1} = Ts,
-    T = meet(T0, T1),
-    meet_types(Vs, Ts#{V:=T});
+    case meet(T0, T1) of
+        T1 -> meet_types(Vs, Ts);
+        T -> meet_types(Vs, Ts#{V:=T})
+    end;
 meet_types([], Ts) -> Ts.
 
 meet([T1,T2|Ts]) ->
     meet([meet(T1, T2)|Ts]);
 meet([T]) -> T.
 
+subtract_types([{V,T0}|Vs], Ts) ->
+    #{V:=T1} = Ts,
+    case subtract(T1, T0) of
+        T1 -> subtract_types(Vs, Ts);
+        T -> subtract_types(Vs, Ts#{V:=T})
+    end;
+subtract_types([], Ts) -> Ts.
+
+%% subtract(Type1, Type2) -> Type.
+%%  Subtract Type2 from Type1. Example:
+%%
+%%     subtract(list, cons) -> nil
+
+subtract(#t_atom{elements=[_|_]=Set0}, #t_atom{elements=[_|_]=Set1}) ->
+    case ordsets:subtract(Set0, Set1) of
+        [] -> none;
+        [_|_]=Set -> #t_atom{elements=Set}
+    end;
+subtract(number, float) -> #t_integer{};
+subtract(number, #t_integer{elements=any}) -> float;
+subtract(list, cons) -> nil;
+subtract(list, nil) -> cons;
+subtract(T, _) -> T.
+
 %% meet(Type1, Type2) -> Type
 %%  Return the "meet" of Type1 and Type2. The meet is a narrower
 %%  type than Type1 and Type2. For example:
diff --git a/lib/compiler/src/beam_validator.erl b/lib/compiler/src/beam_validator.erl
index 1945faba7f..46c689e034 100644
--- a/lib/compiler/src/beam_validator.erl
+++ b/lib/compiler/src/beam_validator.erl
@@ -809,6 +809,14 @@ valfun_4({test,has_map_fields,{f,Lbl},Src,{list,List}}, Vst) ->
     assert_type(map, Src, Vst),
     assert_unique_map_keys(List),
     branch_state(Lbl, Vst);
+valfun_4({test,is_list,{f,Lbl},[Src]}, Vst) ->
+    validate_src([Src], Vst),
+    Type = case get_term_type(Src, Vst) of
+               cons -> cons;
+               nil -> nil;
+               _ -> list
+           end,
+    set_aliased_type(Type, Src, branch_state(Lbl, Vst));
 valfun_4({test,is_map,{f,Lbl},[Src]}, Vst0) ->
     Vst = branch_state(Lbl, Vst0),
     case Src of
@@ -820,20 +828,27 @@ valfun_4({test,is_map,{f,Lbl},[Src]}, Vst0) ->
 	_ ->
 	    kill_state(Vst0)
     end;
+valfun_4({test,is_nil,{f,Lbl},[Src]}, Vst) ->
+    case get_term_type(Src, Vst) of
+        list ->
+            branch_state(Lbl, set_type_reg(cons, Src, Vst));
+        _ ->
+            branch_state(Lbl, Vst)
+    end;
 valfun_4({test,is_eq_exact,{f,Lbl},[Src,Val]=Ss}, Vst0) ->
     validate_src(Ss, Vst0),
     Infer = infer_types(Src, Vst0),
     Vst1 = Infer(Val, Vst0),
-    Vst = branch_state(Lbl, Vst1),
-    case Val of
-        {literal,Tuple} when is_tuple(Tuple) ->
-            Type0 = get_term_type(Val, Vst),
-            Type = upgrade_tuple_type({tuple,tuple_size(Tuple)},
-                                      Type0),
-            set_aliased_type(Type, Src, Vst);
-        _ ->
-            Vst
-    end;
+    Vst2 = upgrade_ne_types(Src, Val, Vst1),
+    Vst3 = branch_state(Lbl, Vst2),
+    Vst = Vst3#vst{current=Vst1#vst.current},
+    upgrade_eq_types(Src, Val, Vst);
+valfun_4({test,is_ne_exact,{f,Lbl},[Src,Val]=Ss}, Vst0) ->
+    validate_src(Ss, Vst0),
+    Vst1 = upgrade_eq_types(Src, Val, Vst0),
+    Vst2 = branch_state(Lbl, Vst1),
+    Vst = Vst2#vst{current=Vst0#vst.current},
+    upgrade_ne_types(Src, Val, Vst);
 valfun_4({test,_Op,{f,Lbl},Src}, Vst) ->
     validate_src(Src, Vst),
     branch_state(Lbl, Vst);
@@ -920,6 +935,25 @@ valfun_4({get_map_elements,{f,Fail},Src,{list,List}}, Vst) ->
 valfun_4(_, _) ->
     error(unknown_instruction).
 
