beam_block: Fix potentially unsafe optimization in move_allocates/1

beam_block has an optimization that only is safe when it is applied
immediately after code generation. That is pointed out in a comment:

  NOTE: Moving allocation instructions is only safe because it is done
  immediately after code generation so that we KNOW that if {x,X} is
  initialized, all x registers with lower numbers are also initialized.
  That assumption may not be true after other optimizations, such as
  the beam_utils:live_opt/1 optimization.

The new beam_reorder pass added in OTP 19 runs before beam_block.
Therefore, the optimization is potentially unsafe. The optimization
is also unsafe if compilation is started from assembly code in a
.S file.

Rewrite the optimization to make it safe. See the newly added comment
for details.

ERL-202

1 files changed, 25 insertions, 2 deletions

diff --git a/lib/compiler/test/beam_block_SUITE.erl b/lib/compiler/test/beam_block_SUITE.erl
index 4bcb252833..9fcb6e497d 100644
--- a/lib/compiler/test/beam_block_SUITE.erl
+++ b/lib/compiler/test/beam_block_SUITE.erl
@@ -21,7 +21,8 @@
 
 -export([all/0,suite/0,groups/0,init_per_suite/1,end_per_suite/1,
 	 init_per_group/2,end_per_group/2,
-	 get_map_elements/1,otp_7345/1,move_opt_across_gc_bif/1]).
+	 get_map_elements/1,otp_7345/1,move_opt_across_gc_bif/1,
+	 erl_202/1]).
 
 %% The only test for the following functions is that
 %% the code compiles and is accepted by beam_validator.
@@ -37,7 +38,8 @@ groups() ->
     [{p,[parallel],
       [get_map_elements,
        otp_7345,
-       move_opt_across_gc_bif
+       move_opt_across_gc_bif,
+       erl_202
       ]}].
 
 init_per_suite(Config) ->
@@ -135,6 +137,27 @@ positive(speaking) ->
 
 paris([], P) -> P + 1.
 
+
+%% See https://bugs.erlang.org/browse/ERL-202.
+%% Test that move_allocates/1 in beam_block doesn't move allocate
+%% when it would not be safe.
+
+-record(erl_202_r1, {y}).
+-record(erl_202_r2, {x}).
+
+erl_202(_Config) ->
+    Ref = make_ref(),
+    Ref = erl_202({{1,2},Ref}, 42),
+
+    {Ref} = erl_202({7,8}, #erl_202_r1{y=#erl_202_r2{x=Ref}}),
+
+    ok.
+
+erl_202({{_, _},X}, _) ->
+    X;
+erl_202({_, _}, #erl_202_r1{y=R2}) ->
+    {R2#erl_202_r2.x}.
+
 %%%
 %%% The only test of the following code is that it compiles.
 %%%