ERL-452: Unnecessary stack allocation in map matches

[OTP-Maintainer]

Original reporter: michalmuskala
Affected version: OTP-20.0
Fixed in version: OTP-21.0
Component: compiler
Migrated from: https://bugs.erlang.org/browse/ERL-452

---

Given a module like the following:

{code:erlang}
-module(test).

-export([test/1]).

test(Map) ->
    X = case Map of
            #{x := X0} ->
                X0;
            #{} ->
                erlang:error({badkey, Map, x})
        end,
    Y = case Map of
            #{y := Y0} ->
                Y0;
            #{} ->
                erlang:error({badkey, Map, y})
        end,
    {X, Y}.
{code}

The compiler will emit the following code:

{code:asm}
{function, test, 1, 2}.
  {label,1}.
    {line,[{location,"test.erl",5}]}.
    {func_info,{atom,test},{atom,test},1}.
  {label,2}.
    {allocate_zero,2,1}.
    {move,{x,0},{y,1}}.
    {test,is_map,{f,4},[{x,0}]}.
    {get_map_elements,{f,3},{x,0},{list,[{atom,x},{x,1}]}}.
    {move,{x,1},{x,0}}.
    {move,{x,0},{y,0}}.
    {test,is_map,{f,6},[{y,1}]}.
    {get_map_elements,{f,5},{y,1},{list,[{atom,y},{x,1}]}}.
    {move,{x,1},{x,0}}.
    {test_heap,3,1}.
    {put_tuple,2,{x,1}}.
    {put,{y,0}}.
    {put,{x,0}}.
    {move,{x,1},{x,0}}.
    {deallocate,2}.
    return.
  {label,3}.
    {test_heap,4,0}.
    {put_tuple,3,{x,0}}.
    {put,{atom,badkey}}.
    {put,{y,1}}.
    {put,{atom,x}}.
    {line,[{location,"test.erl",10}]}.
    {call_ext,1,{extfunc,erlang,error,1}}.
  {label,4}.
    {line,[{location,"test.erl",6}]}.
    {case_end,{y,1}}.
  {label,5}.
    {test_heap,4,0}.
    {put_tuple,3,{x,0}}.
    {put,{atom,badkey}}.
    {put,{y,1}}.
    {put,{atom,y}}.
    {line,[{location,"test.erl",16}]}.
    {call_ext,1,{extfunc,erlang,error,1}}.
  {label,6}.
    {line,[{location,"test.erl",12}]}.
    {case_end,{y,1}}.
{code}

There are extra move isntructions that could be removed by https://github.com/erlang/otp/pull/1506.

What is more concerning is the extra, seemingly completely unnecessary, stack allocation of 2 slots. The function should be able to be compiled without any allocation.

Similar code is emitted by the Elixir compiler faced with the {{map.key}} syntax.

[OTP-Maintainer]

raimo said:

This will have to wait to after the summer vacations.

[OTP-Maintainer]

bjorn said:

https://github.com/erlang/otp/pull/1528