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

1 files changed, 6 insertions, 3 deletions

diff --git a/lib/compiler/src/beam_utils.erl b/lib/compiler/src/beam_utils.erl
index dc8079d0b7..45cdf8a659 100644
--- a/lib/compiler/src/beam_utils.erl
+++ b/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);