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File 2191-Only-compute-vars-and-keys-if-we-have-patterns.patch of Package erlang (Revision 47ed1bf4cc5a95d537125263d3097a97)

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From adbcbda42f316512e212217824d140a130df3d5a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20Valim?= <jose.valim@dashbit.co>
Date: Wed, 1 Jul 2020 20:09:54 +0200
Subject: [PATCH] Only compute vars and keys if we have patterns

The goal of the sys_core_alias pass is to optimize
code like this:

foo({ok, Val}) ->
      {ok, Val}.

into:

foo({ok, Val} = Var) ->
      Var.

To do so, we need to find all patterns and find all
tuples and lists that may mirror their patterns.

Previously, we would compute the keys for each data
structure even if no pattern was stored. This optimizes
it so we only compute the keys if we have a pattern
so far.
---
 lib/compiler/src/sys_core_alias.erl | 82 ++++++++++++++++++++---------
 1 file changed, 57 insertions(+), 25 deletions(-)

diff --git a/lib/compiler/src/sys_core_alias.erl b/lib/compiler/src/sys_core_alias.erl
index 3326c6a2a8..3de8f0f193 100644
--- a/lib/compiler/src/sys_core_alias.erl
+++ b/lib/compiler/src/sys_core_alias.erl
@@ -43,13 +43,29 @@
 -include("core_parse.hrl").
 
 -define(NOTSET, 0).
+-define(HAS_SUBS(Sub), Sub#sub.t =/= none).
 
 -record(sub, {p=#{} :: #{term() => ?NOTSET | atom()},              %% Found pattern substitutions
               v=cerl_sets:new() :: cerl_sets:set(cerl:var_name()), %% Variables used by patterns
-              t=undefined :: term()}).                             %% Temporary information from pre to post
+              t=none :: temp()}).                                  %% Temporary information from pre to post
 
 -type sub() :: #sub{}.
 
+%% temp keeps the stack of substitutions.
+%%
+%% 1. none is the root
+%% 2. temp stores the previous temp value - this is used when
+%%    we update the existing sub instead of replacing it
+%% 3. sub is the previous substitution - this is used when
+%%    we replace the current sub
+%% 4. clause is used when there are clauses with patterns
+%%
+-type temp() ::
+        none
+        | {temp, temp()}
+        | {sub, sub()}
+        | {clause, [term()], [term()], cerl_sets:set(cerl:var_name()), temp()}.
+
 -spec module(cerl:c_module(), [compile:option()]) ->
         {'ok',cerl:c_module(),[]}.
 
@@ -60,7 +76,7 @@ module(#c_module{defs=Ds0}=Mod, _Opts) ->
 def({#c_var{name={F,Arity}}=Name,B0}) ->
     try
         put(new_var_num, 0),
-        {B1,_} = cerl_trees:mapfold(fun pre/2, fun post/2, sub_new(undefined), B0),
+        {B1,_} = cerl_trees:mapfold(fun pre/2, fun post/2, sub_new(none), B0),
         erase(new_var_num),
         {Name,B1}
     catch
@@ -69,25 +85,30 @@ def({#c_var{name={F,Arity}}=Name,B0}) ->
             erlang:raise(Class, Error, Stack)
     end.
 
-pre(#c_let{vars=Vars}=Node, Sub) ->
-    {Node,sub_fold(get_variables(Vars), Sub)};
-
-pre(#c_fun{vars=Vars}=Node, Sub) ->
-    {Node,sub_fold(get_variables(Vars), Sub)};
-
 pre(#c_clause{pats=Pats}=Node, Sub0) ->
-    VarNames = get_variables(Pats),
-    Sub1 = sub_fold(VarNames, Sub0),
-    Keys = get_pattern_keys(Pats),
-    Sub2 = sub_add_keys(Keys, Sub1),
+    case get_pattern_keys(Pats) of
+        [] when ?HAS_SUBS(Sub0) ->
+            VarNames = get_variables(Pats),
+            {Node,sub_fold(VarNames, Sub0)};
+        [] ->
+            {Node,Sub0};
+        Keys ->
+            VarNames = get_variables(Pats),
+            Sub1 = sub_fold(VarNames, Sub0),
+            Sub2 = sub_add_keys(Keys, Sub1),
+            #sub{v=SubNames,t=Temp} = Sub2,
+            Sub3 = Sub2#sub{v=merge_variables(VarNames, SubNames),
+                            t={clause,Pats,Keys,SubNames,Temp}},
+            {Node#c_clause{pats=[]},Sub3}
+    end;
 
