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

0 files changed, 0 insertions, 0 deletions