From 7169258d1315b86622093e1f14faf2267c9c448e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Tue, 13 Sep 2016 08:43:44 +0200
Subject: sys_core_fold: Improve case optimization

The optimization that avoids building a tuple in a case
expression would not work if any clause matched a tuple
as in the following example:

f(A, B) ->
    case {A,B} of
	{<<X>>,Y} ->
	    {X,Y}
    end.

The generated Core Erlang code would look like this (note
the tuples in the case expression and the pattern):

'f'/2 =
    fun (_cor1,_cor0) ->
	case {_cor1,_cor0} of
	  <{#{#<X>(8,1,'integer',['unsigned'|['big']])}#,Y}>
            when 'true' ->
	      {X,Y}
          .
          .
          .
	end

It is expected that the code should look like this (note
that tuples have been replaced with "values"):

'f'/2 =
    fun (_cor1,_cor0) ->
	%% Line 5
	case <_cor1,_cor0> of
	  <#{#<X>(8,1,'integer',['unsigned'|['big']])}#,Y> ->
	      {X,Y}
          .
          .
          .
	end

While at it, also fix bugs in the handling of pattern with
aliases. The bindings were produced in the wrong order (creating
'let's with referring to free variables), but in most cases
the incorrect bindings were discarded later without causing any
harm.
---
 lib/compiler/src/sys_core_fold.erl | 61 ++++++++++++++++++++------------------
 1 file changed, 32 insertions(+), 29 deletions(-)

(limited to 'lib/compiler/src/sys_core_fold.erl')

diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index 79a45b1c7c..b8df4e04f8 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -2018,10 +2018,10 @@ case_opt_lit_1(_, []) -> [].
 %%  the clauses where it is actually needed.
 
 case_opt_data(E, Cs0) ->
-    Es = cerl:data_es(E),
     TypeSig = {cerl:data_type(E),cerl:data_arity(E)},
-    try case_opt_data_1(Cs0, Es, TypeSig) of
+    try case_opt_data_1(Cs0, TypeSig) of
 	Cs ->
+	    Es = cerl:data_es(E),
 	    {ok,Es,Cs}
     catch
 	throw:impossible ->
@@ -2029,44 +2029,47 @@ case_opt_data(E, Cs0) ->
 	    {error,Cs0}
     end.
 
-case_opt_data_1([{[P0|Ps0],C,PsAcc,Bs0}|Cs], Es, TypeSig) ->
+case_opt_data_1([{[P0|Ps0],C,PsAcc,Bs0}|Cs], TypeSig) ->
     P = case_opt_compiler_generated(P0),
-    BindTo = #c_var{name=dummy},
-    {Ps1,[{BindTo,_}|Bs1]} = case_data_pat_alias(P, BindTo, TypeSig, []),
-    [{Ps1++Ps0,C,PsAcc,Bs1++Bs0}|case_opt_data_1(Cs, Es, TypeSig)];
-case_opt_data_1([], _, _) -> [].
+    {Ps1,Bs} = case_opt_data_2(P, TypeSig, Bs0),
+    [{Ps1++Ps0,C,PsAcc,Bs}|case_opt_data_1(Cs, TypeSig)];
+case_opt_data_1([], _) -> [].
 
-case_data_pat_alias(P, BindTo0, TypeSig, Bs0) ->
-    case cerl:type(P) of
-	alias ->
-	    %% Recursively handle the pattern and bind to
-	    %% the alias variable.
-	    BindTo = cerl:alias_var(P),
-	    Apat0 = cerl:alias_pat(P),
-	    Ann = [compiler_generated],
-	    Apat = cerl:set_ann(Apat0, Ann),
-	    {Ps,Bs} = case_data_pat_alias(Apat, BindTo, TypeSig, Bs0),
-	    {Ps,[{BindTo0,BindTo}|Bs]};
-	var ->
-	    %% Here we will need to actually build the data and bind
-	    %% it to the variable.
+case_opt_data_2(P, TypeSig, Bs0) ->
+    case case_analyze_pat(P) of
+	{[],Pat} when Pat =/= none ->
+	    DataEs = cerl:data_es(P),
+	    {DataEs,Bs0};
+	{[V|Vs],none} ->
 	    {Type,Arity} = TypeSig,
 	    Ann = [compiler_generated],
 	    Vars = make_vars(Ann, Arity),
 	    Data = cerl:ann_make_data(Ann, Type, Vars),
-	    Bs = [{BindTo0,P},{P,Data}|Bs0],
+	    Bs = [{V,Data} | [{Var,V} || Var <- Vs] ++ Bs0],
 	    {Vars,Bs};
-	_ ->
-	    %% Since case_opt_nomatch/3 has removed all clauses that
-	    %% cannot match, we KNOW that this clause must match and
-	    %% that the pattern must be a data constructor.
-	    %% Here we must build the data and bind it to the variable.
+	{[V|Vs],Pat} when Pat =/= none ->
 	    {Type,_} = TypeSig,
-	    DataEs = cerl:data_es(P),
+	    DataEs = cerl:data_es(Pat),
 	    Vars = pat_to_expr_list(DataEs),
 	    Ann = [compiler_generated],
 	    Data = cerl:ann_make_data(Ann, Type, Vars),
-	    {DataEs,[{BindTo0,Data}]}
+	    Bs = [{V,Data} | [{Var,V} || Var <- Vs] ++ Bs0],
+	    {DataEs,Bs}
+    end.
+
+case_analyze_pat(P) ->
+    case_analyze_pat_1(P, [], none).
+
+case_analyze_pat_1(P, Vs, Pat) ->
+    case cerl:type(P) of
+	alias ->
+	    V = cerl:alias_var(P),
+	    Apat = cerl:alias_pat(P),
+	    case_analyze_pat_1(Apat, [V|Vs], Pat);
+	var ->
+	    {[P|Vs],Pat};
+	_ ->
+	    {Vs,P}
     end.
 
 %% pat_to_expr(Pattern) -> Expression.
-- 
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