Correct unsafe optimization of '==' and '/='

Since '=:=' is cheaper than '==', the compiler tries to replace
'==' with '=:=' if the result of comparison will be the same.

As an example:

  V == {a,b}

can be rewritten to:

  V =:= {a,b}

since the literal on the right side contains no numeric values
that '==' would compare differently to '=:='.

With the introduction of maps, we will need to take them into
account. Since the comparison of maps is planned to change in 18.0,
we will be very conservative and only do the optimization if
both keys and values are non-numeric.

1 files changed, 5 insertions, 0 deletions

diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index f4f5c3f361..ed8f609082 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -1229,6 +1229,11 @@ is_non_numeric([H|T]) ->
     is_non_numeric(H) andalso is_non_numeric(T);
 is_non_numeric(Tuple) when is_tuple(Tuple) ->
     is_non_numeric_tuple(Tuple, tuple_size(Tuple));
+is_non_numeric(Map) when is_map(Map) ->
+    %% Note that 17.x and 18.x compare keys in different ways.
+    %% Be very conservative -- require that both keys and values
+    %% are non-numeric.
+    is_non_numeric(maps:to_list(Map));
 is_non_numeric(Num) when is_number(Num) ->
     false;
 is_non_numeric(_) -> true.