[erlang-patches] binary matching optimization error

[Christian von Roques]

Using R12B-3 I recently wrote code similar to the following:

-module(bug).
-export([foo/2]).

%% foo(<<1,2>>, 2) should evaluate to <<1,2>>, but evaluates to <<2>>
%% when compiled without no_bsm_opt
foo(Bin, 0) ->
    foo(Bin, 42);
foo(<<_A, Rest/bitstring>>, 1) ->
    foo(Rest, 43);
foo(Bin, _I)  ->
    Bin.


beam_bsm "optimizes" foo/2 to the following:

{function, foo, 2, 2}.
  {label,1}.
    {func_info,{atom,bug},{atom,foo},2}.
  {label,2}.
    {test,is_eq_exact,{f,3},[{x,1},{integer,0}]}.
    {bs_context_to_binary,{x,0}}.
    {move,{integer,42},{x,1}}.
    {call_only,2,{f,2}}.
  {label,3}.
    {test,bs_start_match2,{f,4},[{x,0},2,0,{x,0}]}.
    {test,bs_skip_bits2,
          {f,4},
          [{x,0},
           {integer,8},
           1,
           {field_flags,[{anno,[7,{file,"./bug.erl"}]},unsigned,big]}]}.
    {'%',{bin_opt,[7,{file,"./bug.erl"}]}}.
    {bs_save2,{x,0},{atom,start}}.
    {move,{x,0},{x,2}}.
    {test,is_eq_exact,{f,4},[{x,1},{integer,1}]}.
    {move,{integer,43},{x,1}}.
    {move,{x,2},{x,0}}.
    {call_only,2,{f,2}}.
  {label,4}.
    {bs_context_to_binary,{x,0}}.
    return.


The problem seems to be, that the match state needs to be saved so
that after a tail-recursive call to foo(Rest, 43) the
bs_context_to_binary at {label 2} results in shortened binary and at
the same time the match state must not be saved so that
bs_context_to_binary at {label 4} yields the unshortened binary.

I've come up with a minor modification of beam_bsm, which detects
cases like these and disables "optimizing" them.

Another possible solution could be to push the bs_save2 downwards
until after the match is complete.

	Christian.
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