Enhance optimization of function_clause exceptions

There is an optimization for reducing the number of instructions needed
to generate a `function_clause`. After the latest improvements of the
type optimization pass, that optimization is not always applied.

Here is an example:

    -export([foo/3]).

    foo(X, Y, Z) ->
        bar(a, X, Y, Z).

    bar(a, X, Y, Z) when is_tuple(X) ->
        {X,Y,Z}.

Note that the compiler internally adds a clause to each function to
generate a `function_clause` exception. Thus:

    bar(a, X, Y, Z) when is_tuple(X) ->
        {X,Y,Z};
    bar(A1, A2, A3, A4) ->
        erlang:error(function_clause, [A1,A2,A3,A4]).

Optimizations will rewrite the code basically like this:

    bar(_, X, Y, Z) when is_tuple(X) ->
        {X,Y,Z};
    bar(_, A2, A3, A4) ->
        erlang:error(function_clause, [a,A2,A3,A4]).

Note the `a` as the first element of the list of arguments. It
will prevent the optimization of the `function_clause` exception.

The BEAM code for `bar/4` looks like this:

    {function, bar, 4, 4}.
      {label,3}.
        {line,[{location,"t.erl",8}]}.
        {func_info,{atom,t},{atom,bar},4}.
      {label,4}.
        {'%',{type_info,{x,0},{atom,a}}}.
        {test,is_tuple,{f,5},[{x,1}]}.
        {test_heap,4,4}.
        {put_tuple2,{x,0},{list,[{x,1},{x,2},{x,3}]}}.
        return.
      {label,5}.
        {test_heap,8,4}.
        {put_list,{x,3},nil,{x,0}}.
        {put_list,{x,2},{x,0},{x,0}}.
        {put_list,{x,1},{x,0},{x,0}}.
        {put_list,{atom,a},{x,0},{x,1}}.
        {move,{atom,function_clause},{x,0}}.
        {line,[{location,"t.erl",8}]}.
        {call_ext,2,{extfunc,erlang,error,2}}.

The code after label 5 is the clause that generates the
`function_clause` exception.

This commit generalizes the optimization so that it can be applied for
this function:

    {function, bar, 4, 4}.
      {label,3}.
        {line,[{location,"t.erl",8}]}.
        {func_info,{atom,t},{atom,bar},4}.
      {label,4}.
        {'%',{type_info,{x,0},{atom,a}}}.
        {test,is_tuple,{f,5},[{x,1}]}.
        {test_heap,4,4}.
        {put_tuple2,{x,0},{list,[{x,1},{x,2},{x,3}]}}.
        return.
      {label,5}.
        {move,{atom,a},{x,0}}.
        {jump,{f,3}}.

For this particular function, it would be safe to omit the
`move` instruction before the `{jump,{f,3}}` instruction, but
it would not be safe in general to omit `move` instructions.

0 files changed, 0 insertions, 0 deletions