Fix loading of Core Erlang code for extracting a map element

The following Core Erlang code could not be loaded:

    'f'/1 = fun (_1) ->
                case <_1> of
                  <~{'foo':='foo'}~> when 'true' ->
                    _1
                end

Loading would fail with the following message:

     beam/beam_load.c(2314): Error loading function example:f/1: op i_get_map_element_hash p x a u x:
       no specific operation found

https://bugs.erlang.org/browse/ERL-955

3 files changed, 36 insertions, 4 deletions

diff --git a/lib/compiler/src/beam_ssa_codegen.erl b/lib/compiler/src/beam_ssa_codegen.erl
index c2d5035b19..07f4c8b461 100644
--- a/lib/compiler/src/beam_ssa_codegen.erl
+++ b/lib/compiler/src/beam_ssa_codegen.erl
@@ -1016,6 +1016,14 @@ bif_fail({catch_tag,_}) -> {f,0}.
 next_block([]) -> none;
 next_block([{Next,_}|_]) -> Next.
 
+%% Certain instructions (such as get_map_element or is_nonempty_list)
+%% are only used in guards and **must** have a non-zero label;
+%% otherwise, the loader will refuse to load the
+%% module. ensure_label/2 replaces a zero label with the "ultimate
+%% failure" label to make the module loadable.  The instruction that
+%% have had the zero label replaced is **not** supposed to ever fail
+%% and actually jump to the label.
+
 ensure_label(Fail0, #cg{ultimate_fail=Lbl}) ->
     case bif_fail(Fail0) of
         {f,0} -> {f,Lbl};
@@ -1160,6 +1168,11 @@ cg_block([#cg_set{op=call}=I,
           #cg_set{op=succeeded,dst=Bool}], {Bool,_Fail}, St) ->
     %% A call in try/catch block.
     cg_block([I], none, St);
+cg_block([#cg_set{op=get_map_element,dst=Dst0,args=Args0},
+          #cg_set{op=succeeded,dst=Bool}], {Bool,Fail0}, St) ->
+    [Dst,Map,Key] = beam_args([Dst0|Args0], St),
+    Fail = ensure_label(Fail0, St),
+    {[{get_map_elements,Fail,Map,{list,[Key,Dst]}}],St};
 cg_block([#cg_set{op=Op,dst=Dst0,args=Args0}=I,
           #cg_set{op=succeeded,dst=Bool}], {Bool,Fail}, St) ->
     [Dst|Args] = beam_args([Dst0|Args0], St),
@@ -1606,8 +1619,6 @@ cg_test({float,Op0}, Fail, Args, Dst, #cg_set{anno=Anno}) ->
              '/' -> fdiv
          end,
     [line(Anno),{bif,Op,Fail,Args,Dst}];
-cg_test(get_map_element, Fail, [Map,Key], Dst, _I) ->
-    [{get_map_elements,Fail,Map,{list,[Key,Dst]}}];
 cg_test(peek_message, Fail, [], Dst, _I) ->
     [{loop_rec,Fail,{x,0}}|copy({x,0}, Dst)];
 cg_test(put_map, Fail, [{atom,exact},SrcMap|Ss], Dst, Set) ->
diff --git a/lib/compiler/test/core_SUITE.erl b/lib/compiler/test/core_SUITE.erl
index e5611e99d1..72016c6d76 100644
--- a/lib/compiler/test/core_SUITE.erl
+++ b/lib/compiler/test/core_SUITE.erl
@@ -29,7 +29,8 @@
 	 bs_shadowed_size_var/1,
 	 cover_v3_kernel_1/1,cover_v3_kernel_2/1,cover_v3_kernel_3/1,
 	 cover_v3_kernel_4/1,cover_v3_kernel_5/1,
-         non_variable_apply/1,name_capture/1,fun_letrec_effect/1]).
+         non_variable_apply/1,name_capture/1,fun_letrec_effect/1,
+         get_map_element/1]).
 
 -include_lib("common_test/include/ct.hrl").
 
@@ -57,7 +58,8 @@ groups() ->
        bs_shadowed_size_var,
        cover_v3_kernel_1,cover_v3_kernel_2,cover_v3_kernel_3,
        cover_v3_kernel_4,cover_v3_kernel_5,
-       non_variable_apply,name_capture,fun_letrec_effect
+       non_variable_apply,name_capture,fun_letrec_effect,
+       get_map_element
       ]}].
 
 
@@ -95,6 +97,7 @@ end_per_group(_GroupName, Config) ->
 ?comp(non_variable_apply).
 ?comp(name_capture).
 ?comp(fun_letrec_effect).
+?comp(get_map_element).
 
 try_it(Mod, Conf) ->
     Src = filename:join(proplists:get_value(data_dir, Conf),
diff --git a/lib/compiler/test/core_SUITE_data/get_map_element.core b/lib/compiler/test/core_SUITE_data/get_map_element.core
new file mode 100644
index 0000000000..092b5e71eb
--- /dev/null
+++ b/lib/compiler/test/core_SUITE_data/get_map_element.core
@@ -0,0 +1,18 @@
+module 'get_map_element' ['get_map_element'/0]
+attributes []
+
+'get_map_element'/0 =
+    fun () ->
+      apply 'match_map'/1(~{'foo'=>'bar'}~)
+
+'match_map'/1 =
+    fun (_0) ->
+	case _0 of
+	  <~{'foo':='bar'}~> when 'true' ->
+	    'ok'
+	  %% It will be undefined behaviour at runtime if no
+	  %% clause of the case can be selected. That can't
+	  %% happen for this module, because match_map/1 is
+	  %% always called with a matching map argument.
+    end
+end