Eliminate unnecessary 'move' instructions

The compiler could sometimes emit unnecessary 'move'
instructions in the code for binary matching, for
example for this function:

escape(<<Byte, Rest/bits>>, Pos) when Byte >= 127 ->
    escape(Rest, Pos + 1);
escape(<<Byte, Rest/bits>>, Pos) ->
    escape(Rest, Pos + Byte);
escape(<<_Rest/bits>>, Pos) ->
    Pos.

The generated code would look like this:

{function, escape, 2, 2}.
  {label,1}.
    {line,[{location,"t.erl",17}]}.
    {func_info,{atom,t},{atom,escape},2}.
  {label,2}.
    {test,bs_start_match2,{f,1},2,[{x,0},0],{x,0}}.
    {test,bs_get_integer2,
          {f,4},
          2,
          [{x,0},
           {integer,8},
           1,
           {field_flags,[{anno,[17,{file,"t.erl"}]},unsigned,big]}],
          {x,2}}.
    {'%',{bin_opt,[17,{file,"t.erl"}]}}.
    {move,{x,0},{x,3}}.                              %% UNECESSARY!
    {test,is_ge,{f,3},[{x,2},{integer,127}]}.
    {line,[{location,"t.erl",18}]}.
    {gc_bif,'+',{f,0},4,[{x,1},{integer,1}],{x,1}}.
    {move,{x,3},{x,0}}.                              %% UNECESSARY!
    {call_only,2,{f,2}}.
  {label,3}.
    {line,[{location,"t.erl",20}]}.
    {gc_bif,'+',{f,0},4,[{x,1},{x,2}],{x,1}}.
    {move,{x,3},{x,0}}.                              %% UNECESSARY!
    {call_only,2,{f,2}}.
  {label,4}.
    {move,{x,1},{x,0}}.
    return.

The redundant 'move' instructions have been marked.

To avoid the 'move' instructions, we can extend the existing
function is_context_unused/1 in v3_codegen. If v3_codegen can
know that the match context will not be used again, it can reuse
the register for the match context and avoid the extra 'move'
instructions.

https://bugs.erlang.org/browse/ERL-444

0 files changed, 0 insertions, 0 deletions