beam_utils: Fix check_liveness/3 for receive loops

Sometimes the beam_bool pass wants to know whether an
y register will be killed by the code that follows and
will do (effectively):

  beam_utils:is_killed({y,Y}, Code, L)

When asked to calculate the liveness for an y register,
beam_utils:is_killed/3 will loop forever if the code
includes a receive loop.

Since this rarely occurs, fix the problem in the simplest
and most conservative way.

Reported-by: Christopher Williams

lib/compiler/src/beam_utils.erl

@@ -489,10 +489,13 @@ check_liveness(R, [{bs_context_to_binary,S}|Is], St) ->
 	S -> {used,St};
 	_ -> check_liveness(R, Is, St)
     end;
-check_liveness(R, [{loop_rec,{f,_},{x,0}}|Is], St) ->
+check_liveness(R, [{loop_rec,{f,_},{x,0}}|_], St) ->
     case R of
-	{x,_} -> {killed,St};
-	_ -> check_liveness(R, Is, St)
+	{x,_} ->
+	    {killed,St};
+	_ ->
+	    %% y register. Rarely happens. Be very conversative.
+	    {unknown,St}
     end;
 check_liveness(R, [{loop_rec_end,{f,Fail}}|_], St) ->
     check_liveness_at(R, Fail, St);

lib/compiler/test/compilation_SUITE.erl

@@ -46,7 +46,7 @@ all(suite) ->
      trycatch_4, opt_crash,
      otp_5404,otp_5436,otp_5481,otp_5553,otp_5632,
      otp_5714,otp_5872,otp_6121,otp_6121a,otp_6121b,
-     otp_7202,otp_7345,on_load,string_table,otp_8949_a
+     otp_7202,otp_7345,on_load,string_table,otp_8949_a,otp_8949_a
     ].
 
 -define(comp(N),
@@ -628,5 +628,23 @@ otp_8949_a() ->
 	    end
     end.
 
+otp_8949_b(Config) when is_list(Config) ->
+    self() ! something,
+    ?line value = otp_8949_b([], false),
+    ?line {'EXIT',_} = (catch otp_8949_b([], true)),
+    ok.
+
+%% Would cause an endless loop in beam_utils.
+otp_8949_b(A, B) ->
+    Var = id(value),
+    if
+	A == [], B == false ->
+	    ok
+    end,
+    receive
+        something ->
+	    id(Var)
+    end.
+
 
 id(I) -> I.