+upgrade_ne_types(Src1, Src2, Vst0) ->
+    T1 = get_durable_term_type(Src1, Vst0),
+    T2 = get_durable_term_type(Src2, Vst0),
+    Type = subtract(T1, T2),
+    set_aliased_type(Type, Src1, Vst0).
+
+upgrade_eq_types(Src1, Src2, Vst0) ->
+    T1 = get_durable_term_type(Src1, Vst0),
+    T2 = get_durable_term_type(Src2, Vst0),
+    Meet = meet(T1, T2),
+    Vst = case T1 =/= Meet of
+              true -> set_aliased_type(Meet, Src1, Vst0);
+              false -> Vst0
+          end,
+    case T2 =/= Meet of
+        true -> set_aliased_type(Meet, Src2, Vst);
+        false -> Vst
+    end.
+
 verify_get_map(Fail, Src, List, Vst0) ->
     assert_not_literal(Src),                    %OTP 22.
     assert_type(map, Src, Vst0),
@@ -1509,12 +1543,16 @@ assert_not_literal(Literal) -> error({literal_not_allowed,Literal}).
 %%
 %% term			Any valid Erlang (but not of the special types above).
 %%
+%% binary               Binary or bitstring.
+%%
 %% bool			The atom 'true' or the atom 'false'.
 %%
 %% cons         	Cons cell: [_|_]
 %%
 %% nil			Empty list: []
 %%
+%% list                 List: [] or [_|_]
+%%
 %% {tuple,[Sz]}		Tuple. An element has been accessed using
 %%              	element/2 or setelement/3 so that it is known that
 %%              	the type is a tuple of size at least Sz.
@@ -1535,7 +1573,7 @@ assert_not_literal(Literal) -> error({literal_not_allowed,Literal}).
 %%
 %% map			Map.
 %%
-%%
+%% none                 A conflict in types. There will be an exception at runtime.
 %%
 %% FRAGILITY
 %% ---------
@@ -1548,6 +1586,47 @@ assert_not_literal(Literal) -> error({literal_not_allowed,Literal}).
 %% Such terms are wrapped in a {fragile,Type} tuple, where Type is one
 %% of the types described above.
 
+%% meet(Type1, Type2) -> Type
+%%  Return the meet of two types. The meet is a more specific type.
+%%  It will be 'none' if the types are in conflict.
+
+meet(Same, Same) ->
+    Same;
+meet(term, Other) ->
+    Other;
+meet(Other, term) ->
+    Other;
+meet(T1, T2) ->
+    case {erlang:min(T1, T2),erlang:max(T1, T2)} of
+        {{atom,_}=A,{atom,[]}} -> A;
+        {bool,{atom,B}=Atom} when is_boolean(B) -> Atom;
+        {bool,{atom,[]}} -> bool;
+        {cons,list} -> cons;
+        {{float,_}=T,{float,[]}} -> T;
+        {{integer,_}=T,{integer,[]}} -> T;
+        {list,nil} -> nil;
+        {number,{integer,_}=T} -> T;
+        {number,{float,_}=T} -> T;
+        {{tuple,Size1},{tuple,Size2}} ->
+            case {Size1,Size2} of
+                {[Sz1],[Sz2]} ->
+                    {tuple,[erlang:max(Sz1, Sz2)]};
+                {Sz1,[Sz2]} when Sz2 =< Sz1 ->
+                    {tuple,Sz1};
+                {_,_} ->
+                    none
+            end;
+        {_,_} -> none
+    end.
+
+%% subtract(Type1, Type2) -> Type
+%%  Subtract Type2 from Type2. Example:
+%%      subtract(list, nil) -> cons
+
+subtract(list, nil) -> cons;
+subtract(list, cons) -> nil;
+subtract(Type, _) -> Type.
+
 assert_type(WantedType, Term, Vst) ->
     case get_term_type(Term, Vst) of
         {fragile,Type} ->
@@ -1581,25 +1660,27 @@ assert_type(Needed, Actual) ->
 %%  be executed at run-time.
 
 upgrade_tuple_type(NewType, {fragile,OldType}) ->
-    make_fragile(upgrade_tuple_type_1(NewType, OldType));
+    Type = upgrade_tuple_type_1(NewType, OldType),
+    make_fragile(Type);
 upgrade_tuple_type(NewType, OldType) ->
     upgrade_tuple_type_1(NewType, OldType).
 
