Eliminate crash for map updates in guards

beam_validator would complain that x(1) is uninitialized
in a test_heap instruction when attempting to compile
the following code with sys_core_fold turned off:

  foo(M) when not (M#{true := 0}); [M] ->
    ok.

Simplified, the generated BEAM assembly code looked like
this:

    test is_map BadMap x(0)
    put_map_exact Fail x(0) => x(1) ...
    jump BooleanStuff

  BadMap:
    move ok => x(1)
    jump Fail

  BooleanStuff:
    ...
    move Boolean => x(2)
    jump Build

  Fail:
    move false => x(2)

  Build:
    test_heap 2 3  %% x(0), x(1), x(2) must be live.
    ...

That is, if put_map_exact failed, control would transfer
to the label Fail without initializing x(1).

Fix that by making sure that x(1) is initilized even if
put_map_exact fails:

    test is_map BadMap x(0)
    put_map_exact BadLbl x(0) => x(1) ...
    jump OkLbl

  BadLbl:
    move ok => x(1)
    jump Fail

  OkLbl:
    jump BooleanStuff

  BadMap:
    move ok => x(1)
    jump Fail

  BooleanStuff:
    ...
    move Boolean => x(2)
    jump Build

  Fail:
    move false => x(2)

  Build:
    test_heap 2 3  %% x(0), x(1), x(2) must be live.
    ...

Note that this situation is rare, and that other optimization passes
(beam_dead and beam_jump in particular) will clean up this mess.

1 files changed, 24 insertions, 0 deletions

diff --git a/lib/compiler/test/map_SUITE.erl b/lib/compiler/test/map_SUITE.erl
index c3c4862794..36e82c1459 100644
--- a/lib/compiler/test/map_SUITE.erl
+++ b/lib/compiler/test/map_SUITE.erl
@@ -1287,6 +1287,7 @@ t_guard_update(Config) when is_list(Config) ->
     first  = map_guard_update(#{}, #{x=>first}),
     second = map_guard_update(#{y=>old}, #{x=>second,y=>old}),
     third  = map_guard_update(#{x=>old,y=>old}, #{x=>third,y=>old}),
+    bad_map_guard_update(),
     ok.
 
 t_guard_update_large(Config) when is_list(Config) ->
@@ -1353,6 +1354,29 @@ map_guard_update(M1, M2) when M1#{x=>second} =:= M2 -> second;
 map_guard_update(M1, M2) when M1#{x:=third}  =:= M2 -> third;
 map_guard_update(_, _) -> error.
 
+bad_map_guard_update() ->
+    do_bad_map_guard_update(fun burns/1),
+    do_bad_map_guard_update(fun turns/1),
+    ok.
+
+do_bad_map_guard_update(Fun) ->
+    do_bad_map_guard_update_1(Fun, #{}),
+    do_bad_map_guard_update_1(Fun, #{true=>1}),
+    ok.
+
+do_bad_map_guard_update_1(Fun, Value) ->
+    %% Note: The business with the seemingly redundant fun
+    %% disables inlining, which would otherwise change the
+    %% EXIT reason.
+    {'EXIT',{function_clause,_}} = (catch Fun(Value)),
+    ok.
+
+burns(Richmond) when not (Richmond#{true := 0}); [Richmond] ->
+    specification.
+
+turns(Richmond) when not (Richmond#{true => 0}); [Richmond] ->
+    specification.
+
 t_guard_receive(Config) when is_list(Config) ->
     M0  = #{ id => 0 },
     Pid = spawn_link(fun() -> guard_receive_loop() end),