Merge branch 'john/erts/OTP-18.3.4/minusminus_trapping/OTP-15371' into maint-18

* john/erts/OTP-18.3.4/minusminus_trapping/OTP-15371:
  Optimize operator '--' and yield on large inputs

2 files changed, 42 insertions, 6 deletions

diff --git a/lib/stdlib/doc/src/lists.xml b/lib/stdlib/doc/src/lists.xml
index 89ba5238b5..4a4c170bba 100644
--- a/lib/stdlib/doc/src/lists.xml
+++ b/lib/stdlib/doc/src/lists.xml
@@ -708,12 +708,6 @@ splitwith(Pred, List) ->
 > <input>lists:subtract("123212", "212").</input>
 "312".</pre>
         <p><c>lists:subtract(A, B)</c> is equivalent to <c>A -- B</c>.</p>
-	<warning><p>The complexity of <c>lists:subtract(A, B)</c> is proportional
-	to <c>length(A)*length(B)</c>, meaning that it will be very slow if
-	both <c>A</c> and <c>B</c> are long lists.
-	(Using ordered lists and
-	<seealso marker="ordsets#subtract/2">ordsets:subtract/2</seealso>
-	is a much better choice if both lists are long.)</p></warning>
       </desc>
     </func>
     <func>
diff --git a/lib/stdlib/test/lists_SUITE.erl b/lib/stdlib/test/lists_SUITE.erl
index a0f7fd2744..21b2dd1777 100644
--- a/lib/stdlib/test/lists_SUITE.erl
+++ b/lib/stdlib/test/lists_SUITE.erl
@@ -2692,6 +2692,13 @@ subtract(Config) when is_list(Config) ->
     ?line {'EXIT',_} = (catch sub([a|b], [])),
     ?line {'EXIT',_} = (catch sub([a|b], [a])),
 
+    %% Trapping, both crashing and otherwise.
+    [sub_trapping(N) || N <- lists:seq(0, 18)],
+
+    %% The current implementation chooses which algorithm to use based on
+    %% certain thresholds, and we need proper coverage for all corner cases.
+    [sub_thresholds(N) || N <- lists:seq(0, 32)],
+
     ok.
 
 sub_non_matching(A, B) ->
@@ -2701,6 +2708,41 @@ sub(A, B) ->
     Res = A -- B,
     Res = lists:subtract(A, B).
 
+sub_trapping(N) ->
+    List = lists:duplicate(N + (1 bsl N), gurka),
+    ImproperList = List ++ crash,
+
+    {'EXIT',_} = (catch sub_trapping_1(ImproperList, [])),
+    {'EXIT',_} = (catch sub_trapping_1(List, ImproperList)),
+
+    List = List -- lists:duplicate(N + (1 bsl N), gaffel),
+    ok = sub_trapping_1(List, []).
+
+sub_trapping_1([], _) -> ok;
+sub_trapping_1(L, R) -> sub_trapping_1(L -- R, [gurka | R]).
+
+sub_thresholds(N) ->
+    %% This needs to be long enough to cause trapping.
+    OtherLen = 1 bsl 18,
+    Other = lists:seq(0, OtherLen - 1),
+
+    Disjoint = lists:seq(-N, -1),
+    Subset = lists:seq(1, N),
+
+    %% LHS is disjoint from RHS, so all elements must be retained.
+    Disjoint = Disjoint -- Other,
+
+    %% LHS is covered by RHS, so all elements must be removed.
+    [] = Subset -- Other,
+
+    %% RHS is disjoint from LHS, so all elements must be retained.
+    Other = Other -- Disjoint,
+
+    %% RHS is covered by LHS, so N elements must be removed.
+    N = OtherLen - length(Other -- Subset),
+
+    ok.
+
 %% Test lists:droplast/1
 droplast(Config) when is_list(Config) ->
     ?line [] = lists:droplast([x]),