-upgrade_tuple_type_1({tuple,[Sz]}, {tuple,[OldSz]}=T) when Sz < OldSz ->
-    %% The old type has a higher value for the least tuple size.
-    T;
-upgrade_tuple_type_1({tuple,[Sz]}, {tuple,OldSz}=T)
-  when is_integer(Sz), is_integer(OldSz), Sz =< OldSz ->
-    %% The old size is exact, and the new size is smaller than the old size.
-    T;
-upgrade_tuple_type_1({tuple,_}=T, _) ->
-    %% The new type information is exact or has a higher value for
-    %% the least tuple size.
-    %%     Note that inconsistencies are also handled in this
-    %% clause, e.g. if the old type was an integer or a tuple accessed
-    %% outside its size; inconsistences will generally cause an exception
-    %% at run-time but are safe from our point of view.
-    T.
+upgrade_tuple_type_1(NewType, OldType) ->
+    case meet(NewType, OldType) of
+        none ->
+            %% Unoptimized code may look like this:
+            %%
+            %%   {test,is_list,Fail,[Reg]}.
+            %%   {test,is_tuple,Fail,[Reg]}.
+            %%   {test,test_arity,Fail,[Reg,5]}.
+            %%
+            %% Note that the test_arity instruction can never be reached.
+            %% To make sure it's not rejected, set the type of Reg to
+            %% NewType instead of 'none'.
+            NewType;
+        Type ->
+            Type
+    end.
 
 get_tuple_size({integer,[]}) -> 0;
 get_tuple_size({integer,Sz}) -> Sz;
@@ -1608,6 +1689,17 @@ get_tuple_size(_) -> 0.
 validate_src(Ss, Vst) when is_list(Ss) ->
     foreach(fun(S) -> get_term_type(S, Vst) end, Ss).
 
+%% get_durable_term_type(Src, ValidatorState) -> Type
+%%  Get the type of the source Src. The returned type Type will be
+%%  a standard Erlang type (no catch/try tags or match contexts).
+%%  Fragility will be stripped.
+
+get_durable_term_type(Src, Vst) ->
+    case get_term_type(Src, Vst) of
+        {fragile,Type} -> Type;
+        Type -> Type
+    end.
+
 %% get_move_term_type(Src, ValidatorState) -> Type
 %%  Get the type of the source Src. The returned type Type will be
 %%  a standard Erlang type (no catch/try tags). Match contexts are OK.
@@ -1641,6 +1733,8 @@ get_term_type_1(nil=T, _) -> T;
 get_term_type_1({atom,A}=T, _) when is_atom(A) -> T;
 get_term_type_1({float,F}=T, _) when is_float(F) -> T;
 get_term_type_1({integer,I}=T, _) when is_integer(I) -> T;
+get_term_type_1({literal,[_|_]}, _) -> cons;
+get_term_type_1({literal,Bitstring}, _) when is_bitstring(Bitstring) -> binary;
 get_term_type_1({literal,Map}, _) when is_map(Map) -> map;
 get_term_type_1({literal,Tuple}, _) when is_tuple(Tuple) ->
     {tuple,tuple_size(Tuple)};
@@ -2041,6 +2135,14 @@ return_type_1(erlang, setelement, 3, Vst) ->
 	{integer,I} -> upgrade_tuple_type({tuple,[I]}, TupleType);
 	_ -> TupleType
     end;
+return_type_1(erlang, '++', 2, Vst) ->
+    case get_term_type({x,0}, Vst) =:= cons orelse
+        get_term_type({x,1}, Vst) =:= cons of
+        true -> cons;
+        false -> list
+    end;
+return_type_1(erlang, '--', 2, _Vst) ->
+    list;
 return_type_1(erlang, F, A, _) ->
     return_type_erl(F, A);
 return_type_1(math, F, A, _) ->