-    #sub{v=SubNames,t=Temp} = Sub2,
-    Sub3 = Sub2#sub{v=merge_variables(VarNames, SubNames),
-                    t={clause,Pats,Keys,SubNames,Temp}},
+pre(#c_let{vars=Vars}=Node, Sub) when ?HAS_SUBS(Sub) ->
+    {Node,sub_fold(get_variables(Vars), Sub)};
 
-    {Node#c_clause{pats=[]},Sub3};
+pre(#c_fun{vars=Vars}=Node, Sub) when ?HAS_SUBS(Sub) ->
+    {Node,sub_fold(get_variables(Vars), Sub)};
 
-pre(Node, Sub0) ->
+pre(Node, Sub0) when ?HAS_SUBS(Sub0) ->
     %% We cache only tuples and cons.
     case cerl:is_data(Node) andalso not cerl:is_literal(Node) of
         false ->
@@ -101,13 +122,9 @@ pre(Node, Sub0) ->
                 error ->
                     {Node,Sub0}
             end
-    end.
-
-post(#c_let{}=Node, Sub) ->
-    {Node,sub_unfold(Sub)};
+    end;
 
-post(#c_fun{}=Node, Sub) ->
-    {Node,sub_unfold(Sub)};
+pre(Node, Sub) -> {Node, Sub}.
 
 post(#c_clause{}=Node, #sub{t={clause,Pats0,Keys,V,T}}=Sub0) ->
     {Sub1,PostKeys} = sub_take_keys(Keys, Sub0),
@@ -115,6 +132,15 @@ post(#c_clause{}=Node, #sub{t={clause,Pats0,Keys,V,T}}=Sub0) ->
     Sub2 = sub_unfold(Sub1#sub{v=V,t=T}),
     {Node#c_clause{pats=Pats1},Sub2};
 
+post(#c_clause{}=Node, Sub) ->
+    {Node,sub_unfold(Sub)};
+
+post(#c_let{}=Node, Sub) ->
+    {Node,sub_unfold(Sub)};
+
+post(#c_fun{}=Node, Sub) ->
+    {Node,sub_unfold(Sub)};
+
 post(Node, Sub) ->
     {Node,Sub}.
 
@@ -130,14 +156,20 @@ post(Node, Sub) ->
 sub_new(Temp) ->
     #sub{t=Temp}.
 
-%% Folds the sub into a new one if the variables in nodes are not disjoint
+%% Folds the sub into a new one.
+%% If the variables are disjoint, it means we can continue reusing
+%% the patterns collected so far. Otherwise, it means a variable
+%% was reassigned, so we build a new sub with fresh patterns.
 sub_fold(VarNames, #sub{v=SubNames}=Sub) ->
     case is_disjoint_variables(VarNames, SubNames) of
         true  -> Sub#sub{t={temp,Sub#sub.t}};
         false -> sub_new({sub,Sub})
     end.
 
-%% Unfolds the sub in case one was folded in the previous step
+%% Unfolds the sub in case one was folded in the previous step,
+%% otherwise returns the (root) Sub itself.
+sub_unfold(#sub{t=none}=Sub) ->
+    Sub;
 sub_unfold(#sub{t={temp,Temp}}=Sub) ->
     Sub#sub{t=Temp};
 sub_unfold(#sub{t={sub,Sub}}) ->
-- 
2.26.2

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File 2191-Only-compute-vars-and-keys-if-we-have-patterns.patch of Package erlang (Revision 47ed1bf4cc5a95d537125263d3097a97)

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