Inefficient binary pattern compilation

[michalmuskala]

Describe the bug
Given the following code:

-module(test).

-export([config_value/1]).

config_value(<<"0">>) ->
    <<"0">>;
config_value(<<"1">>) ->
    <<"1">>;
config_value(Bin) when is_binary(Bin) ->
    Bin.

compiled with erlc +to_asm test.erl we can see the following output:

  {label,2}.
    {'%',{var_info,{x,0},[accepts_match_context]}}.
    {test,bs_start_match3,{f,6},1,[{x,0}],{x,1}}.
    {bs_get_position,{x,1},{x,0},2}.
    {bs_match,{f,5},
              {x,1},
              {commands,[{ensure_at_least,8,1},
                         {integer,2,{literal,[]},8,1,{x,2}}]}}.
    {select_val,{tr,{x,2},{t_integer,{0,255}}},
                {f,5},
                {list,[{integer,48},{f,4},{integer,49},{f,3}]}}.
  {label,3}.
    {bs_match,{f,5},{x,1},{commands,[{ensure_exactly,0}]}}.
    {move,{literal,<<"1">>},{x,0}}.
    return.
  {label,4}.
    {bs_match,{f,5},{x,1},{commands,[{ensure_exactly,0}]}}.
    {move,{literal,<<"0">>},{x,0}}.
    return.
  {label,5}.
    {bs_set_position,{x,1},{x,0}}.
    {bs_get_tail,{x,1},{x,0},2}.
    {test,is_binary,{f,1},[{tr,{x,0},{t_bitstring,1,false}}]}.
    return.
  {label,6}.
    {test,is_binary,{f,1},[{x,0}]}.
    return.

There are multiple inefficiencies in this code:

1. The bs_match instructions are split. Rather than {ensure_at_least,8,1} and then {ensure_exactly,0} in branch 3 and 4, I would have expected to see {ensure_exactly,8} in the initial match and no bs_match in the 3 and 4 branch at all.
2. The bs_set_position and bs_get_tail operations in label 5 are entirely redundant - the resulting {x,1} register is not used further down.
3. Fixing 2. will likely allow the compiler to combine the 6th and 5th label into just one, reducing code size.

Affected versions
Tested on 27.1, 28.0.2, and 28.1.1