File 3686-Avoid-redundant-checks-of-the-key-returned-by-ets-lo.patch of Package erlang

From 1dfaa9483a3b36c5b6b7db54a8dae01ffd118c50 Mon Sep 17 00:00:00 2001
From: Robin Morisset <rmorisset@fb.com>
Date: Tue, 11 Oct 2022 14:29:22 +0200
Subject: [PATCH] Avoid redundant checks of the key returned by ets:lookup in
 pg.erl

Part of pg.erl in the stdlib has the following pattern:
[{Key, Value1, Value2}] = ets:lookup(Table, Key)

Because Key appears on the left side, and is already bound,
it results in an equality check between the Key
that is returned by lookup and the one that was passed to lookup.
This is pointless since they are guaranteed to match.

I verified on a small example, and this results in 3 bytecode instructions
and 19 (!) useless x86 instructions.
It can be trivially avoided by using an unbound variable such as _Key:
[{_Key, Value1, Value2}] = ets:lookup(Table, Key)

I don't expect the performance win to be significant, but it is so simple
that I think it worthwhile to land.
---
 lib/kernel/src/pg.erl | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/lib/kernel/src/pg.erl b/lib/kernel/src/pg.erl
index 017b1942b4..aca00d1cb5 100644
--- a/lib/kernel/src/pg.erl
+++ b/lib/kernel/src/pg.erl
@@ -492,7 +492,7 @@ sync_groups(Scope, ScopeMon, MG, RemoteGroups, [{Group, Pids} | Tail]) ->
         {Pids, NewRemoteGroups} ->
             sync_groups(Scope, ScopeMon, MG, NewRemoteGroups, Tail);
         {OldPids, NewRemoteGroups} ->
-            [{Group, AllOldPids, LocalPids}] = ets:lookup(Scope, Group),
+            [{_Group, AllOldPids, LocalPids}] = ets:lookup(Scope, Group),
             %% should be really rare...
             AllNewPids = Pids ++ AllOldPids -- OldPids,
             true = ets:insert(Scope, {Group, AllNewPids, LocalPids}),
@@ -517,7 +517,7 @@ join_local([Pid | Tail], Group, Local) ->
 
 join_local_update_ets(Scope, ScopeMon, MG, Group, Pid) when is_pid(Pid) ->
     case ets:lookup(Scope, Group) of
-        [{Group, All, Local}] ->
+        [{_Group, All, Local}] ->
             ets:insert(Scope, {Group, [Pid | All], [Pid | Local]});
         [] ->
             ets:insert(Scope, {Group, [Pid], [Pid]})
@@ -525,7 +525,7 @@ join_local_update_ets(Scope, ScopeMon, MG, Group, Pid) when is_pid(Pid) ->
     notify_group(ScopeMon, MG, join, Group, [Pid]);
 join_local_update_ets(Scope, ScopeMon, MG, Group, Pids) ->
     case ets:lookup(Scope, Group) of
-        [{Group, All, Local}] ->
+        [{_Group, All, Local}] ->
             ets:insert(Scope, {Group, Pids ++ All, Pids ++ Local});
         [] ->
             ets:insert(Scope, {Group, Pids, Pids})
@@ -534,7 +534,7 @@ join_local_update_ets(Scope, ScopeMon, MG, Group, Pids) ->
 
 join_remote_update_ets(Scope, ScopeMon, MG, Group, Pid) when is_pid(Pid) ->
     case ets:lookup(Scope, Group) of
-        [{Group, All, Local}] ->
+        [{_Group, All, Local}] ->
             ets:insert(Scope, {Group, [Pid | All], Local});
         [] ->
             ets:insert(Scope, {Group, [Pid], []})
@@ -542,7 +542,7 @@ join_remote_update_ets(Scope, ScopeMon, MG, Group, Pid) when is_pid(Pid) ->
     notify_group(ScopeMon, MG, join, Group, [Pid]);
 join_remote_update_ets(Scope, ScopeMon, MG, Group, Pids) ->
     case ets:lookup(Scope, Group) of
-        [{Group, All, Local}] ->
+        [{_Group, All, Local}] ->
             ets:insert(Scope, {Group, Pids ++ All, Local});
         [] ->
             ets:insert(Scope, {Group, Pids, []})
@@ -576,10 +576,10 @@ leave_local([Pid | Tail], Group, Local) ->
 
 leave_local_update_ets(Scope, ScopeMon, MG, Group, Pid) when is_pid(Pid) ->
     case ets:lookup(Scope, Group) of
-        [{Group, [Pid], [Pid]}] ->
+        [{_Group, [Pid], [Pid]}] ->
             ets:delete(Scope, Group),
             notify_group(ScopeMon, MG, leave, Group, [Pid]);
-        [{Group, All, Local}] ->
+        [{_Group, All, Local}] ->
             ets:insert(Scope, {Group, lists:delete(Pid, All), lists:delete(Pid, Local)}),
             notify_group(ScopeMon, MG, leave, Group, [Pid]);
         [] ->
@@ -588,7 +588,7 @@ leave_local_update_ets(Scope, ScopeMon, MG, Group, Pid) when is_pid(Pid) ->
     end;
 leave_local_update_ets(Scope, ScopeMon, MG, Group, Pids) ->
     case ets:lookup(Scope, Group) of
-        [{Group, All, Local}] ->
+        [{_Group, All, Local}] ->
             case All -- Pids of
                 [] ->
                     ets:delete(Scope, Group);
@@ -603,10 +603,10 @@ leave_local_update_ets(Scope, ScopeMon, MG, Group, Pids) ->
 leave_remote_update_ets(Scope, ScopeMon, MG, Pid, Groups) when is_pid(Pid) ->
     _ = [
         case ets:lookup(Scope, Group) of
-            [{Group, [Pid], []}] ->
+            [{_Group, [Pid], []}] ->
                 ets:delete(Scope, Group),
                 notify_group(ScopeMon, MG, leave, Group, [Pid]);
-            [{Group, All, Local}] ->
+            [{_Group, All, Local}] ->
                 ets:insert(Scope, {Group, lists:delete(Pid, All), Local}),
                 notify_group(ScopeMon, MG, leave, Group, [Pid]);
             [] ->
@@ -616,7 +616,7 @@ leave_remote_update_ets(Scope, ScopeMon, MG, Pid, Groups) when is_pid(Pid) ->
 leave_remote_update_ets(Scope, ScopeMon, MG, Pids, Groups) ->
     _ = [
         case ets:lookup(Scope, Group) of
-            [{Group, All, Local}] ->
+            [{_Group, All, Local}] ->
                 case All -- Pids of
                     [] when Local =:= [] ->
                         ets:delete(Scope, Group);
-- 
2.35.3