From 86f82183855d6b21912e0e125b096a28a52bbcf6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Mon, 28 Jan 2019 15:10:34 +0100
Subject: 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.
---
 lib/compiler/test/beam_except_SUITE.erl | 12 ++++++++++++
 1 file changed, 12 insertions(+)

(limited to 'lib/compiler/test')

diff --git a/lib/compiler/test/beam_except_SUITE.erl b/lib/compiler/test/beam_except_SUITE.erl
index da61931136..49acad2bc5 100644
--- a/lib/compiler/test/beam_except_SUITE.erl
+++ b/lib/compiler/test/beam_except_SUITE.erl
@@ -88,8 +88,17 @@ coverage(_) ->
     {'EXIT',{{strange,Self},[{?MODULE,foo,[any],[File,{line,14}]}|_]}} =
         (catch foo(any)),
 
+    {ok,succeed,1,2} = foobar(succeed, 1, 2),
+    {'EXIT',{function_clause,[{?MODULE,foobar,[[fail],1,2],
+                               [{file,"fake.erl"},{line,16}]}|_]}} =
+        (catch foobar([fail], 1, 2)),
+    {'EXIT',{function_clause,[{?MODULE,fake_function_clause,[{a,b},42.0],_}|_]}} =
+        (catch fake_function_clause({a,b})),
+
     ok.
 
+fake_function_clause(A) -> error(function_clause, [A,42.0]).
+
 -file("fake.erl", 1).
 fc(a) ->	                                %Line 2
     ok;						%Line 3
@@ -104,3 +113,6 @@ bar(X) ->					%Line 8
 %% Cover collection code for function_clause exceptions.
 foo(A) ->                                       %Line 13
     error({strange,self()}, [A]).               %Line 14
+%% Cover beam_except:tag_literal/1.
+foobar(A, B, C) when is_atom(A) ->              %Line 16
+    {ok,A,B,C}.                                 %Line 17
-- 
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