beam_bsm: Eliminate unsafe optimization

The following code causes a compiler failure:

first_after(Data, Offset) ->
    case byte_size(Data) > Offset of
	false ->
	    {First, Rest} = {ok, ok},
	    ok;
	true ->
	    <<_:Offset/binary, Rest/binary>> = Data,
	    %% 'Rest' saved in y(0) before the call.
	    {First, _} = match_first(Data, Rest),
	    %% When beam_bsm sees the code, the following line
            %% which uses y(0) has been optimized away.
	    {First, Rest} = {First, Rest},
	    First
    end.

match_first(_, <<First:1/binary, Rest/binary>>) ->
    {First, Rest}.

Here is the error message from beam_validator:

t: function first_after/2+15:
  Internal consistency check failed - please report this bug.
  Instruction: {call,2,{f,7}}
  Error:       {multiple_match_contexts,[{x,1},0]}:

Basically, what happens is that at time of code generation,
the variable 'Rest' is needed after the call to match_first/2
and is therefore saved in y(0). When beam_bsm (a late optimization
pass) sees the code, the use of y(0) following the call
to match_first/2 has been optimized away. beam_bsm therefore
assumes that the delayed sub-binary creation is safe. (Actually,
it is safe, but beam_validator does not realize it.)

The bug was caused by two separate commits:

e199e2471a reduced the number of special cases to handle in BEAM
optimization passed by breaking apart the tail-recursive call
instructions (call_only and call_last) into separate instructions.
Unfortunately, the special handling for tail calls was lost, which
resulted in worse code (i.e. the delayed sub-binary creation
optimization could not be applied).

e1aa422290 tried to compensate, but did so in a way that was not
always safe.

Teaching beam_validator that this kind of code is safe would be
expensive.

Instead, we will undo the damage caused by the two
commits. Re-introduce the special handling of tail-recursive calls in
beam_bsm that was lost in the first commit. (Effectively) revert the
change in the second commit.

ERL-268

1 files changed, 25 insertions, 1 deletions

diff --git a/lib/compiler/test/bs_match_SUITE.erl b/lib/compiler/test/bs_match_SUITE.erl
index 224abf6c29..a9bee888d9 100644
--- a/lib/compiler/test/bs_match_SUITE.erl
+++ b/lib/compiler/test/bs_match_SUITE.erl
@@ -38,7 +38,8 @@
 	 no_partition/1,calling_a_binary/1,binary_in_map/1,
 	 match_string_opt/1,select_on_integer/1,
 	 map_and_binary/1,unsafe_branch_caching/1,
-	 bad_literals/1,good_literals/1,constant_propagation/1]).
+	 bad_literals/1,good_literals/1,constant_propagation/1
+	]).
 
 -export([coverage_id/1,coverage_external_ignore/2]).
 
@@ -768,6 +769,11 @@ multiple_uses(Config) when is_list(Config) ->
     {344,62879,345,<<245,159,1,89>>} = multiple_uses_1(<<1,88,245,159,1,89>>),
     true = multiple_uses_2(<<0,0,197,18>>),
     <<42,43>> = multiple_uses_3(<<0,0,42,43>>, fun id/1),
+
+    ok = first_after(<<>>, 42),
+    <<1>> = first_after(<<1,2,3>>, 0),
+    <<2>> = first_after(<<1,2,3>>, 1),
+
     ok.
 
 multiple_uses_1(<<X:16,Tail/binary>>) ->
@@ -789,6 +795,24 @@ multiple_uses_match(<<Y:16,Z:16>>) ->
 multiple_uses_cmp(<<Y:16>>, <<Y:16>>) -> true;
 multiple_uses_cmp(<<_:16>>, <<_:16>>) -> false.
 
+first_after(Data, Offset) ->
+    case byte_size(Data) > Offset of
+	false ->
+	    {First, Rest} = {ok, ok},
+	    ok;
+	true ->
+	    <<_:Offset/binary, Rest/binary>> = Data,
+	    %% 'Rest' saved in y(0) before the call.
+            {First, _} = match_first(Data, Rest),
+            %% When beam_bsm sees the code, the following line
+            %% which uses y(0) has been optimized away.
+	    {First, Rest} = {First, Rest},
+	    First
+    end.
+
+match_first(_, <<First:1/binary, Rest/binary>>) ->
+    {First, Rest}.
+
 zero_label(Config) when is_list(Config) ->
     <<"nosemouth">> = read_pols(<<"FACE","nose","mouth">>),
     <<"CE">> = read_pols(<<"noFACE">>),