From 7173fb8984d1822cc9c081f094841fb755f800fc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Tue, 21 May 2019 14:48:11 +0200
Subject: Eliminate compiler crash when compiling complex receive statements

Certain complex receive statements would result in an internal
compiler failure. That would happen when the compiler would fail
to find the common exit block following a receive. See the added
test case for an example.

https://bugs.erlang.org/browse/ERL-950
---
 lib/compiler/src/beam_ssa_pre_codegen.erl | 11 +++++++----
 lib/compiler/test/receive_SUITE.erl       | 26 ++++++++++++++++++++++++--
 2 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/lib/compiler/src/beam_ssa_pre_codegen.erl b/lib/compiler/src/beam_ssa_pre_codegen.erl
index bf99e8fc26..9af72afca7 100644
--- a/lib/compiler/src/beam_ssa_pre_codegen.erl
+++ b/lib/compiler/src/beam_ssa_pre_codegen.erl
@@ -1415,12 +1415,15 @@ fix_receive([], _Defs, Blocks, Count) ->
 find_loop_exit([L1,L2|_Ls], Blocks) ->
     Path1 = beam_ssa:rpo([L1], Blocks),
     Path2 = beam_ssa:rpo([L2], Blocks),
-    find_loop_exit_1(reverse(Path1), reverse(Path2), none);
+    find_loop_exit_1(Path1, cerl_sets:from_list(Path2));
 find_loop_exit(_, _) -> none.
 
-find_loop_exit_1([H|T1], [H|T2], _) ->
-    find_loop_exit_1(T1, T2, H);
-find_loop_exit_1(_, _, Exit) -> Exit.
+find_loop_exit_1([H|T], OtherPath) ->
+    case cerl_sets:is_element(H, OtherPath) of
+        true -> H;
+        false ->  find_loop_exit_1(T, OtherPath)
+    end;
+find_loop_exit_1([], _) -> none.
 
 %% find_rm_blocks(StartLabel, Blocks) -> [Label].
 %%  Find all blocks that start with remove_message within the receive
diff --git a/lib/compiler/test/receive_SUITE.erl b/lib/compiler/test/receive_SUITE.erl
index 0038eb1a4b..752491f0f8 100644
--- a/lib/compiler/test/receive_SUITE.erl
+++ b/lib/compiler/test/receive_SUITE.erl
@@ -26,7 +26,7 @@
 	 init_per_testcase/2,end_per_testcase/2,
 	 export/1,recv/1,coverage/1,otp_7980/1,ref_opt/1,
 	 wait/1,recv_in_try/1,double_recv/1,receive_var_zero/1,
-         match_built_terms/1]).
+         match_built_terms/1,elusive_common_exit/1]).
 
 -include_lib("common_test/include/ct.hrl").
 
@@ -47,7 +47,7 @@ groups() ->
     [{p,test_lib:parallel(),
       [recv,coverage,otp_7980,ref_opt,export,wait,
        recv_in_try,double_recv,receive_var_zero,
-       match_built_terms]}].
+       match_built_terms,elusive_common_exit]}].
 
 
 init_per_suite(Config) ->
@@ -427,4 +427,26 @@ match_built_terms(Config) when is_list(Config) ->
     ?MATCH_BUILT_TERM(Ref, <<A, B>>),
     ?MATCH_BUILT_TERM(Ref, #{ 1 => A, 2 => B}).
 
+elusive_common_exit(_Config) ->
+    self() ! {1, a},
+    self() ! {2, b},
+    {[z], [{2,b},{1,a}]} = elusive_loop([x,y,z], 2, []),
+    ok.
+
+elusive_loop(List, 0, Results) ->
+    {List, Results};
+elusive_loop(List, ToReceive, Results) ->
+    {Result, RemList} =
+        receive
+            {_Pos, _R} = Res when List =/= [] ->
+                [_H|T] = List,
+                {Res, T};
+            {_Pos, _R} = Res when List =:= [] ->
+                {Res, []}
+        end,
+    %% beam_ssa_pre_codegen:fix_receives() would fail to find
+    %% the common exit block for this receive. That would mean
+    %% that it would not insert all necessary copy instructions.
+    elusive_loop(RemList, ToReceive-1, [Result | Results]).
+
 id(I) -> I.
-- 
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